Meat sharing among the Gombe chimpanzees: harassment and reciprocal exchange

Sharing food with nonkin is detrimental to a food donor's fitness, unless it is matched by compensatory benefits. I evaluated two explanations for nonkin meat sharing among wild chimpanzees, Pan troglodytes schweinfurthii. Reciprocal exchange proposes that a possessor relinquishes food in exchange for past or future sharing or for items of a different currency (e.g. grooming, alliances or copulations). The second hypothesis is the sharing-under-pressure/harassment model, which proposes that an individual shares to avoid the costs of defending a food item against persistent beggars. At Gombe National Park, Tanzania, I collected dyadic grooming and association data during focal follows of adult male chimpanzees. I videotaped meat-eating bouts, subsequently extracting detailed begging and sharing data. There was mixed support for the reciprocal exchange hypothesis. Sharing with males was not influenced by overall association and grooming rates. Female sexual receptivity did not affect the probability of sharing, nor did sharing increase the probability of mating. Meat possessors shared larger amounts, and were more likely to share actively with frequent female grooming partners. However, this pattern may have resulted from increased harassment by these individuals. In contrast, the sharing-under-pressure hypothesis was consistently supported: the possessor's feeding rate decreased with the number of beggars, the probability of sharing increased with the occurrence and duration of harassment, and harassment decreased following sharing events. I conclude that the pattern of meat sharing among the Gombe chimpanzees is largely explained by the sharing-under-pressure hypothesis, while the significance of reciprocal exchange remains unclear.