ABSTRACT
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.
ABSTRACT
Studies of exercise physiology of anuran amphibians have led to the suggestion that there is a dichotomy between species that depend upon movement to escape from predators and species that utilize static defenses. This generalization has been based upon a limited taxonomic survey and it contrasts with morphological, ecological, and behavioral studies that have revealed diverse and complex interrelationships among these features of anuran biology. We tested the hypothesis of a dichotomy of physiological types among anurans by measuring aerobic and anaerobic metabolism during maximum exercise for 17 species representing seven families and a variety of ecological types and locomotor modes. All degrees of dependence upon aerobic and anaerobic power input were found among the 17 species and the variation did not follow phylogenetic divisions. No single, simple prediction of the predominant source of power utilized for activity by the anurans we studied is possible. Predator avoidance behavior was not significantly correlated with the metabolic pattern. Predatory mode (active versus passive searchers) and mode of locomotion (non-jumpers versus jumpers) were correlated with dependence upon aerobic energy production and with each other. Reproductive behavior is probably another associated factor. The diversity of modes of power input among anurans is great and is intimately linked with numerous features of a species' biology. Single-factor explanations of this physiological characteristic are not appropriate.
