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Food Sharing and Underlying Reciprocity among Chimpanzees
Food sharing was observed in a captive colony of chimpanzees (Pan troglodytes). The exchange of food in the group was characterised by the high frequency of food transfers between a mother-infant pair, the general respect for a possessor's right, as well as the asymmetrical flow of food between different age-sex classes. To investigate the underlying reciprocity in the group, reciprocity correlations (i.e., positive correlations between the frequency with which an individual A performs beneficial acts to B and B does beneficial behaviour to A) were examined among unrelated adults for three behavioural measures: grooming, food transfer, and begging behaviour. The confounding effect of inter-individual association was removed in the all analyses. Only grooming showed a symmetrical distribution of giving and receipt per dyad, whereas neither food transfer nor begging behaviour did. A positive correlation was found between A's grooming B and B's sharing food with A, indicating that the two favours were reciprocally exchanged. This was further supported by a positive correlation between A's grooming B and A's begging for food from B, suggesting that an individual was likely to request food from the recipient of grooming. The tendency for reciprocal exchange of grooming and food was especially pronounced in male-to-female food transfer, which implies the influence of dominance on an individual's choice of currency in social reciprocity. The triadic reciprocity hypothesis, that an adult contributor sharing food with an unrelated immature would receive benefits from the mother of the immature, was also tested. A positive correlation between an adult's sharing food with a non-kin young and the mother's grooming the adult donor was consistent with the hypothesis.
de Waal, F. B. M. (1989). Food sharing and reciprocal obligations among chimpanzees.
Journal of Human Evolution 18: 433-459.