Caitlin O’Connell: University of Southern CaliforniaSocial relationships are an integral part of primate life for humans and non-humans alike, but the extent to which a primate devotes its time and energy to socializing can vary tremendously within and between species. With a semi-solitary social system, orangutans present a unique opportunity to examine both social and solitary conditions within a single population to test predictions regarding the costs and benefits of sociality. While the socioecological model predicts that orangutans display reduced sociality compared to other apes, this should affect individuals differently across life history stages. This research examines the variation in social behavior among age-sex classes in wild orangutans using social interactions, behavioral and hormonal indicators of stress, and intestinal parasites to evaluate the reasons orangutans socialize or remain solitary at different times. Adolescent females were found to socialize the most, to suffer the lowest physiological cost from socializing, and to employ unique behavioral strategies to mitigate potentially risky social situations. My findings highlight the adolescent period as behaviorally distinct and socially rich for female orangutans who face unique challenges as members of a socially dispersed species with high levels of sexual coercion.