• joint commitment
• mutual eye gaze
• Old World monkeys
• play face
• Rapid Facial Mimicry

During risky interactions such as competitive social play, communicative strategies can be extremely useful. Rapid Facial Mimicry (RFM), the automatic mimicry of a play partner's facial expressions (play faces), can contribute to discouraging aggressive escalation and lead to behavioral and/or emotional synchronization. At the same time can be important to establish share engagement and joint commitment to the imminent playful interaction, ensuring that the partner is available and ready to play, and to the conclusion of the session, through the exchange of mutual eye gaze (meg) to limit the risk of aggressive escalation. In this context, we studied RFM and meg in a group of rhesus macaques (Macaca mulatta), specifically testing hypotheses about their presence, function, and potential emotional significance. For the first time in a despotic macaque group, contrary to previous data, we found that RFM during interactions is present and common during play fighting, especially between young individuals of the same age and among male-male dyads. RFM can predict more rewarding sessions and higher arousal of playmates but it turned out to not emotionally affect an external observer who detected the PF of the players. The presence and frequency of RFM prolonged play, likely facilitating motor synchronization. Furthermore, we discovered that mutual eye gaze is present and frequent before and after play sessions, especially those perceived as riskier, potentially requiring more effective communication to avoid misunderstandings. Additionally, we found that the coordination established by this agreement signal before the session is then reaffirmed during the session through RFM.