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

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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.



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  • Comparison of estimated mean copulation rates (copulations/month) in unimale and multimale groups from the GLMM analyses. Error bars represent 95% confidence intervals of the estimated means. (A) All copulations involving the adult female and male group members. (B) Copulations involving dominant male–female dyads only. p<0.01, ∗∗p<0.001.

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  • Mean duration of copulations by males in stable one-male groups and two-male groups, and an unstable three-male grouping. Means are calculated as means of group means. Error bars represent 95% confidence intervals. (A) All copulations involving the adult female and male group members. (B) Copulations involving dominant male–female dyads only. The three-male group was not included in the LMM analysis. p<0.01, ∗∗p<0.001.

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  • Mean duration of copulations in multimale groups where copulations were monopolized by the dominant male (‘Monopolized’) vs. groups where copulations were shared between two males (‘Shared’). Means were calculated as means of individual means for all individuals for which at least two copulations were observed. Error bars represent 95% confidence intervals. p<0.01, ∗∗p<0.001.

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