Here's something you probably didn't expect to see today: a monkey humping a deer. 

Japanese macaques and sika deer live together on the island of Yakushima, and it's been known for a long time that the two species share a close relationship … we just didn't know they were this close.

This new footage, released as part of a report by scientists from France and Japan, is the first record of this unexpected behaviour.

To be clear: there's no mistaking what that monkey is doing. While he doesn't penetrate the deer (that would probably not have been comfortable), he does make sexual motions before ejaculating onto the deer's back. Not only does the first deer not seem to mind (the second deer is not so accepting), but she also licks up the mess – it's rich in protein, I guess?

The monkey was also seen chasing other males away his, um, special friend.

"No ambiguity is possible; it is clearly sexual behaviour," Marie Pelé of Strasbourg, France, lead author of the study, said in a press release.

Inter-species sexual activity may seem strange (and it is), but it's common enough that scientists have come up with a term for it: reproductive interference. However, it's usually seen in domesticated or captive animals, and this is only the second time it has been reported happening in the wild.

The other case is the infamous report of Antarctic fur seals chasing and forcefully mounting king penguins. Unlike the monkey-deer situation, the seals were definitely not amicable, and in at least one case, the seal even followed up the molestation by eating the penguin.

If you're a curious sort of person, you might have a lot of questions at this point. For a start, it's highly unlikely that the macaque is mistaking the deer for a lady-monkey, or that it thinks himself a deer. Scientists suspect that during mating season, when not all males are lucky enough to hook up with a female, they're left with an abundance of hormonal energy. "This young macaque... did not have access to females, but was very excited," Pelé explained. "It took advantage of the presence of the doe."

This sort of hormone-driven "anything will do" situation isn't unique to the macaques. Some male frogs, for example, are known for working themselves into a breeding frenzy during which they'll try their luck with just about anything, even salamanders, inanimate objects, or – just in case you're not weirded out enough yet – dead frogs.

But there may be more to this peculiar pairing. Sika deer and Japanese macaques have a famously close association. The deer follow the monkeys around to eat the food (and poop) they leave behind, and the monkeys are known to groom and even ride around on the deer. The researchers suspect that when two species spend so much time in intimate and even playful association ... well, one thing leads to another.

This sort of play-related sex has been seen in dolphins. Mixed groups of Atlantic spotted dolphins and bottlenose dolphins are known to spend time travelling, playing and chasing each other. Amidst all of this togetherness, males and females (and even males and males) of the two species are known to have sex with each other.

So for a lonely, hormonal monkey during breeding season, it might not be too big of a leap to look to his deer friends for some benefits.

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Top header image: Mark Dumont, Flickr