In the 1990s Stanford observed male chimpanzees hunting colobus monkeys with their bare hands.
The new discovery of chimps hunting with tools is "stunning," Stanford said in a telephone interview.
"Except for one anecdote many years ago, there's never really been any evidence or suggestion that chimps would use weapons when they were hunting," he said.
The earlier anecdote—reportedly based on a single observation—described a female chimpanzee's use of a tool to rouse a squirrel from a tree hollow in Tanzania.
Chimpanzees are well-known toolmakers. In the 1960s primatologist Jane Goodall famously observed chimps using sticks to fish termites out of mounds. (Photo gallery: Jane Goodall encounters chimps that are unafraid of humans.)
And Stanford's research has shown that chimpanzees are highly efficient hunters of colobus monkeys (watch video of chimps hunting colobus monkeys).
"But we've never discovered chimp populations that made the cognitive leap to put those two [skills] together and use weapons to assist in their hunting," Stanford said.
"And clearly this is what these guys are doing."
(Related news: "Chimp 'Stone Age' Finds Are Earliest Nonhuman Ape Tools, Study Says" [February 13, 2007].)
Mothers and Children
What makes the discovery all the more remarkable, project leader Pruetz said, is who the hunters are: predominantly mature females and immatures—youngsters between about two and ten years old.
"We don't think of chimpanzee hunting in terms of the females and immatures," she said.
The new finding shows that females and immatures do hunt. It also suggests that females played a role in the evolution of tool use and hunting among early human ancestral species, she added.
Chimpanzees are modern humans' closest living relatives. And Pruetz's research site is a savannah similar to the open environment that early human ancestors are believed to have moved into millions of years ago.
"Looking at our closest living relatives in a habitat that is fairly similar to what we see characterizing early hominids six million years ago" can help researchers understand early human ancestors' behavior and ecology, she said.
USC's Stanford likens chimpanzees to a window to a past poorly preserved in the archaeological record.
Hunting "is something that the chimps do that almost certainly early, early hominids did too. They were just using a material—wood—that does not leave any archaeological trace," he said.
Putting Too Fine a Point on It?
In their paper, Pruetz and Bertolani describe a deliberate toolmaking process.
The tools, on average, are about 24 inches (60 centimeters) long and 0.4 inch (11 millimeters) around.
The researchers refer to the tools as spears. Pruetz said they differ from throwing spears, in the sense that they are jabbed into tree trunks and branches, not tossed.
USC's Stanford said the word "spear" is an overstatement that makes the chimpanzees sound too much like early humans.
He prefers "bludgeon."
"They seem to be using it to hit the animal hard, and having a point on the end certainly helps," he said.
"But I think it's not clear whether the point that they made is in fact even sharp enough to penetrate the animal."
Jill Pruetz's work with chimpanzees will be featured in an upcoming NOVA/National Geographic special on PBS (airdate to be determined).
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