Sex between snow monkey and deer shows different species may mate if they're 'deprived,' study says

There's a snow monkey in Japan who has got a taste for something a little different.

A Japanese macaque was caught on tape mounting female sika deers, going all the way and trying to actually have sex with an animal they usually just ride like horses. Now a new study based out of the Japan Monkey Centre is trying to figure out what's behind the strange romance.

"Although Japanese macaques are known to ride deer, this individual showed clearly sexual behaviour towards several female deer, some of which tried to escape whilst others accepted the mount," the study says.

The bizarre, unexpected phenomenon of interspecies sex has only been recorded in scientific research once before -- when an Antarctic fur seal was sexually harassing king penguins, according to the new study, published in the academic journal Primates.

This makes the recorded intimacy between deer and snow monkey all the more crucial. Researchers say it's "a key element" in understanding more about the humans who try having sex with non-human animals.

Now primatology researchers involved in the recent study say they can unravel the mystery behind the interspecies sex. The answer? A little something called "mate deprivation," which states that "males with limited access to females are more likely to display this behaviour (namely, sex with other species)," according to the study.

On the Japanese island of Yakushima, a Japanese macaque was seen not only getting on top of a female deer but also releasing some sexual fluids in the process, according to one account in the study, leading researchers to describe the behavior as real sexual activity and not just riding. And the snow monkey made several sexual proposals, some of which were not reciprocated.

"The male then attempted to mount another female deer that did not accept the sexual behaviour as she attempted to escape/remove the macaque by moving, increasing her speed, turning around and displaying threats," the study explains.

And, sort of unfortunately, the bizarre animal sex didn't stop there.

"The animals were monitored for an hour before they disappeared in the forest," the study explains. "A mount was observed every 4–5 min."

In further detail you may or may not want to know, the sexual behavior happening between this snow monkey and its lady deer neighbors wasn't exactly sex ... well, at least in one sense of the word.

Researchers noted that the sexual activity did not involve penetration. Despite the very forward actions of that snow monkey, meanwhile, they say the behavior was not aggressive.

"Unlike the case reported for the Antarctic fur seal and king penguins, the sexual behaviour of the male macaque towards the female deer did not seem to be one of sexual harassment or coercion," the study says.

Rather, the monkey "stayed close to the deer and chased away other male macaques" in a behavior that mirrored the protective practice of mate guarding, the study reports. Aside from the deer-riding, the snow monkeys also drop food from trees that the deer can then eat, and the deer even eat the poop of the monkeys. It all makes for a close relationship the researchers refer to as a "strong cohabitation."

Since the animals are such different species -- varying widely in phylogenetic and morphological terms -- the explanation couldn't just be that they think they're all the same species, according to the researchers. And that's where the theory of so-called mate deprivation comes in, particularly considering the competitive nature of dating for snow monkeys.

"Indeed, this heterospecific sexual behaviour is more likely to be observed in species with higher risk of sexual competition (i.e. multi-male, multi-female breeding) as for other sexual behaviour such as masturbation ... or homosexual behaviour" in the snow monkey, the study says.