Colobus Monkeys

Colobus monkeys and the closely related langurs and leaf monkeys are Old World monkeys in the sub-family Colobinae of the family Cercopithecidae. The primates in this subfamily share a common trait—they lack thumbs or have only small, useless thumbs. (The name colobus comes from a Greek word meaning mutilated.) However, lack of a thumb does not stop them from nimbly moving among the branches. They just grasp a branch between the palm of the hand and the other fingers. The Colobinae are distinguished from the other subfamily of Old World monkeys, the Cercopithecinae, by their lack of cheek pouches, their slender build, and their large salivary glands. They live throughout equatorial Africa, India, and Southeast Asia.

Unlike most monkeys, which are fairly omnivorous, eating both plant and animal foods, many colobus monkeys eat primarily leaves and shoots. In the tropical and subtropical regions in which these leaf-eaters live, they have an advantage over other monkeys in that they have a year-round supply of food. Leaf-eaters also have an advantage in that they can live in a wider variety of habitats than fruit-eaters, even in open savanna. Fruit-eaters, on the other hand, must keep on the move in forests to find a fruit supply in season. Most colobus monkeys also eat fruits, flowers, and seeds when they can find them.

Like all Old World monkeys, colobus monkeys have tough hairless pads, called ischial callosities, on their A black and white colobus monkey in Lake Nakuru National Park, Kenya. © Tim Davis, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission. rear ends. There are no nerves in these pads, a fact that allows them to sit for long periods of time on tree branches without their legs "going to sleep." Some colobus monkeys have loud, raucous calls that they use particularly at dawn. Apparently these signals tell neighboring troops of monkeys where a particular group is going to be feeding that day.

Leaves do not have much nourishment, so colobus monkeys must eat almost continuously, up to a quarter of their body weight each day. Their leafy diet requires them to have stomachs specially adapted for handling hard-to-digest materials. The upper portion of the stomach contains anaerobic bacteria that break down the cellulose in the foliage. These bacteria are also capable of digesting chemicals in leaves that would be poisonous to other primates. Colobus monkeys have large salivary glands which send large amounts of saliva into the fermenting food to help its passage through the digestive system.