The term monkey is commonly applied to a large number of primates, excluding only the tree shrews, the lemurlike forms, the apes, and humans, and therefore embraces an enormous evolutionary and adaptive array of animals. This array, however, can be divided into two main groups--the New World monkeys and the Old World monkeys--which represent quite separate evolutionary histories. These two major divisions--technically called the Platyrrhini (New World monkeys) and the Catarrhini (Old World monkeys, but also including apes and humans)--were established in 1812 by the French zoologist Etienne Geoffroy de Saint-Hilaire. The terms chosen by Geoffroy refer to the external structure of the nose (-rhini is from the Greek word for nose): the Old World monkeys, or catarrhines, are characterized by nostrils that are close together and tend to open downwards, whereas the New World monkeys, or platyrrhines, have nostrils that are wide apart and open toward the sides. Not all monkeys, however, closely fit these distinctions. The two groups also show differences in the bony parts of the ear and in the teeth. In the New World monkeys the ear drum (tympanum) is encased in a bony ring (tympanic ring), and both the eardrum and the bony ring are at the surface of the skull. In the Old World monkeys the eardrum is situated within the skull, and the bony ring is formed into a tube (the external auditory meatus) that leads to the outside. New World monkeys have three premolar teeth on each side of the upper and lower jaws, whereas Old World monkeys have only two premolar teeth in each of these quadrants. The most important distinction between the two groups, however, is their separate evolutionary development. It is thought that both arose from the same primitive lemurlike primates inhabiting the formerly connected land masses of North America and Europe, but that the two groups probably diverged from this common ancestral stock at least 55 million years ago, at about the end of the Paleocene Epoch, with the ancestors of the New World forms moving toward Central and South America and those of the Old World monkeys toward Africa. OLD WORLD MONKEYS Despite their wide geographic distribution, the Old World monkeys are a relatively similar group and are classified as the superfamily Cercopithecoidea, whose members are referred to as cercopithecoids. This superfamily contains a single living family, the Cercopithecidae, which is divided into two subfamilies, the Cercopithecinae and the Colobinae. The cercopithecoids are the only primates, apart from humans, with representatives outside the tropics and subtropics. The Japanese macaque, Macaca fuscata, the stump-tailed macaque, M. arctoides, and the langur, Presbytis entellus, have all been observed living successfully in snow-covered habitats. The subfamily Colobinae, which contains the colobus monkeys, langurs, and other related forms, is primarily Asiatic, with only one genus, Colobus of Africa, being found elsewhere. In contrast, the subfamily Cercopithecinae, containing the baboons, macaques, guenons, and others, is largely African, with two genera, Papio and Macaca, also occurring in Asia, the latter being widespread. Characteristics All Old World monkeys possess special pads on their buttocks, which aid in sitting for long periods of time, as in nighttime sleeping. These pads, called ischial callosities, are thick, horny calluses that cover swellings of the ischial bone at the bottom rear of the pelvis, or hipbone. In virtually all species the forelimbs and hind limbs are almost of equal length, which generally indicates a quadrapedal, or four-legged, locomotion; however, all species climb well. Some cercopithecoids, especially the macaques, show a reduction in the length of the tail. All species except the colobus monkeys have hands and feet of moderate length and with opposable first digits (thumbs and big toes); in the colobus monkeys the thumb is reduced to a small nub or may be totally lacking. Opposability means that the thumb or big toe is set apart from the other digits and can be moved so as to bring its fleshy lower portion into contact with the fleshy lower portions of one or more of the other digits. Opposability is an adaptation for grasping and for the finer manipulation of objects; it also permits eating with one hand, in contrast to the manner of rodents or other animals that must hold food with two hands. Each molar tooth has four projections, or cusps, and is bilophodont, that is, each pair of cusps, front and rear, are joined together by a cross ridge, or "loph." Many species, particularly those in the subfamily Cercopithecinae, show a pronounced sexual dimorphism, the males being considerably larger than the females. The size range among the Old World monkeys is also substantial; fully grown talapoins, Cercopithecus talapoin, may weigh only about 2.25 kg (5 lb), whereas large male mandrills, Mandrillus sphinx, or baboons, Papio ursinus, may tip the scales at 45 kg (100 lb). One of the chief characteristics of the subfamily Cercopithecinae is the cheek pouches, which are lacking in the members of the subfamily Colobinae. Cheek pouches are muscular cavities in the cheeks that extend below the jaws and are used for the temporary storage of food. The most striking specialization of the subfamily Colobinae is the complex stomach, which is divided into pockets, or chambers (sacculations), and banded by two longitudinal muscular strips. The sacculated stomach is involved in the processing of the cellulose contained in leaves eaten by these monkeys. Longevity in the wild is difficult to estimate, but members of both subfamilies have lived from 20 to 30 years in zoos. Usually only single infants are born; twin births do occur but are relatively rare in all species. Old World monkeys share the primate tendency toward lengthening the time an infant is dependent on its mother and the duration of the maturation process; most female cercopithecoids reach sexual maturity only at about 31/2 years of age, and males up to a year or more later. Gestation periods vary; an average for both subfamilies would be about 6 months (170 to 190 days). Females show a regular menstrual cycle of about 30 days; estrus, the period within which ovulation occurs and when the female is fertile, lasts from 5 to 9 days and may or may not be accompanied by external signs such as the swelling and color changes of skin in the genital region. Sexual skin swelling is pronounced in the baboons. NEW WORLD MONKEYS The New World monkeys also constitute a single superfamily, the Ceboidea, but because they are more diverse than the Old World monkeys, their superfamily comprises three families: the Callitrichidae (or Callithricidae), containing the marmosets and tamarins; the Cebidae, containing the capuchins, squirrel monkeys, and others; and the Callimiconidae, containing only Goeldi's monkey, which possesses characteristics of the other two families and has often been classified in one or the other. Characteristics New World monkeys are all arboreal; there are no ground-dwelling species as occur in the Old World. They inhabit forests from sea level to altitudes of about 1,500 m (5,000 ft). New World monkeys are usually slender-bodied animals with slender limbs and long tails. Unlike that of the Old World monkeys, the tail may be prehensile and used as a "fifth hand." New World monkeys lack the buttock pads (ischial callosities) of the Old World species and frequently sleep in a horizontal position. Females also lack the sexual skin around the genitals that enlarges and changes color when the female is in a sexually receptive state, but many females have external genitals which themselves may be enlarged and swollen in appearance. The opposability of the thumb in New World monkeys is more limited and has been termed pseudo-opposability because although there is no rotation at the wrist-thumb joint as in true opposability, a downward movement of the thumb and the bending of the hand result in the meeting of the thumb and one or more fingers for a functionally opposable grip. Marmosets have short thumbs and do not possess even pseudo-opposability, but their hands are long enough to be folded upon themselves to permit grasping and one-handed feeding. Thumb opposability is also lacking in the spider monkeys, Ateles, which have vestigial thumbs or are thumbless, and in the woolly spider monkey, Brachyteles, in which the thumb is also reduced or missing. The big toe of New World monkeys, however, is opposable in the same manner as that of the Old World species. New World monkeys range in size from the pygmy marmoset, Cebuella pygmaea, which may weigh as little as 70 g (2.5 oz), to the hefty howler monkey, Alouatta, which reaches 9 kg (20 lb). The marmosets and tamarins, family Callitrichidae, are small, most of them weighing about 500 g (1 lb), and are strikingly distinguished by possessing clawlike nails on all digits except the first toe and by the presence of only two molar teeth instead of the more typical three on each side of the upper and lower jaws. As in most quadrupeds, the forelimbs are only slightly shorter than the hind limbs. The marmosets are found in both moist and dry forests in the Amazon River basin and differ from the tamarins in being somewhat smaller and in having elongated incisor teeth. They appear to be monogamous, staying with a single mate; females often bear twins, and the male may assist in the birth and carry the young about. Primarily arboreal and diurnal, marmosets supplement their vegetarian diet with insects. Tamarins are found primarily in the rain forests of Central and South America and appear to resemble the marmosets in their diet and activity pattern, although they may form larger social groups. The Goeldi's monkey, Callimico goeldii, the only member of the family Callimiconidae, is generally similar to the marmosets but possesses three molar teeth. Little is known of its behavior in the wild. The members of the family Cebidae--all distinguished by the possession of three molar teeth on each side of the upper and lower jaws and by digits carrying flat nails--cover a great diversity of types; consequently, usually five and recently seven subfamilies have been recognized. The subfamily Aotinae contains the douroucouli and usually the titis, but the latter are now considered by some authorities to constitute a separate subfamily, the Callicebinae. The douroucouli, or owl monkey, Aotus trivirgatus, is the only nocturnal monkey in the world. The squirrel monkeys, Saimiri, usually classified in the subfamily Cebinae, are now also considered by some authorities to form a separate subfamily, the Saimiriinae. There are two species, both feeding primarily on fruit and insects. The uakaris, Cacajao, and the sakis, Pithecia and Chiropotes, are representatives of the subfamily Pitheciinae. They are found from the Guianas down to the Amazon basin. The uakaris, which are the only short-tailed New World monkeys, are almost hairless on the face and top of the head and appear bald. In contrast, the sakis have thick hair on the head, forming a hood or a crest with a coiffured look. The capuchins, Cebus, in the subfamily Cebinae, are famous for their impressive performances in laboratory tests of intelligence. They are found throughout Central America and forested South America, where they live usually in small multimale groups. They apparently have quite complex patterns of social relationships. The howler monkeys, Alouatta, in the subfamily Alouattinae, are one of the best-known New World monkeys. Their name is derived from their impressive roaring displays used in defending their territories. These remarkable vocalizations are produced with the aid of highly specialized structures. The hyoid, a supportive bone at the base of the tongue, is enlarged and formed into an egg-shaped hollow box, which acts as a resonating chamber. The thyroid cartilage of the larynx (voice box) and the lower jaw are also enlarged. The subfamily Atelinae contains the spider monkeys, Ateles, the woolly monkeys, Lagothrix, and the woolly spider monkey, Brachyteles, which are the most specialized in their locomotor apparatus of all the New World monkeys. Like the howlers they possess a fully prehensile tail with a sensitive tactile surface at its tip, but they are much more active and acrobatic. Unlike the quadrupedal howlers, the atelines have rather elongated forelimbs and regularly use their long tail as a "fifth limb" in locomotion; they frequently suspend or propel themselves by their arms or even hang by the tail alone. The spider monkeys and the woolly spider monkey lack thumbs, the hand being essentially modified into a long hook, but the woolly monkeys do possess them. As one might expect, the reproductive biology of the New World monkeys is much more variable than that of the Old World monkeys, although it is generally poorly known. Twin births are the rule among the marmosets and tamarins; single offspring are normal for members of the family Cebidae. There are few indications that any members of the group have a restricted breeding season. Gestation periods range from about 140 days in the marmosets to 225 days in the woolly monkey; most fall in the range of 140-180 days. Sexual maturity is reached in 3 to 4 years among the large forms and in little over a year in some of the smaller ones; pygmy marmosets reach adult size, if not reproductive status, in only 24 weeks. Longevities in the wild are unknown but probably range from about 10 years to upwards of 20. The earliest occurrence of Old World monkeys in the fossil record is arguable. Some authorities would claim that the two genera Parapithecus and Apidium, found in Oligocene deposits, about 30 million years old, in the Fayum area of Egypt, represent the earliest-known catarrhine monkeys, but others believe that this is not the case. In any event, even if these two genera of marmoset-sized animals are early monkeys, they show no close relationship with any living cercopithecoid. The oldest recognizable cercopithecoid monkeys come from Miocene deposits in East Africa and North Africa. Two specimens, a frontal bone and an isolated tooth, are known from the 19-million-year-old site at Napak, Uganda, and have been claimed to show that the two cercopithecoid subfamilies, Cercopithecinae (represented by the molar) and Colobinae (represented by the frontal), were already in existence at that time; this is far from certain, however. Better evidence for this theory may come from the 18-million-year-old site at Rusinga Island, Lake Victoria, where a form known as Victoriapithecus appears to be a member of the Cercopithecinae. Skeletal bones from a nearby, and perhaps somewhat younger, site appear to have belonged to this animal and show some terrestrial adaptations, whereas others may have come from another form, more arboreal and colobinelike. Probably slightly older than these East African fossils are three lower jaws from Wadi Moghara, in Egypt, but although the genus to which they belong, Prohylobates, was clearly a monkey, its precise relationships are unclear. Although the fossil record of the Old World monkeys is patchy at best, good evidence is available that the two modern subfamilies became separated by about 11 million years ago and possibly even as early as 18 to 19 million years ago. The fossil record of the South American monkeys is quite poor. The earliest-known primate from South America is represented by only a single upper-jaw fragment on which the genus Branisella has been based. The relationships of this little known animal are unclear, but it does serve to confirm the presence of primates in South America as early as perhaps 35 million years ago. Branisella is presumably a descendant of one of the groups of primates that flourished in North America during the early Tertiary. Scientists believe that fossil skull fragments discovered in Colombia are from a primitive owl monkey, genus Aotus, that lived between 12 and 15 million years ago. Perhaps the most puzzling of all the extinct New World monkeys is also the most recent. Xenothrix, known only from a lower jaw from a (Late Pleistocene) cave deposit in Jamaica, has been placed alone in its own family, although a link has tentatively been suggested with the titis and the owl monkey. IAN TATTERSALL Bibliography: Ashton, E. H., and Holmes, R. L., eds., Perspectives in Primate Biology (1981); Benirschke, K., ed., Primates (1985); Berger, G., Monkeys and Apes (1984); Bourne, Geoffrey, Primate Odyssey (1974); Bramblett, Claud A., The Patterns of Primate Behavior (1976); Bromley, L., Monkeys, Apes, and Other Primates (1981); Clark, W. E., Antecedents of Man (1971); De Vore, Irven, and Eimerl, Sarel, The Primates (1965); Dukelow, W. R., and Erwin, J., eds., Comparative Primate Biology, vol. 3 (1984); Dunbar, R. I. M., Primate Social Systems (1988); Else, J. G., and Lee, P. C., eds., Priamte Ecology and Conservation (1986) and Primate Evolution (1986); Jolly, Alison, The Evolution of Primate Behavior (1972); Moynihan, Martin, The New World Primates (1976); Napier, J. R. and P. H., eds., The Natural History of Primates (1985) and Old World Monkeys (1970); Time-Life Books, Monkeys and Apes (1976); Richard, A. F., Primates in Nature (1985); Tuttle, Russell H., ed., The Socioecology and Psychology of Primates (1975); Von Hollander, Jurgen, Monkeys, ed. by Hans Reich (1974).