Humans have long been fascinated by monkeys, both as interesting creatures in their own right and as a source of information about the human species. Like apes and humans, monkeys belong to the group of animals called primates. Scientists divide the monkey species in two groups—the Old World monkeys and the New World monkeys.

Where monkeys live

The Old World species are found in Africa and parts of Asia while the New World monkeys are found in the tropical regions of Central and South America. Monkeys are not native to North America, Europe, or Australia.

Most monkeys are arboreal, that is, they live mainly in trees in forests. Some monkeys, however, live on land and occupy open areas. These include the baboons, which prefer the open or rocky terrain. The wooly monkeys are primarily arboreal, but they often come down to the forest floor and walk upright, balancing themselves with their arms. The macaques live mostly on the ground and unlike most monkeys can swim. Proboscis monkeys are also strong swimmers, perhaps the best among the primates.

Some monkeys, like the capuchins, are able to adapt to different kinds of habitats. The langurs of Asia also occupy diverse habitats, ranging from the edge of the deserts to the snowline of the Himalayas. The Barbary apes are primarily native to Asia. A colony, however, has been established and maintained for many years at the Rock of Gibraltar by the British.

Physical features

On the whole, the Old World monkeys are larger than New World monkeys. They have narrow noses and closely spaced nostrils that point downward. Their tails may be very short or absent altogether.

The New World monkeys, in comparison, usually have long tails, which they sometimes use to hold on to branches. These monkeys have broader noses than their Old World cousins and nostrils that are widely spaced and directed outwards.

Monkeys vary greatly in size. The pygmy marmoset monkey, at a length of about 6 inches (14 centimeters) and weighing only 4 ounces (113 grams), is the smallest monkey in the world. The African mandrill monkey is one of the biggest of the monkey group at a length of about 3 feet (1 meter) including the tail stub. It weighs about 43 pounds (19.5 kilograms).

Some monkeys stand out for their unusual physical appearance. The proboscis monkey of Borneo for instance, presents a peculiar appearance with its long, banana-shaped nose. The baboon has a doglike face. The drills and mandrills have prominent ridges on each side of their nasal bones. Male mandrills present a striking appearance with their bright blue muzzle and bright crimson snout. Many guenons of Africa also have brightly colored coats and faces.

Marmosets and tamarins have a soft and sometimes silky coat, and characteristic tufts, or bunches of hair, adorn the head. They are unusual as all the fingers and toes, except the great toe, have claws instead of the flattened nails generally found in other primates. The wooly monkeys of central South America have a distinctive coat: short, thick, and woolly, with a heavy undercoat. The bearded sakis, as their name suggests, have a full, thick beard.

Behavior

Monkey behavior ranges from the friendly to the aggressive towards humans. Baboons are boisterous, cunning, and often fierce. They have been known to raid human settlements and sometimes attack humans. Many macaques are also ill tempered and aggressive.

Among the more friendly monkeys are the sakis. The capuchins are probably the most common monkeys in captivity in the United States and Europe. They are lively and highly intelligent and are often used in films and television.

Monkeys are highly social animals and almost all species live in troops consisting of several females with their young and either a single male or several males. Usually, but not always, the females stay in the troop in which they were born, and so are closely related to each other. Males join new troops upon reaching maturity, so they are unrelated to each other and somewhat hostile.

Most monkey species are diurnal, meaning that they are active primarily during the day. The only exception is the night monkey of South America. Also called the owl monkey, it has large eyes and can see very well in the dark. It moves rapidly along tree limbs and performs remarkable acrobatics.

Squirrel monkeys are among the most vocal of the primates. They are thought to form larger troops than other New World species. Troops containing about 300 individuals have been reported. The howler monkey has a remarkable voice. Its calls, which can be heard 2 to 3 miles (3 to 5 kilometers) away, have been described as deep howls and give these monkeys their name. The mangabeys are unique among monkeys in being virtually mute. They communicate by various facial signals, which include fluttering eyelids. In species with white or light-colored eyelids, the fluttering signals are visible even in the deep shade of an African rain forest.

The diet of most monkeys is based primarily on plant foods, such as fruits, flowers, and seeds. Some monkeys, such as the colobines and the howlers, eat leaves and have specialized digestive tracts to digest them. For this reason, colobines are often called leaf monkeys. Other species also eat insects, birds' eggs, spiders, and small vertebrates (creatures with backbones).

Reproduction and growth

The period of pregnancy in monkeys ranges from roughly five months to more than seven months. Most species produce single offspring, though the marmosets and tamarins sometimes produce twins.

Like apes and humans, monkeys are born with their eyes open. They depend heavily on their parents for care during their early life. In some species, the father provides as much care as the mother. Marmoset and tamarin males, in particular, help the mother during birth and carry the young on their backs, transferring them to the mother for feeding. The olive colobus monkey is unique among primates; the mother in this species carries newborns in its mouth during the first weeks of life.

The young mature at 3 to 4 years of age in most monkey species. Captive monkeys have been known to live for up to 45 years, but their life span in the wild is probably much shorter.

Relationship to humans

Humans have brought many monkey species to the verge of extinction through hunting or destruction of their natural habitat. These species include the black and white colobus monkey, the red colobus monkey, the strikingly colored douc langurs, the snub-nosed langur, the proboscis monkey, and Goeldi's marmoset.

The golden lion tamarin, the golden-headed lion tamarin, and the black lion tamarin also remain highly endangered despite the establishment of captive breeding programs for each species.

Squirrel monkeys are more abundant than other primates in the Amazon region, but cutting down of trees poses a threat to their populations too. Among the rarest and most endangered primates are the wooly spider monkey and the pig-tailed langur.

Because of their similarities to humans, monkeys, especially rhesus monkeys, are often captured for use in biomedical research.