There’s some unusual monkey business happening on an island in Japan.
On December 11, scientists at the University of Lethbridge in Canada published a paper revealing what appear to be sex acts between young female Japanese macaques and sika deer. The wild monkeys were observed mounting the deer in Minoo, central Japan.
Similar monkey-deer interactions on Yakushima Island were reported in January 2017. Locals in Minoo most likely have been observing these behaviors since at least 2014, says study co-author Noëlle Gunst. But while the previous research is based on anecdotal evidence, the current work focuses on the numbers.
According to the team, which also consists of researchers Paul Vasey and Jean-Baptiste Leca, this paper 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 writes in an email.
Wild Japanese macaques have long been known to ride sika deer. The monkeys will sometimes groom their four-legged mounts, while the deer get to eat any fruits the macaques drop and occasionally make meals of the monkeys’ feces. (See a weasel flying on a woodpecker, a seal surfing on a whale, and other animals riding animals.)
The latest study looked at mating season interactions from videos of the behavior and hormone testing from fecal samples. The researchers compared 258 monkey-deer interactions to homosexual contact observed between female macaques in the past.
Based on instances of mounting, thrusting, and vocalizations, the team concludes that these relations were, in fact, sexual. In some cases, the macaques also bit the deer or pulled on their antlers. (If you’re wondering, there is video.)