Does Knowledge of the Partner's Need Affect Food Sharing in Tufted Capuchin Monkeys?

The ability to understand the internal (psychological or physiological) states of others can be adaptive in both cooperative and competitive settings. In this study, we tested whether tufted capuchin monkeys (Sapajus spp.) altered their willingness to share part of their food with a partner in relation to having recently seen it eating. We first gave partners food (banana, carrot, or nothing) that could not be shared with the subjects, and then gave subjects food (apple) that they could share with the partners. During the first phase of the tests, the subjects seemed aware that their partner was eating, but did not show any evident emotional response. Neither having seen the partner eat nor the quality of the food eaten by the partner had any effect on the subject's willingness to subsequently share their food during the second phase of the tests. Our results do not confirm those of a previous similar study, and suggest the effect of having seen the partner eat on subsequent food sharing is weak and/or variable in tufted capuchin monkeys. The ability of animals to understand the internal states of other individuals remains contentious.