LOS ANGELES - Chimpanzees are willing to attack and kill other chimps from neighboring groups to try to expand their territory, according to scientists who have studied a primate colony in Uganda for 10 years.

The chimp behavior could shed light on the evolutionary roots of human cooperation, the researchers said in a study published online yesterday in the journal Current Biology. Other experts said it could help explain the origins of human warfare.

Primatologists have long known that chimpanzees band together in silent, single-file lines to patrol the border between two communities. If they come across an unfamiliar male, they will attack him with their teeth, take turns jumping on his body, causing great harm. If they encounter an unfamiliar female with a baby, they will rip it out of her hands and kill it, sometimes cannibalizing the infant.

"It's dramatic," said Sylvia Amsler, a biological anthropologist at the University of Arkansas at Little Rock who worked on the new study.

What Amsler and her colleagues did not know was why the chimps, who along with bonobos are humans' closest living relatives, engaged in this behavior. Some researchers proposed that the attacks were designed to attract females. Others theorized that contact with scientists had caused once-peaceful chimpanzee communities to engage in warlike behavior.

Scientists did know that chimpanzees benefit when their territory increases, Amsler said. For instance, females give birth more frequently when they have more resources at their disposal.

To determine whether the killings were related to resources, the researchers tracked chimps in a large and prosperous community of about 150 animals (the average chimp colony contains 30 to 80, Amsel said).

The researchers observed 18 killings and found evidence of at least three others - far more attacks than had been witnessed in other chimp colonies. Most of the victims were infants, probably because they were easier to kill, Amsler said.

The study supports the view that chimpanzees are competing for territory, said Michael Wilson, a primatologist at the University of Minnesota. Observations like these could help scientists figure out whether warfare could have predated the development of weapons, agriculture and ideology, he said.

Amsler, however, said the findings could be more useful for understanding the evolution of cooperation among primates, including humans.

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