The ontogeny of play in a highly cooperative monkey, the common marmoset

Abstract Play is widespread in mammals. It is mostly observed in juveniles and has been subdivided in three categories, social, locomotor/rotational, and object play. Although there is no strict consensus on its ultimate function, the dominant idea is that through play juveniles acquire social, technical and cognitive skills for their adult life. In certain species, however, adults remain playful especially with immatures. This pattern can be observed in particular when same-age play partners for immatures are lacking and if adults also invest in caretaking. We studied the ontogeny of play in cooperatively breeding common marmoset twins from the age of two to six months. Social play increased with age and was by far the most prevalent category. Play partners varied with age. Before 19 weeks old, immatures played 54% of the time on average with either one of their parents (in a dyad) and 29% on average after 19 weeks old. Thus, despite the constant presence of a twin, adult-immature play remained considerable, with equal contributions by mothers and fathers and no trade-offs with other care-taking behaviours for either of the parents. However, parents avoided playing simultaneously, presumably to avoid periods with no one vigilant. Together these results show that parents are important play partners for marmoset infants, fathers and mothers alike.