Wild South American monkeys routinely use fist-sized rocks to crack open seeds and to dig in dry Brazilian soil for grubs and edible tubers, researchers report in the journal Science.
The use of primitive tools by apes and even some birds is not unusual, but Antonio C. de A. Moura of Darwin College (search) and Phyllis C. Lee of the University of Cambridge (search), both in Cambridge, England, say in Science that they have now observed capuchin monkeys using rocks to gather food during the dry season in Brazil.
Captive capuchins have been easily taught to use simple tools, but scientists speculated that the wild animal doesn't use tools because food is so abundant there is no need for them, the researchers say.
However, during the dry season in the Serra da Capivara National Park in northeastern Brazil, food for monkeys becomes scarce. As a result, the native monkeys resort to using rocks as food-gathering tools, the authors said.
Lee and de A. Moura report watching the monkeys select a fist-sized hammer rock and then dig in the soil to free tubers and grubs.
"Monkeys typically held the stone with one hand and hit the ground quickly three to six times while simultaneously scooping away the soil with the other hand," the authors wrote.
The monkeys were also seen using stones to crack open seeds, to open hollow branches in a search for insects and to break tubers into bite-sized pieces, the researchers said.
A variety of rocks were used and at least two different groups of monkeys miles apart were seen using stones as food-gathering tools.
Adult capuchin monkeys (search) are small, about 18 inches from head to foot, with a tail of about equal length. They are tree dwellers in the wild, but are popular as domestic pets and performing animals. They once were a favorite of organ grinders. Their hands have opposable thumbs and resemble human hands.
The animal got its name because the hair on its head resembles a skull cap. "Capuce" is French for "skull cap."