Following a report of monkey-deer interactions earlier this year , researchers have now recorded the behaviour in another group of monkeys

Sexual interactions between snow monkeys and sika deer could be a new behavioural tradition within a group of monkeys observed in Japan, researchers have suggested.

While the first report of a male Japanese macaque, or snow monkey, and female sika deer taking to each other was revealed earlier this year, scientists say they are now confident the behaviour is sexual after scrutinising adolescent females suggestively interacting with stags at Minoo in Japan.



“The monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioural tradition at Minoo,” said Dr Noëlle Gunst-Leca, co-author of the study from the University of Lethbridge in Canada.

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While sexual interactions between closely related species have been seen for all manner of animals, from various species of fish to species of baboon, such liaisons are rare, with the sexual assault of king penguins by Antarctic fur seals the only other known example between distant species.

But earlier this year, a study revealed a male Japanese macaque had been filmed mounting a female Sika deer at Yakushima island in southern Japan. Gunst-Leca said it wasn’t clear quite what was going on.

“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. “As a result, even the sexual nature of this interaction was not clearly demonstrated.”



In the latest study, published in the Archives of Sexual Behavior, Gunst-Leca and colleagues describe how they sought to unpick whether the intentions were indeed carnal.

To investigate, the team recorded the behaviour of snow monkeys at Minoo, north of Osaka in Japan. With only adolescent female monkeys spotted mounting deer, the team compared the interactions to sexual interactions between adolescent female monkeys. The latter is a well known practice, and the only available comparison, as during the study adolescent females were not seen performing sexual mounts on adult male monkeys.

In total, the team recorded 12 successful interactions between monkeys, involving six adolescent females, between November 2012 and January 2013 – with a total of 67 mounts by the monkeys. In addition, 13 successful interactions of an apparently sexual nature were recorded between monkeys and deer between early November 2014 and January 2015, involving five adolescent females and a total of 258 mounts.



Analysis of the animals’ behaviour revealed no clear difference between the adolescent female snow monkeys and other females or deer when it came to how often they sought such attentions, mounted their partner, how long they spent on their partner, or even their orientation – although as expected monkeys more often undertook sitting mounts on deer than on other monkeys. Unexpectedly, pelvic thrusting was more common when the partner was a deer.

With no monkey-deer sexual interactions previously having been noticed at Minoo, the team say that the behaviour could be the start of a new custom, adding that adolescent females would watch others on the backs of the deer and try to take their place.



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.



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“It is maybe a new/innovative behaviour that can be socially transmitted and will spread,” he said. “Monkeys do this according to the sex ratio at the reproductive season: if females cannot have access to males, they can have homosexual relations or relations with a deer.”

The team also found that the adolescent females emitted high pitched calls at the deer when gazing at them, and threw tantrums – including body spasms and screams – if the deer walked away, as they do when engaged in sexual interactions with other monkeys.

The successful sexual interactions observed involved a male deer, with the majority involving an adult male: two female deer and three young males courted by the monkeys simply reared up, unseating them.

“Also, heterospecific mounts between Japanese macaques and sika deer have not been observed outside their coinciding mating seasons,” said Gunst-Leca.



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.

Since snow monkeys have also been spotted riding sika deer with no clear sexual agenda – the animals are often found at the same sites since the deer eat food left by monkeys as they forage – it is also possible that adolescent females have developed a taste for the genital stimulation, Gunst-Leca added



“Future observations at this site will indicate whether this group-specific sexual oddity was a short-lived fad or the beginning of a culturally-maintained phenomenon,” the authors note.