The maximum oxygen consumption and aerobic scope of birds and mammals: getting to the heart of the matter

Resting or basal metabolic rates, compared across a wide range of organisms , scale with respect to body mass as approximately the 0.75 power. This rel ationship has recently been linked to the fractal geometry of the appropria te transport system or, in the case of birds and mammals, the blood vascula r system. However, the structural features of the blood vascular system sho uld more closely reflect maximal aerobic metabolic rates rather than submax imal function. Thus, the maximal aerobic metabolic rates of birds and mamma ls should also scale as approximately the 0.75 power. A review of the liter ature on maximal oxygen consumption and factorial aerobic scope (maximum ox ygen consumption divided by basal metabolic rate) suggests that body mass i nfluences the capacity of the cardiovascular system to raise metabolic rate s above those at rest. The results show that the maximum sustainable metabo lic rates of both birds and mammals are similar and scale as approximately the 0.88 +/- 0.02 power of body mass (and aerobic scope as approximately th e 0.15 +/- 0.05 power), when the measurements are standardized with respect to the differences in relative heart mass and haemoglobin concentration be tween species. The maximum heart beat frequency of birds and mammals is pre dicted to scale as the - 0.12 +/- 0.02 power of body mass, while that at re st should scale as - 0.27 +/- 0.04.