Chimp Nut-Cracking Site Offers Clues to Early Tool Use

By Bijal P. Trivedi
National Geographic Today
May 23, 2002
In the dense Taï rain forest in West Africa's Ivory Coast, archaeologists are exploring a site where chimpanzees have used stone tools to crack open nuts for more than a century. Although this archaeological site is only 100 years old, it may serve as a model to help scientists identify ancient chimp sites, forcing them to reconsider when exactly tool use began.

"Our work shows that animals, other than humans, can create archaeological sites and that these can be used to trace ape behavior back in time," said Julio Mercader, a specialist in rain forest archaeology at the George Washington University in Washington, DC. Mercader led the archaeological expedition and is the author of a report which appears in the May 24 issue of the journal Science.

The most primitive human stone tool sites are in Olduvai Gorge, East Africa, and date back 2.6 million years to when people were deliberately modifying their stone tools by flaking the rock to create a razor-like edge. By contrast, chimps don't change the shape of the stones they use as hammers.

"But before 2.6 million years ago we don't know what sort of stone technology or stone use there was," said Mercader.

The stone hammers used by chimps today may be similar to tools used by early humans before they began to intentionally chisel stone tools—sharpening the edges for weapons and knives, said co-author Melissa Panger, who studies primate tool use at GWU.

The new archaeological site in the Taï forest could be used as an example of how more ancient sites might appear, Panger said. Stone assemblages resembling those in the archaeological site in the Taï forest may hint at more primitive tool use predating the more sophisticated tools found in the Olduvai Gorge, she added.

In the Taï forest mother chimpanzees teach their infants the tasty art of nut cracking, which takes about seven years to master—it takes about 2,200 pounds (1,000 kilograms) of skillfully applied force to split the nut without pulverizing it. Between February and August, the prime nut-cracking season, a practiced chimp can break open more than 100 nuts per day obtaining a nutritional kick of 3,000 calories.

Although the nuts are widely available in tropical Africa, nut-cracking behavior has only been documented in chimps from the Western Ivory Coast, Liberia and Southern Guinea-Conakry.

The stone hammers—rocks—acquired by the chimps are carried to specific nut-cracking sites where the chimps pound the golf-ball-size nuts of the Panda oleosa tree to expose the three seeds within.

As the chimps strike the nuts they unintentionally chip off fragments of rock, which then accumulate at these sites. As Mercader excavated a site called Panda 100, he collected nine pounds (four kilograms) of stone chips and 88 pounds (40 kilograms) of old nutshells.

What is particularly striking about the site's layout is the arrangement of stone chips in six distinct locations around the Panda tree. The chimps strike the nuts only in areas focused around an anvil—usually a nook in a tree stump or another immobile object—that can steady the nut as it is being whacked.

Six anvils lie like orbiting satellites around the Panda 100 tree; and concentrated at each anvil site is a collection of stone chips, mapping exactly where the nut-cracking activity had taken place. Some of the stone chips found at Panda 100 resemble artifacts found in the Olduvai Gorge, suggesting, say the authors, that these early humans may have also been cracking nuts.

"Julio's study is very exciting. Now we can compare the earliest human archaeological sites with archaeological sites from our closest relatives," Jeanne Sept, an archaeologist at Indiana University, in Bloomington, said. "Before this discovery we were only able to compare early human sites with other human sites—it was a little like looking in the mirror and it didn't tell us very much."

The earliest known sites of stone-tool use were discovered because they held collections of "deliberately flaked stones" which were recognizable as early tools, Sept said. This study provides a model to look for ancient sites that preceded the stage where humans were deliberately sculpting their tools.

"Mercader's research presents new challenges," Sept said. "Debris that may not have been noticed before, because the rocks were not deliberately flaked and sharpened, may now demand more attention."

Taï Forest is an oasis of virgin tropical forest in West Africa and was declared a world heritage site in 1982. The forest is home to many threatened mammals such as the pygmy hippopotamus and 11 species of monkeys. The forest has also been the site of much primate research over the last two decades.

Christophe Boeshe, a primatologist at the Max Planck Institute for Evolutionary Anthropology of Leipzig, Germany, and the third co-author of the paper, has been studying chimp behavior and chimp tool use in the Taï National Park for more than 20 years.

Since it can take up to ten years for chimps to become habituated to people, Boesch's long relationship with the apes was invaluable, Mercader said. He made it possible to carry on the archaeology without disturbing the chimps, which continued their nut cracking activities at other Panda trees.

Chimps are well known for their use of tools, be it sticks and leaves for retrieving ants from holes, or rocks for cracking nuts. But this is the first time that non-degradable evidence has been found regarding these activities.

"Many scientists have a preconception that nothing is preserved in rain forests and archaeology in these areas is not worthwhile," Mercader said. "Our work proves that this is not true and that discoveries relevant to human history can come from places we do not expect."

Mercader's study was primarily funded by the Max Planck Institute for Evolutionary Anthropology. The George Washington University, National Geographic Society and the National Science Foundation provided additional funding.

This story will be airing May 23 on National Geographic Today.

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