Date of Thesis



Prosociality refers to the behavior of one animal that benefits another. The influence of the strength or weakness of the affiliative relationship between partners on prosociality remains poorly understood. I tested brown capuchin monkeys' tendency to behave prosocially during both a food-sharing task and a prosocial choice task (PCT). In both tasks, each individual was paired with a socially-strong partner and a socially-weak partner. Social index scores, which are based upon the duration of affiliative behavior between individuals, were used to determine socially-strong and socially-weak dyads. During a food-sharing task, one subject was secluded in a compartment containing food, and a partner was confined to an adjacent compartment. Subjects could choose to open an interconnecting door by removing a lock, which could only be accessed by the subject. If the subject opened the door, the partner could enter the subject's compartment and access the food. Of seven monkeys tested, two adult females opened the door more often than expected for a socially-strong partner and opened the door less often than expected for a socially-weak partner. During the PCT, eight subjects were presented with and chose one of the two tokens. By selecting a "selfish" token, only the subject was rewarded. However, selecting the token representing the "prosocial" option resulted in simultaneous rewards for both the subject and his or her partner. Regardless of condition, none of the subjects chose the prosocial token significantly more often than chance. Additionally, there were no significant differences in prosocial token selection across strong and weak social partner conditions. Results provide evidence that some capuchin monkeys are capable of behaving prosocially during a food-sharing task, but not during a PCT. Furthermore, stronger affiliative relationships were associated with increases in prosocial behavior in the food-sharing task. Social index scores are a quantitative measurement of relationship strength and may provide more accurate assessment of the influence of social dynamics on prosociality than qualitative measures, such as kinship.


prosociality, prosocial behavior, food sharing, food-sharing task, door-opening task, prosocial choice task (PCT), capuchin monkeys (Cebus apella)

Access Type

Masters Thesis (Bucknell Access Only)

Degree Type

Master of Science


Animal Behavior

First Advisor

Peter G. Judge