UK researchers have collected the first hard evidence of monkeys using tools, Science magazine reports.
Hard times may make tool use more important
Cambridge researchers observed wild capuchin monkeys in the Brazilian forest using stones to help them forage for food on an almost daily basis.
Scientists have already known for some time that capuchins use tools in captivity, but have only occasionally observed them doing so in the wild.
But the latest findings confirm that the tool use was habitual, or routine.
The monkeys used tools for digging, for cracking seeds and hollow branches, digging for tubers (nutritious plant storage organs such as potatoes that often lie below the ground) and for probing tree holes or rock crevices.
They captured the monkeys on video in the Caatinga dry forests of northeastern Brazil.
Digging was the most frequent type of tool use observed. The monkeys typically held the stone with one hand and used it to hit the ground quickly three to six times, while simultaneously scooping away the soil with the other hand.
In Science, Antonio Moura and Phyllis Lee write that the results suggest wild capuchin monkeys are far more skilled at understanding cause and effect than previously thought.
"We think these findings are extremely important for understanding the role of tools in cognitive evolution," said Mr Moura.
"I'm looking forward to returning to the field to try to specify more about the nature of the tools."
But the Cambridge anthropologists also propose that the monkeys may only use tools under certain ecological conditions, such as the long dry seasons of the Caatinga.
When food availability is stretched, using tools may be crucial, allowing the monkeys to obtain nutritional foods such as tubers that are otherwise inaccessible.
Antonio Moura observed the monkeys using tools a total of 154 times from October 2000 to March 2002.
The routine use of tools is well known in great apes such as chimpanzees. But making the step between these crude tools and those made by early humans required a giant cognitive leap.
Chimps can be shown how to strike flakes of stone from a "core" to use as cutting tools, as our early ancestors did.
But they seem unable to understand that making useful cutting flakes depends on striking the core at the right angle and with the right force - a skill that seemed to come naturally to even the earliest human tool-makers.