WASHINGTON (Reuters) - Chimpanzees have been seen using spears to hunt bush babies, U.S. researchers said on Thursday in a study that demonstrates a whole new level of tool use and planning by our closest living relatives.

A male chimpanzee feeds in Kibale National Park tropical rain forest, 354km southeast of Uganda's capital Kampala, December 2, 2006. Chimpanzees have been seen using spears to hunt bush babies, U.S. researchers said on Thursday in a study that demonstrates a whole new level of tool use and planning by our closest living relatives. REUTERS/James Akena

Perhaps even more intriguing, it was only the females who fashioned and used the wooden spears, Jill Pruetz and Paco Bertolani of Iowa State University reported.

Bertolani saw an adolescent female chimp use a spear to stab a bush baby as it slept in a tree hollow, pull it out and eat it.

Pruetz and Bertolani, now at Cambridge University in Britain, had been watching the Fongoli community of savanna-dwelling chimpanzees in southeastern Senegal.

The chimps apparently had to invent new ways to gather food because they live in an unusual area for their species, the researchers report in the journal Current Biology.

“This is just an innovative way of having to make up for a pretty harsh environment,” Pruetz said in a telephone interview. The chimps must come down from trees to gather food and rest in dry caves during the hot season.

“It is similar to what we say about early hominids that lived maybe 6 million years ago and were basically the precursors to humans.”

Chimpanzees are genetically the closest living relatives to human beings, sharing more than 98 percent of our DNA. Scientists believe the precursors to chimps and humans split off from a common ancestor about 7 million years ago.

Chimps are known to use tools to crack open nuts and fish for termites. Some birds use tools, as do other animals such as gorillas, orangutans and even naked mole rats.

But the sophisticated use of a tool to hunt with had never been seen.

Pruetz thought it was a fluke when Bertolani saw the adolescent female hunt and kill the bush baby, a tiny nocturnal primate.

But then she saw almost the same thing. “I saw the behavior over the course of 19 days almost daily,” she said.

PLANNING AND FORESIGHT

The chimps choose a branch, strip it of leaves and twigs, trim it down to a stable size and then chew the ends to a point. Then they use it to stab into holes where bush babies might be sleeping.

It is not a highly successful method of hunting. They only ever saw one chimpanzee succeed in getting a bush baby once. The apes mostly eat fruit, bark and legumes.

Part of the problem is this group of chimps is shy of humans, and the females, who seem to do most of this type of hunting, are especially wary. “I am willing to bet the females do it even more than we have seen,” she said.

Pruetz noted that male chimps never used the spears. She believes the males use their greater strength and size to grab food and kill prey more easily, so the females must come up with other methods.

“That to me was just as intriguing if not even more so,” Pruetz said.

The spear-hunting occurred when the group was foraging together, again unchimplike behavior that might produce more competition between males and females, she said.

Maybe females invented weapons for hunting, Pruetz said.

“The observation that individuals hunting with tools include females and immature chimpanzees suggests that we should rethink traditional explanations for the evolution of such behavior in our own lineage,” she concluded in her paper.

“The multiple steps taken by Fongoli chimpanzees in making tools to dispatch mammalian prey involve the kind of foresight and intellectual complexity that most likely typified early human relatives.”