Lowland gorillas (Gorilla gorilla gorilla) failed to respond to others' yawn: experimental and naturalistic evidence

Play fighting, a common form of mammalian play, can escalate into aggression if playful motivation is misinterpreted and not shared by players. In primates, playful facial expressions and mimicry can be performed to signal and share playful motivation. Here we compare play facial expressions (play face [PF]: lower teeth exposed; full play face [FPF]: upper and lower teeth exposed) and their mimicry in captive chimpanzees and lowland gorillas, during play fighting. These two species have different social dynamics, with social cohesion being lower—and play possibly riskier—in gorillas than in chimpanzees. Thus, we hypothesized that gorillas would perform redundant PFs more often to avoid misunderstanding (Prediction 1). However, the two species are phylogenetically very close and possess a similar biology. Thus, we hypothesized that both species could perform rapid facial mimicry (RFM: response within 1 s) and delayed facial mimicry (DFM: response occurring between 1 s and 5 s), which may have different roles in play modulation (Prediction 2). Gorillas performed more FPF than chimpanzees and FPFs lasted longer than the less intense PF (Prediction 1 supported). RFM was present in both species, whereas DFM was present only in chimpanzees (Prediction 2 not fully supported). DFM may be performed by chimpanzees to modulate the session at a later stage of the interaction and favor interindividual cohesion. RFM prolonged play sessions and may be performed to communicate playful motivation to the playmate thus demonstrating that animals integrate contextual information into their understanding of others’ states and intentions.