BioEd Online

Capuchins in the dry forests of northeastern Brazil have an unusual approach to food: they have been caught using tools to dig up tubers, a feat previously only seen in humans.

"They're using their minds, not just brute force," claims Phyllis Lee of the University of Cambridge, UK, who reports the discovery with her colleague Antonio Moura in this week's Science¹.

Although many primates, particularly chimpanzees and orang utans, are thought to be good at reasoning things out for themselves, digging for food has never been seen before, in the wild or in captivity.

Several species are known to use 'tools', such as the birds of prey that dash their hard-shelled prey on to rocks to crack them open. But the latest case of tool use differs from many of these examples because it may be based on an understanding of cause and effect.

Moura travelled to the dry caatinga forest in the Brazilian state of Piauí, where he spotted the capuchins (Cebus apella libidinosus) using stones to bash the ground, then scraping away the debris to reach tasty roots and tubers. He captured the behaviour on video ( click for video).

The monkeys did not just use stones for digging. They came in handy for cracking open seeds, branches or cacti, breaking tubers into bite-sized pieces, and even pulverizing hapless lizards. The capuchins also used twigs to probe nooks and crannies for insects.

Paws for effect

Much of this behaviour is similar to that of apes such as chimpanzees and orang utans, who dig the ground with their hands, says Lee. And although capuchins are known to be inventive manipulators of twigs and sticks in captivity, they don't generally manage to do anything useful with them.

So the digging is a delightful surprise, says Lee. "Digging for food with tools was thought to be a very 'human' behaviour," she says. "This suggests that they understand cause and effect."

The fact that digging has not been seen in the species before may be down to the harsh conditions endured by the monkeys in caatinga forests, relative to the lifestyles enjoyed by other capuchins. "There are times of the year when there's nothing out there apart from the odd bug," says Lee.

This lack of easily available food may have forced this particular band of monkeys to be more inventive, she suspects. "For them, life is so hard that they have to do it. Others may be more laissez-faire."

Accidental tools

This is not the only possible explanation, however, says Carel van Schaik, who studies primate tool use at the University of Zurich, Switzerland. Monkeys elsewhere may not use stones simply because they never venture to the ground for fear of predators, or because the forest is too dense.

And he does not necessarily agree that the Brazilian monkeys know exactly what they are doing. Capuchins often crack open large fruits by banging them on branches, and perhaps hitting the ground with stones is simply a variant on this foraging style that happens to pay off, he suggests.

Capuchins' sociable nature may help such strategies to become a part of everyday life, van Schaik says, as monkeys living side by side are likely to adopt each other's skills.