VAGILITY AND LOCAL MOVEMENTS OF POCKET GOPHERS (THOMOMYS AND PAPPOGEOMYS) IN AN ÁREA OF SYMPATRY by STEPHEN LORY WILLIAMS, B.S. A THESIS IN ZOOLOGY Submitted to the Graduate Faculty o£ Texas Tech University in Partial Fulfillment o£ the Requirements for the Degree o£ MASTER OF SCIENCE Approved December, 1973 73 • 1973 l\)o.7j\S ACKNOWLEDGMENTS Cl-õp • ^ I extend my sincere appreciation to my major advisor, Dr. Robert J. Baker, for his advice, encouragement, and assistance with many aspects of this study. I am indebted to Dr. Hugh H. Genoways for generously providing time and suggestions applicable to this study, and to Dr. Joe R. Goodin for providing laboratory equipment for soil analyses. I am also grateful to Drs. Robert J. Baker, Hugh H. Genoways, Robert L. Packard, and Joe R. Goodin for their criticai reviews of this manuscript. Finally, I acknowledge with gratitude financial assistance provided by the International Center for Arid and Semi- arid Land Studies, the Theodore Roosevelt Fund, and the Organized Research Fund of Texas Tech University. 11 TABLE OF CONTENTS ACKNOWLEDGMENTS 11 LIST OF TABLES iv LIST OF ILLUSTRATIONS v I. INTRODUCTION 1 II. METHODS AND MATERIALS 3 III. RESULTS 13 IV. DISCUSSION 33 V. SUMMARY 43 LITERATURE CITED 45 111 LIST OF ILLUSTRATIONS Figure Pa;. 1. Aerial photograph of the study área .... 2. Data form used in recording captures ... 6 3. Individual movements of Pappogeomys castanops and Thomomys bottae in the study área 16 4. Distribution of Pappogeomys castanops and Thomomys bottae in the study área . 2 2 5. Distribution of Pappogeomys castanops and Thomomys bottae in the vicinity of the study área 24 6. Graphical representation of changes in mean soil moisture between June 1971 and May 1972 26 7. Extent of flooding in the study área on 23 August 1971 30 8. Burrow systems eroded by floods 32 V LIST OF TABLES Table Page 1. Sample sizes, means, ranges, and standard errors of movement data, grouped according to age, sex, and species .... 14 2. Leveis of significance and F values derived from comparisons õT movement data grouped according to age, sex, and species 15 IV INTRODUCTION Vagility, the potential or ability to disperse, is a characteristic influencing many biological aspects of a species. The rate and degree of population growth, gene flow, and morphological divergence are some features affected by vagility. Vagility, in turn, may be affected by gross morphology, behavior, physiology, natural history, and population dynamics of the species (Udvardy, 1969). Organisms with limited vagility encounter problems such as range expansion, maintenance of heterozygosity, and complete dependence on local environment. A group of mammals with low vagility are pocket gophers of the family Geomyidae (Patton ejt ai., 1972). These animais are specifically adapted to a herbivorous, fossorial habitus (Patton, 1972) and are restricted to a burrow system. Confinement to a burrow system obviously restricts the vagility of a species. The morphological adaptations of pocket gophers cause them to be awkward and vulnerable outside of their specific habitus. The location selected for this study was an área, described by Reichman and Baker (1972), where Thomomys bottae limpiae and Pappogeomys castanops pratensis occur sympatrically. This área is particularly applicable to this study because Reichman and Baker (19 72) had already noted movements occurring between these populations. In addition this locality was favorable because the habitat was relatively undisturbed, the áreas occupied by pocket gophers are restricted, and both genera could be studied simultaneously under similar environmental conditions. Because members of the family Geomyidae are not known to have subdivided the fossorial habitus (Russell, 1968), the role of vagility is particularly criticai when pocket gopher species are sympatric. METHODS AND MATERIALS This study was conducted 9.0 km N, 9.5 km E Fort Davis, Jeff Davis Co., Texas, between May 1971 and May 1973. Intensive live-trapping (Baker and Williams, 1972) was conducted in an área which extended 250 meters on either end of the zone of sympatry. Trapping was occasionally extended to other áreas to determine the local distribution of both species. Traps were set in áreas where recent mounding activity was indicated. An aerial photograph (Fig. 1), obtained from the Department of Interior Geological Survey, was used to map the zone of sympatry and plot individual capture sites. The distance and direction of any movement could be determined by plotting capture sites on the aerial photograph. This method of recording localities proved to be reasonably accurate (+^ five meters) and easily utilized. In addition, it minimized the problems of working large áreas with irregular terrain. Each pocket gopher captured in the study área was coded according to species (P = Pappogeomys; T = Thomomys), sex (d*;?), sequential collecting number, and age (A = adult; S = subadult, juvenile). Specimens were " p. Cd B o w p a> p ti Cd • o 4-> iH to CO t*^ 3 Oi to O Cd -P.£3 to ^ -H Ü PJ u 0) ,£3 D Cd +J •H <D B ,CJ.£3NO > P fH O Cd >>c tD ,£3 o >s Cd pí >^o Cd n:) Cd 3 P B p CO •HX to to O Cd Cd PU ^ u PU p4bO M-i Cd o to o >^ rH fH bo O Cd .£3 o <D 3 P^PU o* Cd o (D í-i.£3 * bO PU ^ H o o P to -H P 0'H U ^,£3 <l> B PUH P •H PJ -P HH Pj Cd v •H tom ü 0) o P o H P to C o O (ü B (-1 p .. 3 0 p Cd iH Pt PU Cd 3 Cd 0) u bo u Qco •H toe-clipped on the hind feet and released at the site of capture. The species, specimen Identification, sex, reproductive condition, age, molt, weight, burrow description, locality, and collector were recorded on a form used for each capture (Fig. 2). The design of this study incorporated the suggestions of Howard and Inglês (1951) concerning methods of studying fossorial mammals. Although soil type, burrow depth, and burrow diameter were recorded with each capture, excavation of burrow systems was restricted to preserve the habitat of the pocket gophers. The size and extent of burrow systems were determined indirectly by multiple trappings of individual pocket gophers. Sex was verified by checking the pubic symphysis, mammae, or baculum. Ascertaining the reproductive status of live animais in the field is difficult because observations are limited to externai criteria. In female pocket gophers, lactation, signs of nursing, pregnancy near parturition, and changes in the pubic symphysis were used as indicators of the reproductive status. In male pocket gophers, the position of the phallus and testes were noted but were recognized as possible invalid indications of reproductive condition (Miller, 1946). Age determination in both species was primarily based on pelage. Using pelage, individuais were 1 <9 z z <« 1 a: • >• -<1 ^ ^< RUJ 1- o ^ 1 z O z Ji z lü V) "è 15- HÜJ >. < O z~ 1- 3 1- z J 2 J -1 o S < CE CO < < 1 H UJ > P -•. O h- ÜE PH T 0^ ac UJ UJ UJ o LI o o 9 Q. < <»- _l' Z (/) z H > UI -J < ft» ^ X 1- >- o UI 2E C! 1 < < •H o < > -1 3 i w o Tí Ul 1 UJ ^1 1- UI Z O o 1 ã o a. O Lü ^1 ^ z z (U 8 s ã < z f-. J 5, < d i i • o i o -J 1 UJ <o í<Qn .1 £1 oZ UoI 1- oz 1- Ti <D >• o 0o0 (T< tcUJ zUJ £ ^ 1 < -1 > o K UI Z >• UJ z 1 z (/) < < 1 UJ < O 1 UJ (/) CO O <0 I o - C .. 5 z G O z z UI UJ 1 m UJ PUBI MAM PRE 1- 0. z z •M UJ o Q O 1 X) l BURROW o Z z < Q. ^— -^\ UJ • 11 1 J < Syi-\ ^ 3 -JZIZ f \ ^ I E D -» T bo P r T y •H MOL EVIDE SUBA JUVEN e ADUL < -—' o 2-, O (T ( - ^ UJ 5 z ^—2 — o s d u_ ^—, ' • </> o J m o •^ ^ z —^ identified as being a juvenile, subadult, or adult. More specific age determinations were not possible. Weights were not used because of their questioned validity (Hansen, 1960). The home range of pocket gophers is e^sentially restricted to the burrow system (Turner ejt ai., 1973; Wilks, 1963). Because this study is primarily concerned with dispersai tendencies beyond the burrow system the actual área of the home range, as treated by other investigators (Howard and Childs, 1959; Turner ejt ai. , 1973; Wilks, 1963), is not criticai. However, the size of the burrow system, or home range at any specific time, is important in differentiating movements within a single burrow system and movements into new áreas. Movements within a single burrow system could be eliminated if the maximum length for a burrow system was known. An estimate of the size of a burrow system was obtained for each species by determining the maximum distance between capture sites of any individual during a 24 hour period. This time limit was used because it is unlikely that an individual pocket gopher will move into a new área, establish its territory, and build a new burrow system within such a short period of time. Therefore, distances less than the length of a burrow system, as determined above, are considered to be movements within a single 8 burrow system, whereas greater distances are treated as movements into new áreas. This value is arbitrary and subject to error because the actual size of a burrow system cannot be accurately determined without excavating the burrow system or using radio-telemetry.