Chimps show lethal side

Killings sometimes occur in communities across Africa

PORTLAND, Ore. — In a cooperative venture aimed at understanding the most uncooperative of acts, researchers studying different African communities of wild chimpanzees have pooled their data and found that the apes sometimes kill each other nearly everywhere they’ve been studied.

Chimp homicides occurred most frequently in groups with the most adult males, anthropologist Michael Wilson of the University of Minnesota in Minneapolis reported April 12 at the American Association of Physical Anthropologists’ annual meeting.

Wilson persuaded researchers at 10 wild chimp sites, containing a total of 17 communities, to contribute their findings on lethal attacks collected over the past several decades.

Chimps spend most of their time in peaceful pursuits, such as playing, foraging and grooming each other. Yet researchers, beginning with Jane Goodall more than 40 years ago, have described occasional chimp homicides. Some investigators have speculated that these animals get lethally riled up by human intrusions, such as deforestation, hunting and feeding of chimps by eco-tourists.

But the new study found that chimp communities with the most documented killings had no or only rare encounters with humans. Groups of males carried out most killings, and most victims were male adults and infants in neighboring communities.

“The new findings suggest that killing is an evolved strategy, mainly for adult males to eliminate rivals and competitors for mates,” Wilson said.

Researchers documented 86 cases of chimp homicide, either by observing lethal attacks in person or finding dead bodies with fresh wounds, often after hearing attacks in progress.

The highest number of killings in one community, 31, occurred at Ngogo, a site in Uganda’s Kibale National Park. That community contained 38 adult males, the most of any group being studied.

“This is the best data we have on chimp killings,” commented anthropologist and chimp researcher Linda Marchant of Miami University in Oxford, Ohio. “It puts to rest the idea that human intervention causes chimps to kill.”

It’s unclear why some adult males targeted infant males for death, Marchant said at the meeting. This practice may reflect a weeding out of potential competitors to a male chimp’s own offspring, she speculated.

Research in four communities of bonobo chimps has yielded no documented killings, Wilson said. Bonobos have been touted as the peace-and-love alternative to common chimps, although the extent to which bonobos carry out aggressive acts remains poorly understood.

“Bonobos do appear to be more peaceful than chimps,” Wilson said.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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