Fair play and honest signals in immature chimpanzees

In primates, specific facial displays (PF, play face; FPF, full play face) often accompany play bouts and are considered an integral part of play development. In humans, laughter, a universal expression of joy, seems to derive from non-human primate play faces and pant-like vocalizations. Playful facial displays may represent honest signals that express the non-agonistic intent of players in order to maintain the session at a later time. Here, we examine the possible roles of facial displays during play sessions of immature chimpanzees (Pan troglodytes). Infants and juveniles performed PF and FPF with comparable frequency to maintain the playful mood. However, the use of playful expressions was fine-tuned in relation to the asymmetry of the session and of the receiver's identity. In infants, whose play sessions were the most unbalanced, we found a positive correlation between the play face frequency and the asymmetry degree that characterizes each single session. On the other hand, in juveniles, we found that the most of the facial signals was directed towards peers. This result is not surprising also considering the high complexity and competition levels characterizing juvenile play in chimpanzees. Probably, when play becomes more competitive, as it occurs in juvenile chimpanzees and adolescent humans, clear and fair signals are essential to maintain the session and to avoid it turning into aggression. Therefore, like human laughter, playful expressions do not give simple information, but positively influence the receiver's reaction. Independently of the playmate age, in chimpanzees, play facial signals, may have a role in advertising cooperative dispositions and intentions thus increasing the likelihood of engaging in honest social relationships. In social play, there are codes of conduct that regulate what is permissible and what is not. The existence of shared codes in immature individuals might contribute in developing some traits of social morality in adults.