Chimps Dig Tubers, Tool Study Finds

Anne Minard
for National Geographic News
November 13, 2007
The first evidence has been found that chimps use tools to dig for tubers, roots, and bulbs to eat.

The discovery may shed light on how early human ancestors survived the transition from food-rich forests to drier habitats, a new study says.

Anthropologist Adriana Hernandez-Aguilar observed chimpanzees digging in the Ugalla Forest Reserve of western Tanzania. The arid woodland savanna is home to a small population of chimps that have adapted to life beyond their species' typical forest habitat.

Chimps were never directly observed using the food sources. But at 11 dig sites—10 of them directly below chimp nests—Hernandez-Aguilar, of the University of Southern California, found chimp knuckle prints, feces, and chewed wads of fibrous tubers.

And three of the sites contained dirt-caked sticks. Wear patterns suggested the sticks had been used to dig up the foods, which are often protected by a hard crust of earth.

What's more, the chimps are using tubers in the rainy season, when other foods are presumably abundant. This challenges a long-standing theory that chimps use tubers only as backup food sources.

Chimps are known to use potato-like tubers for the nutrients and then spit out wads of fiber. People also use some of the plants as food or medicine.

"Adriana's find was a delightful surprise," said Jim Moore of the University of California, San Diego, in an email. Moore, one of the study's co-authors, has coordinated chimp research in the Ugalla savanna for 18 years.

"The discoveries that they sometimes use tools [to access the underground foods], do so in the wet, resource-rich season, and that some of the plants may have medicinal properties—those were real surprises," Moore added.

The paper appears online this week in the journal Proceedings of the National Academy of Sciences.

Human Origins

Some experts believe access to underground foods helped enable the expansion of early human ancestors into dry, relatively food-poor habitats like the African savanna.

John Kappelman, a professor of anthropology at the University of Texas, says the new discovery supports that theory.

Thirty years ago, Kappelman said by email, he and a colleague "hypothesized that these belowground plant parts would have offered a rich food resource for early humans who were living in dry seasonal habitats such as bushland and savanna.

"Before that, [yam expert] D. G. Coursey suggested that even the crudest form of a digging tool would have put early humans at a marked evolutionary advantage for exploiting these foods," Kappelman added.

"Well, this new work proves Coursey right, because the tools used by these chimpanzees are as simple as simple can be. And because they consist of thin pieces of wood, [they] are unlikely to be the sort of thing that would be preserved in the fossil record."

Breaking Barriers

Previously, humans were thought to be the only primates to dig up tubers using tools—just as humans were once thought to be the only primates to use tools at all.

Jill Pruetz, an Iowa State University primate researcher not affiliated with the new study, said "it suggests to me that we will continue to redefine humans as we find more out about other species, especially our closest living relatives.

"I wouldn't be surprised to hear about more of these kinds of 'discoveries'—ones that cause us to rethink our definition of what a human is."

(Pruetz made headlines earlier this year with her discovery that chimps make "spears" to hunt other mammals.)

Chimps at Risk

The chimps at Ugalla, the study authors say, face environmental challenges similar to those of our human ancestors about five million years ago, when a changing climate delivered a hit to their forests.

But there are other more immediate factors that could be contributing to the Ugalla chimps' decline.

Ugalla is adjacent to several camps housing people from Congo, Rwanda, and Burundi. These refugees may be responsible for heavy poaching reported in the last five years, study co-author Moore said.

Additionally, a proposed new major road will make travel in the region much easier, and "such roads almost always increase pressure on wildlife," Moore said.

"If the refugees are repatriated to their home countries soon and development along the road is regulated, the area is in decent shape and wildlife will recover," he added.

"If those things don't happen, the outlook isn't good."

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