Both chimps and humans are 'natural born killers'
Chimpanzees and humans have one key trait in common - both are natural born killers, scientists have shown.
Evidence suggests our closest animal relatives have an almost psychopathic tendency towards violence and slaughter that is not the result of human interference.
A widely held theory is that chimps only turn on each other when humans disrupt their forest habitats or food supplies.
The new research indicates this is wishful thinking. In reality, chimps fight and kill to get what they want and "eliminate rivals", say the authors.
Competing groups of the animals go to war over resources such as territory, food or mates, the study found.
And, just as in human conflict, it is often the innocent who are the victims. In the violent clashes investigated by the scientists, nursing infants were frequently killed, sometimes after being snatched from their mothers.
US lead scientist Dr Michael Wilson, from the University of Minnesota, said: "This is an important question to get right. If we are using chimpanzees as a model for understanding human violence, we need to know what really causes chimpanzees to be violent."
A 30-strong team of researchers analysed five decades of data from 18 chimpanzee and four bonobo study sites in east, west and central Africa.
Bonobos, another member of the ape genus Pan, are endangered close relatives of chimpanzees said to display positive human behavioural traits such as altruism and empathy.
The study looked at 152 killings by chimpanzees, mostly in east African ape communities, and only one suspected killing by bonobos.
Attackers were usually males acting together, and victims mostly males and nursing infants of other groups who were unlikely to be close kin.
In some cases infants were killed but their mothers spared, despite being at the mercy of the attackers.
Inter-community attacks accounted for 66% of killings and usually victims were strongly out-numbered, typically with odds of eight to one.
A key finding was that killing rates bore no relation to measures of human impacts.
The researchers considered three human variables likely to influence chimp behaviour - artificial feeding of wild apes, the size of a protected area - with those that are smaller experiencing more impacts - and environmental disturbance.
They found that killings increased as the number of males and population density rose, but none of the human factors had any effect.
Co-author Dr David Morgan, from the Lester E Fisher Centre for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago, US, said: "Wild chimpanzee communities are often divided into two broad categories depending on whether they exist in pristine or human disturbed environments. In reality, however, human disturbance can occur along a continuum and study sites included in this investigation spanned the spectrum.
"We found human impact did not predict the rate of killing among communities."
Writing in the journal Nature, the researchers concluded: "Patterns of lethal aggression in Pan show little correlation with human impacts, but are instead better explained by the adaptive hypothesis that killing is a means to eliminate rivals when the costs of killing are low."
In an accompanying commentary, Professor Joan Silk, from the School of Human Evolution and Social Change at Arizona State University, US, wrote: "In 2013 there were 33 armed state-level conflicts around the world. Many of these had persisted for decades, killed thousands of people, and thwarted international peace-keeping efforts. War is certainly a contemporary fixture, but has it always been one? There is vigorous disagreement over the answer to this question ...
"These results should finally put an end to the idea that lethal aggression in chimpanzees is a non-adaptive by-product of anthropogenic (human) influences - but they will probably not be enough to convince everyone.
"Perceptions of the behaviour of non-human primates, particularly chimpanzees, are often distorted by ideology and anthropomorphism, which produce a predisposition to believe that morally desirable features, such as empathy and altruism, have deep evolutionary roots, whereas undesirable features, such as group-level violence and sexual coercion, do not. This reflects a naive form of biological determinism."