Chimpanzee gangs kill for land.

Chimp-on-chimp attacks in the
wild are very common, especially
among small packs of males on
patrol. Now research suggests
the motive for these crimes is to
gain territory.
To understand this violence,
researchers studied a large
group of chimpanzees living in
Ngogo, Kibale National Park in
Uganda. After monitoring the
group for over a decade,
scientists counted 21 chimp-on-
chimp murders.
Of those crimes, the researchers
witnessed 18 directly, and
deduced three from
circumstantial evidence. They
think as many as 13 of the
victims belonged to a single
neighboring group.
"The take-home is clear and
simple," said researcher John
Mitani of the University of
Michigan. " Chimpanzees kill each
other. They kill their neighbors.
Up until now, we have not
known why. Our observations
indicate that they do so to
expand their territories at the
expense of their victims."
After some of these neighboring
competitors were dispatched
with, the researchers observed
the Ngogo chimpanzees
beginning to use a large portion
of new territory to the northeast
of their previous range. That
piece of evidence allowed the
researchers to link the murders
with a motive – that of gaining
new ground.
The scientists think the new land
offers greater access to food,
and potentially to females.
The attacks seem to be triggered
when bands of chimpanzees go
out patrolling into the territory of
a neighboring chimpanzee
group.
"Patrollers are quiet and move
with stealth," Mitani said. "They
pause frequently to scan the
environment as they search for
other chimpanzees. Attacks are
typically made only when
patrolling chimpanzees have
overwhelming numerical
superiority over their
adversaries."
This tends to happen often for
the Ngogo chimpanzees, who
have a particularly large group of
more than 150 individuals —
about three times the number
found in chimp communities
studied elsewhere. That
advantage may explain the
surprisingly high level of violence
observed, the researchers said.
Mitani and colleagues described
the research in the June 22 issue
of the journal Current Biology.