Just like us, our primate relatives seek sexual gratification, though the ways they do so might seem a little strange. Ongoing research on the Japanese macaque (Macaca fuscata), also known as the snow monkey, reveals that it has an, uh, interesting sexual relationship with the sika deer (Cervus nippon) that involves female monkeys riding the sikas like antlered Sybians.

In a paper published Monday in the journal Archives of Sexual Behavior, researchers describe seeing female Japanese macaques rubbing their crotches on the backs of sika deer in a way that suggests sexual gratification. While researchers have known that sexual behavior occurs among female macaques, this was the first systematic observation of inter-species (“heterospecific”) sexual encounters.

First author Noëlle Gunst-Leca, Ph.D., an adjunct assistant professor of psychology at the University of Lethbridge in Canada, told Inverse four reasons why this might happen.

monkey humping
Female Japanese macaques may be practicing for later sexual encounters with their own species.

One reason is that sex can be very dangerous for females, so horny animals are simply seeking a less threatening way to get off. “Small-bodied and vulnerable adolescent female Japanese macaques may prefer to engage in relatively safer sexual interactions with female monkey sexual partners in lieu of riskier sexual interactions with more aggressive male mates,” Gunst-Leca says, explaining that sometimes humping other animals is safer than hooking up with a potentially violent male of your own species.

“Adolescent female Japanese macaques may engage in sexual interactions with passive non-conspecific mates, such as stags, for similar reasons,” she says. “We called this the ‘safe sex’ explanation.”

The second and most likely reason is that females are practicing sex moves in preparation for sex with a male of their species, which is what the research team’s observations seemed to indicate. Over the course of two winters, the researchers watched 13 seemingly sexual interactions between monkeys and deer. These interactions involved five adolescent female macaques forming 14 different partner combinations with various deer.

The researchers observed that these interactions occurred only during the animals’ coinciding mating season in Minoo, Japan, suggesting that they were indeed sexual encounters.

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Adolescent females grind on deer in the absence of conspecific mates.

In the videos, you can see that the deer look surprisingly undisturbed by this behavior. And that’s because they’re actually pretty used to being ridden by monkeys, though it’s usually in a non-sexual way. The macaques and the deer have a symbiotic relationship in which the deer eat fruit the monkeys drop and also allow the monkeys to groom them, so a monkey jumping on a deer’s back is not as freaky as it looks. What’s remarkable, though, is how the monkeys are getting off on the deer. This brings us to one of the researchers’ other hypotheses: This behavior could be a kind of “spill-over” from the deer and monkeys’ non-sexual relationship.

“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,” says Gunst-Leca.

Which brings us to the fourth and final possible explanation: These young females are figuring out their sexuality and relieving some frustration.

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Mature male deer are more likely to let adolescent females use them for sexual gratification.

Since mature males don’t usually accept adolescent females as sexual partners, these young monkeys are left with a couple options when it comes to gratification: They can either play with each other to get off — which they often do — or they can grind on a deer’s back. Most often, it’s mature male deer that allow the female monkeys to have these sexual encounters. Two female deer and three young males reportedly reared up to buck the frisky monkeys off.

A Japanese macaque male and a sika deer.
This male monkey tried to mate with this deer.

This study isn’t the first report of this heterospecific match-up, though it is a new variation on it. In January 2017, researchers reported that a male Japanese macaque was trying to have sex with sika deer. In this particular case, a male was seen mounting female deer and humping them until he ejaculated, though he didn’t appear to successfully achieve penetration. Some of the deer involved were fine with it, while others shook him off. Interestingly, this male macaque seemed to be a marginalized individual, so for him, the sexual frustration explanation could be most fitting, though it might not be the only one.

Gunst-Seca points out that none of these explanations is mutually exclusive: “Each of them could account for part of the phenomenon,” she says. “We argued that the monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioral tradition at Minoo.” Going forward, she and her colleagues will examine whether this behavior is passed down to other monkeys, like a tradition, or dies off.

“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.”