Causes and significance of variation in mammalian basal metabolism.

Mammalian basal metabolic rates (BMR) increase with body mass, whichs explains approximately 95% of the variation in BMR. However, at a given mass, there remains a large amount of variation in BMR. While many researchers suggest that the overall scaling of BMR with body mass is due to physiological constraints, variation at a given body mass may provide clues as to how selection acts on BMR. Here, we examine this variation in BMR in a broad sample of mammals and we test the hypothesis that, across mammals, body composition explains differences in BMR at a given body mass. Variation in BMR is strongly correlated with variation in muscle mass, and both of these variables are correlated with latitude and ambient temperature. These results suggest that selection alters BMR in response to thermoregulatory pressures, and that selection uses muscle mass as a means to generate this variation.