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The Monkey is one of many kinds of small, lively mammals that rank among the most intelligent animals. Scientists classify monkeys--together with human beings, apes, lemurs, and lorises--in the order Primates, the highest order of mammals. The intelligence of monkeys enables them to adapt to a broad range of environments. Their liveliness makes them favorites in zoos. Because of the similarities between monkeys and humans, scientists have used monkeys in research on human behavior and disease. For example, a blood substance called the Rh factor was discovered during experiments with the rhesus monkey.

There are about 200 species of monkeys. Most of them live in tropical regions in Central and South America, Africa, and Asia. Most species live in forests and some spend their entire life in the trees. Some African and Asian species live in savannas (grasslands with scattered trees) and spend most of their life on the ground. But even these monkeys sleep in trees--or on steep cliffs--for protection at night. All monkeys live together in various kinds of groups.

Scientists classify monkeys into two major groups, New World monkeys and Old World monkeys. New World monkeys live in Central and South America, and Old World monkeys are found in Africa and Asia. The two groups differ in several ways. For example, New World monkeys have nostrils spaced widely apart. The nostrils of Old World monkeys are close together. Most kinds of New World monkeys have 36 teeth. Old World monkeys have 32 teeth, as do humans. Some species of New World monkeys can grasp objects with their tail, but no Old World monkey can.

Old World monkeys include baboons, colobus monkeys, guenons, langurs, and macaques. Some Old World monkeys, including colobus monkeys and langurs, are leaf-eating monkeys and live mainly in trees. Many other Old World monkeys live on the ground. Among the monkeys that live on the ground, the males may be twice as large as the females. See the Related Articles at the end of this article for a list of the monkeys about which World Book has separate articles.

Many people believe that apes--chimpanzees, gibbons, gorillas, and orangutans--are monkeys. But monkeys and apes differ in several ways. For example, apes are more intelligent than monkeys. Most monkeys have a tail, but none of the apes do. Monkeys are smaller than most apes. Apes are expert climbers. Monkeys generally run, jump, and leap among tree branches.

Human activities have greatly reduced the number of monkeys throughout the world. Some people hunt monkeys for food. Others catch them for pets. The clearing of land for agricultural, housing, and industrial developments has reduced the amount of living space available to monkeys. A number of species of New World and Old World monkeys are threatened with extinction.

The body of a monkey

All monkeys, including those that live on the ground, are the descendants of monkeys that live in trees. As a result, all monkeys have a body primarily suited for living in and moving through trees. For example, monkeys have long arms and legs that help them climb, leap, and run. They also can use their hands and feet to grasp objects--including tree branches. Most species have a long tail that helps them keep their balance. Some New World monkeys can use their tail like a hand to grasp branches and food while moving through the trees.

Head. Monkeys, unlike many other mammals, depend more on their eyes than their nose to gather information about their surroundings. They have large eyes that face forward. They can see in depth and distinguish colors. Their eyes help them judge distances and tell the size, shape, and ripeness of food.

Some monkeys, including baboons, mandrills, and sakis, have large, heavy jaws and eat grass and leaves. Smaller monkeys, such as marmosets and squirrel monkeys, have smaller, lighter jaws. They eat mostly fruit and insects. Many kinds of Old World monkeys have cheek pouches much like those of hamsters and squirrels. The pouches enable the monkeys to store food temporarily. No New World monkey has these pouches.

Arms and legs. Monkeys usually walk and run on all fours, either on tree branches or on the ground. Most species have legs that are slightly longer than their arms. Many kinds of monkeys can stand and even run on their legs, but only for a short period of time. Monkeys usually stand or run on their legs when carrying food, peering over high grass, or threatening enemies or members of the group.

Hands and feet. Old World monkeys have opposable thumbs--that is, the thumb can be placed opposite any of the other fingers. This enables a monkey to grasp small food items. Most kinds of New World monkeys have thumbs that are only partly opposable. Their thumbs also do not move so freely as those of Old World monkeys. Two kinds of New World monkeys--spider monkeys and woolly spider monkeys--have only tiny thumbs or no thumbs at all. Among Old World monkeys, colobus monkeys have no thumbs. The feet of most monkeys are larger and more powerful than their hands. All monkeys have five toes on each foot. The big toes look and function much like thumbs, giving the monkey an extra pair of grasping "hands." Marmosets and tamarins have claws on their fingers and toes, except for their big toes, which have a nail. All other kinds of monkeys have flat or flattish nails on all their fingers and toes.

Tail. Most monkeys that live on the ground have a shorter tail than do most that live in trees. Arboreal monkeys may have a tail longer than their body. They use their tail for balancing on tree branches. They also use it as an air brake--that is, to slow themselves down when they leap from branch to branch. Some New World monkeys, including howlers, spider monkeys, and woolly monkeys, can grasp objects with their tail. The tail of such monkeys has bare skin at the end. The tail of other monkeys is completely covered with hair.