At the start of this year, the world was left baffled when evidence emerged of a male snow monkey unsuccessfully getting it on with a female Sika deer on Japan's Yakushima Island.
Now another research team has recorded female monkeys humping male deer, and these romps happened in an entirely different group of animals, in another part of the country.
Snow monkeys, or Japanese macaques, are known to cohabit with Sika deer, as the latter are attracted by food scraps discarded by the monkeys on the forest floor.
Researchers have seen monkeys grooming the deer and even riding them, but this sexual stuff is something new.
The single male macaque caught in the act on Yakushima was thought to be a low-status male who couldn't get a female mate and thus reduced his sexual frustration by taking it out on the deer - who seemed unphased by the affair.
But it's hard to judge what might be going on between the species with just one example, so primatologists from the University of Lethbridge in Canada decided to gather more data from the groups they were observing in Minoo, central Japan.
Scientists already know that adolescent female macaques have similar urges - they routinely interact with male and female monkeys in a sexy way, soliciting them for sex and even mounting them.
Having seen adolescent monkey girls mounting male deer in the vicinity, the team decided to compare these two behaviours to see whether the interactions really were meant to be sexual.
After counting a total of 67 mounts between two female monkeys and 258 female-monkey-on-deer mounts, the researchers identified 25 "successful" interactions, defined by three or more mounts within a 10 minute period.
Twelve of these happened between monkeys, and the other thirteen were monkey-deer liaisons. The romps would take anywhere between just a few minutes to as long as two hours.
Mostly, the deer didn't seem to care at all - they nonchalantly stood still and even kept eating, and only a few reared up to dislodge the horny monkeys from their backs.
In a video featuring sample interactions recorded during the study, the whole thing looks fairly innocent, as the monkeys grind away atop bored-looking deer (although the biting and occasional antler-pulling is a bit more disconcerting).
After analysing the behaviours, the team concluded there was no significant difference between monkey-monkey or monkey-deer interactions - so what we're seeing is indeed macaques having sexy time with deer.
But why are they doing this? The short answer is, we still don't really know.
The team hypothesises there could be several reasons for these unproductive affairs - adolescent females could be practising their sex moves before engaging in the real deal, or perhaps relieving sexual tension since they couldn't get a mate of their own species.
It all could be down to surging adolescent hormones, as female monkeys get a taste for what genital stimulation feels like, and decide to partake in it with non-threatening deer, as opposed to potentially aggressive adult male monkeys.
The researchers note they may have seen a larger number of interspecies action because of their observation area - a place in the national park where staff provide food twice a day to prevent monkeys from roaming too far out into the nearby city.
But apart from the possibly increased frequency, the researchers don't think the sexual interactions had anything to do with human influence on the environment, as the two species hang around each other in many parts of the country.
"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 team writes in the study.
The research was published in Archives of Sexual Behavior.