Introduction

Food acquisition and predator avoidance are two major factors proposed to promote social grouping in animals (Krebs & Davies 1987). In the primate literature, these two factors have been presented as alternative hypotheses (Wrangham 1980; van Schaik 1983). Defense against potential infanticide by new males also favors social grouping in primates: females form permanent associations with protective males, including their dependent infants' fathers (Wrangham 1979; van Schaik & Dunbar 1990; Smuts & Smuts 1993; van Schaik & Kappeler 1993, 1997; Sterck et al., 1997). The present volume originated from the recognition of the importance of infanticide avoidance in shaping primate social organization.

Factors favoring the formation of permanent social groups of females and males are not necessarily the same factors influencing the size of the groups. For example, many believe food competition to be the principal factor limiting primate group size (Janson 1988; van Schaik 1989; Isbell 1991; Janson & Goldsmith 1995). Upper group-size limits may be mediated by the maximum daily travel distance individuals can sustain as they forage in groups (Wrangham et al. 1993). In this chapter, we present evidence from red howler monkeys (Alouatta seniculus) that infanticide, too, may play a role in limiting group size in primates (see also Steenbeek, Chapter 7). When infanticide rates increase with the number of reproductive females, females may opt for dispersal, thus keeping total group size small. We propose that indirect evidence previously suggesting the role of food competition in limiting group size might actually reflect infanticide.