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University of Nevada, Reno Homing and Navigation in Chuckwallas

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University of Nevada, Reno Homing and Navigation in Chuckwallas University of Nevada, Reno Homing and Navigation in Chuckwallas (Sauromalus ater) A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Biology by Stephanie Wakeling Dr. C.R. Tracy/Thesis Advisor May 2012 THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by STEPHANIE WAKELING entitled Homing And Navigation In Chuckwallas be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE C. Richard Tracy Ph.D., Advisor Lynn Zimmerman Ph.D, Committee Member Michael Webster, Ph.D., Graduate School Representative Marsha H. Read, Ph. D., Dean, Graduate School May, 2012 i ABSTRACT In this study of chuckwalla homing and movement, we displaced 30 chuckwallas, and a total of 11 chuckwallas were able to return to home successfully. Larger animals were the most successful returners, and they only returned well from the shorter displacement distance of 200 m. Our data provides evidence that smaller lizards may have differential navigation ability, higher resistance to movements caused by conspecifics, or lower motivation to return home. Our study also illustrates displaced lizards have greater daily movements than lizards on their home ranges, and the study increases our knowledge of the size and nature of home ranges of chuckwallas during an activity season. We discuss necessary future research to help us understand movements and propensities of chuckwallas and the factors that may be important in determining movements. ii ACKNOWLEDGEMENTS This thesis would not have come together without the help and support of many people, as well as funding from the Biology Department at the University of Nevada, Reno. Thank you to my Advisor, Dr. Richard Tracy for having confidence in my abilities, and for always caring about my progress and wellbeing, and for all of the time you’ve spent helping me develop my science. To my other close mentors, Dr. Bridgette Hagerty, Dr. Franziska Sandmeier, and Dr. Christopher Tracy, thank you for always being supportive, your willingness to teach me new things, and your incredible talents. Also, to my family and friends, thank you for keeping me sane, and indulging my obsession with lizards. iii TABLE OF CONTENTS Abstract i Acknowledgements ii Table of Contents iii Introduction 1 Methods: Experimental Design 5 Methods: Analysis 7 Results 7 Size 9 Differences in movement patterns 10 Home Range 11 Circular Statistics 12 Figures and Table in Results Figure 1. Paths of Animals that Returned Home 8 Figure 2. Male and Female Snout vent lengths 9 Figure 3. Comparisons of paths 11 Table 1. Circular Statistics by Groups 13 Discussion 14 Homing and Movements 14 Home Range and Social Considerations 17 Social Interactions 18 Management Implications 19 Conclusion 20 Literature Cited 22 iv Appendix I: Animal Movement Paths 24 A. Home Range Paths 24 B. Home Range and Displacement Paths 25 C. 200 meter: Male Non-returners 26 D. 200 meter: Female Non-returners 27 E. 200 meter: Male Returners 28 F. 200 meter: Female Returners 29 G. 400 meter: Male Non-returners 30 H. 400 meter: Female Non-returners 31 I. 400 meter: Male Returner 32 Appendix II: Animal Movement Over Time 33 A. 200 meter: Male Non-returners 33 B. 200 meter: Female Non-returners 34 C. 200 meter: Male Returners 35 D. 200 meter: Female Returners 36 E. 400 meter: Male Non-returners 37 F. 400 meter: Female Non-returners 38 G. 400 meter: Male Returner 39 Appendix III: Chuckwallas Included in the Study 40 1 INTRODUCTION Homing experiments are used to assess the existence, or strength, of philopatry among individuals, and to assess mechanisms by which animals can accurately find their “home.” Typically in homing experiments, individuals are displaced from their sites of capture (defined as “home”), to see if those individuals will return home. Often different circumstances for release are experimentally manipulated to assess the importance of those experimental manipulations in improving or reducing the homing performance of individuals. This experimental approach assumes that displaced animals use the same navigational cues to return home as do dispersing and moving animals when they move in areas outside of their home ranges. Also, the approach assumes that animals have benefits associated with their territories to compel those individuals to return home. Many lizard species have been shown to navigate and return home from varying distances, and under different displacement conditions. Sceloporus orcutti can home from 215 m away; older individuals and males were found to be the best at homing (Weintraub 1970). Sleepy lizards, Tiliqua rugosa, have been found to home from up to 500 m from home, and these lizards can orient in the direction of home from 800 m (Freake 2001). Desert iguanas, Dipsosaurus dorsalis, have been shown to home from distances up to 274 m from their capture sites despite these lizards residing in discontinuous habitat types (Krekorian 1977). In the only previous homing experiment with chuckwallas (Sauromalus ater), male and female chuckwallas were 2 displaced 50 m, 100 m, and 500 m from their home ranges, and they returned home within a day or two. They appeared to return home more effectively on sunny days (Prieto and Ryan 1978). That chuckwalla experiment was conducted on a small number of lizards (n=12, males and females total), and their animals were displaced to unsuitable patches without rock piles (Prieto and Ryan 1978). That experiment provides a framework for determining minimum displacement distances leading to reasonable expectations that chuckwallas will return home. However, it leaves open questions about potential differences in homing ability between the sexes, and about the ways in which the animals move outside of their home ranges. Additionally, the sizes and potential differences of home ranges remain to be studied. Here, we report the results of a homing experiment designed to help our understanding of the movements in chuckwallas. Understanding homing and navigational propensity should help us understand the ability of chuckwallas to move effectively outside the boundaries of their home ranges, and it should help us understand the processes allowing chuckwallas to recolonize new home ranges or territories. While homing, individuals often must move through defended areas occupied by other chuckwallas, and displaced lizards often have to travel through patches of unsuitable habitat. These challenges to movements can be taken to simulate similar challenges chuckwallas face outside of their home range for dispersal, courting, or foraging. 3 Chuckwallas, Sauromalus ater [obesus (Hollingsworth 1998)] are large iguanid lizards with a large, patchy range spanning from the Mojave Desert of southern Nevada as far south as Baja California Sur, east into parts of Arizona, and west into parts of the Mojave Desert in southern California (Ustach and Brodie 2003). These herbivorous lizards are often sexually dimorphic and highly territorial. They exist in habitat that is comprised of rock piles or old lava flows, because they are saxicolous they depend on hiding in cracks among rocks for protection from predators (Berry 1974, Abst 1988, Hollingsworth 1998, Kwiatkowski and Sullivan 2002, Abts 1988). Males are believed to defend large territories that include several females, but those territories generally do not overlap with those of other males (Kwiatkowski and Sullivan 2002). Females reportedly have established territories, but their territories are not necessarily exclusive as they often overlap with both males and females (Berry 1974). Chuckwallas are long-lived (Abst 1988) and little is known about their movements and their propensities to move, including how they navigate among sites, or disperse from natal sites or over-taken territories, from which more dominant animals displace them. Movement behavior of chuckwallas is likely influenced by the nature of the rock piles they inhabit. In areas with more continuous rock formations, individuals may move greater distances more easily, but it may also be less important if there is more suitable habitat available for them to establish territories. In areas with less 4 continuous and smaller rock piles, chuckwallas might be forced to disperse to find suitable habitat where they can establish a territory. Chuckwalla habitat is spatially discontinuous as chuckwallas are only found on rock piles that are often uneven in extent and spacing across the landscape (pers. obs. 2010). Chuckwallas are also very territorial animals, which can affect the ability of a wandering chuckwalla to use rock outcrops during its journey. Chuckwallas have a polygynous mating system (Berry 1974, Kwiatkowski and Sullivan 2002), so according to Greenwood’s prediction of males being the dispersing sex in dimorphic species (1980), male chuckwallas may be more likely to disperse further to find suitable territory space and mates. Females can set up non-exclusive territories within other animals’ territories, but adult males are likely to be “runoff” and pushed out of another male’s exclusive territory. In more homogeneous landscapes, it may be easier for chuckwallas to find refuge from predators and to accomplish behavioral thermoregulation, but there might also be more resistance against movement from other male chuckwallas defending and maintaining territories. Because chuckwallas have a polygynous mating system and males court females for long periods of time (Berry 1974, Doughty et al 1994, Kwiatkowski et al 2002), males may tightly maintain their home as a way to prevent other males from courting and copulating with their females. Thus, we hypothesize that navigation is more important to male chuckwallas, and males displaced from home will return at a greater rate than will females. 5 Other reptiles are known to use a variety of cues in navigating; chemical, visual, and magnetic, so a variety of cues may be important to chuckwallas (Southwood and Avens 2010). If there are gender differences in navigation and movement ability, then those differences are likely to be represented in homing ability.
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