Excuse me, the monkeys are doing what now?!
Primates are a clever bunch. But this one's just plain weird.
Eight years ago, nearly to the day, I wrote a story for National Geographic News about this one monkey (a male Japanese macaque) who got caught on video jumping onto the back of a female Sika deer and, well, humping to completion.
You can watch the behavior below. It’s not overly graphic. But it is a monkey humping a deer. So, you know, think about who you’d be comfortable seeing you watch such a thing if you’re reading this in public or at work.
Now, a dozen years in the science journalism industry have taught me that nature is nothing if not weird. I’ve written about turkeys playing ‘ring around the rosy’ around a dead cat, orcas swimming around with salmon as hats, ants performing their own life-saving amputations, and American crocodiles performing ‘virgin births’.
But back in 2017, the idea that a macaque was seen trying to mate with a deer was sort of just thought to be a one-off. A weird thing that happened and which was also caught on video.
So imagine my surprise when Cédric Sueur, a primatologist at the University of Strasbourg in France, emailed me and said that not only was the monkey-mounting-a-deer thing still happening, but the behavior seemed to be spreading!
Originally, Sueur and his colleagues thought the male was just kind of a loser. He was low-ranking, sort of on his own, and probably without many mating opportunities. At the same time, Sika deer were known to hang around macaque troops, because the monkeys are always dropping stuff out of the trees that the deer like to eat. The deer have also been seen eating the monkeys’ poop. And in turn, the macaques sometimes groom the deer for ticks and other tasty parasites, which they then eat. This is known as mutualism—when two species benefit from the other without negative consequences.
So again, the thought was that this one loner male was sexually frustrated and, let’s say, taking it out on his deer neighbors.
But when Sueur reached out to fellow researchers to see if they had any other videos of this behavior, he unearthed a bunch of new examples spooling out over the years. What’s more, that male — identified by his almond-shaped eyes — eventually leveled up in his rank, acquiring a more dominant status. And yet, he continued to grind on deer. Even weirder, some of the females in his group started doing the same thing, but on male deer.
Are you ready for the best part? Across the ocean, on another Japanese island about 370 miles away from the deer-mounting-monkeys, yet another population of macaques was seen performing the same behavior. Which means there may be two separate and extremely interesting things going on here.
First, if the researchers are correct and Old Almond Eyes is the same monkey as before, it seems the other macaques in his group may have learned the behavior by watching him. This would qualify as what scientists call “social transmission”.
And second, given that the behavior has now been documented in two completely separate populations of monkeys, it suggests that when the conditions are right, monkeys humping deer is just a thing that can naturally happen. And that finding has ramifications for how we think about all kinds of things, including many modern human behaviors we take for granted.
Like, did you ever wonder about the first human to ride a horse? “Maybe it was something like this,” Sueur told me when we talked.
To be clear, Sueur isn’t predicting that Japanese macaques are on their way toward saddling up Sika deer and riding their way toward an inter-species revolution. But the fact is, most of human history happened before writing, so things like the domestication of dogs or the development of horseback riding remain mysteries that we can only guess at.
It’s unlikely that one human just decided to hop on a horse one day, and then that behavior spread across the world. Rather, there was probably a pre-existing relationship between the species. Maybe horses realized they were safer from predators if they stayed near us. Or maybe humans started keeping horses for meat. And then, once there was familiarity, as is the case between macaques and deer, one brave human hoisted himself up onto a horse’s back and hung on for dear life. And quite probably, that scenario played out in multiple places at multiple times before we got to a world where horse-riding became a normalized human behavior.
Now, I’ll bet when you opened this email you didn’t think a monkey trying to have sex with a deer was going to bring us to a better understanding of ourselves, but here we are.
Nature is weird. Science is weird. And I know for sure I’m weird. And if you’re weird, too, well, you’re in the right place.
You can read more about the monkeys and deer over at National Geographic News!
Other stuff!
It’s been a busy start to 2025. In case you missed it, I’ve also gotten to write about:
Rattlesnakes that turn their bodies into rain-collecting pancakes, for The New York Times!
The sex lives of fireflies, stag beetles, and peacock spiders, for National Geographic News!
How panda bears are actually part of international diplomacy, for National Geographic News!
How scientists are trying to save a giant, predatory sea star from extinction, for the January issue of National Geographic Magazine!
Misunderstandings surrounding bald eagles, for National Geographic News!