Turn-taking in grooming interactions of sooty mangabeys (Cercocebus atys) in the wild.

Language is widely regarded as a uniquely human trait, rooted in mechanisms of social interaction such as conversational turn-taking. It has been proposed that turn-taking may not be exclusive to humans but may represent an ancestral trait shared across primates and beyond. Early studies on great apes indicated a specialization in gestural forms of turn-taking, whereas monkeys appeared to rely more heavily on vocal exchanges. However, this ape–monkey difference may simply be due to a lack of research. To bridge this gap, the present study investigated turn-taking behaviors during grooming in an Afro-Eurasian monkey species, sooty mangabeys (Cercocebus atys). We addressed the following three key questions: (1) What turn-taking patterns, if any, characterize grooming interactions in sooty mangabeys? (2) Do sooty mangabeys preferentially use vocal turn-taking? (3) Which elements characterizing turn-taking patterns of sooty mangabeys are shared with human conversational turn-taking? We collected 105.3 h of video footage from 22 adult female sooty mangabeys in Taï National Park (between May 2022 and August 2023), yielding 428 female–female grooming videos, of which 71 were coded. An additional 595 grooming interactions were described directly in the field. Results showed that exchanges involving both actions and signals were the most frequent (e.g., a present gesture followed by grooming). The most common type of signal exchange was gestural turn-taking, a form previously reported only in great apes. The study identified key elements of conversational turn-taking: rapid responses, adjacency pair-like sequences, and participation frameworks. These findings suggest that the fundamental structural principles of human conversation likely predate our species, offering valuable insight into the evolutionary history of language.