Interspecies breeding is a phenomenon found throughout nature but doesn’t usually involve two species so far apart on the genetic spectrum. One specifically adventurous monkey decided to change all that in 2015 when he attempted to mate with a deer.
On Yakushima, an island of Japan, macaques (known more colloquially as snow monkeys) and sika deer have an almost symbiotic relationship. The monkeys drop food from the treetops for the deer and groom them, while the deer allow the monkeys to ride their backs around the forest floor.
When Alexandre Bonnefoy and his team tried to record some of the interactions, he saw their relationship take a bit of a step further. One macaque tried to mate with two different deer, a very rare occurrence in the animal kingdom for two species so far removed. The two species’ genitalia isn’t compatible, however, and both attempts failed.
Interspecies breeding has resulted in many hybrids before, such as Ligers (cross between Lion and Tiger), Beefalo (American bison and domestic cow) and Wholphins (dolphins and false killer whales). Those are each pair much closer genetically than a monkey and a deer.
In fact, it’s an extremely rare occurrence to even see an attempt at mating like this and likely comes from a “mate deprivation” situation, where the monkey had either been isolated from the community or there are not enough females within their area.
The only other comparable instance reported in the wild seems to be that of king penguins being sexually harassed by an Antarctic fur seal. Otherwise, anything close to this has only happened in captivity, and among animals exclusively bred there.
While it’s not known if the specific macaque followed the deer after the video was taken, it was significant enough to publish a journal article on it through Springer. The authors conclude the entry with the statement: “Further studies are necessary across species to identify these types of sexual behavior in order to understand how they might evolve.”