Science : Watch out, bands of marauding chimps are about

1 March 1997

THE adage “safety in numbers” is especially true if your companions are more
alert to predators than you are. In the dense rainforests of the Tai National
Park in Côte d’Ivoire, red colobus monkeys mingle with diana monkeys when
chimpanzees are hunting in their neighbourhood for just this reason, according
to scientists in Germany.

Ronald Noë and Redouan Bshary of the Max Plank Institute for Behavioural
Physiology in Starnberg, Germany have been studying the two species since 1991.
They found that when chimpanzee hunting parties entered their territory, the red
colobus joined forces with the diana.

Behavioural ecologists have argued for years over why different primate
species sometimes group together. The most popular theory is that joining groups
makes it easier to search for food. But although red colobus and diana monkeys
share the same territory, they forage for food separately.

Groups of male chimpanzees hunt most frequently during the rainy season, from
September to November, which is when the two species of monkey tend to pair up.
By broadcasting sounds of various predators over loudspeakers, the researchers
worked out that the chimps were the trigger. They tested the snarls of leopards,
the purr of a diesel generator and other noises, but only recordings of
chimpanzees sent them scurrying for their diana companions.


The researchers believe red colobus monkeys are better protected when they
stick with the diana. The diana are quick to detect marauding bands of chimps,
calling loudly to one another and scampering for cover in the tops of high
trees: they are small and hardly move the branches of the trees in which they
hide. But the relationship could be mutually beneficial, because the red colobus
provide extra targets for predators such as leopards.

The findings “provide the first direct evidence that different species form
groups solely to save themselves from predators”, says Bshary. The research
appears in the Proceedings of the Royal Society B (vol 246, p 253).