Yakushima Island, nestled off the coast of southernmost Japan, is known for its fairytale-like forests and its bubbling, crystal-blue rivers. It’s also the site of a new scientific mystery, one involving a frisky macaque monkey trying to have sex with deer.

The case stoked the intrigue of a few scientists, who published an article on Tuesday in the journal Primates theorizing what the heck is going on. The scientists write that this particular macaque has “showed clearly sexual behavior towards several female deer, some of which tried to escape whilst others accepted the mount.” It is less likely that the deer was sexually interested in the monkey, and more that he was anticipating a ride of a non-sexual sort: On Yakushima Island, macaque monkeys and Sika deer typically live in a peaceful, coexistent relationship where deer eat the food the monkeys drop from trees and the monkeys sometimes ride them just for shits and giggles.

This monkey is looking for a more committed relationship and is, in essence, the worst.

“The male mounted the deer and displayed some copulation behaviors, which included about 15 sexual movements over a period of 10 seconds, before dismounting,” co-author Alexandre Bonnefoy said in a statement. “Ejaculation seemed to have occurred as the deer licked the seminal liquid after the mount. This might indicate that the sperm could be a good source of protein to the deer.”

Note that, while it is not for lack of trying, the monkey has not actually had sex with the deer — which the scientists explain is because of a difference between the animals size and build. The monkey also displays “mate-guarding behavior” with the deer, driving away other males that come near the deer “under his watch.”

These Japanese macaques are way chiller.

The researchers aren’t sure why this is happening, but think that it’s because he’s a peripheral male that has limited access to female macaques. A hormonal surge experienced during the breeding season, as well as the fact that there’s already a sort of relationship with Sika deer, may be the cause of “a sexual manifestation of the known play behavior between Japanese macaques and the deer they are known to sometimes ride.”

It is highly unusual for interspecies mating to occur. Only one other scientific paper has been written about the phenomena, a 2008 study detailing the “sexual harassment” of a king penguin by Antarctic fur seals.

Photos via Wikimedia Commons, Alexandre Bonnefoy