The sound can be heard from a distance. Bang, bang, bang! Clack, clack, clack!
Hairy little arms raise a stone, then bring it down with force on a cashew still in its sack of tough skin and toxic resin. The sound echoes as the stone smashes the nut on a bigger stone used as an anvil. A hungry capuchin monkey wants the meal contained inside.
A new study by scientists at the University of Oxford determined that monkeys have been opening cashews this way in Brazil for seven centuries - long before Christopher Columbus arrived in the Americas to claim it as the New World.
"We think we're just at the beginning," said Lydia V. Luncz, one of the study's authors, indicating that the actual date for this capuchin activity could go much further back. The 1300s just happened to be the radio-carbon-dated time in the layers of soil in which stone tools were found. "We definitely expect this to go beyond 700 years," she said.
A finding that primates used tools is nothing to get excited about. The paper's lead author, Michael Haslam, has previously written about the archaeological evidence of wild macaques in Thailand splitting nuts and shellfish with stone tools. What's new is the finding that this happened outside Africa and Asia.
The Oxford paper, published Monday in the journal Current Biology, suggests somewhat controversially that the finding "prompts us to look at whether early human behavior was influenced by their observations of monkeys using stones as tools," according to the release announcing the study. Specifically, the authors speculate that new arrivals to the New World wielding ornate tools might have learned from monkeys that the naturally disguised cashews could be had for eating.
To comprehend the scientists' meaning, you have to envision the cashew in its natural state. It looks very different from the banana-shaped nut people buy in stores, because it starts life sitting atop a bright-red wild apple. It is encased in a shell so tough that bare hands can't open it and is covered in a fluid toxic enough to make people sick. In other words, at first glance it doesn't look much like food.
How European explorers - or even Brazil's inhabitants there thousands of years earlier - discovered the snack is unknown. But they also discovered fire, and for hundreds of years they simply roasted cashews in their shells and broke them open for a meal.
Capuchins, on the other hand, have fire in their eyes as they hammer away. They were observed for several years doing their thing in Serra da Capivara National Park in the Brazilian state of Piaui. "They are very deliberate with this," Luncz said.
In the way that shoppers thump melons to discern their ripeness, the primates -- often called organ-grinder monkeys -- sometimes tap the nut to find a good one. Then bang, they bring the hammer down. They place the cashew on an anvil surface about four times larger than the stone they hold in their little hands. They arrange it just so, then they arrange their feet just so. Then clack, clack, clack.
"They don't want to hit their feet or their fingers" the way chimpanzees and macaques are known to do in other places, leaping with pain, Luncz said. "They strike down on it. They very carefully peel the shell away and expose the kernel. And they love it. They really get into it. It's rich in protein, and it's a great food source for them, definitely worth the effort."
What the study proves is that capuchins entered the stone age centuries ago and handed that knowledge down from generation to generation, rarely innovating in all that time. Their behavior follows that of West African chimps, which did the same with their technology, according to a groundbreaking 2007 study in the relatively new field of primate archaeology.
"It's just a handful of primates 1/8species3/8 living today out of 350 that show this kind of behavior," Luncz said, adding that human use of tools is thoroughly researched and "we need to look back at other species that used tools."
Mysteries remain, including the question of how the big stone anvils came to rest under the cashew trees. "I don't know the answer to that," Luncz said. "At some point 1/8the monkeys3/8 must've carried them there. Maybe they dragged them. I have not observed one of those sites established from scratch. But it must be monkey made because only monkeys live in that area. Humans don't use rocks."