The face never lies: facial expressions and mimicry modulate playful interactions in wild geladas

Play fighting, the most common form of social play in mammals, is a fertile field to investigate the use of visual signals in animals’ communication systems. Visual signals can be exclusively emitted during play (e.g. play faces, PF, context-dependent signals), or they can be released under several behavioural domains (e.g. lip-smacking, LS, context-independent signals). Rapid facial mimicry (RFM) is the involuntary rapid facial congruent response produced after perceiving others’ facial expressions. RFM leads to behavioural and emotional synchronisation that often translates into the most balanced and longest playful interactions. Here, we investigate the role of playful communicative signals in geladas (Theropithecus gelada). We analysed the role of PF and LS produced by wild immature geladas during play fighting. We found that PFs, but not LS, were particularly frequent during the riskiest interactions such as those including individuals from different groups. Furthermore, we found that RFM (PF→PF) was highest when playful offensive patterns were not biased towards one of the players and when the session was punctuated by LS. Under this perspective, the presence of context-independent signals such as LS may be useful in creating an affiliative mood that enhances communication and facilitates most cooperative interactions. Indeed, we found that sessions punctuated by the highest frequency of RFM and LS were also the longest ones. Whether the complementary use of PF and LS is strategically guided by the audience or is the result of the emotional arousal experienced by players remains to be investigated.