Dr Cédric Sueur of the University of Strasbourg, a co-author of the study released earlier this year, said the monkey-deer liaisons might be a nascent relationship.
Sexual interactions between closely related species uncommon but not unheard of and have been observed in a wide range of animals such as fish and baboons, according to the.
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 in the Archives of Sexual Behaviour.
Wild Japanese macaques have always been known to ride sika deer.
"They were dealing with a single anecdotal event between one individual monkey and one individual deer, and the description they provided was short, vague and out of context", she said.
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. There were 13 successful pairings, and 258 separate instances of mounting documented by the team. "You could argue that they masturbate on the deer's back".
It was found that adolescent females were more likely to hump deer, and both species have mating seasons at the same time. The second theory was the "safe sex" theory, which proposed female macaques might find deer less unsafe than male macaques to have sexual interaction. Gunst says the interactions occurred about once every day and lasted from a few minutes to about two hours.
For the deer, the interaction seems less stimulating. In some cases, the deer just kept on eating.
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.
"Although this phenomenon may be explained as copulation learning, this is highly unlikely".
Monkey Tries to Mate With Deer (Rare Interspecies Behaviour) WATCH: Rare interspecies mating behavior between a macaque and sika deer is captured on camera. Or, it could be a sexual alternative for young females.
Like humans, adolescent female macaques go through a period in which they begin to develop sexual behaviours. Unexpectedly, pelvic thrusting was more common when the partner was a deer.
While it is unclear why the animals would engage in such behaviour, Gunst-Leca said there were several possibilities, including adolescent females practising for sex with other monkeys, that it offers females a safer way of engaging in sexual behaviour than hooking up with aggressive male monkeys, or that adolescent females are often rejected by adult male monkeys, leaving them at a loss for partners of their own species.
"We observed multiple occurrences of free-ranging adolescent female Japanese macaques performing mounts and sexual solicitations toward sika deer", the scientists reported in the study.