North American hylid frogs exhibit patterns of locomotor energetics that are generally similar to those found among most terrestrial vertebrates: steady-state rate of oxygen consumption (VO(2ss)) increases within a range of sustainable speeds, above which VO(2ss) becomes independent of speed and endurance decreases. Among hylid anurans, species differences in the capacity to sustain locomotion are related to the interaction of locomotor cost and the maximal capacity for aerobic energy production during exercise. Hence, analyses of metabolic correlates of behavioral activity should incorporate estimates of economy (C(min)) as well as capacity (VO(2ex)). Phylogenetic analyses using independent contrast algorithms indicate that resting and maximal rates of metabolism are positively correlated, even when the confounding effects of body size and phylogeny are removed. Inferred evolutionary changes in metabolic rate proceeded from low rates in neotropical ancestors to higher rates among descendant species in temperate habitats. Adaptation to facilitate activity at low temperatures may have been responsible for this evolutionary shift. These findings support the "aerobic capacity" hypothesis for the evolution of metabolic rate in which evolutionary increases in metabolic rate are brought about by natural selection favoring increased abilities for sustained activity.