Wild Monkeys Unintentionally Make Stone Age Tools, But Don’t See the Point

Scientists observe a “unique” human behavior in wild animals

Wild capuchins make stone tools, but don't know how to use them. Luciano Marra / Flickr Creative Commons

Smack. Smack. Smack.

The wild-bearded capuchin monkeys of Brazil's Serra da Capivara National Park like to smash rocks. Frequently, these primates will grab a rounded “hammer stone,” or quartzite cobble, and bash it repeatedly against similar stones in a cliff face, shattering it the process. Researchers have puzzled over this strange behavior for decades; they suspect it may have to do with getting at minerals inside the stones or lichen on the surface.

But until recently, nobody—including the monkeys—seems to have paid much attention to the perplexing stone fragments that this behavior leaves in its wake. It turns out those flakes are strikingly similar to those our human ancestors created during the Stone Age, for use as basic cutting and scraping tools. While captive bonobos have been known to produce Stone Age tools as well, creating them in the wild is a behavior previously thought to be unique to humans and their ancestors.

Now it seems that isn't the case. “In no way are they trying to produce the flakes,” explains Tomos Proffitt, a primate archaeologist at the University of Oxford and co-author of a new study published in the journal Nature. "They never use the flakes, they don't care about them." And yet they seem to be producing these primitive tools in large numbers: For their study, Proffitt and colleagues collected and examined more than 100 fragmented stones, including whole and broken hammer-stones and flakes, from the rocky cliffs of Serra de Capivara.

“Completely unintentionally, while they are doing this, the stones are fracturing in the same way as you'd expect an intentionally fractured, hominin-made flake to be,” he says.

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So far only the Serra da Capivara group is known to exhibit this behavior. But depending on how widespread the behavior turns out to be, might it be possible that some of the ancient flakes thought to provide proof of hominin sites are actually the work of monkeys? Nope, says Proffit. Though monkeys have shown themselves capable of producing these artifacts, “the level of complexity we see throughout the known archaeological record is far higher than what we see made here by the capuchins,” he says.

Dennis Stanford, an archaeologist at the Smithsonian’s National Museum of Natural History agrees—though he notes that a few people have been fooled by “stone tools” found out of context. He recounts an instance in which a scientist working in South America found numerous broken rocks that initially appeared to be a notable assembly of old, crude tools. “After some study and examination, it turns out they were all found on llama trails,” he recalls. “The sharp hooves of the lamas crunched up the obsidian on the trails, and made these broken rocks look like tools.”

Still, Stanford explains, the chance that monkey-produced tools will bring into question any key sites of the hominin record is extremely slim. Nor should the existence of monkey-made flakes confuse future archaelogists: “If you're a good archaeologist and you're excavating a site, you'll be able to figure that out,” he says. “There are going to be some fire hearths there, there are going to be some other things on a site that monkeys obviously aren't making.”

But the monkey behavior does raise very fascinating questions about when, how and why hominins might have begun making those types of stone tools in the first place. “The monkeys just happen to choose stones that fracture in a certain way and produce a material that we've always attributed to being intentionally produced by hominins,” Proffitt says. Could humans, too, have originally stumbled into tool-making by accident?

“It raises interesting questions about what may be the minimum levels of cognitive complexity required by hominins to make a sharp cutting edge flake, and how advanced the hand morphology has to be to manipulate stones to produce these things,” he says. “The fact that a monkey can do it is just kind of remarkable really.” In other words: You may not have to be as smart as a human to figure out how to break a rock.

Of course, many animals have been known to use tools. Hawaiian crows (like New Caledonian crows) have recently been shown to probe holes in trees with sticks to extract grubs; sea otters use stones to hammer open the shells of their prey, the abalone. Chimps and these same capuchin monkeys use stones to crack nuts. But actually flaking the stone to create a sharper, better instrument is thought to require greater cognitive resources. 

For years, archaeologists have thought of the ability to produce flakes from the mechanics of conchoidal fracture—breaking stone in layers that produce smooth, curved surfaces like those inside a seashell—to represent a notable advance in hominin development. The meaning of that accomplishment may now may be a bit hazier. Simultaneously, the route by which humans evolved such tools might become clearer. Perhaps fragments produced during this kind of “hammer and anvil” process approximate the way our ancestors first came to use stone flakes. 

“People have hypothesized that this might have been a way that hominins started to understand that if they hit stones together they might produce sharp edges,” Proffitt notes. “Or at least reach a point where there were many sharp edges happening to be lying around, and one particularly clever hominin picks one up and does something with it.”

But enough about us. For scientists who study primates, the question is: why were monkeys smashing rocks in the first place?

It may be a nutritional need, where the monkeys try to get a mineral like powdered silicon from the quartz dust they lick off broken rock. Or perhaps that rough stone dust might feel nice on the tongue, smell pleasant or even help to scour parasites from the animals' intestines, Proffitt speculates. The monkeys could also be trying to break apart and get at the lichen growing on many of these cobbles.

How long the monkeys have been at this business is anyone's guess, but that question should also be explored, adds Hélène Roche of Paris-Nanterre University, in an accompanying News & Views piece in Nature. “Investigating the antiquity of the stone-smashing behavior or trying to determine the behavior’s function and possible role in capuchin evolution are some of the many promising fields of research rippling out from the shattering discovery,” she writes.  

In the meantime, remember: You may not be as unique as you think you are.

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