The capuchins are the first animals of their genus observed using stone tools and only the fourth group of nonhuman primates known to do so.

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For years, scientists at the Smithsonian Tropical Research Institute (STRI) in Panama had whispered about the remote island where monkeys used stone tools. A botanist had witnessed the phenomenon during a long-ago survey, but, being more interested in flora than fauna at the time, she couldn’t linger to investigate. A return to the site would require new funds, good weather for a treacherous 35-mile boat ride, and days of swimming, hiking and camping amid rocky, wave-pounded shorelines and dense tropical forest.

“For a while, it kind of just stayed a rumor,” said Brendan Barrett, a behavioral ecologist at the Max Planck Institute for Ornithology in Germany and a visiting researcher at the STRI.

But when Barrett and his colleagues finally arrived at Jicaron Island in Panama’s Coiba National Park last year, what they found was well worth the effort: White-faced capuchin monkeys were using stones almost half their body weight as hammers to smash open shellfish, nuts and other foods.

“We were stunned,” said Barrett, lead author of a new paper on the discovery posted on the preprint website bioRxiv.

The capuchins are the first animals of their genus observed using stone tools and only the fourth group of nonhuman primates known to do so. Sophisticated, social and tolerant of observation, they also provide scientists with an ideal system for studying what causes a species to venture into the Stone Age — and could help researchers understand how and why our own ancestors first picked up stone tools more than 2 million years ago.

Smashing, cracking

White-faced capuchins, or Cebus capucinus, are found all over the rain forests of Central America. Roughly the size of house cats, with nimble black bodies, long dexterous tails and expressive eyes, they live in matrilineal troops of about 20 animals and practice a variety of complex behaviors. They rub plants over their bodies, potentially as medicine; defend themselves with sticks against snakes; play games by passing one another sticks and stones; and engage in weird “bond-testing” rituals that involve sticking their fingers in one another’s noses and eyes — perhaps the monkey equivalent of a trust fall.

A year of observing the inhabitants of Jicaron Island with dozens of motion-detecting cameras revealed that at least some of these animals are also talented at wielding stone tools. The monkeys were captured collecting large, heavy cobbles from streams and shorelines and carrying them to broad, flat rocks or logs that could be used as “anvils.” Standing on two feet, using their tails to anchor themselves against the ground or a nearby tree, they raised their “hammers” high above their heads and then smashed them down on nuts, crabs, snails and other foods, cracking open hard shells to reveal a tasty morsel.

The monkeys used their stone tools almost every day and often saved stones for repeated use. In one instance, a capuchin arrived at a stone-tool site on the coast carrying an armful of almendro nuts, or “sea almonds.” He dropped them on the ground, and then began bashing them open one by one.

Curiously, only male monkeys were seen wielding stone tools, even though females were often foraging nearby. This can’t be explained by females’ smaller size, given that juvenile males were able to use tools.

Even stranger, tool use was observed only among one group of capuchins occupying a roughly one-mile stretch of shoreline, even though Barrett and his colleagues surveyed the entirety of Jicaron and two nearby islands where the monkeys are also found. In a few cases, the scientists left “experimental” hammers and anvils in spots where no stone tools had been found; the monkeys ignored the artifacts, though a few Homo sapiens were observed using them to crack open coconuts.

Which makes Barrett wonder: Why hasn’t this seemingly significant behavior spread across the island? What’s so special about this single group of tool users?

“That’s what makes this really interesting,” he said. “We’re in a position to actually make these comparisons looking at why this would evolve.”

The paper hasn’t yet been published in a peer-reviewed journal, meaning it hasn’t been officially subjected to the scrutiny of other scientists. But Joan Silk, a primate behavioral ecologist at Arizona State University, said the study was a “careful and nice descriptive analysis of this new observation.”

“It’s just more good natural history to add to what we know about the way animals use tools in the wild,” she said. “And knowing more about tool use in other animals is super interesting because it helps us see how human tool use is different.”

A bigger “tool kit”

Humans and our hominin cousins have been using stones as hammers and anvils for least 3 million years. Until recently, scientists thought we were the only creatures who did so.

But in the past few decades, chimpanzees have been observed using an entire “tool kit,” not just stone hammers but also shovel-like branches for drilling holes into termite nests and straw “fishing probes” to extract the tasty insects. On islands off Thailand, long-tailed macaque monkeys have become such capable tool users that they destabilized the local shellfish population.

And in South America, scientists have found hammer-and-anvil stones used by robust capuchin monkeys that date to at least 700 years ago. In fact, tool use was previously one of the traits used to distinguished that genus, Sapajus, from the smaller capuchins of the Cebus genus, including those in Panama.

That always seemed strange, Barrett said, because the Cebus monkeys of Coiba National Park — an archipelago of more than 100 islands on Panama’s Pacific coast — seemed like great candidates for new tool users.

Given how smart they are, the discovery that Cebus monkeys use stones as hammers is exciting but not particularly surprising, said Erin Marie Williams-Hatala, a paleoanthropologist at Chatham University in Pittsburgh.

But stone-tool use is considered a “benchmark” by scientists because “stone preserves,” she said. “Most of what we know about early humans and their behavior is through the stone record, so there is a tremendous bias toward it.”

She’s just as interested in the array of animal behaviors that can’t be seen in the fossil record — and, indeed, might never have occurred to ancient humans. Orangutans build elaborate nests using engineering know-how that would impress any architect. Some female chimpanzees sharpen sticks with their teeth and use them to impale bush babies for food. Still more chimps use moss or chewed leaves as sponges to soak up hard-to-reach water.

“I would die of dehydration before I thought of that,” she said, laughing.