Immature chimpanzees and children: a comparison of play developmental pathways

The developmental trajectories of social play have evolved in concert with the extension of the immature period in primates. Modality, complexity, playmate choice, and frequency of play can be predictive of its adaptive functions. Evaluating and comparing all these parameters in each age-stage of different primate species is important to formulate hypotheses on the diverse roles of play during individuals' ontogenetic process. This study aims at contrasting a possible ontogenetic pathway of immature chimpanzee play with that of children ones. Our data revealed that, like in children, while solitary play peaks in motor independent infants, social play does not vary in frequency between infancy and juvenility, but it varies in complexity, asymmetry, and playmate choice. Human and non-human primates seem to select peer playmates; accordingly, our data reveals that chimpanzees prefer to engage in play with peers. Moreover, we found that play between infants is more asymmetric and less complex than that between juveniles, who have to restrain themselves in order to maintain the bout symmetric and limit the risk of fighting escalation; such interpretation agrees with data in human youngsters, whose play bouts turn into fights in few cases. The asymmetry and lower complexity of infant play reflect the different degree of motor skill maturity and the difficult in performing self-handicapping. Considering play signals, like laughter in humans, play faces in chimpanzees seem to have a role in advertising cooperative intentions thus increasing the likelihood of engaging in solid social relationships. In conclusion, in humans and chimpanzees different play modalities and signals should indicate different functions for different aims.