Do chimpanzees (Pan troglodytes) spontaneously take turns in a reciprocal cooperation task?

Reciprocity is considered to be an explanation for altruism toward nonkin. Although there have been many theoretical studies and reciprocity is arguably prevalent in humans, little experimental work has investigated the proximate mechanism of reciprocity in nonhuman animals. The authors tested whether pairs of chimpanzees (Pan troglodytes) would achieve reciprocal cooperation, that is, whether chimpanzees take turns playing donor and recipient roles in an other-rewarding token insertion task. In this task, when a chimpanzee inserts a token into a vending machine, a food reward is delivered to another chimpanzee in an adjacent booth. For 3 pairs of chimpanzees, token insertion persisted in an alternate condition, in which the participants necessarily had to insert tokens alternately, but not in a free condition, in which they freely took turns inserting tokens. In the free condition, one of the chimpanzees was observed to perform a possible solicitation toward the partner who had previously inserted fewer tokens. These findings suggest that there is some difficulty in the occurrence of reciprocal cooperation in chimpanzees. Chimpanzees, differently from humans, might play a donor's role only on the partner's request, but not spontaneously.