Are big mammals simply little mammals writ large?

Populations are regulated intrinsically (self-regulated) when the animals lower their rate of increase behaviorally or physiologically as a reaction to rising density. They are regulated extrinsically if the equilibrium is a mechanical consequence of interaction between the population and the organisms providing its food. We suggest that, at least for mammalian herbivores, self-regulation is unlikely to evolve unless the population's intrinsic rate of increase exceeds about 0.45 on a yearly basis. That value corresponds to a body weight of about 30 kg, the intrinsic rate being related inversely to body weight by r m=1.5 W(-0.36) with W in kg.The two dynamic strategies, self-regulation and extrinsic regulation, should enforce a bimodality of the frequency distribution of observed intrinsic rates of increase. This in turn might be reflected in a bimodality of body sizes, the smaller herbivores constituting the lower mode generally showing intrinsic regulation and the larger herbivores of the upper mode generally being regulated by extrinsic mechanisms. There is some empirical support for these predictions but it is by no means clearcut.Mechanisms of self-regulation can evolve either by individual or group selection. Individual selection may act in two ways. By inhibiting their neighbours with some form of interference, individuals may increase their relative fitness without increasing their reproductive rate. Alternatively, individual selection may raise the absolute fitness of individuals and thereby raise the populations's intrinsic rate of increase. The population is destabilized if that process continues beyond a certain threshold and the population is then at significant risk of extinction at the troughs of the consequent oscillations. Selection between such populations will favour those carrying the beginnings of a self-regulating mechanism, and with that mechanism strengthened and fixed by continuing group selection, individual selection is again freed of the dynamic restraints on raising further the intrinsic rate of increase.