A current hypothesis in ecological physiology states that differences among animal species in the physiology of exercise arose in response to natural selection on the capacity to sustain behavior. Animals that typically undergo high levels of sustained behavior are expected to have high capacities for aerobic energy production, whereas animals with low aerobic capacities are expected to sustain only low intensity behaviors and must rely on anaerobic metabolism to fuel higher levels of activity, which are not sustainable. For example, species differences in activity metabolism among anuran amphibians have been related to differences in locomotor, foraging, predator avoidance, and calling behavior. Previous studies did not distinguish among adaptive hypotheses (i.e. recent selection on behavior and physiology) and alternative hypotheses for differences in physiology, such as differences in body size, correlation of physiology with some unexamined trait, or constraints imposed by phylogeny. The proposed research will test for physiology-behavior correlations in a group of closely related species, the North American treefrogs. The adaptive significance of variation in physiology will be assessed by determining the fraction of variation due to phylogeny and body size and the fraction attributable to variation in several variables that have demonstrated or adduced effects on survival and reproduction (e.g., locomotor performance and calling behavior). Correlations that persist after removing the effects of body size and phylogeny will be deemed to represent the results of recent selection.
