Chimpanzee hunting tools.

Pictures of our ancestors often show men hunting with spears, arrows, and other tools. Scientists have long thought that only humans made tools for the hunt. They've also assumed that men did most of the hunting.

Now, for the first time, scientists have observed wild chimpanzees hunting with tools. What is just as surprising, females and young chimps outnumber males in these hunts. The discovery throws into question many assumptions about human evolution.

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Researchers from Iowa State University in Ames and the University of Cambridge in England studied 35 chimps in Senegal, a country in western Africa. The primates live in an area of savannah called Fongoli.

Among other food sources, the Fongoli chimps eat squirrel-size primates called bush babies. Between March 2005 and July 2006, the scientists watched Fongoli chimps use tools to go after bush babies 22 times.

To make their tools, the chimps ripped branches from trees. They usually chose branches that were about 18 inches long. Then they broke off twigs and leaves and often peeled off the bark to make the branches more spearlike. They sometimes used their teeth to make the branches sharper. They then used the tools to stab at holes in tree trunks where bush babies sleep during the day.

Most of these attempts failed. Once, however, the scientists saw a female chimp hit a bush baby with her spear. Then, she grabbed it out of its nest and ate it.

In total, the scientists watched 10 Fongoli chimps hunt with tools. Only one of the hunters was an adult male. The rest were females and juveniles. This finding was unusual because researchers elsewhere have observed only male chimps hunting monkeys. These other males worked in teams and shared their prey with others in their group, but they didn't use tools to catch the monkeys.

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This study points out that females often come up with new ideas and that young primates are often first to pick up on those new ideas.

Spear-hunting behavior is "yet another example of chimpanzee cultures," says anthropologist Linda F. Marchant of Miami University in Oxford, Ohio. Different groups of chimps probably come up with their own customs and behaviors, she says, just as different groups of people do.

The scientists suspect that the chimps in Fongoli are behaving much as our ancestors did 3 million years ago. But wooden tools rarely last as long as fossilized bones do, so archaeologists might never find our ancestors' first hunting spears.--E. Sohn