By Rick Weiss
Washington Post Staff Writer
Friday, February 23, 2007
Chimpanzees living in the West African savannah have been observed fashioning deadly spears from sticks and using the tools to hunt small mammals -- the first routine production of deadly weapons ever observed in animals other than humans.
The multistep spearmaking practice, documented by researchers in Senegal who spent years gaining the chimpanzees' trust, adds credence to the idea that human forebears fashioned similar tools millions of years ago.
The landmark observation also supports the long-debated proposition that females -- the main makers and users of spears among the Senegalese chimps -- tend to be the innovators and creative problem solvers in primate culture.
Using their hands and teeth, the chimpanzees were repeatedly seen tearing the side branches off long, straight sticks, peeling back the bark and sharpening one end. Then, grasping the weapons in a "power grip," they jabbed them into tree-branch hollows where bush babies -- small, monkeylike mammals -- sleep during the day.
In one case, after repeated stabs, a chimpanzee removed the injured or dead animal and ate it, the researchers reported in yesterday's online issue of the journal Current Biology.
"It was really alarming how forceful it was," said lead researcher Jill D. Pruetz of Iowa State University, adding that it reminded her of the murderous shower scene in the Alfred Hitchcock movie "Psycho." "It was kind of scary."
The new observations are "stunning," said Craig Stanford, a primatologist and professor of anthropology at the University of Southern California. "Really fashioning a weapon to get food -- I'd say that's a first for any nonhuman animal."
Scientists have documented tool use among chimpanzees for decades, but the tools have been simple and used to extract food rather than to kill it. Some chimpanzees slide thin sticks or leaf blades into termite mounds, for example, to fish for the crawling morsels. Others crumple leaves and use them as sponges to sop drinking water from tree hollows.
But while a few chimpanzees have been observed throwing rocks -- perhaps with the goal of knocking prey unconscious, but perhaps simply as an expression of excitement -- and a few others have been known to swing simple clubs, only people have been known to craft tools expressly to hunt prey.
Pruetz and Paco Bertolani of the University of Cambridge made the observations near Kedougou in southeastern Senegal. Unlike other chimpanzee sites currently under study, which are forested, this site is mostly open savannah. That environment is very much like the one in which early humans evolved and is different enough from other sites to expect differences in chimpanzee behaviors.
Pruetz recalled the first time she saw a member of the 35-member troop trimming leaves and side branches off a branch it had broken off a tree.
"I just knew right away that she was making a tool," Pruetz said, adding that she suspected -- with some horror -- what it was for. But in that instance she was unable to follow the chimpanzee to see what she did with it. Eventually the researchers documented 22 instances of spearmaking and use, two-thirds of them involving females.
In a typical sequence, the animal first discovered a deep tree hollow suitable for bush babies, which are nocturnal and weigh about half a pound. Then the chimp would break off a branch -- on average about two feet long, but up to twice that length -- trim it, sharpen it with its teeth, and poke it repeatedly into the hollow at a rate of about one or two jabs per second.
After every few jabs, the chimpanzee would sniff or lick the branch's tip, as though testing to see if it had caught anything.
In only one of the 22 observations did a chimp get a bush baby. But that is reasonably efficient, Pruetz said, compared with standard chimpanzee hunting, which involves chasing a monkey or other prey, grabbing it by the tail and slamming its head against the ground.
In the successful bush-baby case, the chimpanzee, after using its sharpened stick, jumped on the hollow branch in the tree until it broke, exposing the limp bush baby, which the chimp then extracted. Whether the animal was dead or alive at that point was unclear, but it did not move or make any sound.
Chimpanzees are believed to offer a window on early human behavior, and many researchers have hoped that the animals -- humans' closest genetic cousins -- might reveal something about the earliest use of wooden tools.
Many suspect that the use of wooden tools far predates the use of stone tools -- remnants of which have been found dating from 2 1/2 million years ago. But because wood does not preserve well, the most ancient wooden spears ever found are only about 400,000 years old, leaving open the question of when such tools first came into use.
The discovery that some chimps today make wooden weapons supports the idea that early humans did too -- perhaps as much as 5 million years ago -- Stanford said.
Adrienne Zihlman, an anthropologist at the University of California at Santa Cruz, said the work supports other evidence that female chimps are more likely than males to use tools, are more proficient at it and are crucial to passing that cultural knowledge to others.
"Females are the teachers," Zihlman said, noting that juvenile chimps in Senegal were repeatedly seen watching their mothers make and hunt with spears.
Females "are efficient and innovative, they are problem solvers, they are curious," Zihlman said. And that makes sense, she added.
"They are pregnant or lactating or carrying a kid for most of their life," she said. "And they're supposed to be running around in the trees chasing prey?"
Frans B.M. de Waal, a primatologist at Emory University, said aggressive tool use is only the latest "uniquely human" behavior to be found to be less than unique.
"Such claims are getting old," he said. "With the present pace of discovery, they last a few decades at most."