Rapid facial mimicry as a regulator of play in a despotic macaque species

Abstract During risky interactions like social play, motor resonance phenomena such as facial mimicry can be highly adaptive. Here, we studied Rapid Facial Mimicry (RFM, the automatic mimicking of a playmate’s facial expression, play faces) during play fighting between young rhesus macaques ( Macaca mulatta ) living in a large zoo-housed group. For the first time and in contrast to previous data on highly despotic-intolerant macaques, we found RFM to be present at high frequency in young rhesus macaques, especially when the trigger was dominant over the responder and when both players were subadults. The hierarchical modulation of RFM may be associated with the increased uncertainty and riskiness of play involving a higher-ranking playmate. This highlights the importance of mimicry in improving communication and coordination during such interactions. Interestingly, RFM prolonged playful sessions, possibly indicating a more effective fine-tuning of motor patterns. Moreover, the occurrence of RFM had an effect on shortening the latency to restart playing after a break, possibly acting as an engine to potentially maintain playmates’ arousal. When investigating if bystanders could replicate play faces emitted by the playing subjects, we failed to find RFM, thus highlighting that being directly involved in the interaction might be crucial for RFM activation in monkeys. Even though further comparative studies should investigate the role of RFM across tolerant and despotic-intolerant species, our findings offer valuable insights into the communicative and adaptive value of motor resonance phenomena in regulating social play in despotic societies. Significance statement In risky interactions involving competition and vigorous physical contact, such as play fighting, replicating partners’ facial expressions can serve as a strategy to convey positive mood and intentions. Here we investigated the presence and possible roles of Rapid Facial Mimicry (RFM) in rhesus macaques ( Macaca mulatta ). For the first time, our study demonstrates that communicative strategies, including Rapid Facial Mimicry (RFM), can be both present and frequent in despotic-intolerant macaque species. We demonstrate that the role of mimicry not only prolongs playful interactions but can also be linked to the reinforcement and/or transmission of playful arousal. Our study shows how the adaptive value of motor resonance phenomena may have driven their evolution to cope with challenges during social interactions also for despotic-intolerant species.