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After spending a few months observing macaque monkeys living in a Japanese forest, a group of scientists found a pattern of behaviour they described as both “unusual” and “intriguing.”

Adolescent female macaques climbed on top of a sika deer and crouched. Then, they moved their pelvises as if thrusting or grinding. They squeaked sexual sounds. They also bit, sniffed and pulled on the deer’s antlers.

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The deer, meanwhile, stood nonchalantly and continued foraging for food.

When the deer walked away, the female monkeys “often displayed sexually motivated tantrums which consist of crouching on the ground, body spasms and screaming, while gazing at the deer,” the scientists wrote in a peer-reviewed study published this week in the Archives of Sexual Behavior.

Scientists found that the interactions, documented in the Meiji Memorial Forest of Minoo Quasi-National Park on the outskirts of the city of Minoo in central Japan, are “sexual in nature” – at least for the monkeys. Japanese macaques are known to ride deer for play or to move from one place to another, but adolescent females appear to have taken this playful interaction a step further. Why they do so remains unclear, but scientists have a few theories.