Physiology mediates how organisms interact with and are constrained by their environment. Understanding the variation in physiological responses to normal and extreme environmental conditions, particularly thermal variation, is critical for predicting how organisms will respond to environmental change. As global temperatures rise, species are experiencing increased incidence of stressful climatic conditions at the local scale, including increased temperatures, increased variance in environmental conditions on seasonal and annual bases, and higher incidence of extreme weather events. Changes in the environment can result in alterations at every level of biological organization, from genetics to physiology to demography, and may have consequences for adaptation to stress and related phenotypes. Moreover, such variation in physiological responses to stress – within and among individuals, populations and species – may influence how populations will adapt, acclimate, or face extinction in rapidly shifting climatic conditions. Climatic variability is particularly significant for ectotherms, as thermal profiles and water availability are direct determinants of their performance and are often intimately linked to life history, making them vulnerable to environmental change.

Using two species of widespread North American ectotherms (garter snakes Thamnophis elegans and Thamnophis marcianus), I examined the broad influence of temperature on aspects of physiology in lab studies, and then investigated climatic influences on physiology in the wild. In the first study, I tested for thermal effects on a number of physiological measures: metabolic rate (V ̇O2), stress physiology (corticosterone levels, and ratio of heterophils to lymphocytes in circulation), and energy availability (glucose and insulin levels) before, during, and after a simulated brumation in the checkered garter snake (Thamnophis marcianus). My results demonstrate not only a temperature dependence across physiological axes, but seasonal variation in thermal responsiveness. In the second study, I found that measures of both whole-animal metabolic rate (V ̇O2) and cellular oxygen consumption rate (OCR) increased with increasing temperature but at different rates. Additionally, within any given temperature the measures of metabolic physiology were not correlated within individuals. These findings show that, cellular metabolism does not directly dictate whole-animal oxygen consumption, yet both have substantial phenotypic plasticity across temperature. In the final study, natural populations of garter snakes (Thamnophis elegans) that are characterized by divergent life-history strategies displayed significant annual variation in measures of baseline and stress-induced plasma corticosterone and blood glucose concentrations, and in their reactivity (the magnitude of change between baseline and stress-induced measures), in an ecotype-dependent manner across a 10-year study. Moreover, the effects of precipitation and temperature also manifested in an ecotype-specific manner. This suggests that physiological regulation of stress via glucocorticoids and glucose may not be the most important mechanism by which garter snakes in this system deal with large-scale environmental perturbations such as drought. Instead, ecotypic differences are likely driving physiological responses to stress, and climate indirectly influences these ecotypes through their distinct life histories.