Aversion to violation of expectations of food distribution: the role of social tolerance and relative dominance in seven primate species

Studies on how animals behave when two partners receive different amounts of food have produced variable results, with individuals responding negatively to specific food distributions in some cases (e.g., when food is distributed unequally between partners), but not in others. In this study, we used a simple experimental approach to (i) assess the strictness of dominance relationships based on the degree of social tolerance and (ii) compare the behavioural responses of seven primate species (chimpanzees, Pan troglodytes; bonobos, Pan paniscus; gorillas, Gorilla gorilla; orangutans, Pan go pygmaeus; brown capuchin monkeys, Cebus apella; spider monkeys, Ateles geoffroyi; long-tailed macaques, Macaca fascicularis), when two partners received different amounts of food and no effort was required. We predicted that negative responses (i.e., refusal to participate in the task or avoidance of proximity to the food source) would be elicited by food distributions that violate the individual expectations based on tolerance levels and subject's dominance rank relative to the partner. In the 'tolerance' task, we found that species with less strict dominance relationships were chimpanzees and bonobos, followed by orangutans, spider monkeys, gorillas, brown capuchin monkeys and long-tailed macaques. In the 'food distribution' task, capuchin monkeys and especially macaques showed their aversion by refusing to participate in most conditions, including the ones with equal food distribution. When dominants received more food than the partner, subjects of all species maintained a comparable amount of proximity to the food source,