As for how the monkeys first learned how to process cashews, the researchers suggest that shelled edibles once helped save the capuchins from starvation.
Co-author Eduardo Ottoni, a professor in the Institute of Psychology at the University of São Paulo, explained that "robust" capuchins split off from slenderer built ones as they left the Amazon forest and reached the Atlantic forest.
"That involved crossing much drier areas where hard-shelled fruit was probably a key resource, hence their robust jaws," Ottoni said. "In such a context, stone-aided nut cracking was arguably very useful."
Capuchins do not just use tools to process cashews. Ottoni said that some select even heavier stones to bust open hard palm nuts in certain regions. The Serra da Capivara monkeys use different stones to dig plant roots and tubers.
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They also use sticks as probes to dislodge prey -- mostly lizards -- from rock cracks. They use sticks to pull spiders out of their burrows and to collect honey. Each tool is carefully selected for size, weight and usefulness to the particular task.
Other monkeys, such as macaques, are also adept at using tools. Last year, for example, Amanda Tan from Nanyang Technological University and her team documented how macaques use one-handed hammering with the points of small tools to crack open oysters with precision at Piak Nam Yai and Thao Islands in Laem Son National Park, Thailand.