Japanese monkeys, the same macaques that became famous for taking hot baths in their snowy habitats, live in a rather interesting relationship with sika deer.

A female Japanese macaque on the back of a male sika deer. Image credit: Noëlle Gunst

Japanese monkeys (Macaca fuscata yakui), also known as macaques, have often been observed riding on top of sika deer (Cervus nippon yakushimae). The two species live together peacefully in the forests of Japan. This close relationship is beneficial for both parties involved: the deer eats the seeds and fruits dropped by the macaques in exchange for the monkeys to groom and remove parasites from their four-legged friends. On occasions, even a small ride is included in the deal for the primates.

However, scientists have discovered that there is more to this animal friendship than meets the eye.

A macaque “riding” a deer. Image credit: Alexandre Bonnefoy

Researchers have noticed, that the dynamics of this mutually beneficial relationship between the monkeys and the deer began to shift into a more sexual approach by the macaques.

One of the first times this behavior was observed by scientists was on Yakushima Island, Japan, in 2015, when a low-ranking male monkey attempted to mate with at least two different female deer. While one of the deer didn’t mind the monkey performing sexual mounts on its back, the other one shook the macaque off and ran away.

According to the authors of the study, the most likely explanation is mate deprivation. This means that males with limited access to females are more likely to display this kind of behavior.

The researchers also released video footage of this unusual interaction:

Between 2012 and 2015, another team of researchers took a deep dive to investigate the odd behavior Japanese monkeys showed. All in all, they recorded a total number of 258 monkey-deer interactions at Minoo, north of Osaka in Japan. Interestingly, though, only young female macaques were spotted mounting sika deer, so, the team compared these to sexual contact between adolescent female macaques (which is very well documented).

Based on these comparisons, the researchers were able to determine that there are no clear differences between female monkey interactions, and the mounting of the sika deer. They also found that successful sexual interactions were only possible with male deer. More precisely, adult male deer didn’t really care what the monkeys were up to on their backs.

Lead author Noëlle Gunst told Inverse that there are different hypotheses about the main factors behind the sexual monkey-deer interactions.

One possible explanation is that male macaques can become pretty aggressive when it comes to intercourse, and it’s safer for females to take care of their needs in different ways. In this case, on the back of sika deer.

Another theory suggests that Japanese monkeys may have experienced a kind of stimulation while riding and grooming the deer.

“Juvenile female macaques may first experience genital stimulation during these heterospecific playful interactions with deer playmates, then, during the surge of sex steroid hormones characteristic of the adolescence period, they may seek similar sexual reward with deer mates, particularly when sexually deprived conspecific male mates,” said Gunst.

Macaques have some unusual habits. Image credit: Edwin1710

It’s also possible that the adolescent female monkeys are figuring out their sexuality, since males don’t normally accept young females as sexual partners. But the most probable hypothesis is that female macaques are practicing for future sexual intercourse with male monkeys – at least this is what the research indicates.

However, the authors note that none of the mentioned explanations are mutually exclusive, and that this behavior could be a new habit among Japanese monkeys.

“Each of them could account for part of the phenomenon,” explains Gunst. “We argued that the monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioral tradition at Minoo.”