The natural selection of metabolism explains curvature in allometric scaling

Lars Witting

Abstract: I simulate the natural selection of metabolism and mass to explain the curvature in the metabolic allometry for placental and marsupial mammals. The simulation model starts with a single ancestor in each clade at the Cretaceous-Palaeogene boundary 65 million years ago. The release of inter-specific competition by the extinction of dinosaurs make it possible for each clade to diversify into a multitude of species across a wide range of empty niches. The selection of mass in these species depends on the net assimilated energy that depends on i) the handling of the resources in the different niches, and on ii) mass-specific metabolism that defines the pace of the handling process. The model is fitted to explain the maximum observed body masses over time and the current inter-specific allometry for metabolism. The selection of mass-specific metabolism is found to bend the metabolic allometry over time, even when all species have the same selection on the per-generation time-scale of natural selection. This is because the smaller species evolve over a larger number of generations than the larger species. The strongest curvature is in the placental clade, where the estimated rate of exponential increase in mass-specific metabolism is 9.3x10^-9 (95% CI: 7.3x10^-9 - 1.1x10^-8) on the per-generation time-scale. This is an order of magnitude larger than the estimate for marsupials, in agreement with an average metabolism that is 30% larger in placentals relative to marsupials of similar size.