A male monkey in Japan has been filmed trying to mate with a female deer—likely the first observation of two distantly related species having a nonviolent sexual encounter.

Japanese macaques and sika deer regularly hang around each other on the island of Yakushima. This association has little to do with camaraderie: The deer know that if they stay close to the macaques, they can scarf up any fruits that the macaques drop from the canopy above.

The deer have also been seen eating the monkeys’ feces, and the primates, in turn, have been seen grooming and even riding the deer, says Cédric Sueur, an animal behaviorist at Hubert Curien Multidisciplinary Institute in France. (See a woodpecker atop a weasel and other animals riding animals.)

These are the interactions wildlife photographer Alexandre Bonnefoy hoped to capture when he visited Yakushima for an upcoming photography book, Saru. But he ended up getting more than he bargained for when he captured a single male macaque mounting two separate female deer.

When Bonnefoy showed the video to Sueur and several other primatologists, the scientists quickly realized nothing like this had ever been seen before on Yakushima—or anywhere, for that matter.

“Heterospecific sexual interaction between non-closely related species is very rare to observe,” says Sueur, senior author of a study published this week in the journal Primates. “This case is only the second one to be reported.”

The other? A study of Antarctic fur seals sexually harassing king penguins (see the graphic videos).

Sexual Frustration

What would lead two distantly related animals to do such a thing?

It could be that the whole episode was merely a case of mistaken identity, but this seems unlikely given that interspecies mating usually happens between closely related creatures—a coyote and a dog, say—and it's unlikely the macaque confused the deer with another monkey, the study says.

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Similarly, it’s possible that the male macaque was using the female deer to practice sex. But this explanation also doesn’t quite fit given that macaques are highly social animals, and there are many opportunities for them to observe and mimic their kin.

Instead, Sueur thinks the most reasonable explanation is that the macaque is a so-called peripheral male—a low position in macaque society that generally does not come with breeding rights.

“As a consequence to not having access to females, these peripheral males could socially learn to have sexual interaction with sika deer in order to decrease their sexual frustration,” says Sueur. (Related: "Why Monkeys in the Middle Are More Stressed")

Monkey See, Monkey Do?

Emily Burdfield-Steel, a behavioral ecologist at the University of Jyväskylä in Finland, agrees the mate-deprivation hypothesis makes sense.

“I would be very interested to see if this behavior spreads to other males in the population,” says Burdfield-Steel, noting that this appears to be what has happened in the fur seal-penguin interactions.

She's less sure whether the monkey's behavior amounts to what's called reproductive interference, which occurs when there's a cost to one or both members of an interspecies sexual experience. (See "Interspecies Sex: Evolution's Hidden Secret?")

For instance, does the monkey's mating behavior prevent the deer from foraging or courting mates? Is the monkey using valuable energy of its own?

Each of these outcomes would be considered reproductive interference, but because there's only one observation of the behavior, it's impossible to know, Burdfield-Steel says.

What Was the Deer Thinking?

The other question on the scientists' minds: Was the sika deer a willing participant in this monkey business?

In one video clip, a female deer clearly rejects the macaque’s advances, while another shows the deer seemingly nonplussed by the monkey's actions. For instance, after the monkey jumped off, the deer turned its head to lick the semen on its back—possibly a protein source.

“The behavior certainly seems less aggressive” than the fur seals forcing themselves on the penguins, says Burdfield-Steel, “probably because macaques do not often kill and eat deer!” Fur seals sometimes kill king penguins, and the sexually aggressive fur seal observed in the previous study ate one of the coerced birds.

Beyond that, Burdfield-Steel says the two instances may be more similar than you think.

“In both cases, males with limited access to females of their own species were observed mating, or attempting to mate, with another species that they already interact with,” she says. “Both species are also known to play, and I think this could also be a potential explanation.”

The deer may also be weighing the risk of being bitten or otherwise attacked for not cooperating, says P. J. Nico de Bruyn, a large mammal ecologist at the University of Pretoria in South Africa and author of the seal-penguin study. Of course, it may also be easier and less taxing for the deer to just let the macaque do its thing.

“As the evidence mounts, excuse the pun, the ability to investigate possible drivers increases,” says de Bruyn.