Scientists Have No Clue Why Female Monkeys are Having Sex with Deer in Japan

Scientists have come across something peculiar going on between two different animal species on an island in Japan.

Young female Japanese macaques have reportedly been performing inter-species sex with sika deer according to a paper published by scientists at the University of Lethbridge in Canada on December 11. Researchers have observed that the wild monkeys were mounting the deer in Minoo, central Japan. Such monkey-deer interactions have also been reported on Yakushima Island back in January 2017.

Noëlle Gunst, one of the study’s authors, noted that locals in Minoo have most likely noticed such behavior since at least 2014. Researchers Paul Vasey and Jean-Baptiste Leca are also part of the current research which is more focused on the figures as previous work has been based merely on anecdotal evidence.

The research team noted that it is the first quantitative study of sexual interactions between a non-human primate and a non-primate species.

“These findings support the view that monkey-deer mounting behavior is a sexual practice during which the adolescent female monkeys probably derive sexual gratification,” Gunst told National Geographic.

The relationship between the two species has long been observed in the wild, with the monkeys mounting the deer from time to time. Meanwhile, the deer would eat leftover fruits the macaques drop and or even consume the monkeys’ feces as food.

In the study, the scientists reviewed footage of mating season interactions and conducted hormone testing from fecal samples. For this, some 258 monkey-deer interactions were compared to past homosexual contact observed between female macaques. The interactions have been concluded by the researchers to be sexual based on the instances of mounting, thrusting, and vocalizations.

Five cases from the 14 different monkey-deer pairings observed showed the female monkeys mounting the same deer partner three or more times in a 10-minute period, making the same mating calls they use with fellow monkeys. Some female macaques were observed to interrupt sex acts between other monkeys and deer pairs.

These interactions, which last from a few minutes to two hours, appear to barely bother the deer most of the time. While some of the deer would attempt to shake off the monkeys from their backs, others would just let them do their thing.

While inter-species sex is not entirely unheard of, such behavior is observed to be more common between anatomically similar animals.

According to the researchers, among the reasons why they think monkeys are pursuing the deer is for young macaques to practice sex. It is also assumed that young females are choosing a safer alternative that their larger male counterparts which can be dangerous.

“Juvenile female macaques may first experience genital stimulation during these heterospecific playful interactions with deer playmates,” Gunst explained. “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 of conspecific male mates.”

The study has yet to determine how long such behavior has been happening in Minoo.

“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,” Gunst further noted.

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