AN EVALUATION OF THE ROLES OF PREDATION RATE AND PREDATION RISK AS SELECTIVE PRESSURES ON PRIMATE GROUPING BEHAVIOR

Establishing the importance of predation pressure in determining prima te social structure has generated a great deal of discussion. However, the substantive issues in this debate have been obscured by a conflat ion of the respective roles of predation risk and predation rate as se lective forces. In addition, the reported relationships between predat ion rate and both group size and body weight are likely to be confound ed by the effects of reproductive rate and activity period. We propose that the level of sustainable predation rate for a species is determi ned by the rate at which it is able to reproduce, and that, within thi s constraint, a species adjusts its body weight and a population its g roup size so as to reduce predation rate to some tolerable level. In e ffect, the observed predation rate is the excess mortality from predat ion that animals are unable to control by adjusting their behaviour. T his implies that there should be no relationship between predation rat e and either group size or body weight, once the effects of reproducti ve rate and activity period are removed. We analyse data from the prim ate literature and show that reproductive rate is indeed the best pred ictor of a species' predation rate, and that the reported relationship s with group size are entirely attributable to a combination of the in clusion of nocturnal species which do not use group size as an antipre dator response and the confounding effects of reproductive rate. We di scuss these findings in the light of current investigations on predati on risk, and conclude that an understanding of the role of predation a s a selective pressure on primates will only be achieved by attempts t o study the factors that are important in determining a primate's perc eived risk of predation.