Mutual gaze and facial mimicry as drivers of shared engagement in macaque play fighting

Human and nonhuman animals engage in complex joint actions, characterized by interactions where individuals coordinate their behaviours in space and time. This synchronization points to the presence of an underlying shared social motivation driving joint actions. However, the use of signals to form, maintain and seal joint commitments during joint actions seems to be exclusive of hominin species. Here, we explored how rhesus macaques, Macaca mulatta , use mutual gaze (MG) to open and close naturally occurring play fighting, and its relation to facial mimicry of playful facial expressions during the main body of the interaction. In macaques, the exchange of MG opening joint play contributed to longer successful playful interactions, possibly signalling a shared motivation and emotional state towards the upcoming playful context. Moreover, playmates exchanged mutual gaze more often when ending a joint action compared with a control condition of nonjoint spatial proximity. MG was exchanged independently of the dominance ranks of the young playmates, but it was particularly prevalent in male–male dyads, suggesting that MG may serve as a low-cost play signal in riskier interactions. Notably, we found an association between MG opening and closing the joint action and events of rapid facial mimicry of play faces during the same joint action. This suggests a pivotal role for rapid facial mimicry in the maintenance and reaffirmation of shared engagement, with a similar function of human colaughter. Our study shows that monkeys exchange cognitively low-demanding signals to start, maintain and dissolve shared engagements in joint actions, highlighting the importance of visual communication in play fighting. Furthermore, we provide insights into the potentially ancestral origins of joint commitment as a process, contributing to a deeper understanding of the evolution of the social minds of primates. • Rhesus macaques exchange signals before, during and after play fighting. • Mutual gaze leads to successful playful interactions and is specifically used in play. • Mutual gaze and facial mimicry are temporally and functionally linked. • Macaques might use mutual gaze to establish and dissolve shared engagements. • Facial mimicry seems to help maintain the commitment to the joint action.