Relationship of Basal to Summit Metabolic Rate in Passerine Birds and the Aerobic Capacity Model for the Evolution of Endothermy

Abstract

The aerobic capacity modelfor the evolution of endothermy contends that endothermy arose as a result of selection for enhanced aerobic capacity rather than for thermoregulatory considerations. An implicit assumption of this model is that basal metabolism (BMR) increases with an increase in sustainable activity (i.e., aerobic capacity). We tested this assumption for passerine birds using individuals of 10 diferent species to determine whether basal and summit (maximum coldinduced) metabolic rates were correlated. Metabolic rates were measured as oxygen consumption by open-circuit respirometry. Regressions of basal and summit metabolism against body mass (log10-log10 transformations) indicated that both BMR and summit metabolism were positively and significantly related to mass. Slopes of the two regression lines were not significantly diferent, which indicates similar scaling properties for BMR and summit metabolism. Residuals of log10-log10 transformations of summit metabolism and BMR against body mass demonstrated that summit metabolism was correlated with BMR independently of mass (r = 0.869). In addition, analyses generating phylogenetically independent contrasts indicated that contrasts of log10 BMR and log10 summit metabolism were both positively and significantly correlated with contrasts of log10 body mass. Residuals from these regression lines demonstrated that the significant positive correlation between BMR and summit metabolism (r = 0.861) remained when effects of both mass and phylogeny were removed. Thus, metabolic rates of the passerines in this study are consistent with the aerobic capacity model for the evolution of endothermy.