Punishment and competition over food in captive rhesus macaques, Macaca mulatta

Current socioecological models predict that clumped food resources influence aggression, yet definitions of clumped resources often include two concepts: time spent at the resource and distance between resources. To disentangle these two aspects of clumping, we conducted an experiment on 15 multimale–multifemale groups (range 50–110 individuals) of captive rhesus macaques at the California National Primate Research Center in which we varied both food size and interfood distance independently to test which factor more significantly influenced two forms of feeding competition, contest competition and punishment. At interfood distances of 1–6 m, two same-sized apple pieces of 1 g, 5 g, 15 g or 30 g were offered to the monkeys simultaneously during each of 297 trials. Aggression was recorded 5 min before the arrival of the food, when the food arrived and 5 min after. Results of the independent effects of food size revealed that larger food size was significantly associated with both contest competition and punishment. Interfood distance was not significantly associated with either contest competition or punishment. These results suggest that the size or handling time of food may be a better predictor of within-group aggression than is the spatial distribution of food.