Monkeys are officially having sex with deer, and it may be a 'new behavioural tradition'

Have you ever become romantically entangled with someone you shouldn't have? Most people out there have developed the hots for someone wholly inappropriate at one point or another and the truth is, the heart wants what it wants and we can't control who we like, no matter how much we wish we could. Two animals who may recognise this feeling all too well are the snow monkey and the sika deer, because as crazy as it sounds, the two species appear to have begun interacting sexually - and scientists claim that the bizarre trend is here to stay.

Reports of a male Japanese macaque, otherwise know as a snow monkey, and female sika deer indulging in some completely and utterly unnatural hanky-panky were published back in January 2017. In only the second recorded example of such relations, a snow monkey was caught red-handed as he was observed repeatedly performing sexual mounts on at least two different female deer. Although one was seen bolting and scarpering, the other appeared to enjoy the interaction, reportedly licking her side after the monkey ejaculated.

Initially, scientists weren't quite sure exactly what was going on between the two distantly related species. Dr Noëlle Gunst-Leca, co-author of the study from the University of Lethbridge in Canada, says: “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. As a result, even the sexual nature of this interaction was not clearly demonstrated.”

But it turns out this rendezvous wasn't some fleeting, meaningless encounter or embarrassing one-night stand. In fact, we can look forward to many more snow monkey-deer trysts in the future, with a study confident that the actions may be the beginning of a brand new behavioural tradition.

In their study, published in the Archives of Sexual Behavior, Gunst-Leca and colleagues wrote of how they sought to decipher whether the animal's intentions were indeed sexually motivated. In order to investigate, the team recorded the behaviour of snow monkeys at Minoo, Japan, and after recognising that it was only adolescent female monkeys which were being spotted mounting deer, the team compared the practice to the sexual interactions between adolescent female monkeys, a well-known practice in the animal kingdom.

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In total, they recorded 12 successful interactions between monkeys, involving six adolescent females between November 2012 and January 2013, with a total of 67 mounts by monkeys. However, 13 successful interactions were documented between monkeys and deer between early November 2014 and January 2015, adding up to a total of 258 mounts; successful sexual interactions observed involved a male deer, two female deer and three young males, all courted by monkeys.

Analysis of the animals’ unusual behaviour revealed that there was no obvious difference between the adolescent female snow monkeys and other females or deer; this included when it came to how often they sought such attention, mounted their partner, how long they spent on their partner, or even their orientation. In addition, the research team discovered 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 - exactly as they do when occupied in sexual interactions with other monkeys.

However, unexpectedly, pelvic thrusting was more common when the partner was a deer, highlighting scientists' conclusion that, with no monkey-deer relations having been previously spotted in Minoo, it may now, in fact, be a new behavioural tradition. Dr Noëlle Gunst-Leca, stated: “The monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioural tradition at Minoo."

So, the real question is: Why? Sexual interaction with related but distinct species is not unheard of, but it is rare, with the sexual coercion of king penguins by Antarctic fur seals being one of the few other known examples. So why are these distantly related species suddenly engaging sexually? Well, researchers have hypothesised that the abnormal goings-on could be the result of mate deprivation, where males lack adequate access to females.

Dr Cédric Sueur of the University of Strasbourg, a co-author of the study released in 2017, shed more light on the situation, claiming: “As a consequence to not having access to females, these peripheral males could socially learn to have sexual interaction with sika deer in order to decrease their sexual frustration.”

He continued: "It is maybe a new/innovative behaviour that can be socially transmitted and will spread. 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.”

So, it looks like our animal friends may be giving into their passions for quite some time to come. At the end of the day, you have to be happy for them. After all, forbidden love really pulls at the heartstrings.