Abstract

Maximal metabolic rate (MMR) of mammals scales differently from basal metabolic rate (BMR). This is first shown by scrutinizing data reported on exercise-induced V?O2max in 34 eutherian mammalian species covering a body mass range of 7 g-500 kg. V?O2max was found to scale with the 0.872 (±0.029, 95% confidence limits 0.813-0.932) power of body mass which is significantly different from the 3/4 power reported for basal metabolic rate. The aerobic scope is higher in athletic than non-athletic species, and it is also higher in large than in small species. Integrated structure-function studies on a subset of 11 species (body mass 20 g-450 kg) show that the variation of V?O2max with body size is tightly associated with the aerobic capacity of the locomotor musculature: the scaling exponents for V?O2max, the total volume of mitochondria, and the volume of capillaries are nearly identical. The higher V?O2max of athletic species is tightly linked to proportionally larger mitochondrial and capillary volumes in animals of the same size class. As a result V?O2max is linearly related to both total mitochondrial and capillary erythrocyte volumes. We conclude that the scaling of maximal metabolic rate is explained by features and mechanisms different from those determining basal metabolic rate. © 2004 Elsevier B.V. All rights reserved.