Play and primates: social, communicative, and cognitive aspects of one of the most puzzling behaviour.

Play is extremely difficult to define and its benefits are not easily detectable. Due to its multifunctional nature, play represents a good opportunity to test some hypotheses on social, communicative, and cognitive aspects of animal and human behaviour. For this reason, comparative studies of social play can make contributions to a wide variety of fields (evolutionary biology, ethology, anthropology, psychology, and neuroscience). Here, we present data published in the last ten years by the primatologists of the Natural History Museum (University of Pisa) on a number of primate species in order to elucidate the importance of studying play behaviour in a comparative perspective. Firstly, we explore the immediate functions of adult social play especially in managing tension situations both within and between group members. Then, we discuss data on the importance of playful signals as tools in limiting competition and increasing cooperation that characterized each social play session. Finally, we provide new data on the presence of facial mimicry during play in a cercopitecoid species, a phenomenon homologous to human laughter contagion. The facial mimicry, up to now demonstrated only in apes and humans, is the expression of emotional contagion, a fundamental building block of empathy. In conclusion, such findings suggest that play behaviour also provides a good opportunity to investigate the affecting mechanisms at the basis of animal social cognition