Most of the deer that the monkeys were seen cavorting with were adult males, although the researchers also recorded encounters with two female deer and three young males. "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", he says.
Researchers found that a video that surfaced earlier this year of a monkey seemingly having a sexual interaction with a deer was actually not an isolated case of unusual monkey behavior.
When the deer walked away, the female monkeys "often displayed sexually motivated tantrums which consisting 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.
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. We observed multiple occurrences of free-ranging adolescent female Japanese macaques (Macaca fuscata) performing mounts and sexual solicitations toward sika deer (Cervus nippon) at Minoo, central Japan.
Scientists observed a total of 170 male and female Japanese macaques.
Adolescent macaques usually climb on top of sika deer to play or be transported. Nearly all of the successful interactions involved an adult male deer or a stag, which seemed to passively oblige.
All told there were 13 successful pairings - and 258 separate mounts. Young female monkeys would identify a "particular target" and make calls that sound identical to the ones in monkey-on-monkey courtship.
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The monkeys were seen climbing onto the backs of deer, thrusting their pelvises and rubbing their genitals on them.
The researchers believe the hormonal surge experienced by the Japanese macaques during breeding season and the close cooperation between these primates and Sika deer culminated in this copulation behavior between two unrelated species.
Sexual interactions between closely related species is uncommon but not unheard of. The second theory was the "safe sex" theory, which proposed female macaques might find deer less risky than male macaques to have sexual interaction. With only adolescent female monkeys spotted mounting deer, the team compared the interactions to sexual interactions between adolescent female monkeys. It's similar to one explanation for why young female macaques have sex with each other.
First, it might be a way for a less-mature monkey to practice for future sex with other monkeys. "We suspect that they could redirect some of their sexual arousal on to members of another species that is readily available in their environment".
It could also be a safer way to engage in sex by avoiding aggressive male monkeys, scientists said. This is what the researchers call the "best of a bad job" hypothesis.
It's also possible that the monkeys just do it for fun or for genital stimulation, but researchers also state that it is entirely possible that this is a cultural practice for the monkeys, not unlike other practices such as bathing in hot springs and playing with snowballs. "You could argue that they masturbate on the deer's back".
Alternatively, the paper notes, it could be a "short-lived fad".