Orang-utans and chimpanzees cooperate strategically based on the partner’s incentives

Chimpanzees, Pan troglodytes, are often regarded as a model species for the evolution of human cooperation. However, whether chimpanzees' decision-making in cooperative contexts differs from that of other, less gregarious great ape species such as orang-utans remains unclear. In this study, the performance of zoo-housed chimpanzees and Sumatran orang-utans, Pongo abelii, was compared in a cooperation task with individual foraging alternatives for both participants. In each trial, one individual had access to a tool that she could either use to retrieve an immediate food reward (thereby forfeiting further tool use) or pass to a partner. Then, the partner could use the tool in a cooperative apparatus, releasing food rewards for both individuals or for an individual foraging option. The quality and presence of food rewards were manipulated in individual and cooperative apparatuses. Chimpanzees and orang-utans cooperated at similar rates, with decisions primarily driven by their own immediate payoffs. However, both species were also more likely to transfer the tool when the partner had an incentive to cooperate. Chimpanzee tool givers were more responsive to recipients’ attention-getting signals than orang-utans. These findings suggest that what distinguishes chimpanzees as a more gregarious species may not be their propensity to cooperate per se but rather their use of communicative strategies to facilitate cooperation.