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http://www.newscientist.com/article/dn9231-spider-monkeys-go-on-the-warpath.html

Spider monkeys go on the warpath
10:00 27 May 2006

RAIDING parties, subterfuge and warfare. Chimps use these tactics,
humans are all too skilled in them, and now they have been observed
for the first time in a non-ape - the relatively little-known spider
monkey.

During a long-term study of the monkeys in Mexico, Filippo Aureli of
Liverpool John Moores University, UK, and his colleagues came upon
small groups of males travelling deep into the territory of
neighbouring monkeys. Spider monkeys (Ateles geoffroyi) usually spend
all their time in the trees, so the researchers were amazed to see
these males creeping along silently in single file on the ground,
looking about them and rarely stopping to feed.

"They were clearly aware that they were on risky territory and behaved
in exactly the way described for groups of male chimpanzees when
searching for members of neighbouring communities to attack," says
Aureli. Four of seven witnessed raids resulted in concerted aggression
against groups of neighbouring monkeys, though none were killed.
Aureli suggests that the raids might be attempts by males to get
covert mating opportunities (American Journal of Physical
Anthropology, DOI: 10.1002/ajpa.20451).

Richard Wrangham, of Harvard University, an expert on chimpanzee
raiding, says we can expect reports of spider monkey raiding parties
killing "foreign" males, as for chimpanzees.

Humans, chimpanzees and spider monkeys have similar social systems,
where communities split up and reform into subgroups of variable
membership. This sort of social system could have formed the basis of
warfare in ancestral humans, says Aureli.