Chimps and Spears

From Washington Post via SF Chron:

Chimps make wood spears, kill smaller animals for food

Females described as problem-solvers in primate culture

Rick Weiss, Washington Post

Friday, February 23, 2007

Chimpanzees living in the West African savannah have been observed fashioning spears from sticks and using the handcrafted tools to hunt small mammals -- the first routine production of deadly weapons ever seen in nonhuman animals.

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, the researchers report in Thursday's online issue of the journal Current Biology. Then, grasping the weapon in a "power grip," they jabbed into tree-branch hollows where bush babies -- small monkeylike mammals -- sleep during the day.

After stabbing their prey repeatedly, they removed the injured or dead animal and ate it.

"It was really alarming how forceful it was," said lead researcher Jill Pruetz of Iowa State University.

The observations are "stunning," said Craig Stanford, a primatologist and USC anthropology professor. "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 several decades, but the tools have been simple and used to extract food rather than to kill for it.

Some chimpanzees slide thin sticks or leaf blades into termite mounds, for example, to fish for the tasty, crawling morsels. Others crumple leaves and use them like 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 expressions 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 co-worker Paco Bertolani of the University of Cambridge made the observations near Kedougou in southeastern Senegal. Unlike other chimpanzee sites 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 what it was for, as well.

But in that instance, Pruetz was not able to follow the chimpanzee to see what she did with it.

Eventually, she and Bertolani documented 22 instances of spearmaking and use, two-thirds of them involving females.

In a typical sequence, the animal would discover a deep hollow suitable for bush babies, which are nocturnal and weigh about half a pound. Then the chimp would break off a nearby branch -- on average about 2 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 tip, as though testing to see if it had "caught" anything.

In only one of 22 observations did a chimp get a bush baby. But that is reasonably efficient, Pruetz said, compared to standard chimpanzee hunting practice, which involves chasing a monkey or other prey, grabbing it by the tail and then slamming its head against the ground.

Chimpanzee behavior is widely believed to offer a window on early human behavior, and many researchers have hoped that the animals -- which are humans' closest genetic cousins -- might reveal something about the earliest use of wooden tools.

Many suspect that wooden tools far predate the use of stone tools -- remnants of which have been found going back 21/2 million years. But because wood does not preserve well, the oldest wooden spears ever found are only about 400,000 years old, leaving open the question of when they first came into use.

The discovery that some chimps make wooden weapons today supports the idea that early humans did, too -- perhaps as early as 5 million years ago, Stanford said.

Adrienne Zihlman, a UC Santa Cruz anthropologist, said the work supports other evidence that female chimps are more likely to use tools than males, are more proficient tool users, 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.

"They are efficient and innovative, they are problem-solvers, they are curious," Zihlman said of females.

And that makes sense, she said.

"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?"