Activity Metabolism of Anuran Amphibians: Implications for the Origin of Endothermy

A model has been proposed for the evolution of endothermy from ectothermic progenitors that assumes a positive correlation between resting rates of oxygen consumption ( $$\dot{V}_{O_{2}}$$ rest) and maximum rates of oxygen consumption ( $$\dot{V}_{O_{2}}$$ max) for species of terrestrial vertebrates. Selection for enhanced aerobic capacity during activity is presumed to have resulted in elevated resting rates of oxygen consumption. The assumption that underlies this evolutionary argument was tested using 17 species of anuran amphibians. These species exhibit considerable variation in both $$\dot{V}_{O_{2}}$$ rest and $$\dot{V}_{O_{2}}$$ max. A highly significant, positive correlation was found between these metabolic parameters. Those species with higher than average maximum rates of oxygen consumption are also characterized by high resting levels. Low $$\dot{V}_{O_{2}}$$ max values were associated with low resting rates. The possibility that metabolic characteristics of anuran amphibians are seasonally variable, potentially giving rise to a spurious relationship between $$\dot{V}_{O_{2}}$$ rest and $$\dot{V}_{O_{2}}$$ max, was also investigated. No significant differences were observed between breeding and nonbreeding individuals in either $$\dot{V}_{O_{2}}$$ rest or $$\dot{V}_{O_{2}}$$ max for three species. The positive correlation between $$\dot{V}_{O_{2}}$$ rest and $$\dot{V}_{O_{2}}$$ max is consistent with evolutionary arguments that view metabolic response to exercise as an important factor in a selection process that led ultimately to the endothermic condition.