Monkeys in Japan may be starting a new behavioral tradition, one that involves inter-species sex. Yep, it seems that jumping on the backs of deer and rubbing their genitals is becoming a learned behavior within one group of Japanese macaques.

It was earlier this year that a male Japanese macaque, also known as snow monkeys, was first observed jumping onto the back of a female sika deer and getting its rocks off, before jumping away and allowing the poor doe to clean up the resulting mess. While sex between closely related species is fairly commonplace, that between distant species has so far been confined to Antarctica fur seals assaulting king penguins.

so, researchers were unsure whether or not this was an isolated action by a rogue male, or something more widespread in monkey society.

The team turned to a population of macaques living at Minoo, just to the north of Osaka in Japan. It turns out that researchers have been recording interspecies trysts between female monkeys and deer since around 2014. In these cases, it is adolescent females that are too young to be chosen by the mature males, jumping on the backs and rubbing themselves against the unfortunate deer.

Over a period of two months, the researchers observed five young female monkeys mounting deer a total of 258 times. The females were seen to rub their genitals on the back of the deer, not unlike now the male was seen to ejaculate on the hapless steed he commandeered.

This particular group of macaques was chosen to be studied for a reason, because the immature females here have also been seen to mount each other, rubbing their genitals together and seemingly stimulating themselves. Noëlle Gunst, who co-authored the study published in the Archives of Sexual Behavior, thinks that this might be a clue as to what exactly is going on here.

The two different behaviors, both mounting other monkeys and the deer, occur at similar frequencies, but only during the mating season. Gunst thinks that the females are probably practicing sexual behaviors that they are still not yet developed enough to do for real, and while it may have initially started off occurring between monkeys, it has now be transferred to the deer. Interestingly, it is only the large stags that allow the young females to do it, with the does and young male deer rearing up and tossing them off if they try their luck.  

What is more, this behavior had never been seen until a few years ago, while now it is far more common. This has raised the possibility that the behavior is novel and that researchers are recording a new tradition or culture come into being, as young females watch older ones hump the deer and then follow suit as they grow up.