Sharing emotions or intentions? A comparison of rapid facial mimicry during play in lowland gorillas and chimpanzees

In primates, playful facial expressions can disclose both emotions and intentions. By employing a comparative approach, we investigated the use of these facial displays and mimicry in chimpanzees ( Pan troglodytes ) and lowland gorillas ( Gorilla gorilla gorilla ). Even though the two great ape species are phylogenetically close, they strongly differ in their social cohesiveness and cooperative propensity, a “social fork” which also translates into differences in playful communication. Contrarily to chimpanzees, gorillas mainly performed Full Play Face (FPF: both upper and lower teeth are exposed in a relaxed way). This FPF was maintained for a longer period than the less evident Play Face (PF: only lower teeth exposed). PFs reveal unambiguous, positive emotions and can elicit in the observer the same emotional state as that of the performer, which is expressed through Rapid Facial Mimicry (RFM). RFM is a rapid and automatic response by which a subject reproduces the facial expression of a fellow within 1 second. We confirmed the occurrence of RFM in both species. Intriguingly, in gorillas the frequency of facial matching response (PF → PF; FPF → FPF) was particularly high. For both chimpanzees and gorillas RFM, and not the mere presence of PF/FPF, exerted a positive effect on play by promoting a longer bout duration. Therefore, in both species, RFM communicated to the performer that the stimulus was perceived and accurately processed, thus it reduced the ambiguity of communication. In conclusion, despite their social diversity, emotional rapid mimicry in gorillas and chimpanzees seems to share a similar function even though some differences in the sphere of intentional play communication also apparently exist.