INTRODUCTION

Why do animals live in groups? Generally, animals are thought to live in groups in order to increase their survival and reproductive success. But group living incurs costs, mainly in terms of higher intraspecific competition and higher risks of parasite transmission. Therefore, the discussion has focused around the benefits that could make possible the evolution of sociality. Three main benefits have been proposed; group living decreases predator pressure (van Schaik, 1983; Dunbar, 1988), allows better exploitation and protection of food resources (Wrangham, 1980; Krebs & Davies, 1993), or results from the need for cooperative behavior, like hunting or communal breeding (Caraco & Wolf, 1975; Emlen, 1991). For more specific situations, other factors have been proposed such as rearing strategies (e.g. lions; Packer et al., 1990). In most cases, observers study animals that already live in groups, making it difficult to investigate the factors that were originally responsible for the appearance of social groups. However, we can study the factors that affect the social groups that we observe in the wild.

Predation pressure has been shown to have a large impact on group size in many animals including species of fish, birds, ungulates and primates (Pulliam & Caraco, 1984; Kummer et al., 1985; Cheney & Wrangham, 1987; Dunbar, 1988; Krebs & Davies, 1993). Similarly, seasonal variation in food supply has been shown to have a marked influence in the fragmentation of hamadryas, olive and gelada baboons and of spider monkeys (Kummer, 1971; Dunbar, 1988; Chapman et al., 1995).