Metabolic scaling is demand driven

Abstract Metabolic scaling is a major regularity in nature. Existing theory explains this scaling in terms of the optimal geometry of resource supply networks. Tests of this theory have focused almost entirely on the mathematical derivation of the scaling exponent and comparisons to the regression value for mainly birds and mammals. However, a focus on the per kg metabolic rate shows that large animals like whales have such a low mass‐specific rate that if supply‐limited, they could not function. Instead, it is proposed that the number of muscle contractions per time is the crucial variable that generates metabolic demand. Larger muscles cannot contract as fast as smaller ones. Larger and more sedentary animals necessarily experience fewer muscle contractions per unit time and thus a lower metabolic demand per unit mass. Data for mammal heart rate and breath intervals match the expected −1/4 per kg scaling. Basal metabolic rate (BMR) scaling is explained by exercise debt, which is the prolonged processing of wastes and repair of muscles that follows any activity, and by organ size and tissue‐level adjustments to exercise demand. Thus, BMR scales with physiological time. This explains how larger animals use less energy per kg without energy delivery being a constraint. More active and smaller animals have more muscle contractions per unit time, thus having a higher metabolic rate per kg even at rest.