Graduate School of Letters

Capuchin monkeys, as well as several other primate species, show food-related tolerance in both captive and wild settings. Although researchers have revealed that past experience affects foodrelated tolerance, it is unclear if and how observing a partner's previous food consumption affects tolerance. This question is important to determine the proximate mechanism of food-related tolerance, which may lead to food sharing, co-feeding, or tolerated taking. We investigated whether perception of another's consumption of food affected the rate of tolerant food transfers among brown capuchin monkeys (Cebus apella). First, in the test condition, subjects observed their partner either eating (Eat-In View) or not eating food (No Eat-In View) through a window. In a control condition, the subjects could not observe the same partner behind an opaque screen, while the partner either ate (Eat-Out of View) or did not eat (No Eat-Out of View). After this, the subjects were provided with food to examine how well they tolerated their partner's access to it through the mesh. Tolerant food transfers were sharply reduced after the subjects had observed their partner eat, but not in the control condition or after they had observed the partner not eating. We consider two possible hypotheses for this behavior, one relates to the internal state of the subject after having seen their partner eat (i.e., increased competitiveness, aggression, or food motivation). The other hypothesis relates to how the subject understands the partner's motivational state after having witnessed food consumption, perhaps by grasping the partner's need.