RESEARCH ARTICLE Influence of Sexual Competition and Social Context on Homosexual Behavior in Adolescent Female Japanese Macaques NOËLLE GUNST*, JEAN-BAPTISTE LECA, AND PAUL L. VASEY Department of Psychology, University of Lethbridge, Lethbridge, Alberta, Canada We explored the role that sexual and social partners play in the expression of female homosexual behavior among adolescent female Japanese macaques at Arashiyama, Japan. Our data fully or partially supported all the predictions related to four non-mutually exclusive hypotheses, namely the “adult male disinterest in adolescent females” hypothesis, the “numerous homosexual adult females” hypothesis, the “safer homosexual interactions” hypothesis and the “same-sex sexual interactions” hypothesis. Our results show that both sexual context (e.g., lack of adolescent female attractivity toward adult males, presence of motivated same-sex sexual partners), and social context (e.g., risk of aggression) help explain the high frequency and prevalence of homosexual behavior in adolescent females in the Arashiyama group of Japanese macaques. As with adult females, whose homosexual consortships do not reflect generalized patterns of social affiliation or kinship, we found that adolescent females’ same-sex sexual partners were neither kin, nor were they non-kin individuals with whom adolescent females were closely affiliated outside of a consortship context. Our study furthers the growing database of female homosexual behavior in Japanese macaques and provides additional evidence that homosexual behavior as expressed by adolescent female Japanese macaques is, like heterosexual behavior, sexual in nature. We discuss the relevance of our findings to a broader comparative approach that may shed light upon the development and evolution of human homosexuality. Am. J. Primatol.77:502–515, 2015. © 2015 Wiley Periodicals, Inc. Key words: sexual development; female heterosexual attractivity; female homosexual receptivity; aggression risk; incest avoidance INTRODUCTION Primates are characterized by the longest periods of juvenescence and adolescence in relation to life span in mammals [Pereira & Fairbanks, 1993]. Previous research clearly shows the need for a period of physical maturation and sensorimotor practice in the development of adult-like sexual behaviors and sexual partner preferences in immature male pri- mates [Gunst et al., 2013; Leca et al., 2014a; Owens, 1976]. In addition, an adequate social environment is needed, with particular social stimuli (e.g., interactions with the mother, other adults, and peers) that contribute to experiential processes supporting learning (i.e., the “learning hypothesis”); [Dixson, 2012; Poiani, 2010]. Adolescence is a key phase for understanding the ontogeny of sexuality in female primates. During adolescence, most females experience their first oestrus period and engage in their first sexual interactions. Here, we defined the oestrus period as a phase of increased female sexual proceptivity, receptivity, and attractivity to males, which in anthropoid primates is not limited to the period of fertility [Dixson, 2012]. The development of sexual behavior results in major differences between ado- lescent and adult females in the frequency and form Contract grant sponsor: Natural Sciences and Engineering Research Council of Canada (NSERC); contract grant sponsor: Alberta Innovates Health Solutions (AIHS); contract grant sponsor: Japan Society for the Promotion of Science (JSPS); contract grant sponsor: American Institute of Bisexuality (AIB); contract grant sponsor: L.S.B. Leakey Foundation; contract grant sponsor: Office of the Dean of Arts and Science; contract grant sponsor: Office of Research Services at the University of Lethbridge Ethical standards: All research methods were approved in accordance with the Guide for the Care and Use of Primates prepared by the Primate Research Institute, Kyoto University.  Correspondence to: Noëlle Gunst, Department of Psychology, University of Lethbridge, 4401 University Drive, Lethbridge, T1K 3M4, Alberta, Canada. E-mail: noelle.gunst@gmail.com Received 27 January 2014; revised 19 October 2014; revision accepted 9 November 2014 DOI: 10.1002/ajp.22369 Published online 16 January 2015 in Wiley Online Library (wileyonlinelibrary.com). American Journal of Primatology 77:502–515 (2015) © 2015 Wiley Periodicals, Inc.