The physiological ecology of differences in body size between two populations of the mottled rock rattlesnake (Crotalus lepidus lepidus)

Variation among populations in growth rate and adult body size is ubiquitous and may influence individual fitness and population dynamics. Recent explanations for geographic variation in body size of squamate reptiles have implicated variation in prey size or availability as a causative factor. Any factor which influences either total energy acquisition or the allocation of energy among competing functions of maintenance (including activity), growth, reproduction, and storage, may potentially influence growth rates of individuals. I investigated the effects of variation in thermal environment as a proximate factor influencing differences in growth rate and age-specific body size observed between two populations of the mottled rock rattlesnake in Big Bend National Park, Texas. Snakes from the Boquillas (BOQ) population grew more slowly and attained smaller adult body sizes than snakes from the Grapevine Hills (GVH) population. I examined the effects of differences in thermal environment between GVH and BOQ on energy acquisition and allocation by quantifying; (1) the amount of time available for foraging (which affects resource acquisition), (2) body temperatures (which affect metabolic rate), (3) the thermal dependence of metabolism, and (4) field metabolic rates, at each site. I constructed annual energy budgets for snakes from each site to investigate relationships between total energy budget size, and the proportional allocation of energy to growth and maintenance. The BOQ environment was significantly hotter than GVH, resulting in decreased amount of time for foraging and higher body temperatures relative to GVH. Despite differences in body temperature, estimated inactive metabolic rates were similar between sites because BOQ snakes had lower mass-specific metabolic rates than their GVH counterparts. Estimated annual energy budget for BOQ snakes was approximately half that of GVH snakes, and BOQ snakes allocated a greater proportion of total available energy to maintenance. Observations were consistent with the hypotheses that (1) BOQ snakes are more food limited than GVH snakes and (2) thermal environment may affect growth by influencing foraging time, body temperatures and metabolism. This study underscores the importance of population-specific physiological data for understanding geographic variation in growth rate, body size and other life history traits