Researchers found that a video that surfaced earlier this year of a monkey seemingly having a sexual interaction with a deer was actually not an isolated case of strange monkey behavior. Upon further study, researchers confirmed the behavior to be sexual in nature, although the reasoning behind it is still unclear.

Monkey Do

In January, a report surfaced detailing interspecies sexual behavior between a male Japanese macaque and a female Sika deer on Yakushima Island in Japan. Although even at the time Japanese macaques were already known to ride deer for transport in return for favors such as grooming, the report gained traction because the behavior seemed more sexual than platonic, suggesting an inter-species sexual relationship.

Researchers believed that the monkey-deer interaction was likely because of mate deprivation and unlikely because of copulation practice. Still, even with the video as evidentiary support, it was not quite clear whether the act was indeed sexual in nature.

Monkey-Monkey And Monkey-Deer Interactions

A new study published in Archives of Sexual Behavior revealed that the incidence reported in January was not a single incident that just happened to be caught on camera, as the behavior was also observed in another group of monkeys in Minoo, Japan. This time, the behavior was observed among adolescent female monkeys and male deer.

Based on researchers' observations, the monkeys were indeed engaging in sexual behaviors as evidenced by the mounting postures, which they also do during homosexual monkey-monkey interactions, as well as the vocalizations that they made. Evidently, at least for the monkeys, the act was indeed sexual in nature.

In comparison, researchers observed 13 successful interactions between monkeys and deer with a total of 258 mounts, whereas they observed 12 successful interactions between monkeys and other monkeys. Between the amount of interactions and the behaviors surrounding them, researchers surmise that this may be a previously unknown behavior that could spread to other populations, especially during mating season if the females do not have males to mate with.

What's With The Interspecies Relationship?

Researchers believe that there are a few possibilities as to why the monkeys engage in this behavior. For instance, this method could be their way of practicing future sex with other monkeys such as larger male monkeys, which are considerably larger than the females. Also, just like the hypothesis of the initial paper, it's possible that the females who engage in this behavior — adolescents — are rejected by the males during mating season and are therefore left with no available partners, which is why they seek out the deers.

It's also possible that the monkeys just do it for fun or for genital stimulation, but researchers also state that it is entirely possible that this is a cultural practice for the monkeys, not unlike other practices such as bathing in hot springs and playing with snowballs. However, it's worth noting that the behavior has only been observed at Minoo, so it's also possible that it's just a fad among monkeys in recent years.