Chimpanzees return favors at a personal cost.

Humans regularly provide others with resources at a personal cost to themselves. Chimpanzees engage in some cooperative behaviors in the wild as well, but their motivational underpinnings are unclear. In three experiments, chimpanzees () always chose between an option delivering food both to themselves and a partner and one delivering food only to themselves. In one condition, a conspecific partner had just previously taken a personal risk to make this choice available. In another condition, no assistance from the partner preceded the subject's decision. Chimpanzees made significantly more prosocial choices after receiving their partner's assistance than when no assistance was given (experiment 1) and, crucially, this was the case even when choosing the prosocial option was materially costly for the subject (experiment 2). Moreover, subjects appeared sensitive to the risk of their partner's assistance and chose prosocially more often when their partner risked losing food by helping (experiment 3). These findings demonstrate experimentally that chimpanzees are willing to incur a material cost to deliver rewards to a conspecific, but only if that conspecific previously assisted them, and particularly when this assistance was risky. Some key motivations involved in human cooperation thus may have deeper phylogenetic roots than previously suspected.