Examining postmounting grooming in male golden snub-nosed monkeys to investigate the functions of same-sex mounts

Same-sex mounting behavior has been documented across primate species and several biological functions, that are often difficult to differentiate, have been proposed. Postmounting affiliative behaviors (e.g., embracing and grooming) have been partly overlooked and their performance may more clearly reveal the function of same-sex mounts for different age-sex classes. Here, considering postmounting grooming behaviors (PMG), we tested the affiliation hypothesis for same-sex mounts in an all-male unit (AMU) of captive golden snub-nosed monkeys (Rhinopithecus roxellana) (n = 7−8) and the needing to learn hypothesis in the juveniles of the AMU (n = 4) and the juvenile male from the neighboring one-male unit in Shanghai Wild Animal Park, China from November 2014 to June 2015. A total of 1986 same-sex mounts were recorded from the AMU individuals and the juvenile of the neighboring one-male unit. We found that neither dyadic proximity-based association nor grooming-based affiliation predicted the occurrence of mounts in the AMU, and PMG was more likely to occur in nontense than tense social contexts, providing no support for the affiliation hypothesis. Although dyadic grooming-based affiliation predicted the occurrence of PMG, it exerted no influence on the occurrence of mounts, which was necessary to support the hypothesis. However, consistent with the needing to learn hypothesis, from ages two to four, juveniles' mounting duration increased and they performed pelvic thrusting during mounting more and more often as they grew older. Reciprocated series mounts were much more common among juveniles than other mounting dyads, providing learning opportunities for both participants. The mounter was more likely to groom the mountee at the end of a mount among juveniles than other mounting dyads, suggesting that juvenile mounters may profit from mounting and groom mountees in return for providing them with opportunities to learn copulatory skills. A hypothesis that merits further investigation.