Red colobus and Diana monkeys often share forest territory in the Ivory Coast’s Taï National Park. They live peaceably together because they have different diets; Dianas eat ripe fruits and insects, while red colobus prefer young leaves, flowers, and unripe fruit. Some biologists have theorized that the alliance protects against predators. A larger group has more eyes to spot a lurking leopard. And if that leopard does catch lunch, lunch is much less likely to be you if you’re in a larger group. Now two behavioral ecologists have found evidence that while predation has indeed forged the alliance, the predators driving the monkeys together are not leopards but chimpanzees.
Chimpanzees hunt for meat late in the rainy season; earlier in the season, when their staple plant foods are less abundant, the chimps must break up into smaller groups and forage over wider areas. Red colobus monkeys--which are about twice as large as the smaller, swifter Dianas, with males weighing up to 25 pounds and females about 18--are a favorite prey because they’re easier to catch. A typical red colobus group suffers almost monthly chimp attacks during hunting season.
To find out if chimps were indeed bringing the monkeys together, Ronald Noë and Redouan Bshary of the Max Planck Institute for Behavioral Physiology in Starnberg, Germany, have been watching two mixed groups since 1991. We saw that in the middle of the year, from June to August, when there is no hunting by the chimpanzees, there was a low association rate between the two groups, Noë says. The association rates go up as soon as the hunting starts.
Bshary also played taped chimpanzee calls for the monkeys. If the monkeys were already together, he found, the calls lengthened their stay; if they were apart, the chimp calls drew them together. When the Diana group was nearby and we played a chimpanzee call, the red colobus would go to them, even if it meant heading in the direction of the chimp call, Noë says. Leopard calls, on the other hand, had no effect on monkey affiliation.
To the red colobus, the active Dianas are ideal sentinels; they spread out and scan the forest floor for predators better than just about any other species, sounding alarm calls when they spot chimps. But what’s in it for the Dianas? We think the main advantage is that they are protected against crowned eagles, Noë says. The colobus are too heavy for the eagles to carry away, he says, and are usually a bit higher up in the trees than the Dianas, and shield them against these eagles.