May 2002

From Max-Planck-Gesellschaft

Chimpanzee stone tool site excavated

Scientists use archaeological methods to investigate how chimpanzees in the West African rainforest crack hard nuts.



Young chimpanzees learn how to use tools to open nuts from their mothers.

Foto: Christophe Boesch (Copyright Boesch. All rights reserved)

Since 1979, Christophe and Hedwige Boesch have been following the chimpanzees of the Taļ National Park, in the rainforest of the Ivory Coast and have shown that juveniles take several years to become proficient at nut-cracking. Mothers share nuts with their offspring while they are learning the behavior . The nuts they crack are actually available throughout tropical Africa, yet nut-cracking behavior has been documented only among chimpanzees from Western Ivory Coast Liberia and Southern Guinea-Conakry. Therefore, nut-cracking is a cultural behavior , which, like human culture, can help distinguish one population from another.

The African rainforest has been a place traditionally avoided by archaeologists because of, among other reasons, the cumbersome logistics of transportation, survey, and isolation. Today, we know that many of our typical assumptions about tropical forest archaeology and the role of rainforests in human evolution and cultural complexity will have to be changed as scientists continue to unveil archaeological evidence from these regions. The excavation of a chimpanzee stone tool site in the Ivory Coast is a good example that discoveries relevant to human ancestry may indeed come from unexpected places.



Julio Mercader excavating at the site

For the chimpanzee archaeology project, the first decision was where to dig. Based on the detailed records of chimpanzee nut-cracking sites, a site was selected where chimpanzees had been seen over the course of many years using stone hammers to crack the very hard nuts of Panda oleosa. Early on in the project it was apparent that having an archaeologist working with primatologists would be beneficial. Christophe Boesch had previously noticed large pieces of stone breaking off hammers. After looking at a chimpanzee hammer, Julio Mercader was quick to point out to Christophe Boesch and Melissa Panger the existence of flake scars. The scars were produced by the removal of stone pieces detached during the cracking of nuts and are as small as a few centimeters.

Using archaeological methods on a non-human species, for the first time the excavations revealed new facets of chimpanzee tool behavior. Especially interesting was the excavation of Panda 100, a large tree where Hedwige and Christophe Boesch have observed for many years chimpanzees cracking nuts with different stones they transported over hundreds of meters from other Panda trees. Due to the low visibility across the rainforest floor chimpanzees have to mentally calculate all of these movements keeping transport costs to a minimum. The archaeological data show that chimpanzees collect rocks from various sources across the landscape and bring them to nut-cracking sites.

This project has confirmed for the first time that archaeology can be successfully applied to the study of past chimpanzee behavior. The distribution of remains is not random, presenting very clear concentrations of nut shells and stone remains at nut-cracking areas. The distribution of these clusters, with well defined edges, closely resemble what archaeologists understand as "activity areas". The unearthed materials include more than 4 kilograms of stone pieces and almost 40 kilos of nut-shell, and demonstrate that, like human sites, it is the repeated occupation of the same spot and the creation of large refuse accumulations that creates what archaeologists call a "site". Because the site of Panda 100 is most likely older than 100 years, this discovery indicates that nut-cracking behavior has been present in this region for many generations.

The stone remains retrieved were unintentionally produced by chimpanzees as they hit rock hammers against wooden anvils. In all, 479 stones pieces were excavated, some from a depth of as deep as 21 cm. A fascinating aspect of this discovery is that the size of the stones, the shape of flakes, and the many small pieces of debris left by chimpanzees are similar to the stones left behind by some of our early ancestors in East Africa during a period called the "Oldowan". Moreover the number of stone pieces per m2, and the size of the stone clusters themselves mimic some assemblages from this period.

Thus we see that chimpanzees produced a visible record of their tool using activities. Archaeology has proven a reliable, efficient, and feasible method to uncover past chimpanzee activities in the rainforest. This type of archaeological application is useful in reconstructing past primate behavior both human and non-human. The results open a new territory for many disciplines, including primatology, archaeology, and paleoanthropology, and indicate the possibility that some of the technologically simplest Oldowan sites could be re-interpreted as nut-cracking sites and more generally that some subsets of artifacts from the more sophisticated Oldowan assemblages could be seen as material proof that early hominids were able to eat nuts contained in hard shells.

The archaeologists will continue the excavation of chimpanzee sites in the African forest and explore the implications of this work in East Africa. "The data highlight how much more we still need to learn about our closest living relative, the chimpanzee, to understand humanity's uniqueness. Every day chimpanzees are being killed in the wild and their forest habitat is being destroyed. All of us have a responsibility to care for other species," says Boesch.











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