NVINOSHill^S S3iiJVHa!l LIBRARIES SMITHSONIAN INSTITUTION NOUHillSP z \ ^ ^ in ':z <n

0 H

O X^osv5^ Z O

2 2 SMITHSONIAN INSTITUTION NOIini81SN8 NVINOSHIIIAIS S3IHVaan LIBRARIE

z

a

c/> S m \ ^ in

NviNOSHiiiAis saiavHen libraries smithsonian institution Noiiniiisi

— w> z in z Sir in

t I ^^’“1 I i ^

^SMITHSONIAN INSTITUTION^NOliniliSNI NVIN0SHillAIS^S3 I U VH 8 n”LI B RAR I E rj ^ . os

O ” Xg>mst^ O w — X^ias!^ O

NVINOSHXIWS SHiaVHan'^LtBRARIES^SMITHSONIAN'^INSTITUTION^NOtiniliSI

; > s _ r- _ z r- _ _ Z _ _ _

> 2 w 5 y!fS55x O

SMlTHS0NIAN“lNSTITUTI0N‘^N01iniliSNl“NWN0SHilWs‘*’ S3 I aVH 3 H ”LI B R AR I E

^ i g %% s g ^ i

g f

i ^ I' 2

NVIN0SHil!AIS^S3 1 /vU 8 n\l B RAR I ES^SMITHSONIAN INSTITUTION NOliniliS?

«— CQ

O —

2 -J

SMITHSONIAN INSTITUTION NOIXIlillSNI NVINOSHXIWS SaiaVUSH LIBRARIE

z r- ,. z —

ro > '

^ ^ ^ — c/,

NVINOSHillAlS S3 I BVa 8 H^LI B R AR I ES^SMITHS0NIAN““|NST!TUTI0N NOlinillSi in z in z \y

D

> ‘•^r^ 2 ^ x^oxix^ 2 ^

^SMITHSONIAN INSTITUTION NOlinillSNI NVIN0SH1IINS^S3 I U VU 8 11^ LI B R AR I E

osHilws S3idviiian lsbrarses Smithsonian institution NUJiimjibNi^Nv </i _ zz (f) zz ^ _ z

c/)

1^1

cz

< °r

^ ' o 5 r o

_ 2 -J 2 -j ^

HsoNiAN INSTITUTION NoiiniiiSNi NviNosHims saiBvaaii libraries si^

r* _ z r- z ^ 2

^ ^ _ m

OSHimS S3 I ava a n”LI B RAR I ES^SMITHSONIAN^mSTITUTION^NOlinillSNI N\

W 2 ^ Z C/5 2

r....^l) 8 X • S fe§. il S S

2 ^ > ><giixvs>>^ 2 "^6. ^

HSONIAN INSTITUTION NOliniliSNI NVIN0SHill^S^S3 I BVB 0 H^LI B RAR I Es'^SIV

CO 3; CO

I f€:;i i

CO

03

Q -N^DX^ ^ O ~ O

0SHilWS^S3 I avy S n"'LI B RAR I ES^SMITHSONIAN"'lNSTITUTION^NOIiniliSNl"'N\)

2: r- 2 r^ z _ ^

O ^SsSrSSv - B . ^ ”

CO

rn x^(osv''j>' ^ ffn

CO E CO t: CO

HSONIAN INSTITUTION NOlifllliSNI NVINOSHillNS SaiBVBan LIBRARIES S^ z \ *2-5 ' ^

X 50

CO ? CO

> ’* 2 Xi^osvji^ > ■

0SHJ.iws‘^S3 1 aVaa n^Li B rar i es”smithsonian^institution NoixniiiSNi_N\

C^ _ 5 CO CO _ 2

HSONIAN INSTITUTION NOlifliliSNI NVINOSHilVVS S31HVaan LIBRARIES Sf

... 2 Z ^

^ .!• O m ^

3

4 03 >

^ N.;,

m \Sioc;>' g rn g _

losHilws S3 1 a va a n“Li b r ar i es‘”smithsonian“institution^ NoiiniiisNi n\

^ ^ •'- s i i ■§

i I /#%\ o »s ^

CO • CO IzSI^ e/i X CO

O

2

^2L£S^ > ‘W' 2 ^ > XlOxixsSX S 2

2 CO ^2 - ^ 2 ^ CO

HSONIAN INSTITUTION NOliniliSNI NVINOSHill^S SBIBVaail LIBRARIES S^

— e/i m

THE

TEXAS JOURNAL

OF

SCIENCE

GENERAL INFORMATION

MEMBERSHIP. — Any person or members of any group engaged in scientific work or interested in the promotion of science are eligible for membership in The Texas Academy of Science. Dues for members are $30.00 annually; associate (student) members, $15.00; family members, $35.00; affiliate members, $5.00; emeritus members, $10.00; life members, 20 times annual dues; patrons, $750.00 or more in one payment; corporate members, $250.00 annually; corporate life members, $2000.00 in one payment. Library subscription rate is $45.00 annually. Payments should be sent to Dr. Michael J. Carlo, P.O. Box 10986, Angelo State University, San Angelo, Texas 76909.

The Texas Journal of Science is a quarterly publication of The Texas Academy of Science and is sent to most members and all subscribers. Changes of address and inquiries regarding missing or back issues should be sent to Dr. Robert D. Owen, Department of Biological Sciences, Texas Tech University, Lubbock, Texas 79409-3131, (806) 742-3232.

AFFILIATED ORGANIZATIONS Texas Section, American Association of Physics Teachers Texas Section, Mathematical Association of America Texas Section, National Association of Geology Teachers American Association for the Advancement of Science Texas Society of Mammalogists

The Texas Journal of Science (ISSN 0040-4403) is published quarterly at Lubbock, Texas U.S. A. Second class postage paid at Post Office, Lubbock, Texas 79402. Postmaster: Send address changes, and returned copies to The Texas Journal of Science, Box 43151, Texas Tech University, Lubbock, Texas 79409-3151, U.S. A.

THE TEXAS ACADEMY OF SCIENCE MOURNS THE DEATH OF

Dr. J. Knox Jones, Jr.

Dr. J. Knox Jones, Jr., Horn Professor of Biological Sciences at Texas Tech University, died on 15 November 1992. Dr. Jones was recognized as the Academy’s Distinguished Texas Scientist in 1992 and he was Editor of the Texas Journal of Science from 1986 until his death. A full obituary will appear in a forthcoming issue of the journal.

I will not attempt to fill Knox’s shoes, but I hope to carefully follow the trail that he blazed.

Frank W. Judd Editor

THE TEXAS JOURNAL OF SCIENCE

Volume 45, No. 1

February 1993

CONTENTS

Systematic status of the deer mouse, Peromyscus maniculatus, on the Llano Estacado and

in adjacent areas. By Timothy W Cooper, Robert R. Hollander, Robert J. Kinucan, and J. Knox Jones, Jr. . . . . . . . . 3

A sensitive spectrophotometric technique for measuring phototaxis in

Chalamydomonas reinhardtii. By R. C. Moyer, Wm. F. Schroeder and J. Taboada . 19

Response of small mammals to conversion of a sand shinnery oak woodland into a mixed mid-grass prairie. By Michael R. Willig, Randall L. Colbert, Russell D. Pettit, and Richard D. Stevens . . . . . . . 29

Food habits of male bird-voiced treefrogs, Hyla avivoca (Anura: Elylidae), in Arkansas.

By David H. Jamieson, Stanley E. Trauth, and Chris T McAllister . . . . . .45

American alligator {Alligator mississippiensis) nesting at an inland Texas site.

By Louise A. Hayes- Odum, Debra Valdez, Marjorie Lowe, Loretta Weiss,

Patricia H. Reiff, and Dennis Jones . . . 51

Individual and secondary sexual variation in the Mexican ground squirrel,

Spermophilus mexicanus. By Franklin D. Yancy, H, J. Knox Jones, Jr.,

and Richard W. Manning . . . . . . . 63

Effect of feed quality on growth of the Gulf of Mexico white shrimp, Penaeus setiferus, in pond pens. By Lori Robertson, Addison L. Lawrence, and Frank L. Castillo . . 69

Effects of microhabitat on nest box selection and annual productivity of eastern bluebirds {Sialia sialis) in southeastern Georgia. By Melissa A. Scott,

Julie L. Lockwood, and Michael P Moulton . . . . . . 77

Mammals from the Beach Mountains of Culberson County, Trans-Pecos

Texas. By Frederick B. Stangl, Jr., Walter W. Dalquest, and Steve Kuhn . . . .87

General Notes

Myotis velifer in the Quitaque local fauna. Motley County, Texas.

By Nicholas J. Czaplewski . . . . . . . 97

Diet of some common insects in the South Llano River.

By Gerado R. Camilo and Michael R. Willig . . . . . 100

Records of five species of small mammals from western Texas. By J. Knox Jones, Jr., Richard W. Manning, Franklin D. Yancy, I I, and Clyde Jones . . . 104

The Red Brocket, Mazama americana (Artiodactyla: Cervidae), in El Salvador.

By James G. Owen and J. Knox Jones, Jr. . . . . . . . . 106

Variation in reproductive characteristics of Poa pratensis across a

successional chronosequence. By Mark A. McGinley . . . . . 107

Instructions to authors ........................ . . . . . . . 109

THE TEXAS JOURNAL OF SCIENCE EDITORIAL STAFF

Editor:

Frank W. Judd, The University of Texas —

Pan American Assistant to the Editor:

Beverley T Gonzales, The University of Texas — Pan American

Associate Editor for Botany:

Chester M. Rowell, Marfa, Texas Associate Editor for Chemistry:

Marvin W. Rowe, Texas A&M University Associate Editor for Mathematics and Statistics:

Patrick L. Odell, Baylor University Associate Editor for Physics:

Charles W. Myles, Texas Tech University

Scholarly papers in any field of science, technology, or science education will be considered for publication in The Texas Journal of Science. Instructions to authors are published one or more times each year in the Journal on a space-available basis, and also are available from the Editor (The University of Texas — Pan American, Coastal Studies Laboratory, Box 2591, South Padre Island, Texas 78597, (210) 761-2644).

The Texas Journal of Science is published quarterly in February, May, August, and November for $30 per year (regular membership) by The Texas Academy of Science. Second-class postage rates (ISSN 0040-4403) paid at Lubbock, Texas. Postmaster: Send address changes, and returned copies to The Texas Journal of Science, Box 43151, Texas Tech University, Lubbock, Texas 79409-3151, U.S.A.

SYSTEMATIC STATUS OF THE DEER MOUSE, PEROMYSCUS MANICULATUS, ON THE LLANO ESTACADO AND IN ADJACENT AREAS

Timothy W. Cooper, Robert R. Hollander,

Robert J. Kinucan, and J. Knox Jones, Jr.

Division of Range Animal Science, Sul Ross State University, Alpine, Texas 79830 (TWC, RJK), Department of Biological Sciences, Central Connecticut State University, New Britain, Connecticut 06050 (RRH), and The Museum and Department of Biological Sciences, Texas Tech University, Lubbock, Texas 79409 (JKJ)

Abstract. — Geographic and nongeographic variation of deer mice, Peromyscus maniculatus, were statistically analyzed to determine the systematic status of populations of this species on the Llano Estacado and in adjacent areas of Texas. Four external and 12 cranial measurements were analyzed. Museum specimens were classified as juveniles, subadults, young adults, adults, or old adults based on tooth wear and pelage characteristics. Only individuals categorized as adults of one age or another were used in statistical analyses. A MANOVA was used to test for sexual dimorphism, which resulted in a nonsignificant {P = 0.083) value, allowing the sexes to be combined. ANOVAs and discriminant function analysis also were used to define taxonomic affinities. From this analysis, it was concluded that P m. pallescens extends from south-central Texas northward and westward, intergrading with P. m. luteus in the Lubbock area and perhaps elsewhere in the southeastern part of the Llano. Mice clearly referable to Peromyscus maniculatus luteus as currently recognized appear to occupy all other areas of the Llano Estacado. Key words: deer mouse; Peromyscus maniculatus\ distribution; systematics; Llano Estacado, Texas.

The deer mouse, Peromyscus maniculatus, with more than 60 recognized subspecies (Hall, 1981), occurs from British Columbia east to Labrador and southward through most of the United States into Mexico as far as Oaxaca. It occupies a wide variety of habitats across this broad range, with some subspecies being more habitat specified than others.

The Llano Estacado is the southernmost extension of the Great Plains. It is an immense plateau lying south of the Canadian River in eastern New Mexico and western Texas (Lotspeich and Coover, 1962). Caprock cliffs delimit its northern and eastern margins and, to a lesser extent, the western edge (Mescalero Ridge). To the south, the Llano merges without sharp contrast with the Monahans Sandhills and adjacent areas. Lacking significant topographic features, the Llano has a relatively uniform climate (Judd, 1970).

The taxonomic status of Peromyscus maniculatus on the Llano Estacado has long been the subject of speculation (Blair, 1954b; Judd, 1970); according to Hall (1981), five subspecies occupy the Llano and areas immediately adjacent to it. Several species of Peromyscus often live together in a relatively small geographic area (Thompson and Conley, 1983), and identification of specimens sometimes poses problems (Comely et al., 1981); differentiation at the subspecific level also can be

4

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

problematic. The paucity of investigations addressing intergradation between subspecies of P. maniculatus that utilize similar habitats contributes to the complexity of the problem (Caire and Zimmerman, 1975). A circuitous pattern of gene exchange may take place between adjacent populations, with little or no direct interbreeding (Blair, 1953), thus adding to the problems surrounding the relationships among, and identity of, subspecies. Blair (1954a) found no evidence of reproductive isolation among P. m. pallescens, P m. blandus, and P m. rufinus. Although more is known about population dynamics and geographic variation in Peromyscus maniculatus than any other species of small mammal (Blair, 1953), its taxonomic status is still uncertain over parts of its distribution, including the Llano Estacado, The only apparent distinct geographic feature that might provide a barrier to gene flow between or among subspecies in the vicinity of the Llano is the edge of the caprock, which is evident to a greater or lesser degree along all but the southern part of the region.

The deer mouse is generally distributed over the Llano Estacado, but rarely is locally common. It is much less abundant overall, for example, than its relative, Peromyscus leucopus. Most often, P maniculatus has been taken in mesquite grassland, in grassy areas having a sandy substrate, such as on the Muleshoe Sandhills, and along overgrown grassy-woody fence rows.

Judd (1970) examined 145 specimens from the Llano and assigned them to the subspecies P. m. luteus. The relative scarcity of P. maniculatus on the Llano Estacado (Blair, 1954b; Judd, 1970; Jones et al., 1988) has made it difficult to assemble representative groups of specimens to determine their taxonomic status. New collections have been acquired since Judd’s work, however, and multivariate statistical techniques have come into general use. The goal of research resulting in this paper was to better define the systematic affinities of Peromyscus maniculatus on the Llano and in adjacent areas through use of multivariate morphometeric analysis.

Historical Taxonomy

Osgood (1905) described Peromyscus luteus, with type locality at Kennedy, Cherry County, Nebraska. Later, he (1909) relegated luteus to subspecific status under P. maniculatus, and assigned 10 specimens from Washburn, Armstrong Co., Texas, to P m. nebrascensis [= luteus^ (see Jones, 1958, for the proper use of the subspecific name nebrascensis). Hall (1981) mapped five subspecies converging in the geographic region on or near the Llano Estacado, but the exact borders of the ranges of these races was unclear. They are listed below, based on Hall’s distributional scheme.

Peromyscus maniculatus blandus Osgood, 1904, was mapped as

PEROMYSCUS MANICULATUS ON THE LLANO ESTACADO

5

occurring from southern Coahuila and Chihuahua northward to a latitude of approximately 34° north. In the United States, R m. blandus is found in the lower Sonoran life-zone of western Texas, north to southern New Mexico (Osgood, 1909). This race evidently occurs throughout the southern Pecos River drainage, an area directly adjacent to the southwestern and western edges of the Llano Estacado.

Peromyscus maniculatus luteus Osgood, 1905, was shown to range from South Dakota southward to the southern edge of the Llano Estacado. Osgood (1909) associated luteus primarily with the sandhill region of western Nebraska and adjacent areas.

Peromyscus maniculatus nebrascensis (Coues, 1877) was reported as extending from southeastern Alberta and southern Saskatchewan, south through the western parts of the Dakotas and Nebraska to the Oklahoma Panhandle, and westward to include the southeastern part of Montana and the eastern parts of Wyoming and Colorado. Hall (1981) even mapped an extension of the distribution of this race into the Texas Panhandle, although no specimens were referenced from that area. Osgood (1909) associated the subspecies now known as nebrascensis with the plains and foothill regions of the Rocky Mountains.

Peromyscus maniculatus pallescens J. A. Allen, 1896, occurs throughout central Texas and approaches the Llano from the southeast. It also has been recorded from Hardeman County, Texas, by Hall (1981), but this was an extrapolation of a county record referenced only to species by Davis (1974). Jones et al. (1987), however, listed P. m. pallescens as a subspecies that might be present in northwestern Texas.

Peromyscus maniculatus rufinus (Merriam, 1890) is distributed from western Colorado southward through the southeastern corner of Utah into most of eastern Arizona and western New Mexico. Hall (1981) illustrated the eastern extent of the range of this subspecies as entering the extreme northwestern edge of the Texas Panhandle, although no specimen records were cited from there. P m. rufinus inhabits areas of transition and boreal vegetation in the southern Rocky Mountains (Osgood, 1909; Wilson, 1968). Wilson found it to be the dominant small mammal of mixed conifer forests above 8500 feet. Dalquest et al. (1990) reported that P m. rufinus and P. m. luteus intergrade in Union County, New Mexico. Judd (1970) opined that habitat limitations prevent P. m. rufinus from entering the Llano Estacado. We concur that P m. rufinus is not found on the Llano, and it is included here for comparative purposes only. As an aside having no relationship to the present study, Hoffmeister (1986) regarded rufinus as a synonym of P m. sonoriensis.

Methods and Materials

Four external measurements (total length, length of tail vertebrae, length of hind foot, length of ear) were obtained from original specimen labels. Weights of pregnant females

6

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

were removed from the data set. No weights were recorded for specimens of P. m. pallescens from Hill County, and that subspecies thus could not be compared with the other groups for that parameter. Inconsistency in recording weight and problems associated with missing values prompted removal of that measure from the analysis. Furthermore, because missing values did not allow us to maintain reasonable sample sizes, external measurements were not used in tests involving multivariate techniques.

Judd (1970) noted considerable variation in pelage color within samples taken from the Llano Estacado, and remarked that many deer mice from that region appeared to be intermediate in color between P. m. pallescens and P. m. nebrascensis, but that some resembled P m. luteus. Although qualitative observations of pelage were made, quantitative pelage characteristics were not assessed in this study.

Twelve cranial measurements were taken (by Cooper) with Fowler digital calipers to the nearest 0.01 mm. These were selected based on other studies of Peromyscus (Osgood, 1909; Fox, 1948; Cockrum, 1954; Judd, 1970; Schmidly, 1973; Comely et al., 1981; Koh and Peterson, 1983). Description of cranial measurements follows:

Greatest length of skull. — Length from anterior margin of nasal bone to posterior projecting margin of occiput.

Breath of hraincase. — Greatest width of braincase measured perpendicular to long axis of skull between posterior margins of zygomatic arches.

Zygomatic breadth. — Greatest distance perpendicular to long axis of skull across the zygomatic arches.

Interorbital constriction. — Least width across frontal bones (that is, least constriction in interorbital region).

Length of rostrum. — Distance from anteriormost projection of nasal bone to lateral junction of lacrimal and maxilla.

Occipital depth. — Distance from ventral plane of auditory bullae to dorsalmost projection of parietals.

Breadth of rostrum. — Greatest distance across rostrum anterior to zygomatic arches.

Condylobasal length. — Greatest distance from posteriormost projection of occipital condyle to anteriormost projection of premaxilla.

Breadth of upper molars. — Greatest distance across outer buccal margins of upper molars.

Length of maxillary toothrow. — Greatest length of alveolar space of the upper molar toothrow.

Length of diastama. — Shortest distance between posterior alveolus of incisor and alveolar space at anterior margin of upper molar row.

Length of palate. — Shortest distance from anteriormost point at posterior border of palate to posterior lip of alveolus of incisor.

All statistical tests were made using SPSS/ PC + statistical package (SPSS, Inc., 1986) for IBM compatible personal computers. Specified tests are discussed in greater detail in the following sections on nongeographic and geographic variation.

Nongeographic Variation

Subspecies of Peromyscus maniculatus have been extensively studied with regard to growth rates and development (see, for example, Svihla, 1934, 1935; Dice, 1936, 1937; Dice and Bradley, 1942). In order to minimize the effects of variation in external and cranial dimensions associated with normal development, we initially excluded juveniles and subadults from our evaluations. Dice (1936) found that mice in laboratory populations of P. m. gracilis had demonstrable growth in both

PEROMYSCUS MANICULATUS ON THE LLANO ESTACADO

7

external and cranial dimensions after the second year of life, although individuals rarely live that long in the wild. Fox (1948), however, found no significant age variation in adult specimens that possessed at least slight wear on the last molar. We assigned specimens of R maniculatus to one of five age classes based on criteria developed by Koh and Peterson (1983).

Juvenile. — Specimens with grayish pelage, M3 not reaching the height of Ml and M2.

Subadult. — Specimens with drab brownish subadult pelage or in the process of molting to or from that pelage, with little or no tooth wear evident.

Young adult. — Specimens with new adult pelage, with some wear on cusps of upper molars.

Adult. — Specimens in adult pelage, with noticeable wear on cusps of upper molars.

Old adult. — Specimens in adult pelage, with substantial wear on upper molars (cusps obliterated).

To assess the amount of morphological variation that was attributable to sexual variation, a sample from Lubbock County was analyzed for secondary sexual dimorphism. Comprised of 32 specimens (17 males and 15 females, all young adults and adults), the Lubbock sample represents the largest available from a single locality. A one-way multivariate analysis of variance was performed, using the program MANOVA from SPSS (SPSS, Inc., 1986) with sex as the main factor. The effect of sex was nonsignificant (P — 0.083), and the sexes thus were combined for subsequent analyses.

A one-way multivariate analysis of variance was performed on the same Lubbock sample, using the program MANOVA with age as the main factor (entire sample made up of young adults and adults). Nonsignificant {P — 0.330) results were obtained for the effect of age, and animals from these two age groups (and old adults) thus were combined in all subsequent analyses.

Geographic Variation

In order to obtain acceptable sample sizes for statistical treatment, mice from some localities were pooled. In forming a priori groups, however, caution was taken not to cross biogeographic barriers in pooling specimens (Thorpe, 1976; Hollander, 1990). In one case, however, the grouped sample representing P. m. blandus crossed such a barrier in that the Pecos River divided localities from which specimens were selected from Pecos and Winkler counties. Individuals we examined from possible areas of intergradation between subspecies were not grouped, but were treated as “unknowns.” The small available sample of P. m. nebrascensis precluded it from the statistical analysis. A listing of the resulting eight a priori groups follows (see Table 1 for a summary of statistics): group 1 — Lubbock County, Texas; group 2 — Sherman and Moore counties, Texas; group 3 — Lamb and Bailey counties, Texas; group 4 — Randall and Castro counties, Texas; group 5 — Cherry and

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Table 1. Summary statistics including sample size (N), mean, standarrd deviation (SD), standard error (SE), and minimum and maximum values.

Group

Mean

SE

SD

Minimum

Maximum

N

Breadth of braincase

GLOC 1

10.84

.06

.35

9.72

11.56

32

GLOC 2

11.13

.04

.20

10.75

11.54

30

GLOC 2

11.12

.05

.23

10.67

11.58

17

GLOC 4

11.15

.09

.32

10.52

11.70

13

GLOC 5

11.35

.06

.26

10.78

11.83

22

GLOC 6

11.49

.05

.26

10.87

11.91

28

GLOC 7

11.50

.07

.33

10.97

12.20

23

GLOC 8

11.05

.08

.44

10.00

11.51

28

Breadth across upper molars

GLOC 1

4.58

.03

.16

4.24

4.94

32

GLOC 2

4.74

.03

.15

4.41

5.09

30

GLOC 3

4.76

.04

.17

4.47

5.09

17

GLOC 4

4.75

.04

.16

4.50

4.98

13

GLOC 5

4.69

.04

.21

4.01

5.13

23

GLOC 6

4.81

.03

.16

4.41

5.03

28

GLOC 7

4.85

.03

.16

4.62

5.24

25

GLOC 8

4.58

.03

.16

4.25

4.89

29

Condylobasal length

GLOC 1

21.72

.14

.80

20.04

23.19

32

GLOC 2

22.59

.11

.62

21.12

23.75

30

GLOC 3

22.51

.16

.67

20.96

23.59

17

GLOC 4

22.81

.13

.48

22.08

23.79

13

GLOC 5

22.22

.16

.76

20.49

24.08

22

GLOC 6

23.1 1

.12

.64

21.68

24.97

28

GLOC 7

23.38

.18

.89

21.47

25.15

25

GLOC 8

21.42

.16

.85

19.26

22.62

27

Length of diastama

GLOC 1

6.27

.06

.36

5.52

7.00

32

GLOC 2

6.51

.11

.60

3.57

7.06

30

GLOC 3

6.52

.09

.38

5.83

7.24

17

GLOC 4

6.66

.08

.28

6.34

7.36

13

GLOC 5

6.52

.08

.40

5.51

7.32

23

GLOC 6

6.88

.06

.32

6.37

7.52

28

GLOC 7

6.67

.08

.41

6.00

7.68

25

GLOC 8

5.93

.06

.31

4.94

6.34

29

Occipital depth

GLOC 1

8.77

.07

.38

8.02

9.59

31

GLOC 2

9.06

.07

.38

8.29

9.82

30

GLOC 3

9.03

.07

.28

8.61

9.51

17

GLOC 4

9.17

.09

.33

8.61

9.84

13

GLOC 5

9.22

.07

.32

8.44

9.68

22

GLOC 6

9.34

.05

.29

8.70

9.79

28

PEROMYSCVS MANICULATUS ON THE LLANO ESTACADO

9

Table 1. Continued

GLOC 7

9.36

.06

.31

8.77

10.04

24

GLOC 8

8.73

.09

.46

I.IA

9.50

28

Greatest length of skull

GLOC 1

23.73

.14

.77

21.48

24.89

31

GLOC 2

24.69

.10

.56

23.83

25.86

29

GLOC 3

24.47

.13

.52

23.40

25.44

17

GLOC 4

24.86

.15

.53

24.10

25.63

13

GLOC 5

24.57

.17

.80

22.84

26.22

21

GLOC 6

25.53

.16

.79

24.18

27.87

25

GLOC 7

25.72

.22

1.06

23.58

27.82

24

GLOC 8

23.67

.18

.93

21.39

25.03

28

Length of rostrum

GLOC 1

8.86

.09

.46

8.05

10.10

28

GLOC 2

9.29

.06

.32

8.79

9.90

29

GLOC 3

9.16

.08

.35

8.48

9.69

17

GLOC 4

9.40

.07

.25

8.89

9.90

13

GLOC 5

9.24

.12

.55

8.27

10.59

22

GLOC 6

9.92

.07

.37

9.23

10.72

25

GLOC 7

9.82

.11

.54

8.83

10.61

25

GLOC 8

8.83

.09

.50

7.79

9.66

28

Length of maxillary too throw

GLOC 1

3.61

.03

.16

3.26

3.98

32

GLOC 2

3.75

.03

.14

3.47

4.04

30

GLOC 3

3.74

.04

.18

3.40

4.0

17

GLOC 4

3.76

.05

.20

3.39

4.0

13

GLOC 5

3.69

.04

.17

3.18

4.04

23

GLOC 6

3.91

.03

.15

3.60

4.28

28

GLOC 7

3.89

.03

.17

3.69

4.44

25

GLOC 8

3.60

.03

.17

3.23

3.96

29

Length of palate

GLOC 1

9.84

.07

.41

9.14

10.63

32

GLOC 2

10.05

.06

.30

9.54

10.64

30

GLOC 3

9.97

.09

.37

9.07

10.37

17

GLOC 4

10.37

.09

.33

9.87

10.85

13

GLOC 5

10.01

.11

53

8.75

11.31

23

GLOC 6

10.51

.07

.36

9.78

11.39

28

GLOC 7

10.36

.08

.40

9.63

11.29

25

GLOC 8

9.50

.08

.44

8.44

10.39

29

Interorbital constriction

GLOC 1

3.76

.03

.16

3.45

4.13

32

GLOC 2

3.93

.03

.14

3.67

4.26

30

GLOC 3

3.86

.02

.10

3.73

4.05

17

GLOC 4

3.91

.03

.11

3.67

4.08

13

GLOC 5

3.94

.04

.17

3.70

4.35

23

10

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Table 1. Continued

GLOC 6

3.94

.05

.27

2.95

4.30

28

GLOC 7

4.01

.03

.14

3.64

4.27

25

GLOC 8

3.88

.04

.23

3.28

4.20

28

Breadth of rostrum

GLOC 1

4.34

.05

.27

3.58

4.81

32

GLOC 2

4.49

.03

.18

4.18

5.01

30

GLOC 3

4.58

.05

.21

4.12

4.80

17

GLOC 4

4.58

.06

.22

4.29

4.96

13

GLOC 5

4.54

.05

.23

4.12

5.12

22

GLOC 6

4.46

.03

.17

4.10

4.85

27

GLOC 7

4.59

.06

.30

3.87

5.11

25

GLOC 8

4.40

.04

.22

3.76

4.90

29

Zygomatic breadth

GLOC 1

12.34

.10

.53

10.95

13.19

32

GLOC 2

12.84

.07

.37

12.13

13.91

30

GLOC 3

13.01

.07

.29

12.57

13.51

16

GLOC 4

12.99

.07

.25

12.50

13.33

11

GLOC 5

13.03

.09

.44

12.28

13.75

22

GLOC 6

13.00

.08

.41

12.15

13.84

28

GLOC 7

13.40

.09

.42

12.47

14.15

24

GLOC 8

12.43

.10

.47

11.05

13.22

20

Thomas counties, Nebraska; group 6 — Lincoln and Otero counties, New Mexico; group 7 — Winkler and Pecos counties, Texas; group 8 — Hill County, Texas.

Groups 1, 3, and 4 are from the Llano Estacado. Group 2 represents a sample from north of the Canadian River in an area were mice possibly could be assignable to P. m. nebrascensis according to Hall (1981). Group 5 represents topotypic material of P. m. luteus from the sandhill region of Nebraska. All specimens of group 6 are from mountainous areas of New Mexico and represent P m. rufinus. Animals in group 7 represent P. m. hlandus from just beyond the southern edge of the Llano. Peromyscus maniculatus pallescens was represented by group 8 — specimens from Hill County, Texas.

The presence of significant geographic variation among groups was tested using multivariate analysis of variance (program MANOVA, SPSS, Inc., 1986). Significant results were obtained {P <0.001) indicating morphometeric differentiation among at least some groups. The 12 cranial and four external measurements then were tested using one-way ANOVAs with the a priori groups as the main effect. If significant differences were detected, the character then was subjected to a Student- Neuman-Keuls multiple range test. All characters displayed significant results {P < 0.05), indicating some morphometeric differentiation between

PEROMYSCUS MANICULATUS ON THE LLANO ESTACADO

11

or among some a priori groups for every character. All characters, except length of hind foot {P = 0.0178) were significant at P < 0.001. The Student-Neuman-Keuls test was selected to delineate nonoverlapping subsets among the a priori groups. Student-Neuman-Keuls procedure is computationally identical to Welsch's set-up procedure, except that a different table of critical values is associated with each test (Sokal and Rohlf, 1981). The summarized results of the Student-Neuman-Keuls multiple range test appear in Table 2. Grouped locality (Gloc) 8 and Gloc 1 were placed in the same subset for 10 of the 16 parameters. Glocc 1 and 5 formed subsets for only three of 16 characters. Gloc 6 and Gloc 7 were grouped in subsets for 14 of 16 characters.

A discriminant function analysis (DFA — program Discriminant, SPSS, Inc., 1986) then was employed to test how well individuals within grouped localities could be distinguished. Because any missing value would result in removal of the entire specimen from analysis, only cranial characters were included in the DFA (Table 3). The resulting 60.37 percent of grouped cases correctly classified indicated that the a priori groups did not particularly well reflect potential biological groupings in the region. If all of the Llano Estacado specimens are grouped with P. m. luteus and analyzed by DFA, the percentage of correctly classified specimens increases to a respectable 77.44 percent. However, in this grouping strategy there are only four samples, which somewhat restricts the resolution capability of the test.

Discussion

Results of the multiple range tests lead us to conclude that most P. maniculatus on the Llano Estacado clearly should be assigned to P m. luteus. Grouped localities 2, 3, and 4 all formed subsets with Gloc 5, for

15 of the 16 characters tested. The strong affinity with the luteus material reaffirms the existing taxonomic status for P. maniculatus from the northern and central Llano Estacado.

However, mice from the immediate vicinity of Lubbock are intergrades between P. m. pallescens, based on cranial measurements and qualitative pelage characteristics, and some mice with relatively dark pelage occur even farther westward, in Hale and Lamb Counties. Specimens from the Lubbock County sample formed subsets with Gloc 5 for only three of the

16 characters tested, but this group formed subsets with Gloc 8 for 12 of the 16 characters. Seven of the 16 characters formed nonoverlapping subsets, indicating a significant difference between the subsets at {P < 0.05). The Lubbock mice also have pelage characteristics that resemble pallescens more than luteus, exhibiting a distinctive dorsal stripe similar to pallescens. Few Lubbock specimens exhibited the pale buffy pelage of topotypic luteus material, but many specimens assigned to luteus from the northern Llano Estacado generally had a darker brown coat than

12

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Table 2. Results of the one-way ANOVAs on 12 cranial and four external characters with the grouped localities (GLOC) as the main effect. Also the results of Student-Newman- Keuls multiple range test (MRT). Asterisks in a column indicate nonsignificant subsets.

Group

MRT

Mean

Range

N

P

Breadth

! of braincase

GLOC 1

*

10.84

9.72-11.56

32

<0.001

GLOC 8

*

11.05

10.00-11.51

28

GLOC 3

*

11.12

10.67-11.58

17

GLOC 2

*

11.13

10.75-11.54

30

GLOC 4

*

*

11.15

10.52-11.70

13

GLOC 5

* *

11.35

10.78-11.83

22

GLOC 6

♦

11.49

10.87-11.91

28

GLOC 7

*

11.50

10.97-12.20

23

Breadth

across upper molars

GLOC 8

♦

4.58

4.25-4.89

29

<0.001

GLOC 1

*

4.58

4.24-4.94

32

GLOC 5

♦

4.69

4.01-5.13

23

GLOC 2

♦

♦

4.74

4.41-5.09

30

GLOC 4

♦

*

4.75

4.50-4.98

13

GLOC 3

*

♦

4.76

4.47-5.09

17

GLOC 6

*

♦

4.81

4.41-5.03

28

GLOC 7

♦

4.85

4.62-5.24

25

Condylobasal length

GLOC 8

♦

21.42

19.26-22.62

27

<0.001

GLOC 1

*

21.72

20.04-23.19

32

GLOC 5

*

22.22

20.49-24.08

22

GLOC 3

*

22.51

20.96-23.59

17

GLOC 2

*

22.59

21.12-23.75

30

GLOC 4

*

*

22.81

22.08-23.79

13

GLOC 6

*

23.11

21.68-24.97

28

GLOC 7

*

23.38

21.47-25.15

25

Length

of diastama

GLOC 8

*

5.93

4.94-6.34

29

<0.001

GLOC 1

*

621

5.52-7.00

32

GLOC 2

*

6.51

3.57-7.06

30

GLOC 5

*

6.52

5.51-7.32

23

GLOC 3

♦

6.52

5.83-7.24

17

GLOC 4

♦

6.66

6.34-7.36

13

GLOC 6

*

6.88

6.37-7.52

28

GLOC 7

*

6.67

6.00-7.68

25

Occipital depth

GLOC 8

♦

8.73

7.74-9.50

28

<0.001

GLOC 1

♦

8.77

8.02-9.59

31

GLOC 3

*

' 9.03

8.61-9.51

17

GLOC 2

*

9.06

8.29-9.82

30

GLOC 4

*

*

9.17

8.61-9.84

13

GLOC 5

*

♦

9.22

8.44-9.68

22

PEROMYSCUS MANICULATUS ON THE LLANO ESTACADO

13

Table 2. Continued

GLOC 6

*

9.34

8.70-9.79

28

GLOC 7

*

9.36

8.77-0.04

24

Greatest length of skull

GLOC 8

*

23.16

21.39-25.03

28

GLOC 1

*

23.73

21.48-24.89

31

GLOC 3

*

24.47

23.40-25.44

17

GLOC 5

*

24.57

22.84-26.22

21

GLOC 2

*

24.69

23.83-25.86

29

GLOC 4

*

24.86

24.10-25.63

13

GLOC 6

*

25.53

24.18-27.87

25

GLOC 7

*

25.72

23.58-27.82

24

length of rostrum

GLOC 8

*

8.83

7.79-9.66

28

GLOC 1

*

8.86

8.05-10.10

28

GLOC 3

*

9.16

8.48-9.69

17

GLOC 5

*

9.24

8.27-10.59

22

GLOC 2

*

9.29

8.79-9.90

29

GLOC 4

*

9.40

8.89-9.90

13

GLOC 7

♦

9.82

8.53-10.61

25

GLOC 6

*

9.92

9.23-10.72

25

Length of maxillary too throw

GLOC 8

*

3.60

3.23-3.96

29

GLOC 1

♦

3.61

3.26-3.98

32

GLOC 5

♦ *

3.69

3.18-4.04

23

GLOC 3

*

3.14

3.40-4.10

17

GLOC 2

*

3.75

3.47-4.04

30

GLOC 4

♦

3.89

3.69-4.10

13

GLOC 7

*

3.89

3.69-4.44

25

GLOC 6

*

3.91

3.60-4.28

28

Length of palate

GLOC 8

*

9.50

8.44-10.39

29

GLOC 1

*

9.84

9.14-10.63

32

GLOC 3

*

9.97

9.07-10.37

17

GLOC 5

♦

10.01

8.75-11.31

23

GLOC 2

♦

10.05

9.54-10.64

30

GLOC 7

*

10.36

9.63-11.29

25

GLOC 4

*

10.37

9.87-10.85

13

GLOC 6

*

10.51

9.78-11.39

28

Interorbital constriction

GLOC 1

*

3.76

3.45-4.13

32

GLOC 3

* *

3.86

3.28-4.20

28

GLOC 8

*

3.88

3.28-4.20

28

GLOC 4

*

3.91

3.67-4.08

13

GLOC 2

*

3.93

3.67-4.26

30

GLOC 6

*

3.94

2.95-4.30

28

<0.001

<0.001

<0.001

<0.001

<0.001

14

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Table 2. Continued

GLOC5

♦

3.94

3.70-4.35

23

GLOC 7

*

4.01

3.64-4.27

25

GLOC 1

*

Breadth of rostrum

4.34 3.58-4.81

32

<0.001

GLOC 8

♦

*

4.40

3.76-4.90

29

GLOC 6

*

*

4.46

4.10-4.85

27

GLOC 2

*

4.49

4.18-5.01

30

GLOC 5

*

4.54

4.12-5.12

22

GLOC 4

*

4.58

4.29-4.96

13

GLOC 3

*

4.58

4.12-4.80

17

GLOC 7

*

4.59

3.87-5.11

25

GLOC 1

*

Zygomatic breadth

12.34 10.95-13.19

27

<0.001

GLOC 8

*

12.43

11.05-13.22

20

GLOC 2

*

12.84

12.13-13.91

30

GLOC 4

*

12.99

12.50-13.33

11

GLOC 6

*

13.00

12.15-13.84

28

GLOC 3

*

13.01

12.57-13.51

16

GLOC 5

*

13.03

12.28-13.75

22

GLOC 7

*

13.40

12.47-14.12

24

GLOC 8

*

Total length

132.80 112-147

25

<0.001

GLOC 1

*

137.69

125-160

29

GLOC 5

*

147.26

133-171

23

GLOC 3

*

147.29

135-158

14

GLOC 2

*

150.41

137-179

27

GLOC 4

*

150.50

141-164

12

GLOC 6

♦ *

154.71

127-189

28

GLOC 7

*

160.17

144-180

23

GLOC 1

*

Length

53.83

of tail vertebrae 41-68

29

<0.001

GLOC 8

*

*

57.04

43-67

25

GLOC 2

*

*

57.41

50-64

27

GLOC 4

*

60.55

57-64

11

GLOC 3

*

60.93

54-68

14

GLOC 5

*

61.70

34-71

23

GLOC 6

*

67.39

52-85

28

GLOC 7

*

68.08

58-82

24

GLOC 1

*

Length of hind foot

17.96 15-20

28

<0.020

GLOC 8

*

18.68

14-21

20

GLOC 2

*

19.04

18-20

27

GLOC 4

♦

*

19.42

19-22

12

GLOC 3

*

*

19.50

18-21

14

GLOC 7

*

*

20.08

18-24

24

PEROMYSCUS M A NICULATUS ON THE LLANO ESTACADO

15

Table 2. Continued

GLOC 5

*

*

20.22

18-22

23

GLOC 6

♦

21.89

18-70

28

Length of ear

GLOC 8

*

14.39

11-18

20

<0.001

GLOC 1

♦

14.50

12-17

28

GLOC 3

*

*

15.14

13-17

14

GLOC 2

*

*

15.22

11-18

27

GLOC 5

*

15.70

14-19

23

GLOC 7

*

15.71

13-18

24

GLOC 4

♦

15.75

13-17

12

GLOC 6

*

17.57

14-21

28

found in those from Nebraska. Based on the resuits of the multiple range tests and qualitative pelage characteristics, we conclude that mice in the Lubbock area are intergrades, which we tentatively assign to luteus at the present time.

Results of DFA indicate that the influence P. m. pallescens diminishes in Hale County, from which four individuals fell out with Gloc 3 and only two with Gloc 8. However, the area of intergradation also may include areas along the eastern edge of the Llano, possibly immediately to the north of Lubbock and certainly to the south, once adequate material is at hand for study. Additional specimens also are needed from the extreme southern and southwestern parts of the Llano to confirm certainly that mice from there should be assigned to luteus. [It should be noted here that even though more than 30 specimens were examined from Andrews and Ector counties, most had not yet been associated with cleaned skulls at the time of our study.]

Using discriminant function analysis, ungrouped specimens were aligned with those groups that they most closely resembled in terms of measurements. Of eight specimens from Garza County, Texas, for

Tabi.f 3. Results of discriminant function classification.

Actual

groups

Predicted

group

membership

1

2

3

4

5

6

7

8

1

14

5

1

2

0

1

0

1

2

1

15

2

2

4

2

1

2

3

1

5

7

0

1

0

1

1

4

1

4

0

5

0

0

1

0

5

1

1

1

2

11

1

2

0

6

0

1

1

0

0

21

1

0

7

1

1

0

1

1

3

15

0

8

4

1

0

0

2

0

1

11

16

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

example, four were distributed through DFA with the Lubbock sample, but two were placed with Gloc 2, and one each with Gloc 3 and 4, all representing areas occupied by luteus. Therefore, Garza County, like Lubbock County immediately to the northwest, appears to represent an area of intergradation between luteus and pallescens, but specimens are here assigned to the former. Of seven unknowns from Hale County, four were placed with Gloc 3, two with Gloc 8, and one with Gloc 7. The individual placed with Gloc 7 is an unusually large specimen, but the others indicate a reduced influence of pallescens in Hale County. Of two specimens examined from Collingsworth County, Texas, one aligned with Gloc 5 and the other with Gloc 8. Due to the small sample size no taxonomic judgement concerning relationships should be made, and more material clearly needs to be obtained from this area of the eastern Panhandle.

Peromyscus maniculatus blandus and P. m. rufinus formed subsets for 14 of the 16 characters tested under the Student-Neuman-Keuls procedure. Although size may be similar in P. m. blandus and P. m. rufinus, pelage, habitat, and other distinctions continue to support their status as distinct races. A list of the total number of specimens examined of the several subspecies follows.

Peromyscus maniculatus blandus Osgood, 1904

Specimens examined (41). — TEXAS. Pecos Co.: 4 mi. S, 14.5 E Imperial, 16. Winkler Co.: Winkler County Airport, 6 mi. S Kermit, 17; 1 mi. S Kermit, 2; 2.5 mi. NE Winkler County Airport, 1; 19 mi. E Kermit, base Concho Bluff, 1; 19 mi. E Kermit, 1; vicinity Wink, 3.

Peromyscus maniculatus luteus Osgood, 1905

Specimens examined (334). — NEBRASKA. Cherry Co.: 2.9 mi. N, 1.1 mi. E Valentine, 4; Valentine National Wildlife Refuge, Hackberry Lake, 11; Rice Lake, 10. Thomas Co.: Nebraska National Forest, Bessey Division, 13.

TEXAS. Andrews Co.: 4 mi. N, 9 mi. W Andrews, 1; 3 mi. N, 11 mi. W Andrews, 6; 18 mi. E Andrews, 1; 7 mi. S, 3 mi. E Andrews, 1. Bailey Co.: 2 mi. S, 10 mi. W Muleshoe, 1; 2.2 mi. S Muleshoe, 3; 5.1 mi. S, 2 mi. W Needmore, 7; 1.7 mi. W Needmore, 1; 5.3 mi. S, 0.8 mi. W Needmore, 3. Briscoe Co.: Los Lingos Canyon, 1. Carson Co.: Pantex Research Farm, 12 mi. E Amarillo, 23. Castro Co.: 8 mi. N, 1.5 mi. W Hart, 8; 4 mi. NW Hart, 9; Dimmit, 6. Collingsworth Co.: 9 mi. E Lutie, 7; 2 mi. N, 9 mi. E Lutie, 4. Ector Co.: 4 mi. W Goldsmith, 1; 9 mi. N Odessa, 5; 4 mi. N Notrees, 3; 1 mi. N Notrees, 1; 3.5 mi. S, 1 mi. E Notrees, 1; 2 mi. N Odessa, 3; 10 mi. E Loop 338, Odessa, 10. Garza Co.: 12 mi. S Post, 2; 6 mi. SE Southland, 1; 1 mi. W Post, 3; 8.5 km NE Southland, 9. Hale Co.: 4 mi. N, 5.5 mi. W Cotton Center 2; 5 mi. N, 12.5 mi. W Hale Center, 3. Hemphill Co.: Howe Wildlife Refuge, 5; Lake Marvin, 12 mi. E Canadian, 3. Hockley Co.: 14 mi. NW Levelland, 3; 8.5 mi. NW Levelland, 4. Lamb Co.: 7.2 mi. S Olton, FM 168, 4; 4 mi. N Fieldton, 3; 6 mi. S Spring Lake, 6; 8 mi. S Spring Lake, 1; 3.5 mi. S Earth, 1; 5.5 mi. N FM 168, 1.7 mi. W FM 1842, 2 mi. N dirt road near Running Water Draw, 3. Lipscomb Co.: 2 mi. N, 8 mi. E Lipscomb, 2. Lubbock Co.: 3.5 mi. N Slaton, 5; 8 mi. W Lubbock on 4th St., 1; 3.4 mi NW Lubbock, 4; Lubbock, 10; 10 mi. SW Lubbock, 1; 7.5 mi. N Lubbock, 15; 5 mi. N Lubbock, 4; 11 mi. S Lubbock on U.S. Hwy. 62, 3 mi. S FM 179, 1 mi. E dirt road, 0.5 mi. S, 1; 8 mi. N, 6 mi. NE Lubbock, 2; 5 mi. N Lubbock Lake, 1; 0.5 mi. N Lubbock Lake, 5; 2.8 mi. E Idalou, 4; 1 mi. N, 5 mi. W Lubbock, 4; 1 mi. N, 1.5 mi. W Lubbock, 4; 7 mi. W

PEROMYSCVS MANICULATUS ON THE LLANO ESTACADO

17

EM 1264 and Loop 289, 10; 4 mi. S, 5.7 mi. E Lubbock, 16. Moore Co.: 4 mi. N, 1 mi. E Dumas, 8. Randall Co.: Palo Duro State Park, 7 mi. S entrance, 1; 9.2 mi. S, 13.7 mi. E Canyon, 1; 1 mi. S Umbarguer, 1; Buffalo Lake Wildlife Refuge, 3. Roberts Co.: 6 mi. N Miami, 2. Sherman Co.: 8 mi. S, 2 mi. E Stratford, 17; 10 mi. N Stratford, 15.

Peromyscus maniculatus nehrascensis (Coues, 1877)

Specimens examined (18). — WYOMING. Unita Co.: 3.6 mi. W, 0.8 mi. N Fort Bridger, 12 (NMSU); 7.1 mi. W, 1.9 mi. S Robertson 6 (NMSU).

Peromyscus maniculatus pallescens J. A. Allen, 1896 Specimens examined {4\). — TEXAS. Hill Co.: 3 mi. S, 0.5 mi. W Hillsboro, 12 (TCWC); 3.1 mi. S, 6.6 mi. W Hillsboro, 5 (TCWC); 3.1 mi. S, 0.9 mi. W Hillsboro, 6 (TCWC); 3.4 mi. S, 1.6 mi. W Hillsboro, 2 (TCWC); 5.8 mi. S, 3.4 mi. W Hillsboro, 1 (TCWC); 5.8 mi. S, 3.1 mi. W Hillsboro, 7 (TCWC); 6 mi. S, 4.2 mi. W Hillsboro, 3 (TCWC); 6 mi. S, 3.9 mi. W Hillsboro, 1 (TCWC); 3.4 mi. S, 1.6 mi. W Hillsboro, 4 (TCWC).

Peromyscus maniculatus rufinus (Merriam, 1890)

Specimens examined {65). — NEW MEXICO. Lincoln Co.: 2.5 mi. W Bonito Lake, 15; 2.5 mi. N Bonito Lake, 5; 4 mi. N, 5 mi. W Bonito Lake, 1; 4.9 mi. E, 1.3 mi. N Sierra Blanca, 4 (NMSU); Oak Grove Camp, Sierra Blanca, 6 (NMSU); 1.7 mi. N, 1.2 mi. E Sierra Blanca, 1 (NMSU). Otero Co.: 20 mi. S Cloudcroft, 12; Lightning Lake, 7; Cloudcroft, 14.

Acknowledgments

Specimens housed in the collection at Texas Tech University are not identified by an acronym, whereas those examined at Texas A&M University (for access to which we are thankful to George Baumgardner and Michael Forstner) are identified by the acronym TCWC. We also thank Charles Thaler and Karl Wilson for access to the collection at New Mexico State University, specimens from which bear the acronym NMSU.

The first author owes special thanks to Mark Lockwood, Michael Forstner, Robert Robertson, Sharon Wyerts, and Judy York, for providing moral support throughout the course of this study, results of which were presented to the Division of Range Animal Science, Sul Ross State University, Alpine, Texas, in partial fullfillment of requirements for the degree of Master of Science. Partial funding in support of the research was provided from the Enhanced Research Program at Sul Ross State. Finally, for continual support in his collegiate and professional endeavors. Cooper wishes to thank Arthur Coykendall and Yvette Truitt.

Literature Cited

Blair, W. F. 1953. Factors affecting gene exchange between populations in the Peromyscus maniculatus group. Texas J. Sci., 5:17-33.

- . 1954a. Tests for discrimination between four subspecies of deer-mice {Peromyscus

maniculatus). Texas J. Sci., 6:201-210.

- . 1954b. Mammals of the Mesquite Plains Biotic District in Texas and Oklahoma,

and speciation in the central grasslands. Texas J. Sci., 6:235-264.

Caire, W., and E. Zimmerman. 1975. Chromosomal and morphological variation and circular overlap in the deer mouse, Peromyscus maniculatus, in Texas and Oklahoma. Syst. Zool., 24:89-95.

Crockrum, E. L. 1954. Non-geographic variation in cranial measurements of wild-taken Peromyscus leucopus noveboracensis. J. Mamm., 35:367-376.

Comely, J. E., D. J. Schmidly, H. H. Genoways, and R. J. Baker. 1981. Mice of the genus Peromyscus in Guadalupe Mountains National Park, Texas. Occas. Papers Mus., Texas Tech Univ., 74:1-35.

Dalquest, W. W., F. B. Stangl, Jr., and J. K. Jones, Jr. 1990. Mammalian zoogeography of a Rocky Mountain-Great Plains interface in New Mexico, Oklahoma, and Texas. Spec. Publ. Mus., Texas Tech Univ., 34:1-78.

18

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Davis, W. B. 1974. The mammals of Texas. Bull. Texas Parks Wildlife Dept., 41:1-294.

Dice, L. R. 1936. Age variation in Peromyscus maniculatus gracilis. J. Mamm., 17:55-57.

- . 1937. Additional data on variation in the prairie deer-mouse, Peromyscus

maniculatus hairdii. Occas. Papers Mus. Zool., Univ. Michigan, 351:1-19.

Dice, L. R., and R. M. Bradley. 1942. Growth in the deer-mouse, Peromyscus maniculatus. J. Mamm., 23:416-427.

Fox, W. 1948. Variation in the deer-mouse (Peromyscus maniculatus) along the lower Columbia River. Amer. Midland Nat., 40:420-452.

Hall, E. R. 1981. Mammals of North America. John Wiley & Sons, New York, 2nd ed., 2:vi + 601-1181 + 90.

Hoffmeister, D. F. 1986. Mammals of Arizona. Univ. Arizona Press, Tucson, xx + 602 pp.

Hollander, R. R. 1990. Biosystematics of the yellow-faced pocket gopher, Cratogeomys castanops (Rodentia: Geomyidae) in the United States. Spec. Publ. Mus., Texas Tech Univ., 33:1-62.

Jones, J. K., Jr. 1958. The type locality and nomenclatorial status of Peromyscus maniculatus nebrascensis (Coues). Proc. Biol. Soc. Washington, 71: 107-1 1 1.

Jones, J. K., Jr., R. W. Manning, R. R. Hollander, and C. Jones. 1987. Annotated checklist of Recent mammals of northwestern Texas. Occas. Papers Mus., Texas Tech Univ., 111:1-14.

Jones, J. K., Jr., R. W. Manning, C. Jones, and R. R. Hollander. 1988. Mammals of the northern Texas Panhandle. Occas. Papers Mus., Texas Tech Univ., 126:1-54.

Judd, F. W. 1970. Geographic variation in the deer mouse, Peromyscus maniculatus, on the Llano Estacado. Southwestern Nat., 14:261-282.

Koh, H. S., and R. L. Peterson. 1983. Systematic studies of deer mice, Peromyscus maniculatus Wagner (Cricetidae, Rodentia): analysis of age and secondary sexual variation in morphometeric characters. Canadian J. Zool., 61:2618-2628.

Lotspeich, F. B., and J. R. Coover. 1962. Soil forming factors on the Llano Estacado: parent material, time and topography. Texas J. Sci., 14:7-17.

Osgood, W. H. 1905. A new name for the Peromyscus nebracensis of certain authors. Proc. Biol. Soc. Washington, 18:77.

- . 1909. Revision of the mice of the American genus Peromyscus. N. Amer. Fauna,

28:1-285.

Schmidly, D. J. 1973. The systematic status of Peromyscus comanche. Southwestern Nat., 17:269-278.

Sokal, R. R., and F. J. Rohlf. 1981. Biometry: the principles and practice of statistics in biological research. W. H. Freeman and Co., San Francisco, 2nd ed., xviii + 859 pp.

SPSS Inc. 1986. User’s Guide. McGraw-Hill Book Co., 2nd ed., xiv + 988 pp.

Svihla, A. 1934. Development and growth of deer mice (Peromyscus maniculatus artemisiae). J. Mamm., 15:99-104.

- - --. 1935. Development and growth of the prairie deer mouse, Peromyscus maniculatus bairdii. J. Mamm., 16:109-115.

Thompson, T. G., and W. Conley. 1983. Discrimination of coexisting species of Peromyscus in south-central New Mexico. Southwestern Nat., 28:199-209.

Thorpe, R. S. 1976. Biometeric analysis of geographic variation and racial affinities. Biol. Rev., 51:407-452.

Wilson, D. E. 1968. Ecological distribution of the genus Peromyscus. Southwestern Nat., 13:267-274.

Present address of Cooper: 9320 S.P.I.D., #3302, Corpus Christi, Texas 78418.

A SENSITIVE SPECTROPHOTOMETRIC TECHNIQUE FOR MEASURING PHOTOTAXIS IN CHLAMYDOMONAS REINHARDT!!

R. C. Moyer, Wm. F. Schroeder, and J. Taboada

Department of Biology, Trinity University, San Antonio, Texas 78212 (RCM, WFS), and Clinical Sciences Division, School of Aerospace Medicine, Brooks AFB, San Antonio, Texas 78212 (RCM, JT)

Abstract. — A sensitive and convenient method was developed for measuring phototaxis of algal cells using a Gilford Response UV-VIS Spectrophotometer with attached printer. Wild type cells of Chlamydomonas reinhardtii were exposed to light continuously for eight minutes at selected wavelengths between 400 and 850 nm. The change in optical density resulting from cells swimming into or out of the light path was continuously recorded and printed. Positive phototaxis, manifest as an increase in OD, was observed for wavelengths between 440 nm and 538 nm with a maximum phototactic response at 503 nm. Other wavelengths induced negtive phototaxis, vacillation between positive and negative phototaxis, or no phototaxis, depending upon conditions of growth and phototaxis assay. The maximum positive phototactic response was +0.72 at 500 nm. A study of various factors that may influence the results of the phototaxis assay was conducted. Several parameters of phototaxis of C reinhardtii measured in the Gilford compared favorably with literature values. Key words: Chlamydomonas reinhardtii', algae; motility; phototaxis; spectrophotometry.

Phototaxis in algae has been studied since 1817 (reviewed by Foster and Smyth, 1980). The techniques that have been developed for measuring phototaxis in microorganisms may be divided into two basic procedures: 1) direct microscopy of individual cells, and 2) spectrophotometric or phototube measurement of populations of microorganisms moving toward or away from a photostimulating light.

Both procedures have advantages and disadvantages. Single-cell observations allow rapid study of behavioral responses and measurement of both speed and directness of movement, but are time consuming and a large number of individuals must be studied to yield statistically valid data. Population methods quantify the movement of the population that is an admixture of directness and speed, but give no direct information on individual cell behavior.

Several effective methods have been developed for measuring population movements. Lindes et al. (1965) developed a phototaxigraph in which an actinic beam is used to photostimulate the algae and an observing beam that presents no stimulus to the algae, but passes through the algal suspension at right angles to the actinic beam to the detection system. Feinleib and Curry (1967) improved on the assay of population movements by adapting two photocells connected in a comparison circuit. Nultsch et al. (1971) and Nultsch and Throm (1975) constructed a monitoring device also based on photocell measurements but possessing

20

THE TEXAS JOURNAL OF SCIENCE- VOL. 45, NO. 1, 1993

the capability of being connected to a continuous culture device from which samples could be withdrawn. Stavis and Hirschberg (1973), in a thorough study of Chlamydomonas phototaxis, adapted a spectrophotometer to use two light sources, an actinic beam and an 800 nm light to measure population movement.

All of these population measuring procedures require construction or modification of pre-existing equipment. In this paper, we describe a simple, rapid, yet sensitive method of measuring phototaxis using an unmodified commercially available instrument, the Gilford computerized “Response” spectrophotometer. The characterization of the phototaxis of Chlamydomonas reinhardtii using the Gilford compares quite well with characterizations found in the literature. In the process of refining this phototaxis assay technique, a simple, rapid, and inexpensive procedure for culturing vegetative cells of C. reinhardtii on a liquid medium overlay of an agar medium base also was developed. The procedure yields cells that are immediately motile and reproducibly phototactic.

Materials and Methods

Algal Strain and Conditions of Growth

Chlamydomonas reinhardtii strain 137c (CC-125 mt+) was obtained from the Chlamydomonas Genetics Center, Department of Botany, Duke University, Durham, North Carolina, and was used for these studies. Minimal (HS) and acetate (HSA) media were prepared by the Harris et al. (1977) formulation modification of the original Sueoka (1960) medium. The algae were generally grown in a liquid medium overlay over an agar base, for example, five milliliters of HS broth over a 25-milliliters HS agar base in standard (100 X 15 mm) Petri plate or three milliliters HSA broth over 8 milliliters HSA agar base slant in a standard (125 X 20 mm) glass screw-capped culture tube. Cultures were inoculated with 10^ algal cells per milliliter or more final concentration of cells in their log phase of growth, and yielded 150 to 600 million cells per plate after two to three days of incubation at 23° C in continuous light at 81 ft. Lamberts. Cultures under continuous light yielded significantly better growth than did identical cultures grown under a 12-hour light-dark cycle.

Cell Harvest and Cell Counts

Cells are harvested from the broth overlay by adding fresh HS or HSA broth and triturating to suspend the cells in a final volume of 7.0 milliliters. The cells were diluted (equivalent to 5 to 7 X lO*” cells per milliliter) in HS broth to a target OD (600) of 0.8 to 1.0 in a lO-mm cuvette (but an OD (600) range of 0. 4-1.2 was satisfactory). This range of dilutions of cells was used to perform time scans (TS) (phototaxis assays) and wavelength (WL) scans. An aliquot of this TS dilution (usually 25, 50, or 100 /il) was removed and diluted into 20.0 milliliters Isoton H (Coulter Diagnostics, Hialeah, Florida). The number of cells and the size distribution of the cells were determined in a Coulter Electronic Cell Counter (Model ZBI) and Channelizer. The size standard was provided by Rhesus monkey red blood cells with a volume of 72 ^lm^ Cell counts and cell size distributions of the algae were performed according to instruction provided by the manufacturer. If the raw count was above 10,000, the count was corrected for coincidence using a chart provided by the manufacturer. Motility of the cells was routinely estimated by visual examination of a hanging drop.

TECHNIQUE FOR MEASURING PHOTOTAXIS

21

Spectrophotometry of the Cell Suspensions

Although any spectrophotometer with suitable absorbance versus time and absorbance versus wavelength capability and a beam distribution of about 20 mm^ would function satisfactorily for these experiments, a Gilford Response UV-VIS Spectrophotometer was used for this work. The spectrophotometer was set to a “continuous Time Scan Mode” and the band width of light was set to a maximum of 4 nm to give adequate light to attract the organisms. A time scan could not be less than four minutes, but for our studies eight minutes was optimum. The first author can provide a protocol to interested individuals wanting to use the Gilford instrument.

Evergreen 201-3125-010 standard (10 mm), four sides clear, disposable cuvettes were used for these studies. The time scans were performed on an algal suspension in a cuvette using the suspending medium, HS or HSA, as a blank. The cell suspensions in the cuvette were mixed using air bubbling from a pipet prior to spectrophotometry time scans. After a time scan was performed on a cell suspension, the cuvette of cells was returned to room light for a 15-minute adaption period before subsequent time scans were performed.

Results

The performance of many phototaxis assays brought out several factors that may influence the results. These factors were: cell motility; handling of the cells from time of harvest to phototaxis assay; wavelength of photostimulating light; length of time scan; concentration of cells; repeating of time scans.

Cell Motility

The majority of cultures harvested from liquid medium overlays were 90 percent or more actively motile. We have found (unpublished data) that cultures of C. reinhardtii grown on plates as described in this paper show a periodic fluctuation in the percentage of motile cells in the population as it progresses through the growth curve.

Handling of the Cells from Time of Harvest to Phototaxis Assay The phototactic response at 500 nm of a C. reinhardtii culture grown on an HS plate in continuous light for three days and then preadapted to light or dark is compared in Figure 1. The culture was harvested as described, then separated into two groups — one handled in the light and the other in the dark — prior to subjecting the cells to spectrophotometer time scan. All other conditions of the two groups were identical. The cells preconditioned in light responded by a roughly linear swimming into the light path with a +0.40 AOD(500). The cells preconditioned in dark responded to the light similarly for about four minutes but then gradually began to plateau in their attraction to 500 nm light. The dark preadapted cells had a AOD(500) of +0.27. Thus, subsequent studies were performed with cells handled in the light.

Several other factors also encouraged the handling of cells in room light. It was easier to manipulate them in room light, and dark-adapted

22

THE TEXAS JOURNAL OF SCIENCE VOL. 45, NO. 1, 1993

AO a

500

.2724

+.3971

Figure 1. Effect of preconditioning of algal cells in dark or light on phototactic response of C. reinhardtii to 500 nm light. A three-day-old HS plate culture grown under continuous light was harvested and diluted with HS broth as described. The harvest was separated into two groups, one preadapted and handled in the light and another in the dark, and both groups of cells were tested for phototaxis to 500 nm light. In the test shown here, light preadapted cells responded to the 500 nm light with a roughly linear profile, whereas dark preadapted cells responded by a linear accumulation in the light followed by a plateau after about four minutes. The AOD(500) for light preadapted cells was +0.3971 and for dark preadapted cells it was + 0.2724.

cells had a tendency to stick to the sides of the cuvette at the entrance and exit of the light beam. By comparison, if the cuvette with the algal suspension was preadapted to the light, then returned to room light after the phototaxis assay, the cells would then release from the cuvette surface spontaneously. It also is shown in Table I that cells preconditioned to light show less fluctuation in repeated phototaxis assays than cells stored in the dark. The magnitude of the AOD(500) varied in different cell harvests between being higher for dark-preadapted or light-preadapted cells. The greater precision of repeated phototaxis assays and the greater ease of handling in room light outweighed any benefit of greater AOD(500) of dark-preadapted cells.

Wavelength of Photostimulating Light Positive phototaxis for strain CC-125 was observed between 440 and 563 nm. Negative or undulating phototaxis was observed below 400 nm and above 600 nm (Fig. 2). The algae responded to wavelengths of 400 and 800 nm with an undulating positive-negative phototaxis initially, then

TECHNIQUE FOR MEASURING PHOTOTAXIS

23

0.0 2.0 4D 6i) 8.0

TME IN MMUTES

AOD

400nm -0.0734

500nm -fO.3976

800nm -0.0328

Figure 2. Time scan profiles of C. reinhardtii to 400 nm, 500 nm, and 800 nm light. A 3-day-old HS plate culture was harvested and after one hour in room light, time scans were performed on the same cell suspension at 400, 500, and 800 nm light. The AOD values were AOD(400) = -0.07, AOD(500) = + 0.40 and AOD(800) = -0.03 for the same cells.

followed by a smoother avoidance of those wavelengths of light. The greatest negative phototaxis we have observed was AOD(683) = -.010. The greatest positive phototaxis we have observed was AOD(500) = +0.72.

Length of Time Scan

The length of the spectrophotometer time scan is determined by how long it takes algae to reach their maximum AOD. In our earlier studies, it required an eight minute exposure to the photostimulating light for cell suspension to reach a maximum AOD(500). In these earlier studies, some of our cultures were contaminated with bacteria. With contaminated algae, the OD(500) was reached in about three minutes with a gradual drop in OD(500) thereafter. We continued to use eight-minute time scans to insure that phototaxis assays were not terminated before maximum OD(500) was reached.

Concentration of Cells

Results of two experiments that addressed the question of the effect of the number of cells per unit volume in the cuvette on the AOD(500) of the cell suspension are shown in Figure 3. A fresh culture was serially

24

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Figure 3. Effect of cell concentration on the magnitude of the AOD(500). A four-day-old HS plate culture was harvested and diluted in HS broth to produce an initial cell suspension of OD(600) = 1.3737 (9.6X10^ cells per milliliter). This was diluted through a series of 10 dilutions with HS to a minimum OD(600) = 0.2803. A phototaxis assay at 500 nm

was performed on each of the dilutions and the AOD(500) plotted ( _ • _ • _ ) against

the cell concentration. The culture dilutions were stored overnight at 5°C and the phototaxis assay performed again with the same dilutions of cells. These data were plotted as AOD(500) versus cell density (0 — 0 — 0). The number of particles per milliliter was counted in triplicate in the Coulter Counter as described in Materials and Methods and the correlation between the average of the cell count and the cell suspension OD(600) plotted.

diluted over a range of 1.7 to 10.8 X 10^ cells per milliliter in HS medium and time scans were performed on each dilution. The dilutions were cold stored in dark overnight and the time scans were repeated. In both instances, the AOD(500) varied linearly with the cell concentration. The magnitude of the AOD(500) of the cold-stored cells was less than the same cells that had been freshly harvested. It is clear from these two experiments that the number of cells entering the photostimulating light path varied directly with cell concentration.

Repetition of Time Scan Responses

It was of interest to determine if a single algal suspension could be used for multiple phototaxis analyses or if fresh cells had to be prepared for each analysis. Cells preadapted to light and dark and held under

TECHNIQUE EOR MEASURING PHOTOTAXIS

25

Table 1. Comparison of the precision of the phototactic response of C. reinhardtii using cells preadapted to the light or dark.'

Cells preconditioned to light

Trial no.

Fresh cell suspension

Cell suspension

AOD(500)

cold stored in light

AOD(500)

1

.34

.17

2

.31

.19

3

.32

.18

Mean

.32

.18

S. D.

.0171

.0074

S. D. (%)

5.3

4.1

Cells preconditioned to darkness

Trial no.

Fresh cell suspension

Cell suspension

AOD(500)

cold stored in dark

AOD(500)

1

.37

.20

2

.49

.19

3

.45

.19

Mean

.44

.19

S. D.

.0617

.0067

S. D. (%)

14.1

3.4

‘Cells were harvested from a three-day-old HS agar plate and diluted in HS broth to produce

a WETS sample with OD(600) = 1.04. The algal cells were

aliquoted into two cuvettes;

one to be exposed continually to light and one to be kept in the dark. Each set of identical suspensions was assayed for phototaxis using HS broth as reference. The data are presented

as AOD(500) and

rounded to two places. After the phototaxis assays were completed,

the cuvettes with the algal suspensions were placed at 5°C in light or dark for 24 hours. The algal suspensions were warmed to room temperature and the phototaxis assays repeated.

The percent S. D.

is the percent variation that the standard deviation varies from the

mean.

those respective conditions were used. A phototaxis assay was performed on a cuvette of cells; then the cells were returned to room light (17.5 ft. Lamberts) or darkness. A minimum of 15 minutes to re-equilibrate was required before another trial could be conducted (cells re-equilibrated less than 15 minutes were not consistent in their phototactic response). The cells always were mixed with a disposable Pasteur pipet prior to each time scan. Three separate trials were performed. Results along with the mean, the standard deviation from the mean, and the percent from the mean that the standard deviation represents all are given in Table 1. These data support the contention that the same suspension of cells may be used for at least three trials without significant decay of the algal cells ability to phototax. Time scans may be repeated over a period of six hours. The data presented in Table 1 also show that light-adapted, one- hour post harvest cells showed a smaller data scatter for replicate runs

26

THE TEXAS JOURNAL OE SCIENCE— VOL. 45, NO. 1, 1993

than did cells preadapted and stored in darkness. Cells can be stored overnight in the cold at 5°C in the light or dark and the cells remain phototactic, but the sample suspension showed a AOD(500) about 50 percent less than that for the preceding day. It also was observed that freshly harvested cells gave high AOD(500) values, but that the cells were quite erratic in repeated phototaxis assays, thus yielding poor precision. Repetition of assays with the same initial dilution cell suspension sample showed that cells must be held for one hour in room light after harvest prior to phototaxis assay in order to yield the data precision shown in Table 1.

Discussion

This paper describes a procedure for assaying phototaxis using an unmodified commercially available computerized spectrophometer. In contrast to previously published methods, this procedure does not use a separate actinic light to photostimulate the cells and a nonstimulating light to measure cell population movements. Instead, it measures the number of cells that swim into or out of a beam of light with reference to a randomly moving background population. The single beam of light serves as both the photostimulating light and the measuring light, which is recorded as change in absorbance or AOD with respect to the absorbance of the randomly moving cell population. Because the phototaxis assay does not employ separate actinic and measuring light beams, it was important to show that this technique could duplicate some landmark experiments on phototaxis available in the literature. It was also important to show what effect certain experimental variations may have on the results of a phototaxis assay.

Our experiments are in accord with those of Stavis and Hirschberg (1973) in that phototaxis is dependent upon motility, but that the control of phototaxis is independent of the control of motility. Because Stavis and Hirschberg (1973) observed that dark-grown Chlamydomonas showed poor motility and phototaxis coefficients, a brief experiment was conducted to determine if the history of light exposure of the cells would affect the results of the phototaxis assays. Our experiments, recorded in Figure 1, show that there are minor differences in shape of the time-scan profile. The magnitude of the AOD(500) varied between being greater in dark or light-preadapted cells from different cell harvests, but experiments reported in Table 1 show that the precision of the data from replicate runs is better for cells maintained in room light. The reason (s) for this variation in AOD(500) in response to light or dark preadaptation is not clear, but is most likely related to effects of culture age. The relationship between AOD(500) and cell concentration shown in Figure 3 also compares well with the similar experiment of Starvis and Hirschberg (1973). Our procedure does not have the advantage of the procedure of

TECHNIQUE FOR MEASURING PHOTOTAXIS

27

Pfau et al. (1983) in that it cannot simultaneously measure the effect drugs have on both motility and phototaxis. Our procedure requires motile cells for phototaxis, although it may be possible eventually to evaluate the motility component by the rate of change of optical density with respect to time. We have observed roughly two classes of motility — rapid and sluggish — but usually the motility is rapid. We have occasionally observed motile cells that were nonphototactic; the reason(s) for this were not apparent. The shape of many of our time-scan profiles was quite similar to those of Pfau et al. (1983) in that the cells reach a maximum AOD in two to four minutes, after which they leave the light and the absorbance gradually decreases. The shape of the time scan profile may vary from experiment to experiment, but is generally uniform for repeated phototaxis assays on the same cell suspension without changes in conditions. We observed that in preadaptation to light or dark, storage overnight in the cold, or exposure to different wavelengths of light induced changes in the AOD as well as in the shape of the time- scan profile (Figs. 2 and 3). Cells cultured under the conditions described here are quite durable in their phototactic ability and repeated spectrophotometry time scans on the same cell suspension may be conducted over a six-hour period or longer without significant decay of phototactic response. The cells must be allowed to recover in room light between scans.

Acknowledgments

This research was supported in part by an Air Force Office of Scientific Research/ Universal Energy Systems minigrant 4962085C0013 to Rex C. Moyer, who was an AFOSR University Resident Research Program Fellow. We thank Mr. Randy Prettyman, Sgt. William Bumgarner, and Sgt. Ray Barger for aid with the Coulter Counter; Ms. Angela Braun, Ms. Wendy Nguyen, and Ms. Dagmar Fertl for technical assistance; Mr. Tom Nixon for setting up the lighted environmental control chambers; Ms. Sylvia Stewart and Ms. Maria Garza for typing the manuscript; and Dr. Elizabeth Harris for helpful discussions, and the algal cultures.

Literature Cited

Feinleib, M. E. H., and G. M. Curry. 1967. Methods for measuring phototaxis of cell populations and individual cells. Physiol. Plant., 20:1083-1095.

Foster, K. W., and R. D. Smyth. 1980. Light antennas in phototactic algae. Microbiol. Rev., 44:572-630.

Harris, E. H., J. E. Boynton, N. W. Gillham, C. L. Tingle, and S. B. Fox. 1977. Mapping of chloroplast genes involved in chloroplast ribosome biogenesis in Chlamydomonas reinhardtii. Molec. Gen. Genet., 155:249-265.

Lindes, D., B. Diehn, and G. Tollin. 1965. Phototaxigraph: recording instrument for determination of rate of response of phototactic microorganisms to light of controlled intensity and wavelength. Rev. Sci. Instr., 36:1721-1725.

Nultsch, W., and G. Throm. 1975. Effect of external factors on phototaxis of Chlamydomonas reinhardtii. I. Light. Arch. Microbiol., 103:175-179.

28

THE TEXAS JOURNAL OF SCIENCE—VOL. 45, NO. 1, 1993

Nultsch, W., G. Throm, and I. Von Rimscha. 1971. Phototaktische Untersuchungen an Chlamydomonas reinhardtii Dangeard in homokontinuierlicher Kultur. Arch. Mikrobiol., 80:351-369.

Pfau, J., W. Nultsch, and U. Riiffer. 1983. A fully automated and computerized system for simultaneous measurements of motility and phototaxis in Chlamydomonas. Arch. Microbiol., 135:259-264.

Stavis, R. L., and R. Hirschberg. 1973. Phototaxis in Chlamydomonas reinhardtii. J. Cell Biol., 59:367-377.

Sueoka, N. 1960. Mitotic replication of deoxyribonucleic acid in Chlamydomonas reinhardtii. Proc. Nat. Acad. Sci., 46:83-91.

RESPONSE OF SMALL MAMMALS TO CONVERSION OF A SAND SHINNERY OAK WOODLAND INTO A MIXED MID-GRASS PRAIRIE

Michael R. Willig, Randall L. Colbert, Russell D. Pettit, and Richard D. Stevens

Ecology Program, Department of Biological Sciences and The Museum (MRW, RLC, RDS), and Department of Range and Wildlife Management (RDP), Texas Tech University, Lubbock, Texas 79409-3131

Abstract. — Shrubs were removed in two areas of a sand shinnery oak woodland via the aerial application of tebuthiuron, an herbicide that eliminates at least 90 percent of the shrub component {Quercus havardii). A circular grid containing 260 traps was established at each of four sites: two tebuthiuron treated areas and two untreated or control areas. Estimates of density (total rodents and Dipodomys ordii alone) based upon the minimum number known to be alive were obtained for each area during each of four phonological seasons. Habitat variables (percentages of bare ground, ground litter, canopy cover, shrubs, forbs, and grasses, as well as vertical height) were determined for tebuthiuron treated and control sites. Statistical analyses revealed that both total rodent density and D. ordii density changed in response to tebuthiuron-related effects. Species composition (proportional species densities) also differed significantly between treated and untreated areas. Seasonal effets on density and species composition occurred and probably were related to behavioral responses of rodents to temperature and precipitation regimes. Key words: small mammal community; rodents; mixed grass prairie; sand shinnery oak woodland; Dipodomys, tebuthiuron.

Habitat characteristics clearly affect rodent populations. It is well documented that a variety of vegetational characteristics affect species composition of local rodent communities as well as the density of component species (Hansen and Ward, 1966; Tietjen et al., 1967; Johnson and Hansen, 1969; Rosenzweig and Winakur, 1969; Parmenter and MacMahon, 1983; Schroder, 1987; Wywialowski, 1987; Abramsky, 1988; Brown, 1989; Brown and Zeng, 1989). The community and population responses of small mammals to vegetational changes have been studied by inducing modifications through the use of chemical treatment (Johnson, 1964; Johnson and Hansen, 1969; Christian, 1977; McGee, 1982; Sullivan and Sullivan, 1982; Zou et al., 1989), fire clearcutting (Gashwiler, 1970; Sullivan, 1979; Van Horne, 1981), and mechanical removal of select plant species (Parmenter and MacMahon, 1983). Percent cover (Allred and Beck, 1963; Brown et al., 1972; M’Closkey, 1975; M’Closkey and Lajoie, 1975; Rosenzweig et al., 1975; Feldhamer, 1979), vegetation-precipitation relationships (Reynolds, 1958; Brown, 1973; Hafner, 1977), foliage height diversity (Allred and Beck, 1963; Rosenzweig and Winakur, 1969; M’Closkey, 1975; Morris, 1979), foliage density (Rosenzweig and Winakur, 1969; Brown et al., 1972; M’Closkey and Lajoie, 1975; Hafner, 1977), and ground litter (Morris, 1979) are

30

THE TEXAS JOURNAL OF SCIENCE—VOL. 45, NO. 1, 1993

among the many variables that have been correlated or associated with changes in small mammal densities and community composition.

Munger et al. (1983) suggested that bushes are favorable resource patches that act as proximate cues for rodent activity, whereas results from several other studies pointed toward cover as the main factor affecting rodent populations in arid and semiarid environments (Rosenzweig and Winakur, 1969; Price, 1978; Thompson, 1982; Parmenter and MacMahon, 1983). Nontheless, Hafner (1977) contended that desert rodent species diversity does not respond to a particular parameter because even animals of the same species are influenced by different environmental factors in different geographical situations. This leads to the question of the importance of shrubs in determining small mammal species composition and abundance in a sand shinnery oak woodland.

Sand shinnery oak {Quercus havardii) predominates on sandy soils in semiarid environments in western Texas, southeastern New Mexico, and western Oklahoma, where it grows on about 2.7 million hectares (Pettit and Jones, 1986). The sand shinnery oak ecosystem is essentially a monoculture, with at least 80 percent of the herbage being oak (Pettit and Jones, 1986). The effect of the shrub component on rodent population parameters has not been studied even though removal of the sand shinnery oak via tebuthiuron treatment drastically alters the habitat by changing cover, food resources, foliage height diversity, and water availability to other plants (Mcllvain, 1954; Elwell, 1964; Rechenthin and Smith, 1967; Robison and Fisher, 1968; Jones and Pettit, 1984; Pettit and Jones, 1986). Herein, we assess the response of small mammals to the tebuthiuron-induced conversion of a sand shinnery oak woodland into a mid-grass prairie. We also attempted to identify the potential ecological variables associated with changes in small mammal population density or community composition. The hypotheses for evaluation were that the removal of the sand shinnery oak would affect a change in the overall abundance of small mammals, and that there would be an alteration in species composition, possibly because of herbicide-induced changes in habitat variables. Moreover, Dipodomys ordii density should change in tebuthiuron-treated areas because kangaroo rats, in general, avoid areas of thick cover and are associated more frequently with sparsely vegetated areas (Rosenzweig and Winakur, 1969; Rosenzweig, 1973).

Materials and Methods

Study Site

The study site is located on the Southern High Plains of Texas (33°22'-33°26' N, 102°37'- 102° 50' W), about 19 miles N and 4 miles E Plains, Yoakum County. The terrain is relatively flat at an elevation of about 1260 meters. The climate is warm-temperate and semiarid, with fluctuating temperatures during the winter. Annual precipitation averages 41

SMALL MAMMAL DENSITIES

31

centimeters, with the majority of the rainfall occurring from May through October (Jones and Pettit, 1984).

The vegetation on the untreated control sites is at least 80 percent (biomass) sand shinnery oak (Pettit and Jones, 1986). Other plants found on the control sites include soap weed ( Yucca angustifolia), sand sagebrush {Artemesia filifolia), and prickly pear cactus iOpuntia polyacantha). The vegetation on the treated site is predominantly grasses and forbs; the more common plants include little bluestem {Schizachyrium scoparium), purple threeawn (Aristida purpurea), annual buckwheat (Erigonum annuum), and fleabane (Erigeron modestus). A complete plant species list for the area is presented in Colbert (1986). Mammals known from the study area include Dipodomys ordii (Ord’s kangaroo rat), Onychomys leucogaster (northern grasshopper mouse), Perognathus flavescens (plains pocket mouse), Peromyscus maniculatus (deer mouse), Reithrodontomys montanus (plains harvest mouse), Sigmodon hispidus (hispid cotton rat), Neotoma micropus (wood rat), Spermophilus spilosoma (spotted ground squirrel), Canis latrans (coyote), Taxidea taxus (badger), Lepus californicus (black-tailed jackrabbit), Sylvilagus audubonii (desert cottontail), Antilocapra americana (pronghorn), and Bos bos (cattle).

Shrub Removal

The study area was chosen because shrubs had been removed via the application of tebuthiuron (Graslan*^; N-(5-(l, 1- dimethylethyl) -l,3,4-thiadizol-2yl)-N,N'- dimethylurea) pellets (20 percent active ingredients) that were aerially broadcast at an application rate of one-half pound per acre. Grid I was treated in May 1982, whereas grid II was treated in May 1980 and retreated in May 1983. The use of tebuthiuron is the only method that essentially eliminates sand shinnery oak for an extended period and does not directly affect other components of the plant community, except for initial damage to forbs (Jones and Pettit, 1984). Tebuthiuron has been documented to kill more than 90 percent of the sand shinnery oak in the community (Pettit, 1975; Jones et al., 1978; Jones and Pettit, 1984). The two treated areas, although different in treatment times, were chosen because of their current similarity in vegetation, previous and present grazing pattern, and topography. Within each treated area, grid sites were selected randomly. Trapping sites were established on two control areas where sand shinnery oak had never been treated with tebuthiuron. All four sites were within a 1500-hectare area.

Rodent Trapping

We used a circular grid to trap rodents. Thirteen lines, each comprising 20 medium-sized Sherman live-traps, radiated from a center stake, with the first trap in each line 2.5 meters from the center stake. The remaining traps were placed at five-meter intervals. This spacing was used in order to 1) insure at least eight to 12 traps per home range in the center of the grid, and 2) facilitate the capture of approximately 60 individuals during a trapping session (Anderson et al., 1983). Preliminary results from trapping a circular grid from 25 May to 15 July 1984 suggested that the previously described trap spacing and grid size were appropriate given the small mammal fauna at the sites. Four permanent grids were established, one in each of the treated areas, and one in each control area. Trapping continued until all animals from the center of each grid were captured in accordance with the recommendations of Anderson et al. (1983). This usually required three or four nights of trapping. Trapping sessions were scheduled during the dark phase of the moon to restrict any variation in activity that might be affected by moon light (Price et al., 1984). Because of limitations in the number of available traps, one control site and one treated site were trapped at the same time. The traps were then moved to the replicate sites for additional trapping. The complete trapping regime required eight or nine days each season.

Trapping was aimed at nocturnal and crepuscular rodents only. Larger mammals such as desert cottontails (Sylvilagus audubonii) and black-tailed jackrabbits (Lepus californicus)

32

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

were excluded because of trap size. Diurnal rodents such as the spotted ground squirrel {Spermophilus spilosoma) were not marked when captured, and thus do not appear in the overall evaluation of rodent density and species composition. A granola mixture was used as bait; cotton Nestlets’' were placed in each trap to prevent rodent hypothermia. Upon initial capture, rodents were uniquely toe-clipped, weighed, and identified to sex and species. Trap location was recorded for initial as well as subsequent captures of previously caught individuals.

Trapping was conducted in four sessions defined by seasonal foliage changes that occur in the area. The first session was in the winter after all leaves had fallen from the oaks (19 to 27 January 1985). Trapping was restricted to only three nights on each grid. The second period occurred during oak refoliation in the spring (13 to 20 May 1985). The third trapping session occurred in the summer during maximum plant development (10 to 17 August 1985). The last period was during the autumn dormancy, after the first frost but before leaf fall (9 to 19 November 1985).

Vegetation Sampling

A summary of vegetation sampling methods is offered by Daubenmire (1968), the original source for most of the following procedures. A permanent line transect was established across each grid to measure the same points every trapping season. A frame of 0.25 square meter was placed at 30 points spaced equidistantly along the transect. At each point, the highest vegetation within the frame was measured for vertical height. Then, using either a Canon AE-1 35-mm or a Minolta XG-7 35-mm camera, and focusing from directly above the center of the frame, a photograph was taken of the quadrat. The resultant slides were projected onto a screen of 0.5 by 0.5 meters that was divided into 100 sections. This enabled an accurate estimation of the percentages of bare ground, ground litter, and canopy cover. Also, a step-point analysis was performed to determine vegetation composition. A transect was walked for 100 steps across each grid. Bare ground, litter, or plant species identity was recorded at the end of each step. If bare ground or litter was recorded, the identity of the closest plant species was also noted. Percentage of shrubs, forbs, and grasses was determined from these data.

Population Estimation

We designed the study to take advantage of features of plotless or distance sampling (Anderson et al., 1983), but because of methodological limitations associated with low capture sucess, limited our analyses to a more simple index of density. The minimum number of individuals known to be alive (Nmm) on each grid per trapping session was used as the index of density. This enumeration method has been used serveral times in studies concerning dispersal (Krebs et al., 1976; Stafford and Stout, 1983; Williams and Cameron, 1984) and demography (Krebs, 1966; Yahner, 1983). Moreover, recent studies indicate density estimates based upon Nmm and more complex procedures (JOLLY method for open populations, CAPTURE method for closed populations) are highly correlated; in fact, the magnitude of these estimates differed by no more than 30 percent (S. Blair, personal communication). Regardless, the bias of underestimating density should be consistent in all treatments, thereby not affecting statistical comparisons.

Statistical Methods

Model 1 two-way (tebuthiuron treatment versus season) Analysis of Variance (ANOVA) was used to compare the relative abundance after arcsine (angular) transformation (Sokal and Rohlf, 1981) of shrubs, forbs, and grasses, as estimated from the step-point analysis, using SAS program Proc GLM (SAS Institute, 1985). Four ecological variables (vertical height, percent bare ground, percent ground litter, and percent canopy cover) were analyzed via mixed-model nested two-way ANOVA, with two grouping factors and one trial factor (BMDP2V — Dixon and Brown, 1979). The two grouping factors were tebuthiuron

SMALL MAMMAL DENSITIES

33

Table I. Pure model 1 two-way (tebuthiuron treatment versus season) ANOVA calculated for percent occurrence of forbs, grasses, and shrubs, respectively.

Source of variation

DF

Sum of squares

Mean squares

F

Significance

Forbs

Treatment (T)

I

0.151

0.151

6.02

0.049

Season (S)

2

0.280

0.140

5.59

0.043

Tx S

2

0.217

0.109

4.33

0.068

Error

6

0.150

0.025

Grasses

Treatment (T)

1

0.405

0.405

27.30

0.002

Season (S)

2

0.419

0.210

14.14

0.005

T X S

2

0.265

0.133

8.95

0.016

Error

6

0.089

0.015

Shrubs

Treatment (T)

1

0.083

0.083

111.11

0.001

Season (S)

2

0.010

0.005

0.67

0.548

Tx S

2

0.046

0.023

3..09

0.120

Error

6

0.045

0.007

treatment and season; the trial factor comprised two grids within each treatment. The vertical height analysis had 30 replications within each grid, whereas each of the other three variables had 19 replications within each grid. Percent bare ground, percent canopy cover, and percent ground litter were subjected to arcsine transformation, in order to meet the assumption of normality and homoscedasticity for ANOVA (Sokal and Rohlf, 1981); tebuthiuron treatment and season were the main effects tested. Population size, as estimated by minimum numbers known to be alive, was analyzed by model I two-way ANOVAs (tebuthiuron treatment versus season), followed by the Welsch Step-Up Procedure, a multiple comparison test (Sokal and Rohlf, 1981). Regardless of the significance of the ANOVA, a series of more powerful a priori comparisons of tebuthiuron treated versus control grid densities were conducted within each season (Sokal and Rohlf, 1981).

A Chi-Square Contingency Test (Zar, 1984) was conducted on the number of initial captures of each species for each season to determine if tebuthiuron treatment affected small mammal species composition. If a significant Chi-square value resulted, the proportional contribution of each species to the overall Chi-square value was calculated as a means of identifying taxa affecting significance.

Results and Discussion

Floral Analysis

Composition. — The two-way ANOVA for shrub abundance (Table 1, Fig. 1) indicated a highly significant difference {P — 0.001) between tebuthiuron treated and untreated areas. Season did not have an effect {P — 0.548), and season affected all tebuthiuron treated and untreated areas equally, as no evidence suggested an interaction {P = 0.120). Grass abundance also showed effects of tebuthiuron treatment (Table 1); however, the treatment effects were dependent on season (the interaction was significant, P — 0.016). In particular, the greater abundance of

34

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Winter

I Grasses ^ Forbs ^ Shrubs

Spring

H Grasses ^ Forbs ^ Shrubs

100

Untreated I Untreated II Treated I Treated II

Fall

H Grasses ^ Forbs ^ Shrubs

100 .1 8 0

Untreated I Untreated II Treated I Treated II

Figure 1. Seasonal comparison (winter, spring, and autumn) of floral composition (percent grasses, forbs, and shrubs) in the tebuthiuron treated and untreated grids within the sand shinnery oak woodland.

grasses in tebuthiuron treated grids is magnified in the winter compared to spring or autumn (Fig. 1). The percentage of forbs was significantly different between tebuthiuron treated and untreated areas {P = 0.049); a consistent difference existed among seasons (P — 0.043) as well (Table 1, Fig. 1). In general, tebuthiuron treatment caused a singificant change in shrub, grass, and forb abundance, whereas seasonal variation existed in forb and grass abundance, but not in shrub abundance.

Habitat variables. — Four habitat variables (percent bare ground, percent canopy coverage, percent ground litter, and vertical height) were chosen to evaluate differences between untreated sand shinnery oak and

SMALL MAMMAL DENSITIES

35

tebuthiuron treated areas of shrub removal. The data for each variable were compared in a mixed model two-way nested ANOVA, evaluating differences related to tebuthiuron treatment, season, and grids within treatment (Table 2). Percent ground litter was not significant for main treatment effects or for interaction terms. Percent bare ground also was not significantly different between tebuthiuron treatments {P — 0.092); however, a significant difference among seasons was detected {P = 0.003). No interaction between tebuthiuron treatment and season occurred {P = 0.253). Vertical height was significantly different only between tebuthiuron treated and control areas (P = 0.012). Percent canopy cover was significantly different between tebuthiuron treated and control areas (P = 0.005), as well as among seasons {P — 0.006). These differences were consistent in that the interaction between tebuthiuron treatment and season was not significant (P = 0.588). Canopy cover and vertical height were the only variables significantly affected by tebuthiuron application, whereas canopy cover and bare ground were the only variables that differed seasonally. No evidence of interaction between tebuthiuron treatment and season existed for any of the four habitat variables.

Faunal Analysis

Species composition. — Rodent species captured in the treated area included D. ordii, O. leucogaster, Perognathus flavescens, R. montanus, N. micropus, S. hispidus, and Peromyscus maniculatus. D. ordii was the most often captured every season, except in the autumn, when O. leucogaster was trapped most often. Both D. ordii and O. leucogaster were captured in every season. S. hispidus was caught only in summer. Perognathus flavescens was trapped in every season except winter. N. micropus and Peromyscus maniculatus were caught only in the spring (one individual of each species). R. montanus was captured in the spring and autumn.

The rodent species trapped in the untreated areas were D. ordii, O. leucogaster, P maniculatus, N. micropus, P flavescens, and Mus musculus. D. ordii and O. leucogaster were caught in every season, with D. ordii being captured most often every season except autumn, when trap success for every species was low. N. micropus was caught in winter, whereas M. musculus was caught only in the autumn. Overall, seven species were captured in treated areas and six species were trapped in control areas. In both areas, D. ordii was captured most often except in autumn. It is possible that O. leucogaster was more abundant in both the treated and control areas in the autumn because of an unusually high population of grasshoppers, one of their chief sources of food (Davis, 1978) in summer and early autumn. All species caught in the untreated areas were caught in the treated areas, with the exception of M.

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

< ^ > <u

5 -

Z o

< i

a;

.■H ^

T3 C w O

C/3 -rt

Cd c/2 (U <u

c/2 lU

60 3 (U c/2 -Q

^ u:

Q c . .

CU OJ

S 53

03 c/2

(U C

h o

:§ i <

UJ ^ ^ (U

^ S

. >»

i ^

^ §

cd

-o -O

o c

£ cd

-a .£

^ .5P

(U

^ f

. 03

CNI O

UJ

_) <L)

SC >

ti

— — o

s: 8

H c/^ H UJ a

SMALL MAMMAL DENSITIES

37

Table 3. Pure model I two-way ANOVA of variance (tebuthiuron treatment versus season) comparing density indexes for all species and only D. ordii based upon minimum number known to be alive (Nmm).

Source

DF

Sums of squares Mean squares

F

Significance

Total Species

Treatment (T)

I

506.25

506.25

27.37

0.001

Season (S)

3

2696.75

898.92

48.59

0.000

T X S

3

244.75

81.58

4.41

0.041

Error

8

148.00

18.50

Dipodomys ordii

Treatment (T)

1

441.00

441.00

22.05

0.002

Season (S)

3

2225.25

741.75

37.09

0.000

T X S

3

367.50

122.50

6.12

0.018

Error

8

160.00

20.00

musculus, whereas 5'. hispidus and R. montanus were the only species from the treated area that were not caught in the untreated area.

In Chi-Square Contingency Tests for spring, autumn, and winter, species composition (the number of individuals captured in summer was too low to perform a meaningful test), data from untreated grids were combined and data from control grids were combined. These combined data sets were tested as untreated and treated areas for each season. Species composition for the autumn season was independent of tebuthiuron treatment — 5.52, df = 2, P > 0.05). No significant difference in species composition existed between treated and untreated areas. In contrast, a highly significant difference in species composition (X^ — 165.23, df = 2; P < 0.001) occurred in winter because of tebuthiuron treatment. More than half the Chi-square value (approximately 56 percent) was attributable to the higher than expected number of O. leucogaster captured in the untreated areas, and about 30 percent was due to Peromyscus maniculatus and N. micropus, which were combined to meet Chi-square grouping rules (Sokal and Rohlf, 1981). The remainder was due to the larger than expected numbers of D. ordii in the treated areas. Species composition in spring was significantly different in treated and untreated areas {X^ = 7.04, df — 2, 0.05 > P > 0.025), also mainly because of the high numbers of O. leucogaster (approximately 71 percent). The remaining 29 percent of the deviation was due to the slightly higher numbers of D. ordii, P maniculatus, R. montanus, P flavescens, and N. micropus in the treated areas.

Species densities. — The way in which tebuthiuron affected rodent density (Nmin) depended upon season (a significant interaction, P = 0.041; Table 3); for the most part, the interaction related to the magnitude of the effect of the conversion, rather than to its direction. Mean densities

38

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Winter

Spring

Untreated Treated Untreated Treated

Summer

Fall

50

40

>.

! _ II I _ II

Untreated Treated

Untreated Treated

Figure 2. Seasonal comparison (winter, spring, summer, and autumn) of densities (Nmm) of all rodents in the tebuthiuron treated and untreated grids within the sand shinnery oak woodland.

of all rodents were never higher in the untreated areas compared to the treated areas (Fig. 2). The two-way ANOVA for abundances (Nmin) of D. ordii (Table 3) produced similar results, with a significant season by tebuthiuron treatment interaction (P = 0.018). Again, the interaction was affected mostly by variation in the degree to which tebuthiuron areas exhibited elevated densities compared to untreated areas. The average density of D. ordii on the untreated areas never exceeded that of the treated areas (Fig. 3). Orthogonal a priori comparisons (Sokal and Rohlf, 1981) of total rodent numbers within each season indicated significant differences between treated and untreated areas in all seasons except summer; whereas, only winter showed a significant difference in D. ordii densities between treated and control areas. The Welsch Step-Up Procedure (Sokal and Rohlf, 1981), although less powerful than either ANOVA or a priori comparisons, revealed seasonal differences in rodent density within treated and within control areas in tebuthiuron treated areas. Winter and summer total density indices were each significantly different {P < 0.05) from all other seasons; whereas autumn and spring total density indices were statistically indistinguishable {P > 0.05). In the control areas, winter total density was significantly different than both spring and summer total density indices, whereas spring, summer, and autumn densities were statistically indistinguishable. In tebuthiuron

SMALL MAMMAL DENSITIES

39

Winter

1 _ II I _ II

Untreated Treated

Spring

Summer

Fall

w 30 c

(U

Q 20

I _ II

Untreated

_ II

Treated

Untreated Treated

Figure 3. Seasonal comparison (winter, spring, summer, and autumn) of densities (Nmm) of Dipodomys ordii in the tebuthiuron treated and untreated grids within the sand shinnery oak woodland.

treated areas, the winter density index for D. ordii was statistically different than that for all other seasons; spring, summer, and fall density indices for D. ordii were statistically indistinguishable. In untreated areas, density indices of D. ordii were statistically indistinguishable.

Overview

Canopy cover and vertical height are important determinants of rodent species composition and abundance in arid or semiarid environments (Rosenzweig and Winakur, 1969; Price, 1978). In our study also, rodent densities were higher in treated areas, which exhibited greater canopy cover and vertical height, than in control areas in each season. D. ordii abundance paralleled seasonal changes in percent bare ground; abundance was highest in the winter, followed by autumn, spring, and summer. This parallels observations that Merriam’s kangaroo rat {D. merriami), a congener of D. ordii, had an affinity for feeding in areas of bare ground (Rosenzweig and Winakur, 1969; Rosenzweig, 1975).

Like other studies involving habitat manipulation (Rosenzweig and Winakur, 1969; Brown et al., 1972; Rosenzweig et al., 1975; Feldhamer, 1979), rodent species composition and density were affected by tebuthiuron-induced changes in sand shinnery oak habitats. Rodent numbers increased in tebuthiuron treated areas. Moreover, seasonal

40

THE TEXAS JOURNAL OE SCIENCE— VOL. 45, NO. 1, 1993

fluctuations in Nmin, whether indicative of behavioral responses or actual changes in density, were different in treated and untreated areas, as indicated by significant interaction terms in the ANOVA and detailed in A priori and A posteriori analyses. In part, these results contrast with those of Parmenter and MacMahon (1983), who found that shrub removal had no effect on Peromyscus maniculatus, Perognathus parvus, Onychomys leucogaster, and Spermosphilus armatus in a sagebrush dominated shrub-steppe ecosystem in southwestern Wyoming. In that study, shrubs were removed manually without subsequent replacement by other plants. In our study, the absolute increase in rodent density may be a response to shrub removal, the subsequent dominance of the grass component, or both. As in the work of Parmenter and MacMahon (1983), species composition did not change in our study. Rosenzweig and Winakur (1969) considered the biotic variability of arid environments to provide many opportunities for specialization. The sand shinnery oak ecosystem, as well as the mid-grass prairie that resulted from tebuthiuron application, was dominated by generalist rodents. D. ordii has been described as a good generalized competitor (Garner, 1974, Parmenter and MacMahon, 1983); the other species caught in this study are plastic with regard to food and habitat requirements (Davis, 1978).

We hypothesize that the rodent community that presently characterizes sand shinnery oak habitats is a relictual subset of the original mid-grass prairie rodent fauna that was sufficiently generalized to survive the transition. More specialized rodents may not have persisted in the shrub- dominated flora and insufficient time has elapsed to obtain a shrub- adapted suite of species. It is not surprising then, to observe no change in species composition as a result of tebuthiuron-induced reconversion to mid-grass prairie. The effect of the drastic alteration in plant species composition and structure was to differentially modify constituent species abundances, rather than alter species composition.

Acknowledgments

We are grateful to E. E. Cheslak, R. K. Chesser, D. A. Hall, J. K. Jones, Jr., and two anonymous reviewers for helpful advice. The Texas Tech Range and Wildlife Department provided some financial support. The Bureau of Land Management and Loyola University of New Orleans provided Sherman traps; field research would not have been possible without such assistance. We are indebted to R. Weeks for her many hours of help in the field; M. Goines, T. Nicholson, M. Van Staaden, M. Gannon, S. Pettit, and D. McCullough also provided much appreciated field assistance. S. B. Cox prepared the figures, and M. P. Moulton assisted with statistical analyses. Support for RDS was obtained through a Graduate Fellowship from the Insitiute for Environmental Sciences, Texas Tech University.

Literature Cited

Abramsky, Z. 1988. The role of habitat and productivity in structuring desert rodent communities. Oikos, 52:107-1 14.

SMALL MAMMAL DENSITIES

41

Allred, D. M., and D. E. Beck. 1963. Ecological distribution of some rodents on the Nevada atomic test site. Ecology, 44:21 1-214.

Anderson, D. R., K. P. Burnham, G. C. White, and D. L. Otis. 1983. Density estimation of small mammal populations using a trapping web and distance sampling methods. Ecology, 64:674-680.

Brown, J. H. 1973. Species diversity of seed-eating desert rodents in sand dune habitats. Ecology, 54:775-787.

Brown, J. H., G. A. Leiberman, and W. G. Dengler. 1972. Woodrats and cholla: dependence of a small mammal population on the density of cacti. Ecology, 53:310-313.

Brown, J. H., and Z. Zeng. 1989. Comparative population ecology of eleven species of rodents in the Chihuahuan desert. Ecology, 70:1507-1525.

Brown, J. S. 1989. Desert rodent community structure: a test of four mechanisms of coexistence. Ecol. Monogr., 59:1-20.

Christian, D. P. 1977. Effects of fire on small mammal populations in a desert grassland. J. Mamm., 58:423-427.

Colbet, R. L. 1986. The effect of the shrub component on small mammal populations in a sand shinnery oak ecosystem. Unpublished M. S. thesis, Texas Tech Univ., Lubbock, 49 pp.

Daubenmire, R. 1968. Plant communities: a textbook of plant synecology. Harper and Row, New York, 300 pp.

Davis, W. B. 1978. The mammals of Texas. Bull. Texas Parks and Wildlife, 41:1-294.

Dixon, W. J., and M. B. Brown. 1979. BMDP-79: Biomedical computer programs P-series.

University of California Press, Berkeley, 880 pp.

Elwell, J. M. 1964. Oak brush control improves grazing lands. Agron. J., 56:41 1-415. Feldhamer, G. A. 1979. Vegetative and edaphic factors affecting abundance and distribution of small mammals in southeastern Oregon. Great Basin Nat., 39:207-221.

Garner, H. W. 1974. Population dynamics, reproduction, and activities of the kangaroo rat, Dipodomys ordii, in western Texas. Grad. Studies, Texas Tech Univ., 7:1-28.

Gashwiler, J. S. 1970. Plant and mammal changes on a clearcut in west-central Oregon. Ecology, 51:1018-1026.

Hafner, M. S. 1977. Density and diversity in Mojave Desert rodent and shrub communities. J. Anim. Ecol., 46:925-938.

Hansen, R. M., and A. L. Ward. 1966. Some relations of pocket gophers to rangelands on Grand Mesa, Colorado. Tech. Bull., Colorado Agric. Exp. Sta., 88:1-20.

Johnson, D. R. 1964. Effects of range treatment with 2, 4-D on food habits of rodents. Ecology, 45:241-249.

Johnson, D. R., and R. M. Hansen. 1969. Effects of range treatments with 2, 4-D on rodent populations. J. Wildlife Manag., 33:125-132.

Jones, V. E., C. H. Meadors, and P. W. Jacoby. 1978. Pelleted herbicides for control of sand shinnery oak {Quercus havardii). Proc. 31st Meeting Soc. Range Manag., 31:59. Jones, V. E., and R. D. Pettit. 1984. Low rates of tebuthiuron for control of sand shinnery oak. J. Range Manag., 37:488-490.

Krebs, C. J. 1966. Demographic changes in fluctuating populations of Microtus californicus. Ecol. Monogr., 36:239-273.

Krebs, C. J., 1. Wingate, J. LeDuc, J. A. Redfield, M. Taitt, and R. Hilborn, 1976. Microtus population biology: dispersal in fluctuating populations of M. townsendii. Canadian J. Zook, 54:79-95.

McGee, J. M. 1982. Small mammal populations in an unburned and early fire successional sagebrush community. J. Range Manag., 35:177-179.

Mcllvain, E. H. 1954. Interim report on shinnery oak control studies in the southern Great Plains. Proc. North-central Weed Control Conf., 11:95-96.

M’Closkey, R. T. 1975. Habitat succession and rodent distribution. J. Mamm., 56:950-955.

42

THE TEXAS JOURNAL OF SCIENCE- VOL. 45, NO. I, 1993

M’Closkey, R. T, and D. T. Lajoie. 1975. Determinants of local distribution and abundance in white-footed mice. Ecology, 56:467-472.

Morris, D. W. 1979. Microhabitat utilization and species distribution of sympatric small mammals in southwestern Ontario. Amer. Midland Nat., 101:373-384.

Munger, J. C., M. A. Bowers, and W. T. Jones. 1983. Desert rodent populations: factors affecting abundance, distribution, and genetic structure. Great Basin Nat. Mem., 7:91- 1 16.

Parmenter, R. R., and J. A. MacMahon. 1983. Factors determining the abundance and distribution of rodents in a shrub-steppe ecosystem: the role of shrubs. Oecologia, 59:145- 156.

Pettit, R. D. 1975. Comparative effects of two pelleted herbicides on a shin oak community. Res. Highlights, Texas Tech Univ., 6:43.

Price, M. V. 1978. The role of microhabitat in structuring desert rodent communities. Ecology, 59:910-921.

Price, M. V., N. M. Waser, and T. A. Bass. 1984. Effects of moonlight on microhabitat use by desert rodents. J. Mamm., 65:353-356.

Rechenthin, C. A., and H. N. Smith. 1967. Grassland restoration. V. Effect on yield and water supply. U. S. Dept. Agric., Soil Conserv. Serv., Temple, Texas, 46 pp.

Reynolds, H. G. 1958. The ecology of the Merriam’s kangaroo rat {Dipodomys merriami Means) on the grazing lands of Arizona. Ecol. Monogr., 28:1 1 1-127.

Robison, E. D., and C. E. Fisher. 1968. Chemical control of sand shinnery oak and related forage production. Brush Research in Texas, PR-2583:5-8.

Rosenzweig, M. L. 1973. Habitat selection experiments with a pair of coexisting heteromyid rodent species. Ecology, 54:145-156.

Rosenzweig, M. L., and J. Winakur. 1969. Population ecology of desert rodent communities: habitats and environmental complexity. Ecology, 44:211-214.

Rosenzweig, M. L., B. Smigel, and A. Kraft. 1975. Patterns of food, space, and diversity. Pp. 241-268, in Rodents in desert environments (1. Prakosh and P. K. Ghosh, eds.), W. Junk, The Hague, The Netherlands, 624 pp.

SAS Institute, Inc. 1985. SAS^ user’s guide: Statistics. Version 5. Cary, North Carolina, 956 pp.

Schroder, G. D. 1987. Mechanisms for coexistence among three species of Dipodomys: habitat selection and an alternative. Ecology, 68:1071-1083.

Sokal, R. R., an F. J. Rohlf. 1981. Biometry. W. H. Freeman and Company, San Francisco, California, 859 pp.

Stafford, S. R., and 1. J. Stout. 1983. Dispersal of the cotton rat, Sigmodon hispidus. J. Mamm., 64:210-217.

Sullivan, T. P. 1979. Demography of populations of deer mice in coastal forest and clear- cut (logged) habitats. Canadian J. Zook, 57:1636-1648.

Sullivan, T. R, and D. S. Sullivan. 1982. Responses of small mammal populations to a forest herbicide application on a 20 year old conifer plantation. J. Applied Ecol., 19:95-106.

Thompson, S. D. 1982. Microhabitat utilization and foraging behavior of bipedal and quadrupedal heteromyid rodents. Ecology, 63:1303-1312.

Tietjen, H. R, C. H. Halvorson, R. L. Hedgal, and A. M. Johnson. 1967. 2, 4-D herbicide, vegetation, and pocket gopher relationships. Black Mesa Colorado. Ecology, 48:634-643.

Van Horne, B. 1981. Demography of Peromyscus maniculatus populations in serai stages of coastal coniferous forest in southeast Alaska. Canadian J. Zool, 59:1045-1061.

Williams, L. R., and G. N. Cameron. 1984. Demography of dispersal in Attwater’s pocket gopher {Geomys attwateri). J. Mamm., 65:67-75.

Wywialowski, A. P. 1987. Habitat structure and predators: choices and consequences for rodent habitat specialists and generalists. Oecologia, 72:39-45.

Yahner, R. H. 1983. Population dynamics of small mammals in farmstead shelterbelts. J. Mamm., 64:380-386.

SMALL MAMMAL DENSITIES

43

Zar, J. H. 1984. Biostatistical analysis. Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 718 pp.

Zou, J., J. T. Flinders, H. L. Black, and S. G. Whisenant. 1989. Influence of experimental habitat manipulations on a desert rodent population in southern Utah. Great Basin Nat., 49:435-448.

FOOD HABITS OF MALE BIRD-VOICED TREEFROGS, HYLA AVIVOCA (ANURA: HYLIDAE), IN ARKANSAS

David H. Jamieson, Stanley E. Trauth,

AND Chris T. McAllister

Department of Biological Sciences, Arkansas State University,

State University, Arkansas 72467 and Renal- Metabolic Laboratory (151-G), Department of Veterans Affairs Medical Center,

4500 S. Lancaster Road, Dallas, Texas 75216 (CTM)

Abstract. — We analyzed stomach contents of 56 male bird-voiced treefrogs, Hyla avivoca Viosca, 1928, collected in late spring and early summer 1991 from sites in central Arkansas. Ants and beetles were the predominant prey items; the next most abundant group was caterpillars. Ants of the genus Cremastogaster were the most common food source. Several ground-dwelling arthropods found in the diet of other species of Hyla were noticeably absent from the diet of H. avivoca. Our data suggest that calling male H. avivoca forage largely on tree-dwelling insects when compared to sympatric Hyla, and this mode may be a reflection of habitat preference and prey availability rather than prey selection. Key words: ants; arthropods; insects; bird-voiced treefrog; Hyla avivoca', food habits; diet.

The bird-voiced treefrog, Hyla avivoca Viosca, 1928, has a sporadic and poorly-documented distribution in three states (Arkansas, Louisiana, and Oklahoma) from which the species is known west of the Mississippi River (Smith, 1966; Dundee and Rossman, 1989; Conant and Collins, 1991). Recent investigations by Trauth and Robinette (1990a, 1990b) into the distribution and life history of Arkansas populations have revealed this frog to be more common in the state than previously understood. The species generally inhabits large rivers, headwater swamps, and swampy floodplains and lakes in Arkansas. Although much has been documented about the natural history and ecology of other Hyla species, little ecological data are available for bird-voiced treefrogs, and nothing, to our knowledge, is known about the diet of H. avivoca.

The diet of gray treefrogs, Hyla versicolor LeConte, 1825, and H. chrysocelis Cope, 1880 (considered by some to be the closest living relatives of H. avivoca), was investigated in Texas by Ralin (1968). He reported that both species fed extensively on click beetles (Coleoptera: Elateridae) and harvester ants (Pogonomyrmex). He also concluded that both species fed not only while perched on the bark and foliage of trees, but also, to some extent, on the ground. Interestingly, harvester ants are not known to occur in trees; rather, they are especially common in cultivated fields and bare sandy areas near roads. In addition, other insect orders found in stomachs included grasshoppers (Orthoptera — only in H. versicolor), flies (Diptera), and caterpillar larvae (Lepidoptera). Brown (1974) studied the food habits of several anurans from southeastern Arkansas. He found that green treefrogs, H. cinerea (Schneider, 1792), primarily fed on insects found on the leaves of plants and included leafhoppers (Homoptera:

46

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. I, 1993

Cicadellidae), acridid grasshoppers, caterpillars, leaf beetles (Chrysomelidae), and spiders (Arachnida). Hyla cinerea consumed mosquitoes (Diptera: Culicidae) and houseflies (Muscidae) in laboratory experiments (Freed, 1980). Freed also reported that prey selection was dependent upon prey species activity rather than size of prey. In a field study. Freed (1982) reported that H. cinerea consumed mostly noctuid caterpillars, cantharid beetle larva, and, to a lesser extent, field crickets (Orthoptera: Gryllidae) and stink bugs (Hemiptera; Pentatomidae).

The purposes of our study are to describe feeding patterns for calling male H. avivoca from central Arkansas and compare our results with what is known of diet in other arboreal treefrogs.

Materials and Methods

Fifty-six adult and juvenile male H. avivoca were collected from six localities in four counties (Conway, Faulkner, Grant, and Monroe) of central Arkansas from late May through mid- July, 1991. All frogs were taken from breeding colonies at dusk or after dark as they called several meters above water from trees (primarily cypress-tupelo gums) and buttonbush shrubs (Cephalanthus). Specimens were placed in plastic bags on ice following capture and processed within 24 hours in the laboratory at Arkansas State University (ASU). Frogs were killed in a dilute chloretone solution and fixed in 10 percent formalin for 48 hours prior to examination. Stomachs were removed and stored in individual vials containing 70 percent ethanol. Food items were removed, counted, and identified using a binocular dissecting microscope and dichotomous keys provided in Creighton (1950), Cook (1953), and Borrer et al. (1989). A food item consisted of a whole specimen or parts representing a whole specimen. Whenever possible, insect taxa were identified to family. Prey availability was not assessed quantitatively; however, some qualitative assessment was made by collecting and observing arthropods in each study area. Voucher specimens of H. avivoca are deposited in the ASU Herpetological Museum (nos. 17766; 17770-77; 17787-88; 17799-17834; 17860-67).

Results and Discussion

Of the 56 stomachs examined, 51 (91 percent) contained food items. There was an average of 3.9 food items per stomach (range one to 14). The total number and percent occurrence of food items are given in Table 1. Ants (Hymenoptera; Formicidae) and beetles (Coleoptera) were the predominant prey items; the next most abundant group was lepidopteran larvae. Of the 75 identifiable ants, 72 (96 percent) were Cremastogaster workers ranging from 2.5 to 4.0 mm in length. According to Cook (1953), these ants often nest under the bark of trees and can be recognized by the unique way they climb trees in straggling files. Ant collections made at one locality revealed that Cremastogaster workers were common on the bark and foliage of trees in a cypress-tupelo swamp. As many as 14 Cremastogaster were found in a single stomach and several stomachs contained only Cremastogaster. Although ant trails were observed on the surfaces of several overhanging tree limbs, it was not determined whether these ants were being consumed as they crawled near the frogs or whether the frogs actively pursued them. Duellman and Trueb (1986) suggested

FOOD HABITS OF HYLA AVIVOCA

47

Table 1. Percent occurrence and total number of food items from stomachs of male Hyla avivoca from Arkansas.

Taxa

Percent

occurrence

in stomachs {N=5\)

Total number of

food items

Arachnida

Acarina

5.4

8

Araneae

1.8

1

Insecta

Unidentifiable

7.1

4

Coleoptera

Cantharidae

1.8

1

Carabidae

1.8

1

Coccinellidae

9.0

5

Cucujidae

5.4

3

Elateridae

5.4

3

Unidentifiable

9.0

5

Hemiptera

Reduviidae

1.8

1

Homoptera

Cicadellidae

1.8

1

Hymenoptera

Formicidae

Cremastogaster

39.3

72

Camponotus

5.4

3

Unidentifiable

28.6

36

Lepidoptera

Geometridae (larvae)

3.6

2

Unidentifiable (larvae)

9.0

5

Unidentifiable (adult)

1.8

1

Odonata

Zygoptera

Unidentifiable

1.8

1

Psocoptera

3.6

4

that some ant specialists, particularly several microhylid species, locate ant trails by olfaction. The frogs may then “sit and wait” to capture ants as they pass by. Although the method of ant capture by H. avivoca remains unclear, the large number of ants in several stomachs suggest that H. avivoca may sit motionless near ant trails and consume individuals of Cremastogaster as they parade by in their characteristic straggling files. However, frogs might locate and raid ant nests under the bark of trees. Our data do not distinguish between these alternatives.

Coccinellids were the most frequently encountered beetles followed by cucujids and elaterids. Coccinellids (ladybird beetles) are predaceous and are abundant on aphid-infested vegetation whereas cucujids (flat bark

48

THE TEXAS JOURNAL OF SCIENCE—VOL. 45, NO. 1, 1993

beetles) occur on and under the bark of trees where they feed on mites and other small insects (Borrer et al., 1989). Ralin (1963) stated that elaterids (click beetles) are generally arboreal and concluded that H. chrysoscelis was a more arboreal feeder than H. versicolor based on a higher percent occurrence of elaterids in the diet of H. chrysoscelis.

Unfortunately, because lepidopteran larvae are soft-bodied and digested quickly, few in our sample were identifiable to family. Lepidopteran larvae were encountered in 1 1 percent of the stomachs of H. avivoca compared to 15 percent reported by Freed (1982) for H. cinerea and 21 percent and 14 percent for H. chrysoscelis and H. versicolor, respectively (Ralin, 1968).

The presence of barklice (Psocoptera) and flat bark beetles indicate that H. avivoca spends some time feeding on insects that occur in the tree- bark crevice microhabitat. Unlike H. avivoca, these insects were not consumed by H. cinerea, H. chrysoscelis, or H. versicolor (Brown, 1974; Freed, 1982).

There was a conspicuous absence of ground-dwelling arthropods in the diet of H. avivoca when compared to other Hyla species. Moreover, data from Table 1 suggest that during the study period, H. avivoca rarely, if ever, fed on the ground. This feeding mode may be a reflection of habitat preference rather then prey selection. Unlike H. chrysoscelis, H. cinerea, and H. versicolor (all of which are found in a myriad of habitats), H. avivoca seems to be restricted to old-growth, cypress-tupelo swamps or similar aquatic habitats in Arkansas. Because of the permanent-water situation associated with these environments, H. avivoca would have to leave the site (or, at least, the inundated areas) in order to encounter ground¬ dwelling insects. Thus, calling males appear to have a foraging tactic that precludes movement away from breeding perches or sites. During the breeding season, calling males appear to specialize on arboreal insects in central Arkansas. However, the diet of calling male frogs may not be typical of the population in general that does not approach a maintenance diet. Thus, additional study utilizing female H. avivoca and males collected outside the breeding season is certainly warranted.

Acknowledgments

We thank J. W. Robinette for field assistance and the Arkansas Game and Fish Commission for Scientific Collecting Permits nos. 34 and 1114 to Trauth and McAllister, respectively. This study was funded by a grant (F90-3) from the Arkansas Nongame Preservation Committee to Trauth.

Literature Cited

Borror, D. J., C. A. Triplehorn, and N. F. Johnson. 1989. An introduction to the study of insects. Saunders College Publ., Philadelphia, 875 pp.

Brown, R. L. 1974. Diets and habitat preferences of selected anurans from southeast Arkansas. Amer. Midland Nat,, 91:468-473.

FOOD HABITS OF HYLA AVIVOCA

49

Conant, R., and J. T. Collins. 1991. A field guide to reptiles and amphibians of eastern and central North America. Houghton Mifflin Co., Boston, 3rd ed., 450 pp.

Cook, T. W. 1953. The ants of California. Pacific Books, Palo Alto, 391 pp.

Creighton, W. S. 1950. The ants of North America. Bull. Mus. Comp. Zool., 104:1-569. Duellman, W. E., and L. Trueb. 1986. Biology of amphibians. McGraw-Hill, New York, 670 pp.

Dundee, H. A., and D. A. Rossman. 1989. The amphibians and reptiles of Louisiana. Louisiana State Univ. Press, Baton Rouge, xi + 300 pp.

Freed, A. N. 1980. Prey selection and feeding behavior of the green treefrog {H. cinerea). Ecology, 61:461-465.

- . 1982. A treefrogs menu: selection for an evenings meal. Oecologia, 53:20-26.

Ralin, D. B. 1968. Ecological and reproductive differentiation in the cryptic species of the Hyla vfr^/co/or complex (Hylidae). Southwestern Nat., 13:283-300.

Smith, P. W. 1966. Hyla avivoca. Cat. American Amph. Rept., 28.1-28.2.

Trauth, S. E., and J. W. Robinette. 1990a. Notes on distribution, mating activity, and reproduction in the bird-voiced treefrog, Hyla avivoca, in Arkansas. Bull. Chicago Herpetol. Soc., 25:218-219.

- . 1990b. Geographic distribution: Hyla avivoca (Bird-voiced Treefrog). Herpetol. Rev.,

21:95.

AMERICAN ALLIGATOR {ALLIGATOR MISSISSIPPIENSIS) NESTING AT AN INLAND TEXAS SITE

Louise A. Hayes-Odum, Debra Valdez, Marjorie Lowe, Loretta Weiss, Patricia H. Reiff and Dennis Jones

Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, Texas 77843 (LAH-M), Houston Independent School District, Houston, Texas (DV, ML), Department of Space Physics and Astronomy, Rice University, Houston, Texas 77251 (LW, PHR), and Brazos Bend State Park, Needville, Texas ( DJ)

Abstract. — Fifteen alligator nests were examined at Brazos Bend State Park, Fort Bend Co., Texas, in 1990. One nest fell victim to a predator, apparently a raccoon. Of 13 nests monitored during incubation, adult alligators were observed in close proximity of 12, and two nests were actively defended by adults. Islands were more commonly used nesting sites (II) than banks (four); only those water bodies without islands contained bank nests. One nest was opened prior to hatching by an adult after approximately 75 days of incubation, and intact eggs removed from the nest hatched four to 12 days later. Key words: Alligator mississippiensis; American alligator; nests; nesting ecology; Texas.

One of the earliest accounts of alligator nests and eggs was by Reese (1907). He discussed time of nesting, nest materials, location, size of nest, numbers and dimensions of eggs, and vocalization of the young before hatching. Mcllhenny (1935) collected similar information in greater depth, had observations on nest defense by the adult, and took nest temperatures. The first quantitative study was by Joanen (1969), who examined 315 nests from 1964 to 1968 in a Louisiana coastal marsh, and is the most comprehensive to date. Other studies were conducted in Georgia (Metzen, 1977; Ruckel and Steele, 1984), South Carolina (Bara, 1972), and Florida (Hines et al., 1968; Fogarty, 1974; Goodwin and Marion, 1978; Deitz and Heinz, 1980). Additionally, there are a substantial number of publications dealing with certain aspects of nesting biology.

In Texas, there have been aerial nests counts conducted by the Texas Parks and Wildlife Department (TPWD) since the 1970s for use as a census tool (Potter, 1975, 1981; Thompson et al., 1984; Johnson et al., 1989). There are limited data for coastal habitat on nesting materials, clutch size, and fertility rates (Johnson et al., 1989; TPWD unpublished data). However, inland sites with heavy tree cover cannot be adequately censused by air, and there is a paucity of information on alligator nesting in this type of habitat in Texas.

In this study, nests were examined in 1990 at Brazos Bend State Park. Prior to the opening of the park in 1984, Onadeko (1983) collected limited data on four nests at Brazos Bend. From 1986 to 1989, dates of nest completion and hatching were noted due to concern for park visitors coming in contact with an adult guarding a nest or hatchlings. Known nest locations were mapped, but no diligent effort was made to find nests located on

52

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

islands not easily visible from shore. The focus of this research was to study aspects of nesting ecology at the park (such as location and composition of nests, time of nesting and incubation period, and adults associated with nests) and correlate it with previous studies from other states. Possible changes and adaptations in nesting due to human contact are discussed.

Materials and Methods

Brazos Bend State Park is located in Fort Bend County, Texas, less than 32 kilometers southwest of the Houston city limits, and consists of 1982 hectares of Brazos River floodplain and upland coastal prairie. The park is on the Texas Gulf Coastal Plain and is included in the Coastal Prairie Vegetational Region (Singleton et al., 1978). Major water bodies in the park are a freshwater marsh (Pilant Lake), three lakes (Elm, Forty Acre, Creekfield), four oxbows (Old Horseshoe, New Horseshoe, Hale, Creekwood), and a slough (Pilant Slough). Tall grass prairie uplands form seasonal wetlands as a result of water-filled depressions and swales. The Brazos River forms the eastern park boundary, and Big Creek and various bayous transect the park. A series of foot trails intersperse the marsh, lakes, and two oxbows, and border at least a portion of their perimeters, sometimes in the form, of levees between adjacent impoundments.

Mixed hardwoods, including water oak {Quercus nigra), pecan {Carya illinoiensis), elm (Ulmus sp.) Shumard oak (Q. shumardii), burr oak (Q. macrocarpa), and various shrubs and vines are associated with the Brazos River bottomland and parts of Big Creek. Gallery forests along the waterways consist of sycamore (Platanus occidentalis), eastern cottonwood (Populus deltoides), and black willow (Salix nigra). Live oak (Q. virginiana), along with vines and Spanish moss (Tillandsia usneoides), characterizes the meander escarpment of the Brazos River.

We estimated the park’s alligator population to be between 1000 and 1500 animals as calculated through censuses, and the nest multiplying factors of Taylor and Neal (1984).

Nests were located by walking the perimeters of the major bodies of water that contained the majority of the alligator popultion, and consecutive numbers were assigned in the order of discovery. Binoculars were used to find and observe nests on islands, and a visit was made at least once to each island by boat (nonmotorized) when possible for closer examination of known nests and to find additional nests not visble from the shore. Nesting materials, condition, and appearance of the nests, as well as the presence, size, and behavior of adults associated with the nests were noted. Nests accessible on foot were visited one or more times per week; a 35-mm camera and two VCR camcorders were used to document visual data.

Results

Location of Nests

Fifteen nests were discovered at Brazos Bend State Park in 1990. A summary of pertinent information for each nest is listed in Table 1. Nest location is shown in Figure 1. All nests were shaded by at least partial tree cover. Eleven nests (73 percent) were on islands and four nests (27 percent) were bank nests. Bank nests were accessible by foot and were found only on the lakes without islands — Creekfield, which has a long, narrow shape, and New Horseshoe and Creekwood, both oxbows. Islands were rectangular-shaped spoilbanks of varying length and width, except for some islands at Pilant Lake, which were circular mounds built for alligator basking. Some of the islands in the center of Elm Lake (location

NESTING IN THE AMERICAN ALLIGATOR

53

Table 1. Summary of nest information for 1990 at Brazos Bend State Park.

Nest

Location

Island /bank

Nest composition

Size class of adult at nest

Fecal meterial on nest

1

Forty Acre

island

dirt with vines and blackberry

1.8 -2.1 m

yes

2

Forty Acre

island

dirt with vines and blackberry

2.4 - 2.7 m

yes

3

Forty Acre

island

dirt with vines and willow

1.8 -2.1 m

no

4

Creekfield

bank

dirt with sticks and blackberry

1.8 -2.1 m

no

5

Elm

island

dirt with some vegetation

1.8 -2.1 m

yes

6

N. Horseshoe

bank

Johnson and paspalum grass

1.8 -2.1 m

no

7

N. Horseshoe

bank

Johnson and paspalum grass

2.1 -2.4 m

no

8

Pilant

island

dirt with some vegetation

2.4 - 2.7 m

unknown

9

Pliant

island

dirt with sticks

2.7 -3.0 m

unknown

10

Pilant

island

dirt with sticks

2.4 -2.7 m

unknown

11

Forty Acre

island

dirt with some vegetation

not applicable

unknown

12

Elm

island

egret guano/ dirt

unknown

no

13

Elm

island

dirt with some vegetation

unknown

yes

14

Elm

island

unknown

grass

no adult

no

15

Creekwood

bank

dirt with sticks

not applicable

unknown

of nests 12, 13, 14) were utilized as an egret rookery, and areas heavily used by the egrets had a spongy substrate due to decomposed guano. There were no signs of alligators (trails, footprints, or nests) on islands or parts thereof where dense vegetation existed; extremely overgrown areas also lacked egrets. One island had steep banks, and was not selected for use by alligators. Islands with more favorable conditions consistently had evidence of utilization by alligators in some form.

Nest Composition and Appearance

The nests were composed either of dirt with little or no vegetation, or grass with a paucity of dirt. All nests were directly associated with the amount and type of vegetation in the immediate area. The vegetation encountered in dirt nests was Louisiana blackberry {Rubus louisianus), black willow (S', nigra), assorted vines, and sticks. The grass nests were composed of paspalum {Paspalum floridanum) and Johnsongrass {Sorghum halepense). One unusual dirt nest (no. 12) consisted largely of decomposed egret guano. It was compact, 137 centimeters in diameter by 45 centimeters

54

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Figure 1. Location of alligator nests built in 1990 at Brazos State Park, Fort Bend Co., Texas. Nests were numbered in order of discovery.

high, and it was opened to confirm that it did contain eggs. Of the 15 nests, 12 were dirt (80 percent) and three were grass (20 percent). Four of the 12 nests intact at the time of discovery (33 percent) had white fecal material on top of the nest. Nests, 4, 6, and 12 were opened to check for eggs and stage of development, and nest 4 was found to contain fire ants. Nests on islands were not excluded from the possiblility of invasion by fire ants, because these ants were encountered even on islands located in the center of lakes.

Nest Predation

Predation, probably by a raccoon, destroyed one nest. It was located on the end of an island, approximately six meters from the shoreline. Low water levels coupled with its proximity to shore presumably enhanced its susceptibility to predation.

Time of Nesting and Incubation

One of the authors (DJ) has monitored time of nesting and incubation periods since 1987. He noted that nesting usually occurs from late June to early July at Brazos Bend, with the earliest nest from the period 1987 to 1989 being discovered on 17 June 1987, shortly after egg deposition. In 1990, a nest was discovered on 26 May and two additional nests were found about 1 June. Joanen and McNease (1979, 1989) found a direct correlation between the March - April - May ambient temperatures and time of nesting. When nesting took place early, they noted that temperatures

NESTING IN THE AMERICAN ALLIGATOR

55

Table 2. Mean monthly temperatures for 1990 compared to 1987-1989 and the 30- year average for Brazos Bend State Park.

Month

Temperatures (°C)

1990

1989

1988

1987

30- year average

March

17.2

16.3

16.3

14.9

16.1

April

20.8

20.8

19.8

19.6

20.4

May

25.6

25.4

23.1

25.1

23.8

June

29.3

26.6

26.9

27.4

27.0

July

27.8

28.0

29.1

28.8

28.4

August

29.5

27.6

29.6

30.1

28.1

for those months were correspondingly higher. In a comparison of 1990 mean monthly temperatures for those months with the 30-year average (Houston Area National Weather Service data), it was found that temperatures were higher for each of the three months in 1990 (Table 2). It should also be noted tht the previous winter temperatures were colder than usual, 23 December 1989 was the coldest day with a low of -13.9° C. The usual low based on the 30 year average was 5.6° C (Houston Area National Weather Service data). When comparing temperatures for March to May for the years 1987, 1988, and 1989 to the 30-year average, the year 1989 exhibits temperatures above the 30-year average for all three months. A possible explanation for the lack of early nesting for that year was that drought conditions existed from 1988 until spring 1989, and nesting activity was depressed. Joanen and McNease (1989) also reported an association between water levels and the degree of nesting, with drought resulting in greatly decreased nesting.

Observed incubation periods for 1990 were longer (more than 70 days) than for those known from 1987 to 1989 at the site for at least the three nests for which approximate incubation starting dates were recorded. The incubation period normally extends from late June through early September. Nest 4 was completed and contained eggs by at least 26 May. When the nest was opened on 2 July ( ~37 days) the opaque banding had only covered one half of the total egg length. According to the staging table for alligator embryos via banding by Ferguson (1985), this corresponded to 20 days. On 9 August, after at least 75 days of incubation, the female opened the nest before most, perhaps all, of the young were ready to hatch. The young appeared fully developed in regards to external morphology, but were unable to hold themselves up and possessed a substantial amount of yolk within the yolk sac. Of the 31 intact eggs removed from the nest, one egg was rotten, one egg was infertile or early dead, one hatched on 13 August (-79 days), another hatched on 14 August (-80 days) and the hatchlings died soon afterwards, and the rest of the eggs hatched from 17 to 21 August ( -83 to 87 days) with viable young. The hatchlings were released at Creekfield

56

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Lake within a few meters of their mother, and as of late May 1991 at least 15 were still with her.

Nests 6 and 7 were discovered about 1 June with eggs. On 2 July (-30 days) an egg was removed from nest 6 and opaque banding covered one third of the total egg length. This degree of banding was comparable to 12 days for the staging table by Ferguson (1985). On 16 August (-75 days) young were heard vocalizing at both nests. Eggs in nest 6 hatched successfully on 17 August (-76 days) and those in nest 7 on 19 August (-78 days).

Nests 1, 2, 3, and 5 hatched out sometime between 19 and 31 August as numerous hatchlings were observed at the water’s edge near each nest site. The Creekwood nest (no. 15) was discovered at the end of August because of the presence of hatchlings. Nests at Pilant Lake and the central islands of Elm Lake were too remote for hatching time or hatching success to be determined easily.

Adults Associated With Nests

Of the 13 intact nests, an adult was sighted at 12 (92 percent) or immediately adjacent to the nest at least once. This exception was nest 14, which was visited only once. Adults sighted near two nests splashed and quickly submerged in the water so their size could not be determined. The total length of adults associated with the other 10 nests were 1.8 - 2.1 meters (50 percent), 2.1 - 2.4 meters (10 percent), 2.4 - 2.7 meters (30 pecent), and 2.7 - 3.0 meters (10 percent).

The nests that could be monitored closely were nos. 4, 6, and 7 as they were accessible on foot. Nest 4 at Creekfield Lake, which was prematurely opened, was located adjacent to the adult’s small circular pond. As the water in this shallow 3.2-hectare lake receded, the pond dried up and the adult moved to the less than 1.0 hectare of remaining water located in the central part of the lake in late July. The only other alligators seen in the lake were four subadults. The adult was seen at the lake, but not at the nest, on each visit. Throughout the incubation there was evidence that an adult visited the nest consistently during the night or early morning hours. The nest appeared to be “tidied” regularly. On 2 July (-37 days incubation), there was the imprint of a tail dragged over the nest, and large sticks had been placed on top of the nest on 6 July (-41 days incubation). Beginning on 5 August (-71 days incubation), a small indentation (approximately 20 centimeters in diameter) was noted on the east side of the nest, facing the water and alligator trail. It looked as though an alligator had rubbed the area smooth with its snout. On 6 August, the indentation increased to about 50 centimeters in diameter. On 7 August, it had been enlarged in length to 60 centimeters and was shaped like the snout of an alligator.

This nest was checked periodically on 7 and 8 August for changes or for vocalizations by young; as of the last visit to the nest, just after dusk

NESTING IN THE AMERICAN ALLIGATOR

57

on 8 August, neither had occurred. On 9 August at 0830, the nest was found open on the east side. There were intact eggs visible in the nest cavity and a few outside along the nest mouth. Three cracked eggs, four dead embryos, and five eggshells were strewn outside the nest. Fire ants were swarming in the vicinity of the nest, especially on the dead alligators and partially opened eggs. Inasmuch as there were four dead alligators but five eggshells (all with evidence of containing well-developed embryos), at least one alligator was unaccounted for. On the same morning as the nest opening, the adult that had been observed throughout the study was located in Creekfield Lake and was joined by a second adult perhaps as long as 3.0 meters. Both alligators had just their heads above the water, were swimming back and forth quickly, and appeared alert and wary.

Nest 6 and 7 were adjacent to each other but separated both visually and physically by thick bushes. They were located at the 3.5-hectare New Horseshoe Lake, which was characterized by steep sides and deep water except in the vicinity of the nests. An adult was present at nest 6 on three of nine visits, and at nest 7 on one of nine visits. The adult at nest 6 came up from the water side of the nest by way of a well worn path, stood beside the nest with its body inflated and mouth gaping, then lunged toward the human intruders, performing a high walk for a short interval. This basic sequence was repeated for each visit to the nest when the alligator was present. For the first two encounters, those present backed away from the nest a substantial distance (at least 15 meters). After the lunge portion of the third encounter, one investigator backed up approximately 1.5 meters, then remained stationary. The alligator stopped moving but maintained an inflated posture and was still in this stance at the time all present left the nest site five minutes later.

The single encounter with the adult at nest 7 resulted in a similar behavioral sequence as displayed by the adult at nest 6, even to stopping forward movement when confronted with a human that did not retreat from the nest site. The only difference in behavior noted between the adults at the two nests was that the alligator at nest 7 did not gape and lunge immediately upon reaching the nest. The animal maintained a defensive posture at the nest until one of us moved to the side to view the animal, which immediately resulted in a lunge. The adults also responded if humans were standing at the outside of the oxbow arm directly across the water from both nests. Two adult alligators were sighted in the water near the center and went to the nests immediately after becoming aware of the presence of humans.

On 7 August (-67 days incubation), nest 7 had an indentation similar to that described for nest 4. On 16 August, chirping of the young was clearly heard from both nests 6 and 7. No adult was sighted in the vicinity of either nest. On 17 August, nest 6 had hatched out, but nest 7 still was intact with the young vocalizing and no adult was visible. The same was true on 18 August. The young were liberated from the nest by an adult

58

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

on 19 August. Because adults had aggressively defended the nests previously, this suggests that they were not in the vicinty when the young were vocalizing until the time that they opened the nests. Both adults had previously been sighted sufficient distances from the nests that they would have been unaware of human intrusion to their nests from visual or auditory cues.

Discussion

The choice of islands over banks as nesting sites may possibly function in increasing nest security from predation and human disturbance. An island may serve as a deterrent to, but not total protection from, predation as evidenced by the island nest that was destroyed during this study. The remoteness (distance) of an island from shore and water levels probably determine how safe it is from predators. Because public use of boats is prohibited within the park, islands do effectively deter human contact with nests.

In this study the majority of nests (92 percent) had adults associated with them, but only two of 13 (15.4 percent) nests were actively defended by an alligator. Kushlan and Kushlan (1980) found that about 43 percent of the nests they surveyed had an adult nearby, Goodwin and Marion (1978) observed an alligator at 21.4 percent of nests they studied and Metzen (1977) noted that females were usually in the vicinity of their nests. Joanen (1969), Metzen (1977), Goodwin and Marion (1978), Deitz and Hines (1980), Kushlan and Kushlan (1980), and Ruckel and Steele (1984) found none to 15 percent that defended or closely attended their nests. The literature is not always clear or consistent in terminology used in recording association of alligators with nests. Moreover, data bias (even within a study) due to such factors as unequal number of visits to nests, the use of airboats or marsh buggies (Deitz and Hines, 1980), and different habitat types, make it difficult to quantify and compare information.

Our data suggest that water levels, habitat, and size of the territory of the attending adult may influence movements of the adult and its proximity to the nest. The nest at Creekfield Lake had the total water-filled area greatly reduced in diameter and depth during the nesting season, restricting the adult to a small central pond where it could easily be seen and in turn it could easily monitor the nest. In contrast, adults at New Horseshoe Lake inhabited a deeper body of water that was not subject to decreased area on a seasonal basis, and they were seen at distances more than five times as great from the nest as in comparison to the adult at Creekfield (their home ranges could have been considerably larger).

The adults that displayed aggressive behavior at nests 6 and 7 were located at a lake that is popular for bank fishing, and thus they were habituated to people. Some alligators at this lake are known to take fish off baited hooks and stringers. Deitz and Hines (1980) found evidence suggesting that alligators that become habituated to humans, but are not harassed.

NESTING IN THE AMERICAN ALLIGATOR

59

tend to remain with the nest and even defend it. They had five of 1 1 nests (45.4 percent) attended; our rate was even greater with 12 of 13 nests (92.3 percent) attended. Elsey et al. (1990) reported that approximately 80 percent of their captive penned alligators defended nests. In an examination of four nests at Brazos Bend prior to opening of the park, Onadeko (1983) did not observe adults in the vicintiy of them. This supports the contention that active nest defense at New Horseshoe Lake was due to frequent human contact and may even indicate that the number of adults seen in the vicinity of nests at the park is higher than it would be otherwise. The adult at nest 6 displayed the same aggressive behavioral sequence each time it was present during our visits, which is in agreement with Kushlan and Kushlan (1980) that an adult behaves consistently in its response to the nest being approached.

When comparing the usual peak nesting periods recorded by Joanen (1969), Bara (1972), Metzen (1977), and Goodwin and Marion (1978), they fall collectively into the time span of mid-June through the first week of July. Although the bulk of nesting at Brazos Bend has a slightly later time frame in comparison (late June through the second week of July for the discovery of completed nests), the early nesting dates recorded in 1990 would be considered early also for the sites those researchers studied.

That an incubation of more than 70 days is not normal for most nests is supported by Mcllhenny (1935), Joanen (1969), and Goodwin and Marion (1978), who found incubation periods varying between 59 and 65 days. Two possible explanations of the observed extended incubation relate to monthly air temperatures during the incubation period (Table 2). The July 1990 mean temperature was lower than that of the 30- year average and 1987 to 1989 mean temperatures for July. Also, because nesting was one month early, air temperatures during the first month of incubation were lower than if eggs had been laid in June or July.

When opening nest 4 on 2 July, this dirt nest did not seem moist relative to other nests that had been opened previously. Mcllhenny (1935) monitored a “fairly dry” dirt nest in which it took more than 100 days for the embryos to reach full term, and attributed the lengthy development to the lack of green nest material. Chabreck (1973) noted a lower temperature in nests made of soil or partially decomposed vegetation. Nests 6 and 7 were both grass nests so the latter explanation would not be relevant to them. Joanen (1969) did find that eggs in some nests that were frequently opened during research activities took 75 to 80 days to hatch. Nests 4 and 6 each were opened a single time for approximately one minute and only a small portion of the nest cavity was exposed, so this would be expected to have had little effect on the incubation period.

Regarding the extra eggshell found at the Creekfield nest, we believe it to have contained a young alligator. The youngster may have been taken to the water by the adult, or if dead, eaten. A possible explanation for

60

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

the early nest opening is that because the incubation period was at least 75 days, the adult opened the nest even though vocalizations of the young were not heard. Based on his experience, Joanen (personal communication) felt that the adult heard at least one youngster vocalizing and took whatever young were ready to hatch to the water, abandoning the rest. The large alligator first sighted at the nest immediately after the nest opening may have been the male parent. It is possible that he came to the lake to open or assist in opening the nest and that both adults were protecting at least one hatchling.

Acknowledgments

We are grateful to D. Riskind and R. Trippet of the Texas Parks and Wildlife Department for their cooperation with this project. Thanks is extended to C. Torres, F. Hoot, J. Beatty, C. Sumners, J. Lowe, G. Valdez, and D. Oro for their assistance with various aspects of the field work, and to T. Joanen and R. A. Odum for reviewing the manuscript. This research was supported under NSF grant TPE89-55157.

Literature Cited

Bara, M. O. 1972. Annual progress report for January 1, 1972, through December 31, 1972.

South Carolina Wildlife and Marine Res. Dept., Columbia.

Chabreck, R. H. 1973. Temperature variation in nests of the American alligator. Herpetologica, 29:48-51.

Deitz, D. C., and T. C. Hines. 1980. Alligator nesting in north-central Florida. Copeia, 1980:249- 258.

Elsey, R. M., T. Joanen, L. McNease, and V. Lance. 1990. Stress and plasma corticosterone levels in the American alligator — relationships with stocking density and nesting success. Comp. Biochem. Physiol., 95A:55-63.

Ferguson, M. W. J. 1985. The reproductive biolgy and embryology of crocodilians. Pp. 329- 491, in Biology of the Reptilia, (C. Cans, F. S. Billet, and P. F. A. Maderson, eds.), John Wiley and Sons, New York, 14:1-763.

Fogarty, M. J. 1974. The ecology of the Everglades alligator. Pp. 367-374, in Environments of South Florida: present and past (P. J. Gleason, ed.), Mem. Miami Geol. Surv., 2:1- 482.

Goodwin, T. M., and W. R. Marion. 1978. Aspects of the nesting ecology of American alligators {^Alligator mississippiensis) in north-central Florida. Herpetologica, 34:43-47.

Hines, T. C., M. J. Fogarty, and L. C. Chappell. 1968. Alligator research in Florida: a progress report. Proc. Southeastern Assoc. Game Fish Comm., 22:166-180.

Joanen, T. 1969. Nesting ecology of alligators in Louisiana. Proc. Southeastern Assoc. Game Fish Comm., 23:141-151.

Joanen, T., and L. McNease. 1979. Time of egg deposition for the American alligator. Proc. Southeastern Assoc. Game Fish Comm., 33:15-19.

- . 1989. Ecology and physiology of nesting and early development of the American

alligator. Amer. Zool., 29:987-998.

Johnson, L. A., A. Cooper, B. Thompson, and R. Wickwire. 1989. Texas alligator survey, harvest, and nuisance summary 1988. Pp. 36-72, in Crocodilian Congress on production and marketing, 147 pp.

Kushlan, J. A., and M. S. Kushlan. 1980. Function of nest attendance in the American alligator. Herpetologica, 29:256-257.

NESTING IN THE AMERICAN ALLIGATOR

61

Mcllhenny, E. A. 1935. The alligators’ life history. Christopher Publ. House, Boston, 117

pp.

Metzen, W. D. 1977. Nesting ecology of alligators on the Okefenokee National Wildlife Refuge.

Proc. Southeastern Assoc. Game Fish Comm., 31:29-32.

Onadeko, S. A. 1983. Status of the American alligator and potential resource management problems at Brazos Bend State Park. Unpublished M. S. thesis, Texas A&M Univ., College Station, 111 pp.

Potter, F. E,, Jr. 1975. American alligator study. Spec. Rept., Texas Parks Wildlife Dept., 24 pp.

- . 1981. Status of the American alligator in Texas. Spec. Rept., Texas Parks Wildlife

Dept., 49 pp.

Reese, A. M, 1907. The breeding habits of the Florida alligator. Smithsonian Misc. Coll., 48:381-387.

Ruckel, S. W., and G. W. Steele. 1984. Alligator nesting ecology in two habitats in southern Georgia. Proc. Southeastern Assoc. Game Fish Comm., 38:212-221.

Singleton, R. L., Jr., R. K. Scott, E. C. Liu, and D. W. Koenig. 1978. Development plan and program for Hale Ranch State Park. Texas Parks Wildlife Dept., 159 pp.

Taylor, D., and W. Neal. 1984. Management implications of size-class frequency distributions in Louisiana alligator populations. Wildlife Soc. Bull., 12:312-319.

Thompson, B. C., F. E. Potter, Jr., and W. C. Brownlee. 1984. Management plan for the American alligator in Texas. Texas Parks Wildlife Dept., 81 pp.

Current address of Hayes-Odum: Department of Space Physics and Astronomy, Rice University, Houston, Texas 77251.

INDIVIDUAL AND SECONDARY SEXUAL VARIATION IN THE MEXICAN GROUND SQUIRREL, SPERMOPHILUS MEXICANUS

Franklin D. Yancey, II, J. Knox Jones, Jr., and Richard W. Manning

The Museum and Department of Biological Sciences, Texas Tech University, Lubbock, Texas 79409-3191

Abstract. — One hundred fifty-three adult specimens (73 males and 80 females) of the Mexican ground squirrel, Spermophilus mexicanus, from the eastern part of the Edwards Plateau in Texas were analyzed to ascertain differences in size related to age and sex. Variation in cranial dimensions was found to be dependent on age, and was highly affected by sex. Key words: morphometries; secondary sexual and age variation; ground squirrel; Spermophilus mexicanus; Texas.

Where it occurs, the Mexican ground squirrel, Spermophilus mexicanus (subgenus Ictidomys), frequently is a common and widespread species. It ranges from central Texas and southeastern New Mexico southward to central Mexico (Hall, 1981). In a part of its distribution in Texas and adjacent New Mexico, it barely overlaps the range of the closely related Spermophilus tridecemlineatus, and hybrids between the two have been reported (Cothran, 1983). Nonetheless, hybrid sites are rare and genetic exchange between the two taxa is limited and locally restricted, no doubt the result of secondary contact of populations once geographically isolated.

This ground-dwelling sciurid is abundant in many places, especially in parks and cemeteries, on golf courses, along highway rights-of-way, and the like. Nonetheless, few assessments of morphologic variation in the species have been reported since Howell’s (1938) review of the ground squirrels. Cothran’s (1983) important study involved primarily variation between two different species of Spermophilus, and no other large series of S. mexicanus have been studied (Young and Jones, 1982). Our work was focused on nongeographic variation and secondary sexual dimorphism at the population level.

Using the Texas Tech University Center at Junction as a focal point, students in various courses and others have collected, over the years, a number of Mexican ground squirrels in Kimble and surrounding counties on the eastern edge of the Edwards Plateau, near the eastern terminus of the range of the species. Of the two recognized subspecies, the northern race, S. m. parvidens, occurs in that region.

Materials and Methods

From the collections of The Museum at Texas Tech University, we were able to accumulate 158 specimens, 76 males and 82 females, mostly conventional museum skins accompanied by skulls, but including a few complete skeletons, all collected in the months

64

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

of April and May in the years 1973 to 1991. Almost half were obtained by students in classes in mammalogy supervised by Jones and Manning in the last three years of that period. April and May were chosen because females are not often visibly pregnant by the end of that time frame, and because all animals in the population are adults (born at least in the previous year). The sample contained material from the following counties: Gillespie (four specimens); Kerr (three); Kimble (117); Mason (27); McCulloch (two); Menard (one); Sutton (four).

For external measurements, we did not apply standard statistical tests simply because it was evident that many of the measurements had been taken in different ways by inexperienced collectors. For example, measurements of length of ear ranged from four to 16 mm. We selected 19 specimens of each sex for which we felt measurements were reasonably accurate, and those data are presented to help establish parameters for external size of S. m. parvidens.

Cranial measurements were taken by the same person (Yancey) using a pair of Fowler digital calipers calibrated to 0.01 millimeter. Most cranial measurements were recorded for most specimens (143 of 158 for mastoid breadth being the fewest). All morphometric characters were conventional and thus no description of them is needed in most cases. They were greatest length of skull (GLS), condylobasal length (CBL), zygomatic breadth (ZB), least postorbital constriction (POC), least interorbital constriction (IOC), mastoid breadth (MB), greatest alveolar breadth across upper molars (ABTR); alveolar length of maxillary toothrow (MAXTR), depth of skull (DS) — taken with skull on microscope slide to topmost point of cranium, then subtracting depth of the slide, and alveolar length of mandibular toothrow (MANTR). Univariate and multivariate statistical tests were performed using SPSSX statistical packages on a VAX 860, and were made available through Academic Computing Services at Texas Tech University.

Results and Discussion

We initially attempted to assign our spring-taken specimens to discrete age groups based primarily on wear on the upper cheekteeth. It seemed reasonable that, because mexicanus hibernates, animals taken shortly after termination of hibernation might evidence wear patterns that would allow assignment of them to a specific age group (yearling, two years old, and so on) at least through the first few years of life.

Our expectations were only partly fulfilled. Based on wear patterns involving the protocone and metacone on the Ml and M2, but taking into account also wear on P3, P4, and M3, we finally established three categories: 1) young adults (yearlings) — slight to moderate wear on labial cones of Ml and M2 but cones well formed and discrete, and P3 and M3 showing little wear; 2) adults — labial cones still evident but with heavy wear, P4 and M3 worn and P3 showing some wear; 3) old adults — labial cones almost completely worn away in terms of recognition, all cheekteeth flattened or nearly so with wear. We likened these three groups to first year animals, those two years old, and squirrels older than two years, but this is only an educated guess on our part. The young adult category totaled 123 of 158 specimens, adults 30, and old adults five. Old adults were excluded from statistical analysis because of the small sample size.

Cranial measurements were analyzed using multivariate analysis of

VARIATION IN SPERMOPHILUS MEXICANUS

65

variance (MANOVA). There was a significant difference between specimens in age groups 1 and 2 (P = 0.012) and between the sexes (P < 0.001), but there was no interaction between age and sex (P = 0.481).

Univariate analysis of variance (ONEWAY) was used to test each dependent variable by age group and sex. Males of age group 2 were significantly larger than those of age group 1 in three cranial dimensions, ZB, IOC, and MB. Females of age group 2 were significantly larger than females of age group 1 in only two cranial dimensions (ZB and DS).

Males of age group 1 were significantly larger than females of the same group in all cranial dimensions except POC and MAXTR. Males of age group 2 were significantly larger than females of that age group in all cranial dimensions except POC, MAXTR, and MANTR (Table 1).

Discriminant function analysis (DISCRIMINANT) was employed to ascertain whether cases entered as “unknowns” could be correctly classified as to age group. Significant differences were detected between age groups of males (P = 0.0011) and females (P = 0.0061). This test correctly classified 76.47 percent of the males and 67.50 percent of the females as to age. Therefore, age should be taken into account in any analysis of geographic variation in S. mexicanus.

Principle component analysis (FACTOR) was used to determine which variables explain the greatest amount of total variation in the sample. CBL, ZB, and POC explain 66.0 percent (males) and 73.1 percent (females) of the variation in age group 1 (Table 2), and 80.2 and 70.6 percent, respectively, in age group 2.

Using discriminant function analysis with sex as the main criterion, individuals of age group 1 were correctly classified 82.02 percent of the time, and those of age group 2 correctly classified 82.76 percent of the time. Thus, size is not independent of sex, and the sexes should be treated separately in any future taxonomic considerations of S. mexicanus.

As previously noted, we did not include external measurements with any statistical package because of the broad variation in those taken by different collectors. However, in order to provide descriptive data on these dimensions, we selected 19 specimens of each sex, all of which had complete external measurements, from among those collected in 1989, 1990, and 1991. For all measurements, the sample mean of males was noticeably larger than that of females, except for weight (in which the sexes were essentially the same). Excepting weight in females, our measurements accord fairly well with those given for fewer animals of each sex by Edwards (1946). Average external dimensions (mm) and weights (grams) of males, followed by those of nonpregnant females (minima and maxima in parentheses) are as follows: total length, 327.4 (311-362), 313.9 (292-333); length of tail, 125.3 (110-147), 120.9 (103-134);

66

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Table I. Variation in cranial dimensions (mm) of males and females of age groups one and two in Spermophilus mexicanus.

Sex

Age

N

Mean ± 1 SD

Minimum-

maximum

Coefficient of variation

Greatest length of skull

M

1

58

46.69 ± 0.95

44.60 -

48.53

2.02

2

11

47.20 ± 1.45

43.66 -

49.08

3.07

F

1

58

44.90 ± 1.07

42.38 -

47.47

2.24

2

18

45.20 ± 0.98

43.45 -

46.95

2.17

Condylobasal length

M

1

60

, 42.94 ± 0.97

40.84 -

44.77

2.25

2

12

43.52 ± 1.62

39.47 -

45.87

3.73

F

1

62

41.22 ± 1.22

37.38 -

43.83

2.96

2

18

41.59+ 1.01

39.62 -

43.38

2.43

Zygomatic breadth

M

1

59

27.94 + 0.68

26.56 -

29.50

2.43

2

12

28.77 + 0.88

26.35 -

29.68

3.07

F

1

62

26.91 + 0.85

24.78 -

28.89

3.15

2

18

27.63 + 0.83

26.43 -

30.00

2.99

Postorbital constriction

M

1

61

13.15+ 0.55

11.90-

14.86

4.17

2

12

12.85+ 0.61

12.05-

13.70

4.71

F

1

62

13.03+ 0.53

11.72-

14.32

4.08

2

18

12.90+ 0.61

11.82-

14.15

4.69

Interorbital constriction

M

1

59

9.97+ 0.42

9.15 -

10.95

4.21

2

12

10.43+ 0.59

9.71 -

11.89

5.58

F

1

62

9.60+ 0.52

8.45-

10.72

5.45

2

18

9.79+ 0.49

8.53-

10.26

4.98

Mastoid breadth

M

1

58

21.26+ 0.46

19.92-

22.18

2.14

2

11

21.78+ 0.77

19.94-

22.70

3.52

F

1

55

20.80 + 0.58

19.37 -

22.36

2.81

2

15

20.99 + 0.67

19.81 -

21.98

3.20

Alveolar breadth across

upper molars

M

1

61

11.84+ 0.31

10.97 -

12.72

2.61

2

12

12.01 + 0.33

11.34-

12.61

2.76

F

1

62

11.70+ 0.30

10.63 -

12.41

2.60

2

18

11.74+ 0.34

11.17-

12.11

2.85

Length of maxillary toothrow

M

1

61

8.67+ 0.28

8.06-

9.52

3.18

2

12

8.64+ 0.25

8.29-

9.02

2.95

F

1

62

8.58+ 0.29

7.93-

9.21

3.43

2

18

8.50+ 0.40

7.77 -

9.26

4.69

VARIATION IN SPERMOPHILUS MEXICANUS

67

Table 1. Continued

M

1

61

Length of mandibular toothrow

8.12 ± 0.38 7.11 -

9.59

4.67

2

12

8.16+ 0.23

7.82-

8.53

2.79

F

1

62

7.95+ 0.34

7.22-

8.70

4.25

2

18

7.95+ 0.37

7.28-

8.52

4.70

M

1

60

Depth of skull

19.51 + 0.50

18.35-

20.62

2.56

2

12

19.72+ 0.53

18.61 -

20.37

2.70

F

1

62

18.95+ 0.53

17.58-

20.50

2.81

2

18

19.31 + 0.49

18.61 -

20.46

2.52

Table 2. Results of principle component analysis (FACTOR) for 10 cranial characters for males and females of Spermophilus mexicanus from age groups 1 and 2. Unique variation attributed to each variable and the cumulative percentage of \ ariance attributed

to that factor (and those abbreviations.

that precede it in

the table)

are giv

en. See

text for

Variable

Age group 1

Age group 2

Males

Females

Males

Females

Unique

variance

Cumulative

variance

Unique

variance

Cumulative

variance

Unique Cumulative

variance variance

Unique Cumulative variance variance

CBL

38.9

38.9

44.3

44.3

41.3

41.3

41.1

41.1

ZB

15.7

54.7

16.9

61.2

22.4

63.7

18.9

60.0

POC

1 1.3

66.0

11.9

73.1

16.5

80.2

10.6

70.6

IOC

8.8

74.8

7.7

80.8

9.1

89.3

9.6

80.2

MB

7.2

82.0

6.3

87.1

6.3

95.6

7.0

87.2

ABTR

5.2

87.2

5.6

92.7

3.6

99.2

5.2

92.9

MAXTR

5.0

92.1

2.8

95.5

1.0

100.0

3.4

96.3

DS

3.6

95.8

2.1

97.6

0.4

100.0

2.1

98.4

MANTR

3.6

100.0

0.9

100.0

0.8

100.0

0.7

100.0

length of hind foot, 43.1 (41-45), 42.8 (41-45); length of ear, 12.3 (10-16), 11.5 (9-15); weight, 249.5 (200-290), 249.8 (190-274).

Acknowledgments

It is a pleasure to acknowledge the use of facilities at the Texas Tech Center at Junction, and the helpful assistance of the staff at the Center. Many persons were responsible for collection of specimens and we thank them collectively for their efforts.

Literature Cited

Cothran, E. G. 1983. Morphologic relationships of the hybridizing ground squirrels Spermophilus mexicanus and 5". tridecemlineatus. J. Mamm., 64:591-602.

Edwards, R. L. 1946. Some notes on the life history of the Mexican ground squirrel in Texas. J. Mamm., 27:105-1 15.

Hall, E. R. 1981. The mammals of North America. John Wiley & Sons, New York, l:xv + 1-600 + 90.

68

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Howell, A. H. 1938. Revision of the North American ground squirrels, with a classification of the North American Sciuridae, N. Amer. Fauna, 56:1-256.

Young, C. J., and J. K. Jones, Jr. 1982. Spermophilus mexicanus. Mamm. Species, 164:1-4.

Present address of Manning: Department of Biology, Southwest Texas State University, San Marcos, Texas 78666.

EFFECT OF FEED QUALITY ON GROWTH OF THE GULF OF MEXICO WHITE SHRIMP, PENAEUS SETIEERUS,

IN POND PENS

Lori Robertson, Addison L. Lawrence, and Frank L. Castille

Texas A&M Shrimp Mariculture Project, Texas Agricultural Experiment Station, 4301 Waldron Road, Corpus Christi, Texas 78418 (LR), and Texas A&M Shrimp Mariculture Project Texas Agricultural Experiment Station,

PO. Box Q, Port Aransas, Texas 78373 (ALL, ELC)

Abstract. — To evaluate the effect of feed quality on growth and survival of Penaeus setiferus in an intensive culture system, a trial was conducted using commercially manufactured feeds varying in total protein and squid meal content. Juveniles (3.6 grams initial weight) were stocked at 40 per square meter in bottomless pens of one cubic meter in an earthen pond and were fed different quality feeds for 56 days. The effect of feed quality on instantaneous growth rate of P setiferus was significant {P = 0.0001). Shrimp fed high quality feed (50 percent protein- 15 percent squid meal) grew faster than those fed medium (40 percent protein- five percent squid meal) and low (30 percent protein only) quality feed, and shrimp fed medium quality feed grew faster than those fed low quality feed. Survival was not affected by feed quality {P — 0.8715) and averaged 90.3 percent. Weekly growth, final weight, and harvest biomass of fed shrimp ranged from 0.93 to 1.04 grams per week, 11.0 to 11.9 grams, and 398 to 421 grams per square meter, respectively. Unfed shrimp at the same density grew 0.47 grams per week, had 95 percent survival and were harvested at a final weight of 7.3 grams and biomass of 279 grams per square meter. It was estimated that natural forage contributed <52 percent to shrimp growth. Unfed shrimp stocked outside the pens in the open pond at a density of 0.5 per square meter grew 1.56 grams per week, had 92.3 percent survival, and were harvested at a weight of 16.1 grams and biomass of eight grams per square meter.

Results showed that P. setiferus has the potential to grow at commercially acceptable rates and indicated that quality of presented feed was important for growth of the species under intensive culture conditions even when natural foods were available. Key words: feed quality; pond culture; Penaeus sp.

Historically, results achieved with the Pacific white shrimp, Penaeus vannamei in pond culture have surpassed those obtained with the Gulf of Mexico white shrimp, P. setiferus (Parker et al., 1974; Chamberlain et al, 1981; McKee, 1986). Consequently, P. vannamei is the most widely cultured marine shrimp in the United States. Yet, continued progress of the shrimp culture industry in this country is limited at times by an inadequate supply of postlarval P. vannamei, and is threatened by the possibility of import restrictions. For these reasons, it is prudent to reevaluate the native white shrimp as an alternative culture candidate.

Although past experience with P. setiferus has not been encouraging, current pond production technology may improve results. Recently, production of P setiferus in experimental growout trials in South Carolina was comparable to that achieved with P vannamei (Sandifer et al., 1990; Browdy et al., 1991). Experience with P. setiferus in the 1950s, 1960s, and 1970s generally relied on low quality feeds, many of which

70

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

were not formulated specifically for marine shrimp; poor water stability also was a problem. Although the nutritional requirements specific to P. setiferus still have not been adequately defined, vast improvements have been made in formulating feeds. Pond management strategy also has evolved in the United States from semi-intensive to intensive and super¬ intensive systems. It is generally believed that the greater the culture intensity the greater the role of presented feeds in supplying the required nutrients for growth and survival. Thus, although the nutritional requirements of shrimp may not change with increased intensity, the nutritional quality of feeds is more critical.

The present trial was undertaken to determine the effect of different quality feeds, varying in protein level and squid meal content, on growth and survival of P. setiferus. In this study, use of a higher quality feed under more intensive growout conditions than previously investigated with this species is examined.

Materials and Methods

A feeding trial was conducted with P. setiferus (3.6 grams initial mean weight) stocked at a density of 40 shrimp per square meter in bottomless pens of one cubic meter in an 0.1- hectare earthen pond. The pen system used has been described previously (Robertson et al., 1992).

Shrimp were fed 50, 40, and 30 percent protein commercially manufactured feeds (Rangen, Inc., Buhl, Idaho), which will be hereafter referred to as high, medium, and low quality feeds, respectively. The high quality feed contained 50 percent protein with 15 percent squid meal as fed, the medium quality feed had 40 percent protein and five percent squid meal, and the low quality feed had 30 percent protein and no squid meal. Each of the three experimental treatments consisted of 12 replicate pens situated diagonally in a block design. Additionally, an “unfed” pen of shrimp adjacent to the experimental block design was included to estimate the potential contribution of natural foods to shrimp growth. Another group of unfed shrimp was stocked outside the pens in the open pond at 0.5

shrimp per square meter to estimate potential low density growth. Rations were fed four

times daily at 0100, 0700, 1300, and 1900 hours at an estimated feed rate of four percent body weight per day.

One month before filling the pond, approximately 400 grams per square meter of lime was tilled into the pond bottom inside each pen. The 0. 1 -hectare pond was fertilized at the

time it was filled eight days before stocking with 2.5 kilograms of urea, one liter of

phosphoric acid, and 0.6 kilogram of sodium silicate. After five weeks, it was fertilized again with three kilograms of urea and one liter of phosphoric acid. Seawater for the pond was pumped daily from the Laguna Madre. The seawater was filtered to 400 yu as it entered the pond and was diluted with municipal freshwater as needed. Dissolved oxygen level and percent saturation were measured daily at 0700 and 1900 hours. Salinity, minimum and maximum temperatures, and Secchi disk readings were measured once daily. Total ammonia, nitrite, and pH determinations were made six times throughout the trial. The pond was harvested after 56 days and shrimp in each pen were counted and group weighed. Survival, instantaneous growth rate (IGR), final weight, weekly weight gain, and final biomass were determined for each pen. Comparisons among fed treatments were made on the basis of survival and IGR:

1GR= ln(W,/Wo)/ (trto),

FEED QUALITY AND GROWTH OF SHRIMP

71

Table 1. Water quality during a feed quality growth trial with R setiferus.

Parameter

N

Mean + S.D.

Range

Salinity (ppt)

55

32.4 + 2.2

28-42

Dissolved oxygen (mg/ 1)

A.M.

55

4.6 ± 0.6

3.1-5.9

P.M.

52

6.9 ± 0.6

5.9-8. 8

Oxygen saturation (%)

A.M.

55

68.0 ± 7.4

50-84

P.M.

51

I1I.9 + 8.9

94-144

Temperature (°C)

Minimum

55

26.8 ± 1.2

24-29

Maximum

52

31.4± I.l

28-33

Secchi (cm)

48

77.2 ± 15.6

45-90

pH

6

8.12 + 0.19

7. 8-8.4

Ammonia (mg/ 1)

6

0.055 ± 0.052

0.007-0.125

Nitrite (mg/ 1)

6

0.022 + 0.010

0.006-0.032

where Wi is weight at ti and Wo is weight at to. Multiplying the IGR by 100 gives the percent body weight change per day. Growth and survival differences due to feed quality were tested for statistical significance by analysis of variance (ANOVA) followed by a Student-Neuman-Keul multiple range test where appropriate. Survival data were transformed (arcsine) prior to statistical analysis but are presented as percentages for clarity.

Results

Water quality parameters monitored throughout the trial are summarized in Table 1. Salinity averaged 32.4 parts per thousand, mean daily low temperature was 26.8° C, and daily high temperature was 31.4°C. Morning dissolved oxygen level and percent saturation averaged 4.6 milligrams per liter and 68 percent, respectively. Afternoon dissolved oxygen level and saturation were 6.9 milligrams per liter and 112 percent, respectively. Water was exchanged at a rate of 1 1 percent daily.

The effect of feed quality on instantaneous growth rate of P. setiferus was significant (P = 0.0001). Shrimp fed high quality feed (50 percent protein- 15 percent squid meal) grew faster than those fed medium (40 percent protein- five percent squid meal) and low (30 percent protein only) quality feed, and shrimp fed medium quality feed grew faster than those fed low quality feed. Growth increased significantly from 2.01 percent to 2.11 percent to 2.15 percent per day for shrimp fed low, medium and high quality feeds, respectively (Fig. 1).

Survival was not affected by feed quality {P = 0.8715) and averaged 90.3 percent. Weekly growth, final weight, and harvest biomass of fed shrimp ranged from 0.93 to 1.04 grams per week, 11.0 to 11.9 grams, and 398 to 421 grams per square meter, respectively (Table 2). Unfed shrimp stocked at the same density (40 per square meter) in a pen grew 0.47 grams per week, had 95 percent survival, and were harvested at a final weight of 7.3 grams and biomass of 279 grams per square meter.

72

THE TEXAS JOURNAL OF SCIENCE— VOL. 45, NO. 1, 1993

Instantaneous Growth Rate (%/day)

30/0 40/5 50/15

Feed Quality (% protein /% squid meal)

Figure I. Effect of feed quality on instantaneous growth rate (IGR) of P. setiferus. Bars represent means plus or minus one standard deviation. Lowercase letters denote statistical differences (SNK, P — 0.05).

Comparing the biomass gain of unfed shrimp (137 grams per square meter) with the average biomass gain of the fed groups (266 grams per square meter) gives 52 percent as an estimate of the contribution of natural forage to shrimp growth in this experiment. Unfed shrimp stocked outside the pens in the open pond at a density of 0.5 per square meter grew 1.56 grams per week, had 92.3 percent survival, and were harvested at a weight of 16.1 grams and biomass of eight grams per square meter.

Discussion

Ammonia and nitrite levels were within safe limits as determined for other penaeids (Wickens, 1976; Chen and Lei, 1990) and the other water quality conditions during the trial with the exception of salinity were

FEED QUALITY AND GROWTH OF SHRIMP

73

Table 2. Weekly growth, final weight, final biomass, and survival of P. setiferus fed different quality feeds. Values are means plus or minus one standard deviation.

Feed quality (% protein/

% squid meal)

N

Weekly

growth

(g)

Final

weight'

(g)

Final

biomass^

(g/m^)

Survival

(%)

30/0

12

0.93 ± 0.07

11.0 + 0.5

398 + 25

90.4 + 4.6

40/5

12

1.00 + 0.06

11.6 + 0.5

421 + 26

90.8 + 4.8

50/15

12

1.04 + 0.1

11.9 + 0.8

406 + 74

90.0 + 7.8

'initial weight was 3.6 ± 0.2 grams for all treatments.

^Initial biomass was 142 grams per square meter for all treatments.

within the ranges considered favorable for growth of some commercially important penaeids (Hanson and Goodwin, 1977; Clifford, 1985). Although R setiferus inhabits a wide range of salinities in nature, reduced salinity appears to be better for pond growout, despite conflicting reports of the best salinity. Zein-Eldin (1963) reported the lack of an effect of salinity on growth of postlarval P. setiferus. Hysmith and Colura (1976) reported improved growth of P. setiferus during growout in reduced salinity, whereas Johnson and Fielding (1956) reported that white shrimp grew better in 34 parts per thousand than in 18.5. Even though salinity during the present trial (32.4 parts per thousand) may not have been ideal, the growth rate achieved with P. setiferus at a low density of 0.5 per square meter (1.56 grams per week) is among the highest reported for the species.

Dietary intake of amino acids is essential to shrimp growth. Generally, the quality of formulated feed increases as the amount of protein and squid meal increases because the amino acid profile (quality and quantity) of the feed is improved. Commercially manufactured feeds are typically formulated so that overall feed quality increases with percent protein.

It has been shown in the present study that growth of P setiferus increased as feed quality increased. The highest quality feed tested (a combination of 50 percent protein and 15 percent squid meal) resulted in the best growth. These are higher values than previously reported in studies in which percent protein and percent squid meal were examined independently. Andrews and coworkers (1972) reported the optimal dietary protein level