Field Metabolic Rate, Water Flux, and Energy Budgets of Mottled Rock Rattlesnakes, Crotalus lepidus, from Two Populations

Abstract

Geographic variation in the life histories of terrestrial vertebrate ectotherms may, in part, be explained by geographic variation in energy acquisition and patterns of energy allocation. Comparative study of ecological energy budgets may therefore indicate whether populations differ because of differences in total acquired energy or because of differences in the proportional allocation of total acquired energy among maintenance, growth, and reproduction. I used the doubly labeled water method to measure field metabolic rates and water fluxes of mottled rock rattlesnakes (Crotalus lepidus) from two populations, Boquillas (B) and Grapevine Hills (G), in Big Bend National Park, Texas. Average daily field respiration (DFR) was significantly lower at site B than at site G. With DFR data, I constructed energy budgets to test the hypotheses that snakes from B were more food limited than snakes from G (and thus, had smaller total energy budgets) and that proportional allocation of energy among inactive metabolism, activity, and growth was different between these populations. If my DFR measurements are representative of true population values, then I estimate that snakes from site B have annual energy budgets that are approximately half the magnitude of the annual budgets of snakes from site G and that site B snakes appear to allocate a greater proportion of their total budget to inactive metabolic costs. Energy budgets constructed in this study aid in understanding lower growth rates and smaller adult body size exhibited by snakes from site B, relative to snakes from site G.