Monkeys

A garden-variety term commonly applied to an evolutionary grade of primates, excluding the prosimians (lemurlike and tarsierlike forms), the apes, and humans. This array of monkeys are typically divided into two main groups—the New World monkeys and the Old World monkeys—that reflect not only their current geographic distributions but also, more important, their independent evolutionary histories. These two main divisions can be technically divided into two infraorders called Platyrrhini (which contains only the New World monkeys) and Catarrhini (which contains the Old World monkeys, apes, and humans) within the Hyporder Anthropoidea. These infraorders were established in 1812 by the French zoologist E. G. Saint-Hilaire and chosen simply in reference to the external structure of the nose. Although not all monkeys exhibit these features, Saint-Hilaire felt that the Catarrhines are characterized by nostrils that are close together and tend to open downward, whereas Platyrrhines have widely separated nostrils that tend to open toward the sides. In addition, these infraorders are further characterized by differences in the bony parts of the ear and in their dentition.

Platyrrhines have an eardrum (tympanum) that is encased in a bony ring called the tympanic ring, and both are located at the surface of the skull. New World monkeys also have three premolar teeth on both sides of the upper and lower jaws, giving them thirty-six teeth. Catarrhines, on the other hand, are characterized by a tympanum that is situated well inside the skull and is connected to the outside by a bony tube called the external auditory meatus. Old World monkeys also have only two premolars in each quadrant of the mouth, giving them thirty-two teeth. Despite these general differences, the more important distinction between Platyrrhines and Catarrhines is their evolutionary trajectories. It is thought that both groups arose from a common primitive lemurlike primate at a time when landmasses representative of present-day North America and Europe were still connected. However, by the end of the Paleocene Epoch (55 million years ago), these landmasses had separated, thereby sparking the evolutionary divergence of the two main monkey groups, with Platyrrhines moving toward modern-day Central America continuing into South America, and Catarrhines moving into modern-day Europe continuing into Africa.

New World Monkeys

There are approximately sixteen genera and fifty species of living New World monkeys recognized today distributed throughout the tropical regions of Central and South America. Although new species of monkeys are being discovered as more research is undertaken in the New World, these same environments are fast becoming overdeveloped and destroyed because of human encroachment. This destructive activity has put many more species on the brink of extinction. Pri-matologists consider New World monkeys to be the most diverse in terms of diet, locomotor habits, and overall morphology, but more research is needed. On account of the little that is known about New World monkeys, taxonomic ranking seems always to be in constant revision. Currently, New World monkeys are recognized to constitute a single superfamily called Ateloidea. Within Ateloidea, New World monkeys constitute two families: (1) Atelidae, which contains two subfamilies: Atelinae, containing the spider monkeys, the Muriqui, the howler monkeys, and the wooly monkeys; and Pitheciinae, containing the night monkey, the titi monkeys, the bearded sakis monkeys, and the uakaris monkeys; (2) Cebidae, which also contains two subfamilies: Cebinae, containing the capuchin monkeys and the squirrel monkeys; and Callitrichinae, containing the marmosets and the tamarins.

New World monkeys are characterized by all members being arboreal; no living species of New World monkeys has adopted a terrestrial way of life. They inhabit tropical forests from sea level to forested regions approximately 2,500 m (8,500 ft) in altitude. Most New World monkeys are slender-bodied animals with lean limbs and long tails. In a few species of New World monkeys, the tail is prehensile and is generally used as a "fifth hand" to aid in grasping branches when traveling from one treetop to another, or when foraging for food. New World monkeys lack the buttock pads, known as ischial callosities, typical of the Old World monkeys. Females generally lack the sexual skin around the genitals that enlarges and changes color during estrus (when females are sexually receptive). However, many females have external genitalia that may become enlarged and swollen in appearance during estrus.

New World monkeys also lack a true opposable thumb (pollex), or they are very limited in the degree of opposability—the pollex is set apart from the other digits and can be moved to bring its fleshy lower portion into contact with the fleshy lower portions of one or more of the other digits. This pseudo-opposability reflects the lack of rotation at the wrist-thumb joint (metacarpotrapezium joint complex), but there is a downward movement of the pollex and the bending of the hand, which results in the meeting of the pollex and one or more digits for a functionally opposable grip. Marmosets generally lack even this pseudo-opposability, because their pollices are relatively shorter and their hands are not long enough to bend and meet the pollices—which explains why marmosets never eat with only one hand. Spider monkeys have either a vestigial pollex or lack one altogether, yet these larger New World primates are equipped with prehensile tails. Moreover, all New World monkeys have opposable greater toes (hal-lices) that aid in grasping branches during locomotion. New World monkeys range in size from the pygmy marmoset, Cebuella pyg-maea, which typically weighs as little as 70 gm (2.5 oz), to the large spider monkey, Ateles, which weighs in at 10 kg (22 lb). Overall, we know less about Platyrrhines than all other primates, including the prosimians.

Members of Callitrichinae are generally small, weighing no more than 500 gm (1 lb), and are distinct from other New World monkeys by having laterally compressed nails (which are clawlike) on all digits except their hallices. Marmosets and tamarins also lack a third molar, giving them a tooth count of thirty-two, making them similar in number to Old World primates but not in tooth types. Marmosets inhabit moist and dry forests along the Amazon River basin and are the smallest of this group. Living in monogamous pairs, they typically feed on insects and fruit. Females generally give birth to twins, after which it is the job of the male parent to care for the young.

Members of Cebinae, known as squirrel monkeys (Saimiri), are somewhat larger than the callitrichs (up to 100 gm) and generally live in polygamous groups of up to fifty individuals. Squirrel monkeys have a thigh that is shorter than their shin, which aids in generating more force when leaping. They feed on fruit and insects but can survive by consuming only insects. The larger Cebid (Cebus) weighs up to 4 kg (8 lb), is skilled in leaping, and has developed a partially prehensile tail to aid during its acrobatic leaping. Their tails are unlike those of the Atelinids, in that only the tip of the tail has grasping capabilities. They too live in polygamous groups of up to thirty individuals.

Members of the Atelinae subfamily are the largest of all Platyrrhines and live in polygamous groups of twenty individuals; however, roaming groups more often typically contain only four to eight individuals. Spider monkeys tend to live in mature moist forests and eat fruit and leaves, enjoying a higher percentage of ripe to unripe fruit . Howlers, on the other hand, prefer undisturbed dry to wet forests and woodland savannas and tend to eat more unripe fruit than spider monkeys. Members of Pitheciinae include the colorful nocturnal owl monkey that lives in pair-bonded social groups, and the diurnal titi monkeys that also live in small, monogamous groups of no more than six individuals. Both eat fruit, leaves, and insects. The owl monkey occasionally feasts on small vertebrates and eggs.

Old World Monkeys

There are approximately fifteen genera and eighty-two species of living Old World mon keys recognized today, distributed throughout Africa and Asia from dry grasslands and savannas to rain forests and snowy mountains. Unlike the New World monkeys, a wealth of knowledge is available for Old World monkeys, because of their distribution in areas of high human population. That being the case, seldom are new monkey species discovered, but dozens are flagged for extinction if something isn't done to alleviate the pressure caused by competition with humans. Despite their wide geographic distribution, Old World monkeys are a relatively similar group that constitutes one superfamily called Cercopithecoidea. Within Cercopithecoidea, Old World monkeys constitute only one family, the Cercop-ithecidae, which is further divided into two subfamilies: (1) Cercopithecinae, containing the guenons, macaques, and baboons; and (2) Colobinae, containing the leaf monkeys and colobines.

Old World monkeys are characterized by many species having adopted a terrestrial manner of life. They inhabit most of Africa in a variety of environments, but water seems to be a common limiting factor for those living in drier grasslands and savannas. They also inhabit higher latitudes, with some macaque species keeping warm during snowy days by sitting in naturally heated rock-filled pools of water.

Unlike their cousins, the New World monkeys, Old World monkeys do not have prehensile tails. In fact, many species have either flimsy, uncontrollable tails or extremely reduced tails. Old World monkeys have ischial callosities—thick, rugged calluses that cover the ischial bone (the bony portion of the buttocks); they are used for sitting for long periods of time with little discomfort, such as sleeping in trees or on rock ledges. In almost all species of Old World monkey, forelimbs are similar in size to the hind limbs, which may

Japanese Macaque (Michael Freeman/CORBIS)

reflect their quadrupedal locomotor skills. All species except the colobus monkey have feet and hands of moderate length, and opposable thumbs and great toes. The colobus typically has a reduced pollex almost to the point of being just a nub. The opposable pollex in Old World monkeys allows for finer manipulation of objects and eating with one hand. Cerco-pithecinae are distinct from Colobinae on account of their cheek pouches (muscular cavities that extend below the jaw), which are used to store food for consumption at a later time. Colobinae are distinguished from other Old World monkeys by their complex stomach, which is partitioned into several chambers and equipped to break down usable nutrients from their cellulose-rich diet. Old World monkeys have bilophodont cheek teeth (molars with two pairs of cusps evenly dis tributed on the tooth surface, and each pair connected by a cross ridge or loph). Some species of Cercopithecinae are highly sexually dimorphic (that is, the males are much larger than the females). Old World monkeys vary in size, ranging from 2 kg (5 lb) in adult talapoins to adult male mandrills, which may weigh up to 45 kg (100 lb).

Members of Cercopithecinae known as mangabeys are medium-size monkeys generally restricted to forest dwelling. Mangabeys have large incisor teeth that allow them to exploit hard seeds that are not accessible to other species. Guenons are the African long-tailed monkeys that are typically brightly colored and generally live in forest or scrub savanna. Males are much larger than females. Male vervets are easily recognized by their bright blue scrotum and red penis and perineal patch.

All species are typically omnivorous, with eclectic diets that include fruit, insects, eggs, rodents, shoots, flowers, leaves, and buds. Macaques are to some extent terrestrial monkeys that live in large groups with multiple males. Baboons are the largest members of the Cercopithecinae and are terrestrial. They live in extremely large close-knit groups of up to 300 individuals, moving across the landscape like regimented soldiers. When an outside male tries to enter the group, he is faced with several family-oriented subgroups that must be penetrated if he is to be accepted.

Members of Colobinae include the proboscis monkey, which lives in Borneo tidal mangrove swamps. This monkey, Nasalis, is the largest of the Asian colobines (weighing up to 22 kg [52 lb]) and is distinct from all other primates with its large, protruding, fleshy nose. The langurs (Semnopithecus) reach weights similar to that of Nasalis and are highly adapted to living in mountainous regions, such as the troops living in the Himalayas. The true colobines (genus Colobus) live in the equatorial regions of Africa. Population densities vary greatly, with some species (Presbytis) congregating in groups of up to 125 individuals per square mile, while much larger groups are found in the Hanuman langurs, whose groups have up to 2,000 individuals per square mile. Smaller groups are typical in less forested regions.

Fossil Monkeys

The oldest-known New World monkeys are rare. Branisella consists of several jaw fragments recovered from Oligocene deposits in present-day Bolivia that date to 27 million years ago. Other fossils—Dolichocebus and Tremacebus—are of Oligocene age but found in Argentina; absolute geological dates are not available. Tremacebus is represented by an almost complete skull and jaw fragment, exhibiting both lower and higher primate characteristics, and is thought to be ancestral to Aotus (the owl monkey). Dolichocebus is represented by a highly distorted skull and a few isolated teeth, and is thought to be ancestral to the squirrel monkey. These phylogenetic associations should be considered tenuous at best.

Perhaps the earliest occurrence of an Old World monkey in the fossil record includes Parapithecus and Apidium, discovered in Oligocene deposits in the Fayum region of present-day Egypt and dated at 30 million years ago. Authorities still argue over their evolutionary relationship to other catarrhines. However, a frontal bone and tooth recovered from 19-million-year-old deposits in Uganda show that a common ancestor to both subfamilies (ceropithecinae and colobinae) was already present in the fossil record. This is further supported by the presence of Victori-apithecus, a member of cercopithecinae, recovered in 18-million-year-old deposits on Rusinga Island in East Africa.

—Ken Mowbray

See also: Great Apes; Homo Sapiens; Primates Bibliography

Fleagle, John. 1988. Primate Adaptation and Evolution. New York: Academic; Napier, John. 1980. Hands. London: Allen and Unwin; Napier, John, and Prue Napier. 1985. The Natural History of the Primates. Cambridge: MIT Press; Schwartz, Jeffrey, Ian Tatter-sall, and Niles Eldredge. 1978. "Phylogeny and the Classification of the Primates Revisited." Yearbook of Physical Anthropology 21:95-133; Szalay, Fred, and Eric Delson. 1979. Evolutionary History of the Primates. New York: Academic.

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