Predation risk as an influence on group size in cercopithecoid primates: implications for social structure

Abstract

Predation pressure has long been proposed as a determinant of mammalian social systems. Group size and composition were compared for 121 populations of cercopithecoid primates, from 39 species of 13 genera, living under low, moderate or high predation risk. In confirmation of previous studies, predation risk was found to have a major effect on group size, with populations under high predation pressure living in significantly larger groups than those at lower risk. However, there were differences in social structure between the predation risk categories. Unimale groups were smaller, had fewer females than multimale groups and were infrequently found under conditions of high predation risk. Predation risk had marked effects on the composition of multimale groups. Under conditions of high predation risk there was a disproportionate increase in the number of adult males over that predicted by the number of females present or by group size. At low predation risk there were fewer males than predicted on the basis of female group size. Together, these results suggest that male strategies to monopolize females depend both on the females' grouping patterns and on the needs of both sexes to maximize group size under high predation pressure.