Mating in the presence of a competitor: audience effects may promote male social tolerance in polyandrous siamang (Symphalangus syndactylus) groups

Audience effects on sexual behavior, including changes in copulation frequency and duration in the presence of conspecifics, have been reported in multimale-multifemale groups of several primate species. We examined the interaction of male sexual behavior with group composition and within-group mating pattern in a population of siamangs (Symphalangus syndactylus) containing unimale-unifemale groups and multimale-unifemale groups using mixed models. Aggression in a sexual context was never observed. As predicted, however, copulation rates were lower and copulations were significantly shorter in duration in two-male groups than in unimale groups, even if copulations involving subordinate males were excluded. Dominant males monopolized copulations with the group female in most groups, but copulations were shared among males in three stable two-male groups. When both resident males copulated with the group's female, there was no evidence that copulating pairs moved to secluded areas, and the duration of copulations did not differ between males. These results are consistent with the hypothesis that male male tolerance in multimale siamang groups is facilitated by adjustments to sexual behavior in the presence of a sexual competitor.