The evolution of placental mammal body sizes: evolutionary history, form, and function.

The unimodal, right-skewed distribution, most frequently identified in contemporary descriptions of placental mammal body size distributions, masks an underlying multidistribution structure; a long-term evolutionary process that has generated a concatenation of two or three frequency distributions specific to locomotory modes (plantigrade, digitigrade and unguligrade). The Afrotropical assemblages are bimodal, with a tendency towards trimodality, whereas the Nearctic assemblage is unimodal. However, mixtures of two and three normal distributions fitted the Nearctic data well, suggesting a multidistribution structure masked by disproportionate species numbers within locomotory modes. Differences in proportional species numbers within modes between assemblages may reflect the evolutionary history of form and function. However, common interassemblage predictions of such proportions in contemporary distributions may be disguised by the relative severity of the Pleistocene megafaunal extinction (patterns supported by the fossil record), geographical scale, and taxonomic composition. A species gap occurs at body sizes around 1 kg at the interface between the largest plantigrade mammals and the smallest digitigrade mammals, coincident with the minimum interspecific variance of basal metabolic rate. In terms of the evolution of the optimal body size in the trade-off between mortality and production, there may be good historical and evolutionary reasons why we should not expect optimization to produce the same results in different zoogeographical assemblages. Moreover, the evolution of diverse mammalian forms and functions, especially with respect to predator-prey interactions and diet, render a single body size optimum untenable in the search for an energetic definition of fitness.