Introduction

Mountain gorillas exhibit pronounced sexual dimorphism. The males are twice as large as the females and possess several traits, such as long canines, associated with fighting ability (Harvey et al., 1978; Stewart & Harcourt, 1987; Plavcan & van Schaik, 1992). Across species, sexual dimorphism is associated with intense male competition for access to females and polygynous mating (Rodman & Mitani, 1987), although the relationship is not straightforward across taxa (van Hooff & van Schaik, 1994).

In mountain gorillas, the most conspicuous male competition takes place during inter-group encounters (Harcourt, 1981), and males seek these encounters because they are the occasion to attract females (Sicotte, 1993; Watts, 1994a). Indeed, females transfer between groups in gorillas, and only do so during inter-group encounters (Harcourt, 1978). During these encounters, males display at each other by beating their chest and sometimes fight with their opponents (Harcourt, 1978; Sicotte, 1993). The aggression displayed by the males is more intense when the number of females that can transfer between the two units is high (Sicotte, 1993). Males that succeed in forming and maintaining a group can have a long tenure and will have mating access to several females over a long period. Others are not so successful in attracting and retaining females (Stewart & Harcourt, 1987; Robbins, 1995; Watts, 2000).

This intense male-male competition to attract and retain females, as well as males- differential success in this competition, does not remove the possibility for active female mate choice (Smuts, 1987).