The effects of social context and food abundance on chimpanzee feeding competition

Feeding competition is thought to play a role in primate social organization as well as cognitive evolution. For chimpanzees (Pan troglodytes), social and ecological factors can affect competition, yet how these factors interact to affect feeding behavior is not fully understood; they can be difficult to disentangle in wild settings. This experiment investigated the differential effects of food quantity, the presence of a co-feeding partner, and the contestability of a food patch on feeding rate. We presented tolerant pairs of chimpanzees from a semi-captive social group with an apparatus comprising a matrix of transparent tubes between two adjacent rooms, of which, either all (abundant condition) or only a small proportion (scarce condition) were baited with peanuts. Dyads were either grouped into the competitive treatment, in which peanuts were accessible from both sides of the apparatus simultaneously, or the non-competitive treatment, in which the peanuts were pre-divided; half of the tubes were accessible to one chimpanzee from one side, and the other half were accessible only from the opposite side of the apparatus. We compared dyadic tolerance levels with individual feeding rates across quantity conditions and between competitive treatments. While tolerance and food quantity had no effect on feeding rate, partner presence significantly increased feeding rate relative to individual feeding. This increase was much larger when the dyads directly competed over the peanuts than when they were co-feeding on a pre-divided set of peanuts. Thus, in a co-feeding situation, the presence of another individual and, to an even larger extent, the contestability of the food source play a larger role in chimpanzee feeding behavior than dyadic tolerance or food quantity. These findings highlight the relative impact of social facilitation and direct competition on co-feeding behavior between pairs of chimpanzees.