Deconstructing Sexuality in Primates: Analyzing Same-Sex Sexual Behavior in Wild White-faced Capuchins in Guanacaste, Costa Rica Open Access

Abstract

Same-sex sexual behavior (SSB) is widespread across animals yet remains under-documented and poorly theorized in many taxa, especially New World primates. Capuchins provide a key test case as highly social and exhibit rich sociosexual repertoires, but most reports of SSB are anecdotal and rarely quantified alongside different-sex behavior (DSB). In this thesis, I investigate the prevalence, age-sex patterning, and potential functions of SSB in wild white-faced capuchins (Cebus imitator) at Capuchinos de Taboga, Costa Rica. Using ad libitum records of sexual behavior and associated vocalizations collected between March and December 2024, we classified 145 sexual events from 63 individuals by age-sex categories and compare rates of SSB and DSB. We then fitted zero-inflated negative binomial mixed models to test how age, sex, and type of sexual dyad (same- vs different-sex) predict individual variation in overall sexual behavior and in mounting specifically, with individual ID as a random effect. We documented 73 SSB and 72 DSB events. Relative to adults, juveniles and subadults showed markedly lower rates of DSB but a strong bias toward SSB: juveniles engaged in SSB at roughly nine times the rate of DSB, and subadults at about twice the rate, whereas adults showed similar rates of SSB and DSB. Mounting displayed a parallel pattern, with juveniles strongly favoring same-sex mounting and adults showing no clear preference. Sex was not a significant predictor of dyad type, indicating that males and females follow broadly similar age-structured patterns. These results provide only partial support for a “heterosexual practice” hypothesis, under which SSB functions mainly as rehearsal for later heterosexual mating. Juveniles do show elevated SSB, but adults of both sexes continue to engage in SSB at rates comparable to DSB despite abundant heterosexual opportunities. Our findings are more consistent with SSB as a flexible sociosexual tool that serves multiple functions across development, including practice in juveniles and enduring roles in social play, bonding, and conflict management in adulthood. The study offers one of the first systematic quantifications of capuchin SSB relative to DSB and underscores the need to integrate SSB into broader models of primate social and sexual behavior. 

Table of Contents

          Overview: Sexual Selection and Historic Bias against SSB......................................1

                  Current Dilemma: The Self-reinforcing Negative Cycle of SSB Research..................3

Hypothesis Testing: Fitting SSB into Evolutionary Framework..................................5

 

RESEARCH QUESTIONS...............................................................................................13

                  

METHODS....................................................................................................................14

                  Study Site and Subjects......................................................................................14

                  Data Collection..................................................................................................15

                  Descriptive Analysis...........................................................................................16

                  Statistical Analysis.............................................................................................17

 

RESULTS......................................................................................................................18

                  Age and Sex Effect on Preference for SSB vs DSB..................................................18

                  Model Results on All Sexual Behavior...................................................................22

Model Results on Mounting.................................................................................25

 

DISCUSSION................................................................................................................28

                  Heterosexual Practice.........................................................................................28

                  Social Bonding...................................................................................................30

                  Connecting with Other Hypotheses.....................................................................31

                  Limitations and Future Directions........................................................................34

 

References..................................................................................................................36

About this Honors Thesis

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School	Emory College
Department	Anthropology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Evolution and Development Anthropology, Physical
Keyword	same-sex sexual behavior white-faced capuchins non-human primates
Committee Chair / Thesis Advisor	Benitez, Marcela, Emory University
Committee Members	Lonsdorf, Elizabeth, Emory University Massa, Megan, Emory University Lindo, John, Emory University

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