Not just for fun! Social play as a springboard for adult social competence in human and non-human primates

Play is one of the most difficult behaviors to quantify and for this reason, its study has had a very rocky history. Social play is ephemeral, difficult to distinguish from the other so-called serious behaviors, not so frequent (especially in sexually mature subjects), fast, and complex to describe. Due to its multifaceted nature, it has often been considered as a wastebasket category that has included all kinds of the behaviors not showing any immediate, obvious goal. Yet, play is widespread across the whole primate order and can have a strong impact on the development of cognitive, psychological, and social skills of many species, including humans. Unlike functional behaviors that are specifically and economically performed to reduce uncertainty and to increase the opportunity to gain resources, play seems to introduce and increase uncertainty, creating new challenges for the animals. For this reason, social play has been hypothesized to be the engine of behavioral innovation in ontogeny. From the first mild and gentle interactions with the mother to the most sophisticated play fighting sessions and acrobatic action sequences with peers, play represents for juveniles (and not only for them!) a window onto the social and physical environment. In this review, I focus on social play and its relation to adult social competence. By playing, juveniles acquire competence to manage interactions with conspecifics, enlarge their social networks, and test their physical power and motor skills (i.e., long-term benefits). At the same time, I propose the view that play—due to its plastic and versatile nature—can be used in an opportunistic way, as a joker behavior, throughout life to strategically obtain short-term or immediate benefits. I put forward the hypothesis that, during ontogeny, the joker function of play can be modulated according to the differing inter-individual relationships present in the diverse societies, characterizing the primate order