Jean-Baptiste Leca, Ph.D.

Assistant Professor
Board of Governors Research Chair (Tier II)
Department of Psychology
University of Lethbridge
Canada

Non-conceptive sex culture in Japanese macaques

We investigated the ontogeny and causal mechanisms underlying another non-adaptive behavior, namely, homosexual behavior in female Japanese macaques, and its relationship to a purportedly adaptive behavior, namely, female-male mounting.

We showed that the emergence of both conceptive and non-conceptive sexual behaviors in female Japanese macaques can be traced back to adolescence, with a major developmental threshold occurring at four years of age
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However, we found marked differences in the development of heterosexual and homosexual behaviors that I explained in terms of aggression risk, social facilitation and sexual reward
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After testing a series of hypotheses about the influence of female-biased sex ratios on females’ sexual partner preferences, our results supported the “bisexual preference” hypothesis and suggested that when a female targets another female as a mate, it is an active choice of a female sexual partner over male alternatives
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We also found substantial intergroup differences and covariation in the frequency and form of female-female mounting [VIDEO] and female-male mounting [VIDEO].


Our results supported the view that these two forms of non-conceptive sexual behaviors were developmentally and evolutionarily linked, and might be cultural practices arising in groups when certain socio-demographic conditions are met.

Whereas male mounting posture should be optimal (i.e., precisely coordinated and invariant) in order to achieve penile intromission during heterosexual copulation, female mounting is less functionally constrained, which allows for more flexible and arbitrary behavioral patterns.


We showed that the customary occurrence, high prevalence, and great diversity of female-female and female-male mounts at Arashiyama may be the result of combined favorable socio-demographic conditions, namely few resident males, most of them being old, sexually under-motivated, and less aggressive and controlling than the average male Japanese macaques living in the other study groups at Minoo and Jigokudani. 


We argued that although genetic explanations for such intraspecific variation cannot be rules out, arbitrary behavioral patterns such as intergroup differences in female mounting postures in Japanese macaques could be purely cultural, as any alternative explanation is difficult to imagine.

We provided the first report of male homosexual consortships and mounts within male-male dyads in a free-ranging all-male group of Japanese macaques, at Minoo, central Japan. Male homosexual interactions shared most of the behavioral components that have been reported to characterize heterosexual and female homosexual consortships in this species (e.g., male-male solicitations, mounting postures, body orientations, inter-mount activities, and third-party male intrusions).


We argued that research on male homosexual behavior in all-male groups of primates under natural settings, and even more interestingly, studies of male bisexual behavior expressed in the dual socio-demographic context of all-male and mixed-sex groups of primates may provide insights into the developmental processes, causal mechanisms, adaptive significance, and phylogenetic pathways that characterize male bisexuality in humans.