Do same-sex mounts function as dominance assertion in male golden snub-nosed monkeys (Rhinopithecus roxellana)?

It has been hypothesized that same-sex mounts can reflect the hierarchical relationship in a mounting dyad and it is widely deemed that mounting and being mounted are demonstrations of dominant and subordinate status, respectively. In this research, we aimed to test whether same-sex mounts function as dominance assertion in male golden snub-nosed monkeys (Rhinopithecus roxellana). We investigated this behavior in eight-individuals, captive all-male unit (AMU) in Shanghai wild animal park, China. Behavioral observations were conducted with a total of 1,855 mounts recorded from November, 2014 to June, 2015, during which the alpha male was replaced in the beginning of April. In support to the dominance assertion hypothesis, we found that during the entire study period the higher-ranking male was more likely to be the mounter than the lower-ranking one, except the mounts that happened among juveniles in peaceful and playful social contexts. Our study indicates that the hierarchical relationship of a mounting dyad can be influenced by the age-class of the participants and the social context where mounting occurs. We suggest that same-sex mounts might have different functions in different age groups and be multifunctional in a species.