Sexual mount of a Japanese macaque male on two different deer. (A–C) Sequence of a sexual mount on a first deer that accepted being ridden. (D) The macaque tried to mount a second deer, but the latter clearly did not accept the mount. Pelé, M., Bonnefoy, A., Shimada, M. et al./Primates

Researchers have published their observations of a rare example of interspecies mating behavior as a male Japanese macaque, or more popularly known as the snow monkey, has been documented while attempting to have sex with female sika deer on Japan’s Yakushima Island.

In a video of the incident that occurred in November last year, the snow monkey can be seen performing sexual mounts on at least two different female deer. Although one deer can be seen dashing away, another female “seemed to accept to be ridden by the male macaque," and eventually licked the sperm on the monkey's back, after the macaque dismounted. The report notes that the behavior of the deer appears to indicate the sperm is a "good source of protein."

The monkey is also seen becoming aggressive and territorial as he chased away other male monkeys that came near its “mate,” though the researchers noted that the monkey did not express any aggressive behavior toward any of the deer.

“No ambiguity is possible, it is clearly sexual behavior,” Marie Pelé, an animal behavior expert at the University of Strasbourg in France and the lead author of the report, said, according to the Guardian.

“This macaque was a non-troop adult male, in other words low in hierarchy. ... He was therefore probably either peripheral or belonged to a group of peripheral males, as other males were observed in the vicinity of the deer,” Pelé explained.

The findings of the paper titled "Interspecies sexual behavior between a male Japanese macaque and female sika deer," published in the journal Primates, theorize that this strange behavior may be because of "mate deprivation.”

“The most realistic hypothesis would be that of mate deprivation, which states that males with limited access to females are more likely to display this behavior,” the researchers conclude.

Another alternative idea is that the snow monkey is learning to copulate and perhaps failed to identify the deer as a member of another species.

Interspecies sex between distantly related species is considered to be highly uncommon. The only other recorded example has been the 2014 report of fur seals forcing themselves on to king penguins in Antarctica before killing and eating them.