as it happens

'They have a monkey on their back and they seem to like it': Why are Japanese macaques humping deer?

A University of Lethbridge researcher tells As It Happens why monkeys are fooling around with deer in Japan.
A team of Canadian researchers are studying sexual interactions between deer and monkeys. (Dr. N. Gunst and Dr. J.-B. Leca)
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Story transcript

Female monkeys in Japan are taking out their sexual frustration on deer.

That's just one of the details in a new study published in the journal Archives of Sexual Behaviour.

We're trying to make sense of this weird behaviour.- Professor Jean-Baptiste  Leca

The interactions between the Japanese macaques, also known as snow monkeys, and sika deer were documented by University of Lethbridge researchers near Osaka, Japan.

"We're trying to make sense of this weird behaviour," Jean-Baptiste Leca, an assistant professor in the department of psychology, told As It Happens host Carol Off.

Leca said the team witnessed female adolescent monkeys producing high-pitch calls and throwing tantrums when the deer would move away. But, he said, "the most impressive is the sexual mounts that [the monkeys] perform on the deer's back."

"They thrust their genital area on to the back of the male deer," he said. "You could argue that they masturbate on the deer's back."

For the deer, the interaction seems less stimulating. The monkeys groom them and pick skin parasites off their backs. So, while there may be hygienic benefits — there doesn't seem to be any sexual gratification for the deer. 

"They have a monkey on their back and they seem to like it. … They don't move much. Sometimes they even keep on foraging. It's almost like they pretend that nothing is really happening."

One of the explanations for the behaviour is that adolescent female monkeys aren't the preferred partners of adult male monkeys.

"The adolescent females, during the mating season, they are very sexually excited — they are all over the place. But they don't have a lot of success with adult males. We suspect that they could redirect some of their sexual arousal on to members of another species that is readily available in their environment."

Leca said adult male monkeys can often be aggressive during the mating season toward adolescent female monkeys. So, this could also be a way for those younger monkeys to have "safe sex."  

Another explanation is that the monkeys are practicing for sexual interactions within their own species.

Jean-Baptiste Leca and Noëlle Gunst, another author of the study. (Jean-Baptiste Leca)

Leca said it's not clear yet why the monkeys started exhibiting this behaviour.

"It could be that one young female came up with this weird sexual interaction with a deer someday and it spread socially among young females."

"Only future research and longer-term observation will allow us to check to see whether this behaviour is a short-lived fad, something that's going to disappear, or if it's the beginning of a culturally maintained phenomenon."

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