Sexual relations between a monkey and a deer uncovered by researchers

The seemingly consensual encounter is a rare example of sex between distant species.

While photographing monkeys on a Japanese island, Alexandre Bonnefoy saw something he didn’t expect: a male snow monkey engaging in sexual behavior with female sika deer. His coauthors on a paper detailing the encounter say this interaction is one of only two documented cases of sexual relations between two very different species in the wild. The other is the sexual harassment of king penguins by Antarctic fur seals.

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Japanese snow monkeys, perhaps best known for their affinity for thermal baths, live side-by-side with sika deer on the island Yakushima. The two species have a cooperative relationship. “The deer eat seeds dropped by macaques on the ground, as well as their feces. Macaques may groom the deer for parasites such as lice, which are rich in proteins,” explains Cédric Sueur, another of the paper’s authors. The monkeys have also been known to ride deer like horses. “They don’t travel the kinds of distances humans do on horseback, but the similarity is there,” Sueur told us. It was this behavior Bonnefoy was on the island to document.

In the sexual encounter described in the paper, the macaque mounted a deer, displaying an assortment of mating behaviors and ejaculating on the deer’s back. There was no penetration, likely because of the animals’ different sizes and builds. The monkey also displayed mate guarding behavior, chasing away other peripheral males. The deer accepted the behavior, says Sueur. A second deer was less accommodating. “She increased her movements and speed, turned on herself, and displayed some threats,” Sueur explained.

Typically, interspecies sexual relations occur either in closely related species or, more rarely, in distant species bred in captivity, says Sueur. That’s what sets this encounter apart. Sueur notes that the behavior is that of an individual monkey, not a widespread practice: “It’s unique to this monkey. We do not see any other behavior like this on Yakushima.”

As for what inspired the act, the authors believe that a limited access to females could be to blame. “Japanese macaques are described as intolerant species, meaning that a low number of adult males—if any at all—are allowed to stay in a group with the dominant male” says Sueur. Seemingly, that practice led one monkey to get creative.


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This discovery was made while the researchers were working on the Saru Project, a book on Japanese macaques for the general public. Read our interview with them to learn more about the project and other fascinating—if less racy—macaque behaviors.