The ecology of movement and site selection in desert rattlesnakes (Crotalus mitchellii and Crotalus ruber) of the southwestern United States

Theories of animal movement often suppose that animals travel in “optimal” patterns that improve efficiency at locating resources. Empirical studies support such “optimality theory” as it applies to foraging movements made by animals. Little attention has been paid, however, to how the distribution of non-food resources influences animal movement patterns. Resources such as mates and sites with suitable micro-climates differ from food in their importance to animals and in how their availability varies in space and time. We may expect that they differ as well in their effects on animal movements.; This dissertation investigates how the spatial distribution of non-food resources affects movements in the rattlesnakes Crotalus mitchellii and C. ruber in the Colorado Desert of the southwestern United States. Two distinct but complementary field studies were conducted. First, movements of individual snakes were monitored continuously via radio-telemetry for 1–4 years each. Second, individuals without radio-transmitters were studied via mark-recapture to assess the spatial distribution of individuals more generally. The data on snake movement and site selection were then compared to custom models of spatial variation in the availability of suitable hibernating sites, potential mates, and shelter sites with acceptable degree of exposure to solar radiation. These comparisons sought to measure how the distributions of these resources influenced the snakes' movements.; Chapter 1 reveals that, though suitable hibernating sites are distributed widely, some desert snakes may repeatedly use particular ones. These individuals are migratory, traveling from hibernacula to summer activity areas each spring and then back to hibernacula in fall. Chapter 2 shows that, at least in Crotalus mitchellii, male movements are influenced by spatial variation in the probability of encountering females. Males concentrate their activity during the mating season, but not otherwise, in regions with higher female encounter probability. Chapter 3 reveals that snakes seeking shelters from summertime heat or hibernacula may not evaluate, at least not as optimality theory predicts, the degree of potential sites' exposure to insolation. These snakes do not generally choose low-irradiance summer shelters, sometimes preferring highly exposed sites. They also do not generally choose high-irradiance hibernating sites, sometimes preferring poorly exposed sites.