EFFECTS OF GEOGRAPHICALLY VARIABLE THERMAL ENVIRONMENT ON BIOENERGETICS OF MOTTLED ROCK RATTLESNAKES

Geographic variation in thermal environment may significantly influence the energetics of ectotherms, and thus may serve as a mechanism for geographic variation in growth and reproduction. Thermal environment affects body temperature (T-b) that, in turn, has consequences both for the amount of time available for surface activities such as foraging, and for physiological processes such as metabolism I studied effects of differences in thermal environment on T-b, surface use, and resting metabolism of mottled rock rattlesnakes (Crotalus lepidus) from two populations (Boquillas: BOQ, and Grapevine Hills: GVH) in Big Bend National Park, Texas. Snakes in these populations differed in growth rate and adult body size, and these differences may be related to the influence of thermal environment on energetics. During summer months, operative environmental temperatures at BOQ were hotter than at GVH. Hotter BOQ environmental temperatures resulted in approximate to 64% less time available for diurnal surface use (e.g., foraging) at BOQ relative to GVH. Hotter BOQ environmental temperatures also constrained the average T-b of BOQ snakes at night to be from 2 degrees C to 4.5 degrees C hotter than GVH snakes during July and August. Despite hotter T-b, estimated resting metabolic expenditures of BOQ snakes during July and August were similar to estimated expenditures of GVH snakes because BOQ snakes had lower mass-specific metabolic rates. Results of this study were consistent with the hypothesis of greater energy limitation at BOQ, and underscored the importance of population-specific data for understanding geographic variation in energy acquisition and allocation.