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.

Deer Mates

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.)

The researchers observed 14 different monkey-deer pairings. In five cases, female monkeys mounted the same quadruped partner three or more times in a 10-minute period and made the same calls heard when monkeys mate with each other. In other cases, female macaques interrupted sex acts started between other monkeys and deer. Gunst says the interactions occurred about once every day and lasted from a few minutes to about two hours.

For the most part, the deer didn't seem to care. Some of the deer shook the monkeys off their backs, but others stood passively while the monkeys thrusted. In some cases, the deer just kept on eating.

Monkey Motivation

Interspecies sex is not unheard of; 10 percent of animal species are known to hybridize. But this practice is more common in animals that are anatomically similar. Since monkeys and deer are so physically different, it's highly unlikely the macaques confused the deer with a potential primate mate.

"Heterospecific sexual interaction between non-closely related species is very rare to observe," Cédric Sueur, who published the previous study on monkey-deer relations, told National Geographic in January.

Monkey Tries to Mate With Deer (Rare Interspecies Behavior) WATCH: Rare interspecies mating behavior between a macaque and sika deer is captured on camera. Video courtesy Alexandre Bonnefoy

The researchers say there are a couple reasons why the monkeys might be pursuing the deer. For one, this could be a way for young macaques to practice sex. Or, it could be a sexual alternative for young females. Small female monkeys are sometimes rejected by potential sexual partners, and intercourse with their larger male counterparts can be dangerous. Performing on the deer may be a more attractive option for the monkeys.

"Juvenile female macaques may first experience genital stimulation during these heterospecific playful interactions with deer playmates," Gunst notes. "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."

It's unclear, however, how long this specific type of behavior has been happening in Minoo. The unlikely pairings could be a peculiarity that only recently started.

"Future observations at this site will indicate whether this group-specific sexual oddity was a short-lived fad," Gunst says via email, "or the beginning of a culturally maintained phenomenon."