de Waal, F. B. M. 1989. Food sharing and reciprocal obligations among chimpanzees. Journal of Human Evolution 18(5), 433-459.

Food sharing has been proposed as the propeller of hominid evolution, particularly of the development of systems of mutual social obligation. Yet, food sharing in our closest relative has never been subjected to a rigorous analysis of reciprocity. Provisioning of branches and leaves to nineteen chimpanzees (Pan troglodytes) in an outdoor corral resulted in 4,653 interactions over food. Food trials were characterised by increased levels of aggressive as well as appeasement behavior. Food exchanges among the nine adult group members (one male, eight females) were remarkably balanced per dyad, and sharing of individual A with B correlated positively with sharing of B with A (r = 0.552). After adjustment of the data for effects of proximity and dominance relationships, a significant degree of reciprocity remained. Individual A's sharing with B on a particular day, however, could not be predicted on the basis of B's sharing with A on the previous trial day. Social grooming earlier during the same trial day did have an effect. Individual A was more likely to share with B after B had groomed A, but less likely after A had groomed B. This suggests a turn-taking rule in the exchange of social favors, which prevents one-sided accumulation of benefits. Individuals who were reluctant to share (i.e., showed a low rate of food distribution) had a higher probability of encountering aggression when they themselves approached food possessors.