Face off! Incongruent facial response and counter-mimicry in chimpanzees: rare but present

Automatic, rapid facial mimicry consists in the replication (within 1 s) of an observed facial expression and supports motor and emotional inter-individual alignment. Facial expression responses have also been reported as non-matching (incongruent: different-expression response to the one observed; counter-mimicry: distinct expression with opposite valence to the one observed), linked to the mirror neuron system flexibly and modulated by experience and socioecological context. In non-human primates, incongruent responses are understudied, and counter-mimicry has not yet been explored. We analysed 962 h of play and aggression video from four captive chimpanzee groups (Mona foundation, Spain; Vallée des Singes and Beauval zoos, France) collected over three years. Facial expressions analysed included play-face/full-play-face, silent bared-teeth, screamface, and pout-face. Rapid responses were classified as congruent (mimicry) or non-matching, i.e. incongruent response or counter-mimicry. We used GLMMs to assess whether each type of response (by a potential responder) occurred more frequently when observing (vs. not observing) the interactant’s facial expression (potential trigger).We applied a Random Forest classifier to evaluate whether the responder’s facial expression could be predicted from the trigger’s expression. Chimpanzees exhibited both mimicry and non-matching facial responses. Mimicry (83%) was the most frequent during play. Non-matching responses — though less common — were present during aggression (incongruent response; 8%) and during borderline play (counter-mimicry; 9%), e.g. play teasing/aggressive play. In non-matching responses, the responder’s expression was weakly predicted by the trigger’s expression, suggesting high individual or contextual modulation. In conclusion, chimpanzees can respond to others’ facial expressions not only through motor matching, but also via non-matching patterns. The presence of rapid counter-mimicry in chimpanzees suggests that the evolutionary roots of non-matching responses predate the divergence between the human and Pan lineages.