Capuchin monkeys use stone tools to crack open cashew nuts — and expand the field of non-human archaeology

The semi-arid region of northeast Brazil offers little for a monkey to eat, except for a variety of hard-shelled fruits and seeds. So when capuchins arrived in this region about half a million years ago, they needed to find a way to access the only food available to them.

One ancient bearded capuchin looked at a cashew fruit and figured out how to crack it open with a rock. In doing so, he or she may have made monkey history by opening up a brand-new habitat for the species.

“It may be that part of the reason that capuchins were able to colonize this area is that they found a technological solution — stone tool use — to overcome these plant defenses,” said University of Oxford primate archaeologist Michael Haslam.

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Haslam, who heads Oxford’s Primate Archaeology project, led a new study that suggests the monkeys have used tools in Brazil for at least 700 years, and possibly for much longer.

Haslam and his colleagues from Oxford and the University of São Paulo discovered dozens of stone hammers and anvils in northeastern Brazil’s Serra da Capivara National Park. The find, described Monday in Current Biology, represents the oldest non-human tools to be found outside of Africa, and the oldest known tools not belonging to humans or chimpanzees.

“If tool use was indeed part of that colonizing process, then we can hypothesize that this behavior may be hundreds of thousands of years old,” Haslam said. “Of course, the only way to know for sure is to keep digging.”

Most capuchins live in forests, such as the Amazon, where they don’t regularly encounter the same variety of hard-shelled foods, Haslam said.

Cashews contain a corrosive oil to defend against would-be predators like monkeys or humans. Brazilian capuchins have learned different ways to avoid the yucky stuff and get to the seed in the middle. Members of one population, found in Fazenda Boa Vista, rub the fruits on a rough surface, making a hole to reach the kernel. At Serra da Capivara, the monkeys use stone tools to prepare their meal.

Capuchins choose tools based on size and material. Hammers should still be small enough to swing in their tiny monkey palms and are usually made of a hard quartzite. The heavier sandstone anvils serve as a flat surface to strike the cashew.

Once they find a suitable cashew-cracker, they don’t want to lose it. So the monkeys stash little tool caches underneath the fruit trees, where they can find them again, Haslam said.

“It becomes like a set of cutlery at a restaurant — [the tools] are just sitting there for you,” Haslam said in a video presenting the research.

To find the ancient monkey tools, the researchers looked near modern “processing sites” and near groves of cashew trees, as the growing conditions for the plants would not be too different now than in the recent past.

Researchers knew the assorted stones were tools because they had dark cashew residue on their surfaces (which was later carbon dated) and by tell-tale signs of damage. Just as modern capuchins do, the ancient monkeys also kept their tools organized in neat little piles.

The 700-year-old artifacts indicate a 100-generation tradition of tool use in capuchins. In human years, that would be the equivalent of roughly 2,500 years.

And in those 100 generations, capuchin technology — unlike that of humans — hasn’t changed much.

This suggests the monkeys have developed a close-to-optimal system, Hallam said. That is, it works well enough that they have no need for an upgrade.

The age and consistency of the tools “also tells us that they are very good at transmitting the behavior over and over again through the generations,” Haslam said.

The first humans arrived in modern-day Brazil about 20,000 years ago. It’s possible that when prehistoric people began eating cashews about 7,000 years ago, they learned how to harvest them by watching the capuchins. However, humans generally would prepare the cashews by roasting them, not whacking them, the study authors noted.

“Humans get a lot of clues about their environment from the behavior of other animals,” Haslam said. “It may be that the highly visible and noisy cashew processing by capuchins could have acted as a guide to those people.”

Because of the patchiness of the capuchin fossil record, this idea is more speculative than anything else, Haslam said, at least until scientists can fully explore the monkeys’ archaeological record.

“We’re standing at the starting line of non-human archaeology, a field that now includes sites of chimpanzees, macaques and capuchins,” he said. “Who knows what we’ll discover as the work continues?”

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