The current paradigm in animal organismal biology links morphology, performance, behavior, and Darwinian fitness in a sequence of proximate causal relations. First, the combination of lower-level organismal traits (e.g., leg length, heart mass, muscle enzyme activities) generate the performance level observed (e.g., maximal sprint speed, maximal oxygen consumption during exercise [V̇O2max]), then maximal performance levels constrain behavioral options, and the consequences of behavioral choices influence fitness. Because behavior and performance are the last steps leading to fitness, selection acts most directly on them than on lower-level (subordinate) traits. Performance and behavior are, therefore, pivotal points in understanding animal adaptation. A primary determinant of an animal's aerobic performance capacity is V̇O2max which sets the limit to the intensity of physical effort that can be sustained over prolonged periods of time. Only during relatively rare occasions (e.g., during predator-prey interactions) do animals perform at levels above V̇O2max; therefore, most activities will be constrained by their capacity to consume oxygen. Many key events involve mostly aerobic levels of activity (e.g., foraging, patrolling territories), making V̇O2max a likely target for natural and sexual selection.
This dissertation investigated V̇O2max in two contexts. In chapters one and two I put V̇O2max in an evolutionary context, evaluating the coadaptation of V̇O2max and ecological traits among lizard and mammal species. These studies revealed that V̇O2max is weakly positively correlated with home range area among species of mammals, that viviparous lizards have lower V̇O2max than non-viviparous species, and that varanids, helodermatids, and skinks have higher V̇O2max than other groups of lizards.
In the third chapter, I use a mechanistic approach to elucidate the proximate causes and consequences of V̇O2max at the level of among-individual variation. I found that V̇O2max in adult male Sceloporus occidentalis lizards from Hampton Butte, OR during the breeding season is positively related to hematocrit (or hemoglobin) levels in the blood and that V̇O2max is positively related to territorial behaviors (number of push-ups using two or four legs per bout of push-ups).