Towards the complexity of laugh communication in great apes: exact facial replications in laugh faces of orangutans and chimpanzees.

Exact facial replications, where individuals match the same facial variant type (e.g., Duchenne smiles) of others, play a central role in everyday social interactions of humans. Such replications help to improve an understanding of others by allowing individuals to be emotionally more in tune with them and/or to predict their behaviours. In this study, we tested for exact facial replications in orangutans and chimpanzees - the great apes phylogenetically farthest from and closest to humans, respectively. We tested in 96 subjects (from altogether eight social groups) whether such facial replications occurred within three seconds for laugh faces, homologues of human smiles/laugh faces. We focused on two variant types (laugh faces with and without the upper teeth exposed), using a two-step method. Our study revealed that both the orangutans and chimpanzees showed exact facial replications for laugh faces where the upper teeth were not exposed, i.e., the variant type that was least associated with rough/risky play. Such variant preference for exact replications might help being emotionally more in tune with others and/or predict their behavioural actions - either one could lead to important advantages, such as prolonging play (a correlation found for the studied orangutans). These findings infer an evolutionary continuity of exact facial replications across great apes (and humans) and a complexity for pre-human laugh communication. Laugh faces of ancestral apes must have already been complex in form and function 10-16 million years ago, and later within the hominin lineage become even more effective tools of everyday social communication.