[THS] Chimps, Too, Wage War and Annex Rival Territory

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June 21, 2010
Chimps, Too, Wage War and Annex Rival Territory
By NICHOLAS WADE

AGGRESSION A young male chimp in Uganda’s Kibale National Park leaps on the
body of a victim killed in an attack. Photo: John Mitani

Every day, John Mitani or a colleague is up at sunrise to check on the action among
the chimpanzees at Ngogo, in Uganda’s Kibale National Park. Most days the male
chimps behave a lot like frat boys, making a lot of noise or beating each other up.
But once every 10 to 14 days, they do something more adult and cooperative: they
wage war.

A band of males, up to 20 or so, will assemble in single file and move to the edge of
their territory. They fall into unusual silence as they penetrate deep into the area
controlled by the neighboring group. They tensely scan the treetops and startle at
every noise. “It’s quite clear that they are looking for individuals of the other
community,” Dr. Mitani says.

When the enemy is encountered, the patrol’s reaction depends on its assessment of
the opposing force. If they seem to be outnumbered, members of the patrol will
break file and bolt back to home territory. But if a single chimp has wandered into
their path, they will attack. Enemy males will be held down, then bitten and battered
to death. Females are usually let go, but their babies will be eaten.

These killings have a purpose, but one that did not emerge until after Ngogo chimps’
patrols had been tracked and cataloged for 10 years. The Ngogo group has about
150 chimps and is particularly large, about three times the usual size. And its size
makes it unusually aggressive. Its males directed most of their patrols against a
chimp group that lived in a region to the northeast of their territory. Last year, the
Ngogo chimps stopped patrolling the region and annexed it outright, increasing their
home territory by 22 percent, Dr. Mitani said in a report being published Tuesday in
Current Biology with his colleagues David P. Watts of Yale University and Sylvia J.
Amsler of the University of Arkansas at Little Rock. Dr. Mitani is at the University of
Michigan.

The objective of the 10-year campaign was clearly to capture territory, the
researchers concluded. The Ngogo males could control more fruit trees, their females
would have more to eat and so would reproduce faster, and the group would grow
larger, stronger and more likely to survive. The chimps’ waging of war is thus
“adaptive,” Dr. Mitani and his colleagues concluded, meaning that natural selection
has wired the behavior into the chimps’ neural circuitry because it promotes their
survival.

Chimpanzee warfare is of particular interest because of the possibility that both
humans and chimps inherited an instinct for aggressive territoriality from their joint
ancestor who lived some five million years ago. Only two previous cases of chimp
warfare have been recorded, neither as clear-cut as the Ngogo case.

In one, a chimp community first observed by Jane Goodall in Tanzania’s Gombe
National Park split into two and one group then wiped out the other. But the chimps
had been fed bananas, to enable them to be observed, and some primatologists
blamed the war on this human intervention. In a second case, in the Mahale
Mountains National Park of Tanzania, Toshisada Nishida of Kyoto University noticed
that a chimp group had disappeared, presumably killed by its neighbors, but he was
not able to witness the killings or find the bodies.

Dr. Mitani’s team has now put a full picture together by following chimps on their
patrols, witnessing 18 fatal attacks over 10 years and establishing that the warfare led
to annexation of a neighbor’s territory.

The benefits of chimp warfare are clear enough, at least from the perspective of
human observers. Through decades of careful work, primatologists have documented
the links in a long causal chain, proving for instance that females with access to more
fruit trees will bear children faster.

But can the chimps themselves foresee the outcome of their behavior? Do they
calculate that if they pick off their neighbors one by one, they will eventually be able
to annex their territory, which will raise their females’ fertility and the power of their
group? “I find that a difficult argument to sustain because the logical chain seems
too deep,” says Richard Wrangham, a chimp expert at Harvard.

A simpler explanation is that the chimps are just innately aggressive toward their
neighbors, and that natural selection has shaped them this way because of the
survival advantage that will accrue to the winner.

Warfare among human groups that still live by hunting and gathering resembles
chimp warfare in several ways. Foragers emphasize raids and ambushes in which few
people are killed, yet casualties can mount up with incessant skirmishes. Dr.
Wrangham argues that chimps and humans have both inherited a propensity for
aggressive territoriality from a chimplike ancestor. Others argue the chimps’ peaceful
cousin, the bonobo, is just as plausible a model for the joint ancestor.

Dr. Wrangham’s view is that since gorillas and chimps are so similar, their joint
ancestor, which lived some seven million years ago, would have been chimplike and
therefore so would the joint ancestor of chimps and humans when they parted ways
two million years later. “So I think it’s very reasonable to think this behavior goes
back a long way,” he said, referring to the propensity to wage war against one’s own
species.

Dr. Mitani, however, is reluctant to infer any genetic link between human and chimp
warfare, despite the similarity of purpose, cost and tactics. “It’s just not at all clear to
me that these lethal raids are similar sorts of phenomena,” he said. More interesting
than warfare, in his view, is the cooperative behavior that makes war possible.

Why do chimps incur the risk and time costs of patrolling into enemy territory when
the advantage accrues most evidently to the group? Dr. Mitani invokes the idea of
group-level selection — the idea that natural selection can work on groups and favor
behaviors, like altruism and cooperation, that benefit the group at the expense of the
individual. Selection usually depends only on whether an individual, not a group,
leaves more surviving children.

Many biologists are skeptical of group-level selection, saying it could be effective only
in cases where there is intense warfare between groups, a reduced rate of selection
on individuals, and little interchange of genes between groups. Chimp warfare may
be constant and ferocious, fulfilling the first condition, but young females emigrate to
neighboring groups to avoid inbreeding. This constant flow of genes would severely
weaken any group selective process, Dr. Wrangham said.

Samuel Bowles, an economist at the Santa Fe Institute who has worked out
theoretical models of group selection, said the case for it “is pretty strong for
humans” but remains an open question in chimpanzees.

Chimp watching is an arduous task since researchers must first get the chimpanzees
used to their presence, but without inducements like bananas, which could interfere
with their natural behavior. Chimpanzees are immensely powerful, and since they can
tear each other apart, they could also make short work of any researcher who
incurred their animosity.

“Luckily for us, they haven’t figured out that they are stronger than us,” Dr. Mitani
said, explaining that there was no danger in tagging along behind a file of chimps on
the warpath. “What’s curious is that after we do gain their trust, we sort of blend into
the background and they pretty much ignore us.”

Source: The New York Times
http://www.nytimes.com/2010/06/22/science/22chimp.html?ref=science