UCLA

Answers to many of the most important questions about the evolution of primate social strategies still elude us, due to the difficulties in extracting information about primates’ decision-making processes and the fitness consequences of their behavioral choices. These sorts of questions cannot be answered experimentally: the answers must be inferred from the behavioral patterns of wild populations in long-term studies. It is only now that sufficiently sophisticated methods are being developed to allow us to make precise inferences from these messy observational data sets. In this thesis I will ask questions about how wild primates make decisions about allocations of favors to social partners, and about the fitness consequences of their behavioral choices.

In the following chapters, I examined: (1) reciprocity in capuchin monkeys’ social relationships; (2) the relationship between adult female capuchins’ sociality and longevity; and (3) the decision rules that capuchins use when soliciting an ally in a coalitionary fight. I used behavioral, genetic, and demographic data of wild white-faced capuchin monkeys (Cebus capucinus). The data came from 11 capuchin social groups and were collected over a 19-year period at Lomas Barbudal Biological Reserve and surrounding private lands in Guanacaste, Costa Rica. To answer the questions above, I used innovative statistical techniques currently unrepresented in the primatological literature. To analyze reciprocity and the fitness benefits of dyadic relationships, I used the Social Relations Model (Kenny 1994). To model capuchin monkey choices of allies in coalitionary aggression, I employed conditional logistic regression. Both of these modeling techniques allow me to take into account the particularities of a dataset collected in the wild.

My findings show that capuchin monkeys reciprocate in grooming and coalitionary support exchanges. When choosing allies in coalitionary aggression, capuchins use both rank and relationship quality of the potential partner to make a decision. Finally, sociality is associated with fitness outcomes in female capuchins: More social females tend to live longer than less social ones. Together, these chapters will provide insights into capuchin monkeys’ social relationships, decision making and social relationships’ effects on fitness.