Cooperation in brown capuchin monkeys: social tolerance, individual traits, and partner sensitivity in a Stag Hunt...

Mutualism with free partner choice is considered a key foundation in the evolution of cooperative behaviour, yet many experimental studies restrict partner choice or provide few alternative options, limiting our insight into the factors shaping cooperative decisions. We investigated cooperation in two groups of captive brown capuchin monkeys (Sapajus spp.) using an apparatus implementing the Stag Hunt paradigm. Individuals could freely choose between a cooperative option, a "Stag box" requiring simultaneous lever pressing by two individuals to obtain high-value rewards, and four individually operable "Hare boxes" that yielded lower-value rewards. Monkeys were free to select partners and interact with any boxes in six sessions. Most individuals (East: N = 22; West: N = 18) engaged in successful cooperative bouts in the Stag box and also obtained rewards individually from the Hare boxes. Generalized linear mixed models showed that dyadic tolerance significantly predicted the likelihood of cooperation in both groups. Individual characteristics also influenced engagement: exploratory individuals and dyads involving juveniles participated in more cooperative interactions. Further analyses of pressing behaviour indicated that the partner presence effect differed across contexts and groups. In the East Group, partner presence increased Stag-box pressing but lowered Hare-box pressing, whereas in the West Group, Stag-box pressing increased across sessions but did not differ depending on whether a partner was present or absent. These findings highlight the importance of integrating social relationships and individual traits to understand cooperative decision-making in primates and demonstrate how paradigms allowing free partner choice can reveal the social dynamics underlying cooperation.