Female Macaques Found Having “Sexual Interactions” With Deer in Japan

Japanese macaque hanging around a hot spring in Nagano, Japan..

Nature is wild, bizarre, and strange. There are numerous species that baffle us because of what they eat, how they behave, or where they live. The ability of organisms to adapt and survive in diverse and adverse environments has pushed the development of peculiar traits we see. Caterpillars basically melt themselves down and rebuild into butterflies. Some plant species have developed the ability to be carnivorous.

Among the more unusual behaviors in the natural world is species-species interactions that delve into the sexual aspects of organisms. Sexual interactions play a crucial role within species to promote mating and reproduction. This is the basis of “survival of the fittest”. Populations that are able to mate and successfully reproduce have their genetic information carried into the next generation. Populations that are unable to mate and reproduce do not have successive generations to carry on their genetic information.

Normal mating that occurs within a species is called homospecific sexual interactions and promotes the survival of a species. Behaviors that go beyond that are called heterospecific sexual interactions and includes different types of sexual activities between members of different species. These sexual activities can include copulation, mating rituals, or even mate guarding.

Researchers from the University of Lethbridge in Canada wanted to explore monkey-deer interactions of Japanese macaques and sika deer because of anecdotal evidence indicating a heterospecific sexual interaction between the two species.

Japanese Macaques

Japanese macaques are native to Japan and sometimes referred to as snow monkeys. They are primates that belong to the Old World Monkeys, which includes species belonging to the family Cercopithecidae and are native to Africa and Asia. Of the four major islands of Japan, snow monkeys are located on only three (Honshu, Shikoku, and Kyushu) of the four (Hokkaido).

They have thick fur,  which is usually differing colors of brown or grey, and are relatively small as males are about 25 lb and females are about 18.5 lb. The three islands they are native to span varying temperatures and environments. As we approach the northern regions, which are colder, the average size of the snow monkeys increase and as we approach the southern regions, which are warmer their size decrease.

While we generally imagine monkeys to be jumping and hanging around in trees, Japanese macaques spend their time on trees, land, and even swimming. They are known for sitting around in hot springs. Like other primates, they are also omnivorous and capable of eating many different plants, insects, and soil. They are not picky eaters.

When it comes time to mate, the males and females pair bond with each other for over two weeks as they feed, mate, and travel together. Males engage in mate guarding, protecting their females from other males that might want to engage in mating. Females are known for having up to four different mates as they engage in consortships. Female macaques have also been known to engage in same-sex mounting behaviors and show a preference for being bisexual.

Sika Deer

Sika deer are native to East Asia and have been introduced to different parts of the world. They are also known as spotted deer because of the distinct white spots on their fur. They have relatively large populations in Japan and are found throughout it. Their lifestyle varies among individuals as there are groups consisting of a single sex and groups with males and harems of females. They can be found active during the day, except when encountering human disturbances. They also experience some migratory habits, especially those in mountainous regions. They have also been known to interbreed with other deer species, such as the red deer.

Sika deer with its notable spots, which remains into adulthood.

Deer Mates

Since 2014, locals at Minoo, Japan had reported seeing macaques mounting sika deer. In the wild, Japanese macaques have been known to ride sika deer to the point that they have formed a somewhat symbiotic relationship. The macaques would groom the deer and the deer would feed on fruits and nuts that the macaques leave behind as well as eat their feces. So, interactions between the two species were fairly common.

Japanese macaque mounted on a sika deer. Credit: Noelle Gunst

To understand the sexual interaction between the two species, the researchers examined footage of the macaque’s behaviors during mating seasons and took fecal samples to test hormone levels.  They then compared 258 monkey-deer interactions to homosexual female monkey interactions, because of the known homosexual behavior within macaques, to see if there were any similarities between the two.

The researchers concluded that some interactions between monkeys and deer were sexual in nature because of their similarity to the sexual interactions among homosexual monkeys as they entered the mating season. They also noted that this sexual interaction was a one-way thing because the deer were uninterested or attempted to hinder these sorts of behaviors. Given the frequency of the heterospecific sexual interactions, the researchers gave thoughts on why the behavior would occur.

Normally, a small percentage of species engage in heterospecific sexual interactions. These interactions are mainly done with closely related species and may even produce some hybrid species. There have been instances of tigers and lions mating to create ligers. It is extremely rare to see these sexual interactions between species that look nothing alike and are not closely related.

The researchers believe that it could be that younger members of the species are using the deer as practice for the real thing. Because they are known to ride and interact with the deer, the researchers surmise that the younger members experience genital stimulations by accident as they interact with deer. As they approach sexual maturity, they may associate genital stimulation with the deer and proceed to do the sexual interactions we observe. Older members of the macaques might use deer mating because they have been rejected by potential mates and see the deer as their only other option.

Animal behaviors are constantly changing or coming to light as we observe and explore their habitats. What started out as anecdotal evidence from locals became a quantified and observed proof of a rare behavior. Perhaps there are more behaviors that we have yet to find and will continue to surprise us.

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