A major study suggests that killing
among chimpanzees results from normal competition, not human
interference.
18 September 2014
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Groups of male chimpanzees patrol the
borders of their territory in single file
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Apart from humans, chimpanzees are the
only primates known to gang up on their neighbours with lethal
results - but primatologists have long disagreed about the underlying
reasons.
One proposal was that human activity,
including destroying habitats and providing food, increased
aggression.
But the new findings, published in
Nature, suggest this is not the case.
Instead, murder rates in different
chimp communities simply reflect the numerical make-up of the local
population.
The international study was co-written
by more than 30 scientists and gathers data from some 426 combined
years of observation, across 18 different chimp communities.
A total of 152 killings were reported.
This includes 58 that were directly observed by researchers; the rest
were counted based on detective work - tell-tale injuries or other
circumstances surrounding an animal's death or disappearance.
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Violence most often occurs between male
chimps,
at the fringes of established community territories
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Interestingly, the team also compiled
the figures for bonobos, with strikingly different results: just a
single suspected killing from 92 combined years of observation at
four different sites. This is consistent with the established view of
bonobos as a less violent species of ape.
Killing the competition
The researchers' global compilation of
chimp violent crime statistics allowed them to consider what
conditions in a community produce a higher murder rate.
Chimpanzees
live in well-defined colonies, and groups of males patrol the borders
of each colony's territory. This is where violent conflicts are known
to arise, particularly if a patrol encounters a single chimp from a
neighbouring community - but never before has this much data on the
lethality of those interactions been combined in a single study.
When the scientists compared the
figures across chimpanzee research sites, they found that the level
of human interference (e.g. whether the chimps had been fed, or their
habitat restricted) had little effect on the number of killings.
Instead, it was basic characteristics
of each community that made the biggest difference: the number of
males within it, and the overall population density of the area.
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Chimpanzees and bonobos are our closest
living evolutionary relatives
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These parameters link the violence to
natural selection: killing competitors improves a male chimp's access
to resources like food and territory - and crucially, it will happen
more frequently when there is greater competition from neighbouring
groups, and when the males can patrol in large numbers, with less
risk to their own survival.
"It's a natural behaviour - it's
not something that we've induced by disturbance or intervention,"
explained Dr Susanne Shultz, an evolutionary biologist at the
University of Manchester.
Dr Shultz was not involved in the
study, but told BBC News the scale of the collected data was
impressive.
"There's a real effort to look
across a really wide range of populations, and the results are very
compelling and very thorough," she said.
Violent debate
In an accompanying commentary for
the journal Nature, Prof Joan Silk from Arizona State University said
the results "should finally put an end to the idea" that
violence in wild chimpanzees was a product of human interference.
She
suggested that our perceptions of our evolutionary cousins can
sometimes be distorted, because we want to believe that it is the
nice behaviours, not the nasty ones, which have deep evolutionary
roots.
There is no need to cling to such
ideas, Prof Silk argues: "Humans are not destined to be warlike
because chimpanzees sometimes kill their neighbours."
Prof John Mitani, a behavioural
ecologist at the University of Michigan and one of the study's
authors, agrees. "There is considerable variation in rates of
killing by chimpanzees living in different populations, so even in
chimpanzees killing is not inevitable," he said.
"And, of course, we are humans and
not chimpanzees. We have the ability to shape and alter our behaviour
in ways that they can't."
Prof Frans de Waal, an animal behaviour
expert from Emory University in the US, said the new study was an
important contribution.
"I'm very glad they're publishing
this," he told BBC News. It answers a "long, long history
of resistance", Prof de Waal explained, to the idea of natural,
inter-community violence in chimpanzees.
"It has always been contentious -
we've had meetings where people screamed at each other.
"What this paper does is, instead
of getting into the ideology and the history of these arguments...
they have just taken the data and analysed it, and said: Where do the
chips fall?"
The chips, in this case, appear to fall
in favour of a natural history of violence.
But rather than having deep
implications for human nature, the authors of the new study suggest
that chimpanzee homicide - which previous research has
estimated to occur at a similar rate to that seen in hunter-gatherer
human societies - goes up and down as a simple consequence of
competition for resources.
(source: BBC)
