Why do animals engage in homosexual acts?

Female macaques observed having sex with deer

A strange monkey show is going on on a Japanese island.

On December 11th, scientists from the University of Lethbridge in Canada published a treatise on what appears to be sexual activity between young female Japanese macaques and Sika deer. The wild monkeys in Minō, Japan, have been observed to mount the deer.

In January 2017, there were reports of similar interactions between monkeys and deer from Yakushima Island. Study co-author Noëlle Gunst says Minō residents have likely been observing this behavior since at least 2014. Previous research was based only on anecdotes, but the current study works with numbers.

According to the team, which also includes researchers Paul Vasey and Jean-Baptiste Leca, the paper marks the first quantitative study of sexual interactions between a non-human primate and a non-primate species.

"The findings support the view that climbing behavior between monkeys and deer is a sexual practice in which adolescent female monkeys are likely to experience sexual satisfaction," Gunst wrote in an email.


It has long been known that wild Japanese macaques ride sika deer. Sometimes the monkeys also groom their four-legged mounts, while the deer eat the fruit that the macaques drop or occasionally consume their faeces.

The most recent study looked at video-recorded interactions during the mating season and hormone tests on fecal samples. The researchers compared 258 monkey-deer interactions with homosexual acts seen in the past between female macaques.

Based on the climbing behavior, the pushing movements and the vocalizations, the team concluded that these acts are indeed sexual in nature. In some cases, the macaques also bit the deer or tugged on their antlers.

The researchers observed 14 different such interactions. In five cases, female macaques mounted the same four-legged partner three or more times within ten minutes, making the same sounds as when macaques mated. In other cases, female macaques interrupted sexual activity between other monkeys and deer. According to Favor, such interactions occurred about once a day and lasted anywhere from a minute to two hours.

For the most part, the deer didn't seem to let that bother them. Some shook the monkeys off their backs, while others just stood passively while the monkeys shifted on them. Others just kept eating calmly.


Sex acts between different species are not new: around ten percent of animal species are known to interbreed with other species. However, such behavior is more likely to occur in anatomically similar animal species. Because the physiology of monkeys and deer is so different, it is highly unlikely that the macaques confused the deer with other animals.

"Sexual interactions between animals of different species that are not closely related are seen extremely rarely," Cédric Sueur told National Geographic in January. He published an earlier study on the relationships between monkeys and deer.