Physiology is the mechanistic link between how an individual organism experiences its external environment and higher-level biological effects. Since natural selection acts on whole phenotypes, understanding the evolution of physiology requires integration with other traits, such as behavior and life-history, at both the phenotypic and genetic levels. Accumulating evidence suggests that within-individual plasticity of physiological and behavioral traits is limited, resulting in stable individual phenotypes and differences among individuals. Using the model systems of two widespread North American vertebrates (the garter snakes Thamnophis sirtalis and T. elegans), the research presented here quantifies physiological variation across levels of biological organization: measures of multiple traits within individuals and repeated measures over time to assess within-individual correlations among markers; comparisons of populations adapted to different environments to assess evolutionary potential; and measures of among-individual differences in physiological and behavioral traits and subsequent consequences to reproductive output and fitness.

In the first study, we measured biomarkers of the stress response (stress hormone: plasma corticosterone; energy mobilization: plasma glucose; immune cell distribution: leukocyte ratios) in T. sirtalis from geographically disparate regions in response to a standardized restraint protocol. Snakes from mountain populations exhibited higher initial baseline concentrations of corticosterone but snakes from plains populations maintained elevated corticosterone and plasma glucose levels for a longer duration before returning to baseline levels, if at all. In the second study, divergent life-history ecotypes of T. elegans elevated corticosterone and glucose concentrations as well as metabolic rate non-linearly at high temperature extremes, while the hormone insulin responded to temperature in a pattern suggesting that it plays a role in the thermal stress response beyond glucose regulation. The ecotypes did not differ in their response, indicating that neither local adaptation nor physiological plasticity allows populations from warmer environments to develop higher thermal tolerances. In the final study, we found that reproductive female T. elegans with matched behavioral and physiological phenotypes (either ‘high reactive-high reactive’ or ‘low reactive-low reactive’) gave birth to offspring in better body condition that, in turn, grew faster and lived longer than offspring from mothers with other phenotype combinations.