Same-sex and immature sexual behaviour repertoire in a wild group of robust capuchin monkeys

In primates, same-sex and immature sexual behaviour is widespread and can include mounting and genital presentation. These patterns can be observed in multiple social contexts and across all ages, and can serve functions such as appeasement, reconciliation, practice and dominance assertion. In this study, we investigated same-sex and immature sexual behaviour in a group of robust capuchins (Sapajus nigritus) living in an Atlantic forest fragment and urban areas. We predicted that in order to practice sex, sexual behaviour in same-sex dyads and/or dyads that included immatures would resemble the adult heterosexual repertoire for solicitation through courtship displays, mounts and post-copulatory display. We also predicted that immature individuals would engage more frequently than adults in sexual interactions, in order to practice sex. We conducted all-occurrence observations of sexual behaviour in the study group (28 individuals: four adult males, one sub-adult male, eight adult females, eight juveniles and seven infants) from September 2016 to August 2017. Sexual interactions that included at least one immature individual in the dyad and/or occurred between individuals of the same sex were infrequent (N = 52, 0.13 interactions/hour), but much more frequent than heterosexual sexual interactions between adults in the group (N = 4, 0.01 interactions/hour). The same-sex and immature sexual repertoire resembled the described heterosexual patterns for the same species from the literature. Individuals displayed solicitation behaviours in "one-way courtship", usually followed by a two-way courtship and then mounts, but post-copulatory behaviour was never observed. Except for the alpha male, all age-sex classes engaged in sexual interactions in same-sex dyads or in dyads that included immature individuals. We found no difference in frequency of participation across age classes, however, male-male dyads engaged more frequently in sexual interactions and may be practicing sex and courtship behaviours. Mounts are unlikely to be a form of dominance assertion as the alpha male did not participate, subordinate adult males did not engage in mounts with other subordinate adult males, juveniles mounted adult males and vice versa, and there were mount switches (taking turns as mounter and mountee) regardless of the initial mounter's age. Contrary to the post-conflict context observed in Cebus, most mounts in this study were preceded by play. Combining our study with additional evidence for the genus, same-sex mounts and mounts that include immatures seem to occur most commonly in affiliative contexts within Sapajus. Sexual behaviour functions in Sapajus require additional investigation, especially among adult males and juveniles of both sexes.