Introduction to the paper: In primates, playful interactions are often accompanied by specific facial expressions (van Hooff and Preuschoft, 2003). In the great apes, these expressions can be performed in two different ways: play face, where the mouth is opened with only the lower teeth exposed, and full play face, where the mouth is opened with upper and lower teeth exposed (Loizos, 1967).The use of the two expressions is plastic as a function of different factors such as play intensity (van Lawick-Goodall, 1968), playmate identity (Flack et al., 2004), and context (Palagi and Mancini, 2011). Playful facial displays may have a role in signaling non-agonistic intent to a playmate and in expressing emotion both during social and solitary play sessions (Palagi, 2006; van Hooff and Preuschoft, 2003).The non-human primate play face is homologous with human laughter (van Hooff and Preuschoft, 2003) which, across the diverse cultures, is the external manifestation of joy and happiness (Sauter et al., 2010). Primate laughter is considered a multifunctional behavior, because it reduces stress in subjects that face new situations, mitigates social ambivalence within a group, and triggers play interactions (see Gervais and Wilson, 2005 for an extensive review). If in the great apes, like it occurs in humans, playful facial expressions cover different roles and convey different information, we expect that they vary in frequency and form in relation to play intensity (Contact or Locomotor-Rotational play), playmate identity (sex of playmate), and number of players (dyadic or polyadic bouts).To test these hypotheses we collected data on two chimpanzee colonies (Pan troglodytes) and contrasted our findings with those coming from human literature on play facial communication.
Primate play laughing: A comparison between immature great apes and humans
