Politics

Did Humans Learn to Eat Cashew Nuts from Capuchin Monkeys?

The long history of capuchins’ usage of stone hammers poses more questions about how tool use originated and spread in the New World.

A wild bearded capuchin using a stone tool to crack open cashew nuts in Serra da Capivara National Park in northeast Brazil. Credit: Michael Haslam/Primate Archaeology Project

A wild bearded capuchin using a stone tool to crack open cashew nuts in Serra da Capivara National Park in northeast Brazil. Credit: Michael Haslam/Primate Archaeology Project

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Evidence of the human use of tools stretches back three million years. For a long time, scientists believed the use of implements separated animals and humans. In the mid-19th century, naturalists reported that animals used objects as well. But it wasn’t until 1960, when Jane Goodall observed chimpanzees fishing ants with grass stems, that our view of the animal kingdom changed dramatically. Since then we know of a whole range of animals from crows to dolphins that use tools to get their way.

The bearded capuchin monkeys of Brazil hammer hard nuts with rocks to crack them open. They don’t just smash the nuts with all their strength. Instead, they take care to place the round nuts so they don’t roll away when struck and use large flat rocks as anvils. They hammer these nuts with moderate force, and by gauging the condition of the nut, they struck the next blow. Archaeologists recently discovered that the primates have been using this technique for hundreds of years. This is the first evidence of historic tool use by a non-human primate in the New World.

Cashew trees grow wild in Brazil. When they fruit, wild capuchins exploit the seasonal bonanza. The nuts are covered by a fibrous shell that exudes a caustic resin. When this substance touches the lips, it causes blisters. Humans char the shells before eating the kernels. Since capuchins haven’t figured out the use of fire, they developed different ways of dealing with the corrosive shells. At Fazenda Boa Vista, capuchins abrade a hole in fresh nuts, when the shells are spongy, by rubbing them on a rough tree branches. They smash dry nuts with stones.

However, the capuchins at Serra da Capivara National Park in northern Brazil use rocks to crack open the shells of fresh and dry nuts. Since the area doesn’t have stones of the size they need, the monkeys bring them from a dry streambed nearby. They choose specific types of stones for anvils and hammers and lug them back to the base of trees. After feasting on the nuts, they abandon their tools at the site and over time, piles of such rocks collected at these spots. When they need the tools again, the monkeys pick up their favourite stones from these piles.

How long have bearded capuchins used tools? Researchers from the UK and Brazil, led by primate archaeologist Michael Haslam, recently published the results of their investigation.

“We know so much about the technological evolution of humans but so little is known about the evolutionary history of other primates,” Lydia Luncz of the University of Oxford, U.K. and an author of the paper, told The Wire. “To find answers to the question – why are us humans so technologically advanced – we need to compare the evolution of our own behaviour with the evolution of other primates. We use stone tools as a window into past behaviour of wild primates.”

In May 2013, the researchers mapped four locations within Serra da Capivara National Park where capuchins had built up piles of discarded stone tools. The stones were larger than others, marked by dark residues, and worn from use. The monkeys don’t have a size preference alone. They choose particular kinds of rocks: smooth, hard quartzite for hammers and flat sandstones for anvils.

By weighing these stones, the researchers estimated their average weights. Anvils weighed about 600 grams on average while hammers were 200 grams. The average weight of stones naturally strewn in the area was no more than 60 grams.

Five months later, the researchers excavated one of the four sites by hand, using trowels and shovels, to find ancient tools. They uncovered 69 stone tools within a 35-square metre hole that was 70 centimetres deep. Using gas chromatography mass-spectrometry, they confirmed the dark residues were derivatives of cashew resin such as cardanol 3 and cardanol 4.

Charcoal found with the stones was ideal for carbon-dating. There was no human activity at the site and the charcoal was a result of natural wildfires. The rock tools were of three distinct ages. Modern ones littered the surface. Below were stones dating as far back as the 17th century. Rocks from the 13th to 15th centuries lay buried at the bottom. The oldest rocks revealed a history of 600 to 700 years of tool use by capuchins. The monkeys were pounding cashew nuts with stones long before the European colonisation of South America. These are the oldest tools wielded by animals outside Africa.

Were the cashew trees so old that a hundred generations of capuchins returned to the same spot?

“Individual cashew trees don’t last that long,” replied another author, Tiago Falótico of University of São Paulo, Brasil. “We know from pollen evidence that cashew trees were present in that valley for at least 7,000 years. We assume that places with clusters of cashews in the present would be likely to have had cashews in the past too.”

Remarkably, a hundred generations of capuchins didn’t change the basic parameters of their tools. The oldest tools were of similar weight and material as modern tools. Just like chimpanzees, these capuchins are also conservative, reluctant to make any changes.

The team also dug at Fazenda Boa Vista, where the capuchins’ preferred method was rubbing cashew nuts on rough surfaces, but found little. “We believe that it has to do with the availability of suitable stone hammers,” says Luncz. “The capuchins there crack open hard oil palm nuts which require large tools to open them. Occasionally, they also crack open dry cashew nuts. Those tools are very rare in the landscape. Therefore, they are very valuable, get re-used intensively and never enter the archaeological record. We tried to excavate oil palm tools because this kind of behaviour occurs so much more often. However, we underestimated the re-use frequency of these tools.”

Another primate with a long tradition of using tools is the chimpanzee. Archaeologists excavated stones from sites in Côte d’Ivoire that had been used 4,300 years ago. Before hunter-gatherer humans settled in hamlets and began farming, chimpanzees started using these rocks to crack open nuts. After this discovery, scientists wondered if use of objects was invented by an ancestor of humans alone or a common ancestor of humans and chimpanzees. Or did ancient chimpanzees watch humans and imitate them? Perhaps the use of technology evolved independently in primates and humans.

Only humans and bearded capuchins have figured out how to process cashew nuts. In Africa, chimpanzees entering cultivated cashew plantations eat the cashew apple but discard the nuts.

In Asia, long-tailed macaques of Thailand use stones to crack open oysters, marine snails, crabs, and nuts. In another recently published paper, Haslam and his team say the macaques choose their tools carefully too, sometimes using the same rock to open several oysters. However, these tools weren’t antiques; they were less than 50 years old.

The long history of capuchins’ usage of stone hammers poses more questions about how tool use originated and spread in the New World.

“This is an exciting, unexplored area of scientific study that may even tell us about the possible influence of monkeys’ tool use on human behaviour,” said Michael Haslam of the University of Oxford, and the main author of the paper, in a press release. “For example, cashew nuts are native to this area of Brazil, and it is possible that the first humans to arrive here learned about this unknown food through watching the monkeys and their primate cashew-processing industry.”

People living in the national park area ate cashews at least 7,000 years ago. To prove whether humans learned to open cashew nuts from watching monkeys, the team would have to find tools used by capuchins of even older provenance. As primate archaeology evolves as a field, it may reveal more surprises.

The study was published in the journal Current Biology on July 11, 2016.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.