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From the tiny, playful marmosets of the Amazon rain forest to the formidable, cunning baboons of the African savannah, monkeys hold a particular appeal for humans. Along with apes, monkeys are a popular attraction in zoos, where their antics and their reputation for cleverness endear them to visitors. More controversial is the extensive use of monkeys in biomedical research because of their anatomical, biochemical, and immunological similarities to humans. Despite their close relationship to these remarkable animals, humans pose the greatest danger to the survival of the monkeys, as habitat destruction and hunting have pushed some species to the brink of extinction.

     Monkeys belong to the order Primates, which also includes the prosimians, apes, and humans. All monkeys are categorized as either Old World or New World primates. Although scientists differ in their opinions about some classifications for the families and subfamilies of monkeys, all classification schemes recognize the distinction between these two groups. Most authorities believe that these taxa had a common ancestor in Africa and that the precursors of the New World monkeys crossed the South Atlantic and colonized South America during the Eocene epoch (approximately 38 to 54 million years ago). During this period Africa and South America were much closer together than they are presently, with many small islands between them. The animals could have traversed the oceanic gaps if they became isolated on drifting masses of vegetation. As the continents drifted farther apart and the small islands were covered by water, the two groups were separated and therefore prevented from interbreeding. Their different
Monkeys are found throughout the tropics of Central and South America, Africa, and Asia. They are not native to North America, Europe, or Australia, however. A few species of monkeys, such as the rhesus monkey (Macaca mulatta) of northern China and the Japanese macaque (M. fuscata), occupy temperate habitats. Most monkeys are arboreal forest dwellers, but some of the Old World species are terrestrial and occupy open areas such as savannah grassland.

     Reproduction is nonseasonal in both Old World and New World monkeys, but most species bear their young during periods of the year that are optimal for survival. Seasonal timing may be governed by annual wet-dry cycles. Monkeys have gestation periods ranging from roughly five months to more than seven months. Most species have single young, though the marmosets and tamarins sometimes produce twins.

     Like apes and humans, monkeys are born with their eyes open. They are highly dependent on their parents for care during their early life. In some species, the father provides as much care as the mother. The young mature at 3 to 4 years of age in most species. Captive monkeys have been known to live for up to 45 years, but their life span in the wild is probably much shorter.

     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, have specialized digestive tracts that are adapted for digesting leaves. Other species include insects, eggs, and small vertebrates in their diets.

     Like the apes, most monkeys have opposable thumbs and great toes. In addition, some species have a prehensile, or grasping, tail, which they use to cling to branches. Color vision, acute hearing, and some form of vocalization are also characteristic of both Old World and New World monkeys.

Old World Monkeys

     The Old World monkeys, along with the apes and humans, belong to the infraorder Catarrhini. The noses of catarrhines are narrow, and the nostrils are closely spaced and point downward. (The prefix 'cata' means 'down.') The Old World monkeys are generally larger than New World monkeys. The tails of catarrhines are never prehensile and may be very short or absent altogether. Catarrhines are native to most of sub-Saharan Africa and portions of Asia and the East Indies.

     All Old World monkeys belong to the family Cercopithecidae. Cercopithecids are diurnal, meaning that they are active primarily during the day. The forelimbs are usually shorter than the hindlimbs, and the five digits of the hands and feet have flattened nails rather than the claws found on some New World monkeys. All cercopithecids have ischial callosities, or brightly colored patches of tough, leathery skin on their rumps, which are used in dominance and sexual displays. Cercopithecids exhibit sexual dimorphism, or physical differences between the sexes, which is manifested generally in size but sometimes as distinct coloration or, as in baboons, development of the canine teeth.

     The Old World monkeys are divided into two subfamilies, the Cercopithecinae and the Colobinae.

The Cercopithecinae

     The cercopithecines are a highly adaptable group that can occupy a variety of habitats. Some are arboreal, but others are at home on the forest floor or in open land. The cercopithecines have a snout that accommodates the strong chewing muscles and molars required for an omnivorous diet. Another adaptation of the group is cheek pouches inside the cheeks, which enable them to store a newly found meal and therefore avoid having to linger in dangerous feeding places.

Baboons, drills, and mandrills.

 environments selected different traits, and the two groups evolved into distinct taxa. (See also Evolution.)
Baboons (genus Papio) are terrestrial monkeys found predominantly in open or rocky terrain in Africa. They have doglike faces and walk on all fours. Baboons are boisterous, cunning, and often fierce animals that have been known to raid human settlements for food and occasionally attack humans (see Baboon). The drills and mandrills (genus Mandrillus) of West Africa are large monkeys characterized by prominent ridges on each side of the nasal bones. Mandrills provide an excellent example of the sexual dimorphism found in some primates: the male's fearsome face is an extraordinary combination of a brilliant blue muzzle and bright crimson snout, while the female's face is a dull brown. This difference in body coloration between males and females, called sexual dichromatism, is found particularly in species in which the male must go through an elaborate courtship period to win a mate.

Macaques.

These monkeys (genus Macaca) are primarily Asian. Many live mostly on the ground, and, unlike most monkeys, some can swim. Many are ill-tempered and aggressive. The rhesus monkey (M. mulatta), a macaque of northern India and China, is often exhibited in zoos and used widely in medical research. The Barbary ape (M. sylvanus) of northern Africa is also a macaque. A colony of these monkeys was established and has been maintained for many years on the Rock of Gibraltar by the British.

Guenons.

     These are common, medium-sized monkeys (genus Cercopithecus) of Africa. Primarily arboreal, most species spend their days chattering in treetops. Guenons characteristically have long arms, legs, and tails; small, round heads; and whiskers and beards. Many have brightly colored coats and faces.

Mangabeys.

     The mangabeys (genus Cercocebus) are large, slender-bodied monkeys of African forests. Less aggressive than baboons and macaques, mangabeys are unique among monkeys in being virtually mute. They communicate by complex 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. Mangabeys form social groups that may include more than one family.

The Colobinae

     The colobine subfamily includes the colobus monkeys of Africa and the langurs of Asia. The colobines are sometimes called leaf monkeys because of their primarily folivorous diet. Their stomachs,like those of cows, consist of several compartments. This adaptation enables them to digest the tough cellulose in leaves. (See also Evolution.)

Colobus monkeys.

hese monkeys are divided into three genera. The black and white colobus monkeys belong to the genus Colobus. The extraordinary coat, or pelage, of these monkeys was prized in the international fur trade of the 19th century, which led to a drastic decline in their population that continued with human destruction of their habitat in the 20th century. The red colobus monkeys of the genus Piliocolobus have also declined in numbers because of deforestation and hunting. Although colobus monkeys generally inhabit rain forests, some species of the red colobus are found in mangroves and floodplains; in Senegal and Gambia, they are often found in savannah woodlands. The olive colobus monkey, Procolobus verus, is the sole member of its genus. In this species, newborns are carried in the mother's mouth during the first weeks of life, which is unique among monkeys and apes.

Langurs.

     These are long-limbed, long-tailed monkeys of Asia. They are classified in seven genera and occupy diverse habitats that range from the desert edge to the snowline of the Himalayas. One species of large langurs is suspected by some naturalists of being the legendary 'abominable snowman.' The legend presumably arose from the observation of partially melted, and thus enlarged, footprints in the snow, which appear to have been made by a much larger primate.
The langurs of the genus Presbytis are arboreal forest-dwellers found on the islands of Sumatra, Java, and Borneo; one species also inhabits parts of the Malay Peninsula. The Hanuman langur (Semnopithecus entellus), the sole member of its genus, is well-adapted to a range of environments and inhabits the Indian subcontinent in semi-desert regions, in tropical forests, and in alpine scrubland. It is considered sacred by the Hindus of India and is fed regularly by the people of Jodhpur. The brow-ridged langurs (genus Trachypithecus) are found in the forests of the Indian subcontinent and in parts of Southeast Asia. The strikingly colored douc langurs (genus Pygathrix), which are endemic to Vietnam, Laos, and Cambodia, were pushed to the brink of extinction by years of war in the region and by intensive hunting. Hunting also endangers the snub-nosed langurs (genus Rhinopithecus), which inhabit the mountains of China, Tibet, and northern Vietnam, because their coats are valued for their supposed medicinal purposes. The pig-tailed langur (Simias concolor), found only on the Mentawai Islands off the coast of western Sumatra, is considered one of the most seriously endangered mammals in the world. Its large size makes it particularly conspicuous to hunters.

A member of the leaf-monkey family that deserves special mention is the peculiar-looking proboscis monkey (Nasalis larvatus) of Borneo with its long, banana-shaped nose. The males grow to be quite large, weighing as much as 50 pounds (23 kilograms). Proboscis monkeys are usually found near fresh water, often in lowland rain forests or mangrove swamps. Proboscis monkeys are strong swimmers, perhaps the best among the primates. Clear-cutting of mangroves has endangered the species.

New World Monkeys
The New World,or neotropical, monkeys belong to the infraorder Platyrrhini. They are distinguished from the Old World primates primarily by their noses, which are broad and have widely spaced, outwardly directed nostrils. ('Platy' means 'flat,' and 'rhin' means 'nose.') Most New World monkeys are fairly small, with tails that are usually long and sometimes prehensile. This adaptive trait enables these arboreal monkeys to hang by their tails from tree branches while feeding. Unlike their Old World cousins, the New World monkeys lack ischial callosities. With the exception of the owl monkey, the New World monkeys are diurnal. They are found only in tropical regions of Central and South America.

     The classification of New World monkeys at the family level has been a source of continuing debate among researchers. The most commonly accepted scheme classifies these primates in two families: the Callitrichidae, which includes the marmosets and tamarins, and the Cebidae, which includes the other genera.

The Callitrichidae

     The family Callitrichidae contains the smallest monkeys in the world: the marmosets and the tamarins. They are found only in the rain forests of Central and South America. The coat is soft and sometimes silky, often with characteristic tufts adorning the head. Callitrichids move along horizontal branches in a quadrupedal run that is often interrupted by leaps. All of the digits except for the great toe have claws instead of the flattened nails found in other primates. The callitrichids do not have opposable thumbs.

     Callitrichids are either monogamous or polyandrous, meaning that a female can have one or more than one mate depending on the species. Females produce one to three young a year, and twins are common. Males assist during birth and carry the young on their backs, transferring them to the mother for feeding.

Marmosets.
These monkeys of the Amazon Basin belong to the genus Callithrix, though some authorities place the pygmy marmoset in its own genus, Cebuella. The pygmy marmoset is the smallest known monkey, weighing approximately 4 ounces (113 grams) as an adult. In 1998 scientists announced that they had discovered a new marmoset species in the Brazilian Amazon: the dwarf marmoset, Callithrix humilis, weighing about 12 ounces (340 grams). Marmosets are omnivorous; their diet includes insects, spiders, fruits, small vertebrates, birds' eggs, and tree sap.

     Goeldi's marmoset (Callimico goeldii) is placed in the family Callitrichidae in many classification schemes, but other authorities classify it with the Cebidae. Some contend that the species is neither a callitrichid nor a cebid because it exhibits characteristics of both taxa. Its facial appearance and claws are similar to those of the callitrichids, but its dental formula and skull structure resemble those of the cebids. This rare monkey is also unusual in that it is often found in the understory of the forest rather than in the canopy, where the other New World monkeys are found. The Callimico is endangered because of habitat destruction and hunting.

Tamarins.

     The tamarins are classified in the genera Saguinus and Leontopithecus. Saguinus has a wide geographic range, inhabiting tropical forests, open woodlands, and secondary growth forests in parts of Colombia, Ecuador, Peru, Bolivia, Brazil, the Guianas, and southern Central America. The diet is similar to that of the marmosets, consisting of fruit, insects, spiders, small vertebrates, and tree sap. Many tamarin species are polyandrous-the female has several mates within the family troop.
The genus Leontopithecus contains the three lion tamarin species, which are found only in the lowland rain forests of southeastern Brazil. The name lion refers to the distinctive mane on the shoulders of these monkeys. Despite the establishment of captive-breeding programs for each species, the golden lion tamarin (L. rosalia), the golden-headed lion tamarin (L. chrysomelas), and the black lion tamarin (L. chrysopygus) remain highly endangered.

The Cebidae

     The highly diverse monkeys of the family Cebidae inhabit rain forests from southern Mexico to northern Argentina. They are characterized by long limbs and digits with nails rather than claws. Most cebids, which range in size from approximately 612 to 33 pounds (3 to 15 kilograms), are considerably larger than any callitrichid. All species except the uakaris have a long, heavily furred tail; in some genera the tail is prehensile with a soft, sensitive underside that is naked near the tip. All species other than the night monkeys are diurnal. The diet is generally omnivorous and based primarily on fruits and insects, but the howler monkeys are folivorous.

Night monkeys.
The night, or owl, monkeys of South America (genus Aotus), which are also called douroucoulis, are the only nocturnal monkeys in the world. With large eyes and excellent night vision, night monkeys are well adapted to their nocturnal niche. They move rapidly along tree limbs and exhibit remarkable acrobatics, though the tail is not prehensile. Night monkeys are threatened by habitat loss, hunting, and capture for use in biomedical research.

Squirrel monkeys.

     These monkeys (genus Saimiri) are found in forests and in some cultivated areas of South America and along the Pacific coast of Costa Rica and Panama. Squirrel monkeys are among the most vocal of the primates and are thought to form larger troops than other New World species; troops containing about 300 individuals have been reported. Squirrel monkeys are more abundant than other primates in the Amazon region, but deforestation remains a threat to their populations.

Howler monkeys.

     The howlers (genus Alouatta) are among the largest of New World primates. Head and body length is roughly 24 to 36 inches (61 to 91 centimeters), and the tail is about the same length; adult weights range from 9 to 22 pounds (4 to 10 kilograms). The howler is best known for its remarkable voice, which is the result of the angle of the large lower jaw and a greatly enlarged hyoid bone. Its calls, which can be heard 2 to 3 miles (3 to 5 kilometers) away, have been described as deep howls and gave these monkeys their name. The howler is distinguished from other New World monkeys by its specialized stomach, similar to that found in colobine monkeys, which is adapted for digesting leaves.

Titi monkeys.

     In much of their behavior and appearance, the titis (genus Callicebus) resemble the night monkeys. The titis are diurnal, however, and their preferred quadruped gait is slower and more cautious than the swift, acrobatic movements of the night monkeys. Titis live in small family troops consisting of a monogamous adult pair and their offspring. When titi monkeys sit side by side, their long tails often intertwine.

Spider monkeys.
Spider monkeys (genus Ateles) are exceptionally acrobatic, exceeded in their agility perhaps only by the gibbons. The limbs and prehensile tail of the spider monkey are unusually long relative to its body length. The tail is often used as a fifth arm to pick up objects and swing through trees in the uppermost canopies of rain and montane forests. Their large size and noisy behavior make spider monkeys particularly vulnerable to hunters.

Capuchins.

     Perhaps the most familiar of the New World monkeys are the capuchins (genus Cebus). These vivacious and highly intelligent animals are probably the most common monkeys in captivity in the United States and Europe. Capuchins are favored by organ-grinders and are often used in films and television. All of the capuchins are adaptable and flexible in habitat selection, and one species, C. apella, has the largest range of any New World monkey.

Wooly monkeys.

     As their name implies, these monkeys (genus Lagothrix) of central South America have a distinctive coat: short, thick, and woolly, with a heavy undercoat. They are fairly large among American monkeys, weighing between 12 and 24 pounds (5.4 and 10.9 kilograms). Although they are primarily arboreal, they frequently descend to the forest floor and walk upright, balancing themselves with their arms. They use their extremely long, prehensile tail as an brace while standing.

Muriqui.

     The muriqui (genus Brachyteles), or wooly spider monkey, is one of the rarest and most endangered primates. The sole species, B. arachnoides, is found only in the Atlantic coast forests of southeastern Brazil. The muriqui is the largest of the New World monkeys, with an average weight of 26 to 33 pounds (12 to 15 kilograms). Its thumbs and hands are particularly well-adapted for swinging from branch to branch, enabling the muriqui to travel more rapidly than other arboreal primates.

Uakaris.

     The uakaris (genus Cacajao) are the only South American monkeys with short tails. The face and the top of the head are naked and colored red, white, or black depending on the species. The habitat of uakaris is more restricted than that of most South American primates; they are found mainly along small rivers and lakes. They are quieter than most monkeys and occupy the highest reaches of the canopy.

Sakis and bearded sakis.

The sakis (genus Pithecia) are fairly common in the middle levels and underbrush of forests of the Amazon Basin and the Guianas. They generally live in small family groups, though some appear to be solitary. Despite their nervous and melancholy appearance, they have gentle personalities and make good pets when tamed. The bearded sakis (genus Chiropotes) are not as well known as the sakis. Unlike the sakis, they occupy the canopy of rain forests. They are distinguished by a full, thick beard.