Functional Diversifications of Contact Calls in a Multi-level Primate Society

Abstract The ability to cultivate social relationships through language is human uniqueness. However, despite small repertoire sizes, nonhuman primates could use vocalization for social purposes too. Vocal communication could be evolutionarily ancient, and likely evolved hand-in-hand with complex, dynamic social structures. By studying the vocal communication in wild golden snub-nosed monkeys ( Rhinopithecus roxellana ), a species of Asian colobine living in 4-layer multi-level societies with fission-fusion dynamics, we unraveled the complex social functions of contact ‘coo’ calls. Contact calls helped cultivate both within- and between-unit relationships, and depending on the sexes, reproductive states, and social roles of callers and recipients, were also functionally diverse. Within units, calls replaced grooming and were supplementary to social proximity. Males directed more calls to lactating females, who were unavailable for grooming and away from unit centers due to extended foraging and childcare. Higher-ranking males called more frequently but did not receive more calls than lower-ranking ones. Considering their larger social network sizes and leadership roles, contact calls likely facilitated inter-unit proximity and were used for advertising intrasexual affiliation and alliances. By comparing the network of contact call exchange with that of social proximity and grooming, we identified the inter-unit social relationships that were otherwise inconspicuous and, more importantly, the potential mechanisms for maintaining male-male alliances and unit and band cohesion. Together with previous findings, we conclude that contact calls are economical and low-risk social tools for reassuring and advertising affiliation, resolving uncertainties, and promoting cohesion and alliance formation.