Mimic Me While Playing! Social Tolerance and Rapid Facial Mimicry in Macaques (Macaca tonkeana and Macaca fuscata)

Social play and tolerance are positively correlated and playful signals are more freely expressed in egalitarian than in despotic species. Macaque species are organized along a continuum from intolerant to tolerant social systems and, for this reason, they are good models to test some hypotheses about the possible linkage between communication and tolerance. We compared facial playful communication in 2 macaque species at opposite ends of the continuum: despotic Japanese macaques, Macaca fuscata, and tolerant Tonkean macaques, Macaca tonkeana. We predicted that tolerance has favored larger facial display repertoires and playful cooperative tendency. Moreover, we evaluated if tolerance could also reflect in the selection for emotional sharing expressed through rapid facial mimicry (RFM). RFM is an automatic, unconscious, and fast (<1 s) motor mirror response which favors emotional communication between subjects. Although M. fuscata and M. tonkeana performed play faces (PF) at comparable levels, only Tonkean macaques showed the phenomenon of RFM. The playful sessions characterized by RFM lasted longer than those characterized by the presence of playful signals perceived by a playmate but not followed by mimicry. Interestingly, the duration of playful sessions in Tonkean macaques overcame that of the sessions in Japanese macaques. It is likely that RFM improves communicative exchanges between the playmates and, at the same time, promotes behavioral coordination and cooperation in the sequence of actions. The tolerant nature of Tonkean macaques, also expressed in play, can foster RFM which, at the same time, positively enhances the propensity to cooperate in a sort of positive feedback.