FIELD METABOLIC-RATE, WATER FLUX, AND ENERGY BUDGETS OF MOTTLED ROCK RATTLESNAKES, CROTALUS-LEPIDUS, FROM 2 POPULATIONS

Geographic variation in the life histories of terrestrial vertebrate e ctotherms may, in part, be explained by geographic variation in energy acquisition and patterns of energy allocation. Comparative study of e cological energy budgets may therefore indicate whether populations di ffer because of differences in total acquired energy or because of dif ferences in the proportional allocation of total acquired energy among maintenance, growth, and reproduction. I used the doubly labeled wate r 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 a t site G. With DFR data, I constructed energy budgets to test the hypo theses that snakes from B were more food limited than snakes from G (a nd thus, had smaller total energy budgets) and that proportional alloc ation of energy among inactive metabolism, activity, and growth was di fferent between these populations. If my DFR measurements are represen tative of true population values, then I estimate that snakes from sit e B have annual energy budgets that are approximately half the magnitu de 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 inact ive metabolic costs. Energy budgets constructed in this study aid in u nderstanding lower growth rates and smaller adult body size exhibited by snakes from site B, relative to snakes from site G.