Why Are Diurnal Primates Living in Groups?

There are two main competing theories on the evolution of group living in diurnal nonhuman primates. The first theory claims that predation avoidance favours group living, whereas there are only disadvantages to feeding in a group and feeding competition increases with group size. The second theory claims that there is a feeding advantage to group living deriving from communal defence of high-quality food patches and that predation is not important. These theories have not yet been rigorously tested. In this paper a critical test is proposed: the theories differ in the predicted relationship between a female's birth rate and the size of the group in which she lives (Fig. 1). An additional test is concerned with the predicted relationship between population density relative to food availability and average group size. Finally, a critical test is proposed of the hypothesis that increasing group size should lead to reduced predation risk by comparing demographic patterns between areas where predators are still present and where they have disappeared. A total of 23 data sets on 13 species were extracted from the literature and supplemented with four unpublished data sets. In all three tests the results provide strong support for the predation-feeding competition theory and are clearly unfavourable for the theory postulating feeding advantages to group living. Such feeding advantages may, however, gain prominence under some conditions.