Japanese monkeys, famed for enjoying hot baths in their snowy surroundings, have an unusual relationship with sika deer.
Japanese monkeys, often known as macaques, have been seen riding on top of sika deer on several occasions (Cervus nippon yakushimae). In the Japanese woodlands, the two species coexist happily. This tight bond benefits both parties: the deer consumes the seeds and fruits dropped by the macaques in exchange for the monkeys grooming and parasite removal from their four-legged buddies. A little ride is often included in the arrangement for the primates.
The dynamics of this mutually beneficial interaction between the monkeys and the deer began to transform into a more sexual approach by the macaques, according to researchers.
On Yakushima Island, Japan, in 2015, a low-ranking male monkey sought to mate with at least two separate female deer, which was one of the first occasions this behavior was documented by scientists. While one of the deer seemed unconcerned with the monkey executing sexual mounts on its back, the other shook the monkey off and bolted.
The most plausible explanation, according to the study’s authors, is mate deprivation. Males who have restricted access to girls are more prone to engage in this type of activity.
Video of this unexpected interaction was also published by the researchers:
Another group of researchers looked at the strange conduct that Japanese monkeys displayed between 2012 and 2015. In total, they counted 258 monkey-deer encounters in Minoo, Japan, which is located north of Osaka. Only juvenile female macaques were seen mounting sika deer, which the researchers equated to sexual intercourse between teenage female macaques (which is very well documented).
The researchers were able to conclude from these studies that there are no significant differences between female monkey interactions and sika deer mounting. They also discovered that only male deer could have effective sexual relations. Adult male deer, to be more accurate, were unconcerned with the monkeys on their backs.
There are several assumptions concerning the major causes for the sexual monkey-deer interactions, according to lead author Nolle Gunst.
One theory is that male macaques can be violent when it comes to intercourse, making it safer for females to take care of their requirements in alternative ways. On the back of a sika deer, in this example.
Another explanation proposes that while riding and grooming the deer, Japanese monkeys were stimulated in some way.
“Juvenile female macaques may first experience genital stimulation during these heterospecific playful interactions with deer playmates, then seek similar sexual reward with deer mates during the surge of sex steroid hormones characteristic of the adolescence period, particularly when sexually deprived conspecific male mates,” Gunst said.
Males seldom accept young females as sexual partners, therefore it’s probable that the teenage female monkeys are figuring out their sexuality. But, according to the research, the most likely explanation is that female macaques are rehearsing for future sexual intercourse with male monkeys.
The authors point out, however, that none of the theories are mutually exclusive, and that this behavior might be a novel habit among Japanese monkeys.
“Each of them might be responsible for a portion of the event,” Gunst says. “We proposed that the monkey-deer sexual encounters described in our research might represent the early stages of a new Minoo behavioral tradition.”