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.

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