Introduction

Competitive relationships result from competitive interactions over resources that can affect survival and reproduction. For male mammals, prospective mates are one such resource. For female mammals, food is more important than mates as a contestable resource (Trivers 1972). Its importance has been recognized by three ecological models that invoke either food distribution (van Schaik 1989, Wrangham 1980) or both food distribution and abundance (Isbell 1991) to explain variation in competitive relationships within and between groups of female primates (see also Isbell and Van Vuren 1996, Sterck et al. 1997, van Hooff and van Schaik 1992, Wrangham 1987). For both sexes, survival is also affected by other factors, such as disease and predation, and it is possible that individuals also compete for resources that in some way minimize exposure to disease or predators. In fact, one of the three models (here called the ‘predation hypothesis’) considers predation to be more important than food in its effects on the grouping behavior of females (van Schaik 1989).

The predation hypothesis assumes that predation ultimately forces females to live together and that variation in predation pressure causes variation in spatial cohesion within groups and among species (Sterck et al. 1997, van Hooff and van Schaik 1992, van Schaik 1989). Where predation pressure is high, females are predicted to decrease interindividual distances and thus live in groups that are spatially cohesive.