An Evaluation of the Roles of Predation Rate and Predation Risk as Selective Pressures on Primate Grouping Behaviour

Abstract

Summary Establishing the importance of predation pressure in determining primate social structure has generated a great deal of discussion. However, the substantive issues in this debate have been obscured by a conflation of the respective roles of predation risk and predation rate as selective forces. In addition, the reported relationships between predation rate and both group size and body weight are likely to be confounded by the effects of reproductive rate and activity period. We propose that the level of sustainable predation rate for a species is determined by the rate at which it is able to reproduce, and that, within this constraint, a species adjusts its body weight and a population its group size so as to reduce predation rate to some tolerable level. In effect, the observed predation rate is the excess mortality from predation that animals are unable to control by adjusting their behaviour. This implies that there should be no relationship between predation rate and either group size or body weight, once the effects of reproductive rate and activity period are removed. We analyse data from the primate literature and show that reproductive rate is indeed the best predictor of a species' predation rate, and that the reported relationships with group size are entirely attributable to a combination of the inclusion of nocturnal species which do not use group size as an antipredator response and the confounding effects of reproductive rate. We discuss these findings in the light of current investigations on predation risk, and conclude that an understanding of the role of predation as a selective pressure on primates will only