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Chimpanzees can be lethally violent to each other but this stems from an inherent streak and not, as some have suggested, from human interference, a study said on Wednesday, AFP reports.
Zoologists, led by the famed Jane Goodall, have speculated for years on the causes of "chimpanzee wars" among Man's genetically-closest relatives.
One theory is that the apes are made more aggressive as a result of human influence: loss of habitat or food creates ever-greater competition for resources.
But new research, published in the journal Nature, said coordinated violence by Pan troglodytes is an evolutionary strategy.
Chimps kill to wipe out rivals, thus gaining territory, mates, water or food, it suggests. In Darwinian terms, they seek an advantage to help them survive and hand on their genes to future generations.
The evidence comes from an examination of five decades of research into 18 closely-studied chimpanzee communities in African forests.
The researchers pored over 152 killings by chimps, most of which were carried out by males acting together.
The groups would often band together to carry out murderous raids on another community, typically killing rival males and infants who were not genetically related.
They sometimes snatched babies from nursing mothers to slaughter them but spared the females.
The investigators had to determine whether these acts were driven by hunger, human disturbance or deforestation and whether the protected area the chimps inhabited was large or small.
Most of the killings occurred in east African communities that were least affected by human interference of any kind.
"Wild chimpanzee communities are often divided into two broad categories depending on whether they exist in pristine or human-disturbed environments," said David Morgan, a specialist in ape conservation at Lincoln Park Zoo in Chicago, Illinois, who has studied chimps in central Africa for 14 years.
"Study sites included in this investigation spanned the spectrum. We found human impact did not predict the rate of killing among communities."
