THE EVOLUTION OF ENDOTHERMY - TESTING THE AEROBIC CAPACITY MODEL

One of the most important events in vertebrate evolution was the acqui sition of endothermy, the ability to use metabolic heat production to elevate body temperature above environmental temperature. Several verb al models have been proposed to explain the selective factors leading to the evolution of endothermy. Of these, the aerobic capacity model h as received the most attention in recent years. The aerobic capacity m odel postulates that selection acted mainly to increase maximal aerobi c capacity (or associated behavioral abilities) and that elevated rest ing metabolic rate evolved as a correlated response. Here we evaluate the implicit evolutionary and genetic assumptions of the aerobic capac ity model. In light of this evaluation, we assess the utility of pheno typic and genetic correlations for testing the aerobic capacity model. Collectively, the available intraspecific data for terrestrial verteb rates support the notion of a positive phenotypic correlation between resting and maximal rates of oxygen consumption within species. Inters pecific analyses provide mixed support for this phenotypic correlation . We argue, however, that assessments of phenotypic or genetic correla tions within species and evolutionary correlations among species (from comparative data) are of limited utility, because they may not be abl e to distinguish between the aerobic capacity model and plausible alte rnatives, such as selection acting directly on aspects of thermoregula tory abilities. We suggest six sources of information that may help sh ed light on the selective factors important during the evolution of hi gh aerobic metabolic rates and, ultimately, the attainment of endother my. Of particular interest will be attempts to determine, using a comb ination of mechanistic physiological and quantitative-genetic approach es, whether a positive genetic correlation between resting and maximal rates of oxygen consumption is an ineluctable feature of vertebrate p hysiology.