Introduction

Models that seek to address the costs and benefits of social groupliving often posit resource competition as one disadvantage of social living (Alexander 1974, Janson and Goldsmith 1993, Terborgh and Janson 1986, van Schaik 1983, van Schaik and van Hooff 1983, Wrangham, 1980). Such models differ in the importance of interversus intragroup competition, and on what factors encourage individuals to tolerate living in larger groups. ‘Predation–intragroup feeding competition’ models (PFC) (Alexander 1974, Terborgh and Janson 1986, van Schaik 1983, van Schaik and van Hooff 1983) all focus on intragroup feeding competition as the ultimate cost for group living, which intensifies as group size increases. For such models, increased predator detection and hence a reduction in predator risk is the driving force behind larger groups, and group size fluctuates as a response to these two factors. Indeed, van Schaik (1983: p. 138) has posited, ‘group living is disadvantageous with respect to feeding and that the avoidance of predation confers the only universal selective advantage of group living in diurnal primates.’ There have been few direct tests of the PFC model for primates, but larger groups of wild long-tailed macaques were better at predator detection (van Schaik et al. 1983) and smaller groups of yellow baboons did exhibit behaviors more conducive to monitoring predators or predator escape (Stacey 1986). It is clear, however, that there is no simple relationship between predation and group size (Boinski and Chapman 1995, Treves 1999). As such, there is a need for more specific quantitative analyses to determine the relationship between group size and predator pressure.