Physiological constraints on the ecology of activity-limited ectotherms

Organismal exchanges of heat, water, and metabolizable energy with their environment are important influences on their behavior, physiology and, life history. Where extreme climates and resource-poor environments restrict an organism's overall activity budget, physiology---through performance, tolerance, acclimation, and tradeoffs---becomes a primary influence on that organism's ecology. Vertebrate ectotherms exemplify these physiological constraints because their body temperatures are closely tied to thermal and hydric exchanges with their environment; they are typically highly tolerant of a broad range of conditions; and they exhibit boom-and-bust cycles related to release from constraints. This dissertation examines physiological constraints on wood frogs (Rana sylvatica) at the extreme southwestern edge of their range, desert tortoises (Gopherus agassizii) which spend >98% of their lives in burrows, and the eggs of leatherback turtles (Dermochelys coriacea) which incubate for months on a tropical beach during the dry season. The distribution of microhabitats available to each focal ectotherm delineated where and when these organisms could be surface-active, what constituted adequate refugia for intolerable surface periods, and why these activity-limited organisms behave differently from conspecifics elsewhere. Direct measurements of physiological performance (metabolic rates, locomotion) over the focal organism's tolerated range of body temperatures and hydrations assessed the consequences of thermal and hydric exchanges with suboptimal microhabitats. These exercises constitute a quantification of important influences on the physiological ecology of activity-limited vertebrate ectotherms. Comparative analysis of the allometry of physiological traits in a variety of mammals served to elucidate the functional and phylogenetic constraints on physiological adaptation. This quantitative and model-based approach is useful for quantifying risk (e.g., desiccation risk), estimating processes that are hard to measure (e.g., metabolic heating of sea turtle eggs buried 1 m deep in sand), and predicting responses to future changes (e.g., global warming). These qualities recommend this approach as a practical tool in conservation biology.