Visitors to the Minoo reserve in Japan's Osaka prefecture have long observed female adolescent macaques mounting and humping adult male deer; in a fascinating paper (Sci-Hub open access link) in Archives of Sexual Behavior, three psychology researchers from the University of Lethbridge in Alberta, Canada, conduct a careful study of these behaviors ("the first quantitative study of heterospecific sexual behavior between a non-human primate and a non-primate species"), and, through a set of naturally occurring experiments, formed an evidence-supported picture of what's going on here.
They conclude that the adolescent females need to practice sex in order to navigate the complex situations that will allow them to reproduce as adults, but that the adult males can be violent and are not very interested in adolescent females (the researchers hypothesize that this is the reason the adolescent females also simulate sex with one another). The deer, meanwhile, have a symbiotic relationship with the monkeys because they get to forage for food that monkeys knock out of the trees when they climb them.
Adolescent and female deer, however, generally don't tolerate monkeys that want to ride them.
Third, monkey-deer sexual interactions may be a substitute for homospecific sex in the absence of conspecific sexual part- ners. It is known that immature female Japanese macaques are not the preferred sexual partners adult males and are routinely rejected by them (Gunst et al., 2015 ; Leca et al., 2015 ). Consequently, adolescent females may seeks tag mates as an outlet for sexual frustration(i. e., ''homospecific sex deprivation' 'hypothesis). This''best-of-a-bad-job'' situation for individuals with limited, or not, access to homospecific mates may also account for the sexual harassment of king penguins by Antarctic fur seals (De Bruyn et al., 2008 ; Haddadetal., 2015).
Fourth, monkey-deer sexual interactions may be a developmental by-product of non-sexual activities between the two species, including play mounting and foraging associations. Immature Japanese macaques occasionally mounts ika deer either in a social play context (''rodeo-style'') or in a locomotion context (''riding- style'') at multiple field sites across Japan, including Minoo (N. Gunst, personal observation), Arashiyama (J. -B. Leca, personal observation), and Yakushima (A. Sawada, 2015, personal communication). Juvenile female macaques may first experience genital stimulation during these heterospecific playful interactions with deer playmates; then, during the surge of sex steroid hormones characteristic of the adolescence period, they may seek similar sexual reward with deer mates, particularly when sexually deprived conspecific male mates. Monkey-deer sexual interactions are likely facilitated by the spatio-temporal association that occurs between the two species within a foraging context (Koda, 2012). These interactions may be further facilitated due to grooming-related hygienic benefits that the deer mates obtain (i. e., ''developmental by-product''hypothesis; cf. Leca et al., 2014c for homosexual mounts between juvenile and adult male Japanese macaques at Minoo).
Fifth, the monkey-deer sexual interactions reported here may reflect the early stage development of a new behavioral tradition at Minoo. Monkey-deer sexual interactions were clearly observed by third-party adolescent females who sometimes performed intrusions on the monkey-deer consortships. Observational learning and social tolerance toward unusual sexual interactions are likely conducive to the expression, spread, and maintenance of other non-conceptive sexual behaviors in this primate species (Leca et al., 2014a). Monkey-deer sexual interactions had never been noticed at Minoo before 2014 (S. Suzuki, 2015, personal communication). Future observations at this site will indicate whether this group- specific sexual oddity was a short-lived fad or the beginning of a culturally maintained phenomenon (i. e., ''cultural heterospecific sex''hypothesis).
Deer Mates: A Quantitative Study of Heterospecific Sexual Behaviors Performed by Japanese Macaques Toward Sika Deer [Noëlle Gunst, Paul L. Vasey and Jean-Baptiste Leca/Archives of Sexual Behavior] (Sci-Hub/open access mirror)