Japanese monkeys have a close relationship with deer there, often riding them for transportation, while providing the deer grooming or food in exchange. But scientists have recently noticed something else: Female Japanese macaques are climbing onto the deer’s backs and grinding.

Researchers observed the adolescent monkeys mounting, thrusting, and making sounds that were typical during sexual interactions between monkeys. Most deer were nonchalant, continuing to eat or stand passively during the thrusting. In some cases when the deer walked away, the female monkey “displayed sexually motivated tantrums which consisted of crouching on the ground, body spasms and screaming, while gazing at the deer,” according to the researchers.

For the first time, scientists at the University of Lethbridge in Canada have done a quantitative study into the sexual behavior of macaques toward sika deer in Minoo, Japan. Their findings are published in the journal Archives of Sexual Behavior.

They have several theories about why juvenile female macaques are pursuing deer. It could be a way for the young monkeys to practice sex. Or it could provide them with an outlet for sexual frustration in the absence of mature adult male partners, who routinely reject immature female Japanese macaques.

Watch our video to see the monkeys in action.