The setting seems straight out of the movie Princess Mononoke by Hayao Miyazaki. The Japanese director was largely inspired by it to reproduce the moss forests where the plot of his masterpiece takes place. The volcano island of Yakushima is a marvel of biodiversity: while turtles come to lay eggs on its warm climate beaches, it can snow at the same time at its summit. It is at the heart of this dreamlike landscape that an astonishing cultural behavior was observed between two species which have, at first glance, nothing in common: macaques and sika deer practice… rodeo. Here, the monkeys enjoy climbing on the backs of deer and staying there for a few seconds, or even a few minutes for the most tolerant ungulates. A tradition well known by researchers and residents of the island, which nevertheless remains difficult to observe.
As surprising as it may seem, this behavior continues to intrigue scientists. In January 2017, a video of a young Japanese macaque having sex with a doe on this Japanese island went around the world. These images documented, for the first time, interspecies sexual practice in a primate, except humans. An exceptional event, we thought, although we now know that it constitutes a relatively common practice. Nearly eight years later, two new studies published in scientific journals Primates And Cultural Science published at the end of December 2024 offer avenues for reflection around this behavior, the sexual aspect of which is in reality only one component among others in the relationship between macaques and deer. Why do macaques do this, and why do deer accept it? What connection should we make with Man? A few days later, on January 7, another scientific analysis published again in Primates attempts to clarify another mystery: what do these marks left by the simple handling of stones and chalk on the ground by these macaques mean? The Frenchman Cédric Sueur, ethologist at the CNRS, specialist in collective behavior among primates, and at the origin of these publications, going so far as to wonder if humans had not started drawing by accident. “This offers a fascinating window into the involuntary emergence of drawing in our evolutionary history,” he says. Interview.
L’Express: Nearly eight years passed between the first observation of a sexual act between a Japanese macaque and a sika deer, which then went around the world. Two recently published studies show, in particular, that this behavior is only one component of the links that unite monkeys and ungulates on the island of Yakushima. What are your conclusions?
Cédric Sueur: In fact, macaques have been known to rodeo on deer in Yakushima for quite some time. The problem is that there has never been a long-term scientific study that would attempt to understand the benefits of exhibiting this behavior for macaques and deer. And that’s what we wanted to do. So I asked Japanese researchers to send me their videos. We were able to analyze more than twenty of them which show “rodeos” which extend over several years. In reality, you would think that this is an easily observable behavior, but that is not the case at all. I recently went to Yakushima for five days, and I was only able to observe it for two short seconds. So it’s something quite rare.
What is interesting about our findings is that sexual behavior is only one aspect of the interactions between these two species. It was observed that young males on the fringes of the group engaged in this practice on does because they did not have access to females, reserved for the dominant male, during the breeding season. Regarding females, these are again young individuals who are still adolescents and rejected by males who satisfy their desires in this way. It is therefore a lack of access to reproduction that creates this behavior. But they know very well that it is a deer, and not a macaque! Moreover, this sexual frustration manifests itself in different ways, since we also observe females rubbing against each other but also face to face, like bonobos. Males can also engage in masturbatory practices.
But isn’t sex the only explanation for this “rodeo” type behavior?
No, actually, there is not sexual intercourse every time. It has also been observed that the deer tries to attract the monkey on its back in order to play with it; but also a thermoregulatory behavior when the macaque lies completely on the back of the deer so that they warm each other up. There is also grooming which allows one to feed on fleas, and the other to get rid of parasites. Furthermore, we realized that, while some deer accept without problem having the macaque on their back, others are very aggressive. It’s unclear why they refuse to be mounted, if it’s just a matter of personality or affinity. Finally, we wanted to test the hypothesis of movement: do monkeys use ungulates to travel long distances, like we do with a horse? But we have not observed this on the ground.
You can only watch monkeys rodeoing deer on Yakushima Island?
Not necessarily. It’s true that it’s very well known in this region, they even advertise it in stores. But near Osaka, Canadian researchers also observed monkeys exhibiting this behavior.
Is this an ancient behavior, or have macaques only recently decided to mount deer?
That’s an interesting point. In China as in Japan, we will find many figurines which show macaques riding on horses. Macaques represent the messengers of the gods, and they form the link between horses and humans. For example, I have a figurine at home that is a little over 100 years old where you can see this. On Japanese parchments dating from the 12th and 13th centuries, we also have a series of somewhat anthropomorphized animals, including a macaque sitting on a deer. Does this mean this is old behavior? We don’t know. It may also be the fruit of the imagination of the designers and sculptors of the time.
Macaques on Yakushima Island, Japan.
© / Biosphoto via AFP
It is often said that “man descended from apes”, but it would be more correct to say that we too are primates. What link do you make between this behavior and our species?
Why did deer and macaques come together on this Japanese island specifically? In reality, sika deer no longer have a natural predator since the disappearance of the Japanese wolf at the very beginning of the 20th century. They therefore reproduced, and are now overpopulated. Consequence: they lack food because no plants grow on the ground, to the point that some are forced to eat their excrement. Getting closer to the monkeys therefore allows them to benefit from the leaves or fruits that fall from the trees where the macaques live. The monkeys take advantage of this to feed on their lice or to keep warm. It’s a win-win in a way. All this makes us think of the rapprochement between Man and horse, or between Man and wolf, which will ultimately lead to a process of domestication. Both species found it of interest.
But that’s not all. In Saudi Arabia, for example, groups of baboons regularly steal puppies. They accustom them to living in their group, feed them, and the adult dogs protect them from other canids who would come and attack the baby baboons. There is also almost a process of domestication which would be at work, without the aspect of genetic modification of course as was the case between man and the wolf.
It has long been believed that culture is reserved for humans. We now know that most animals exhibit so-called “cultural” behaviors. Can we talk about culture, in your opinion, when we observe Japanese macaques?
What is a culture? It is a behavior present in one or more populations, absent in one or more other populations, and which cannot be explained by an ecological difference. We are therefore perfectly in this situation. It is known to be present in Yakushima and near Osaka, but is not observed in other places in Japan where there are deer and monkeys living together. The question now is to know how this behavior originated, and to observe whether it spreads or not.
It is now necessary to incorporate cultural behaviors into conservation programs for animal species. Today, we only talk about the number of individuals and genetic heritage when we want to save a species, but saving this cultural heritage in animals is just as essential. We realized that a program to save a species can fail if we do not take this aspect into account.
In another study that you published on January 7you study the practice of drawing among Japanese macaques. With this question: what if humans had started drawing by accident?
The use of complex technologies by humans (Homo sapiens) and their ancestors is a key feature of our evolution, marked by the appearance of stone tools 3.3 million years ago. These technologies reflect cognitive complexity and advanced understanding of materials and mechanisms. The study of current primates, particularly those that use stones, provides insight into the evolution of human behavior.
In particular, macaques’ manipulation of stones suggests that some complex behaviors in humans, such as the creation of sharp and bifacial tools, may have arisen unintentionally. Japanese macaques from Shodoshima have been observed leaving marks on the ground using stones and chalk. By analyzing this manipulation of stones, I propose that drawing in humans may have arisen involuntarily; these unintentional origins of marking behaviors may represent early precursors to human drawing.
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