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

Play is mostly observed in juveniles in mammals, and the type of play (social, locomotor and object play) tends to mirror adult function. In some species, also adults play with immatures, in particular if same-aged play partners are lacking and adults also invest in caretaking. We studied the ontogeny of play in cooperatively breeding common marmoset groups composed of parents and twin offspring between the age of two to six months. Social play was by far the most prevalent and increased with age. Adults were important play partners: Before 19 weeks old, the play partners of immatures was an adult in 54% of the time spent playing socially. After week 19, this proportion decreased to 29%. The rest of the social play time was spent playing with their twin. 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 (i.e., carrying and food sharing) for either of the parents. Notably, parents avoided playing simultaneously, presumably to avoid periods when no one could be vigilant. Together, these results resonate strongly with the highly interdependent and cooperative lifestyle of common marmosets. Highlights Social play is the predominant form of play in marmosets and increases between the age of 2 to 6 months Parents are important play partners for immatures during ontogeny The odds of playing with the twin rather than a parent increases with age Mothers and fathers play at similar amounts with the immatures Mothers and fathers take turns and almost never play simultaneously with the immatures