The functions of non-reproductive mounts among male Barbary macaques (Macaca sylvanus).

Same-sex, non-reproductive mounts have been observed in a number of primate species and in various social contexts. However, the function of non-reproductive mounts is still largely unknown. We aimed to test whether non-reproductive mounts function to assert dominance and as appeasement behavior in male Barbary macaques. We analyzed post-mount behavior in 54 macaques belonging to two captive groups at Trentham Monkey Forest in Staffordshire, using 10 min post-mount/matched-control focal sessions collected either on the mounter or the mountee. In support of the dominance assertion hypothesis, the higher-ranking male within a mounting pair was more likely to be the mounter than the mountee, and to mock bite the lower-ranking male. In support of the appeasement hypothesis, the former mounting partners were more likely to exchange grooming and to have a lower frequency of self-scratching (a measure of social tension) after a non-reproductive mount than in control sessions. Our study indicates that non-reproductive mounts have different and not mutually exclusive functions and can modulate the quality of social interactions among group members. We discuss the possible factors that can affect the occurrence of non-reproductive mounts within and between species.