Food sharing: a model of manipulation by harassment

Most analyses of food-sharing behavior invoke complex explanations such as indirect and delayed benefits for sharing via kin selection and reciprocal altruism. However, food sharing can be a more general phenomenon accounted for by more parsi-monious, mutualistic explanations. We propose a game theoretical model of a general sharing situation in which food owners share because it is in their own self-interest—they avoid high costs associated with beggar harassment. When beggars harass, owners may benefit from sharing part of the food if their consumption rate is low relative to the rate of cost accrual. Our model predicts that harassment can be a profitable strategy for beggars if they reap some direct benefits from harassing other than shared food (such as picking up scraps). Therefore, beggars may manipulate the owner’s fitness payoffs in such a way as to make sharing mutualistic. Key words: food sharing, harassment, manipulation, mutualism, game theory. [Behav Ecol 13:393– 400 (2002)] Red colobus monkeys (Colobus badius) are an importantand desirable source of protein for chimpanzees (Pan troglodytes; Goodall, 1986). A monkey kill attracts beggars that sidle up to the carcass owner, extend their hands, vocalize, and sometimes grab a scrap of meat. In some situations, the carcass owner may share with the beggar, breaking off a piece of meat and handing it to a beggar. Why would the owner give away food that it could consume? Food sharing, defined here as joint use of a monopolizable food source, can be as active as this chimpanzee example or as passive as a lioness allowing another lioness to feed on her gazelle carcass. Either way, sharing appears to exemplify ani-mal altruism because one individual accepts a fitness cost while another receives a fitness benefit. Although sharing may be altruistic at the time of the sharing event, mechanisms such as kin selection (Hamilton, 1964) and reciprocal altruism (Trivers, 1971) may provide indirect or delayed benefits to th