Evolution and systematics

The higher primates (suborder Anthropoidea) are divided into the broad-nosed monkeys of the New World (infraorder Platyrrhini) and the narrow-nosed monkeys and apes of the Old World (infraorder Catarrhini). The Old World monkeys and apes, which occur in Africa, Asia and Southeast Asia, are uniformly characterized by a dental formula of (I2/2 C1/1 P2/2 M3/3) X 2 = 32, differing from all New World monkeys by reduction in the number of premolars from three to two in each tooth row. All Old World monkeys and apes have trichromatic color vision comparable to that of humans. As a group, the Old World monkeys are distinguished from the apes by the presence in both upper and lower jaws of four-cusped molars with the cusps linked in pairs to form transverse cutting ridges (bilophodonty). Furthermore, all Old World monkeys possess well-developed hardened sitting pads (ischial callosities) on the buttocks, supported by broad, roughened bony flanges (ischial tuberosities) on the pelvis. Among the apes, only the gibbons show a similar development. The Old World monkeys are divided into two main groups, the cheek-pouched monkeys (subfamily Cercop-ithecinae) and the leaf-monkeys (subfamily Colobinae). Defining features of these two groups are related to their feeding habits. Whereas all cercopithecine monkeys possess cheek pouches for the temporary storage of food, leaf-monkeys have a complex, multi-chambered stomach as an adaptation for digestion of plant cell walls in their leaf-rich diet with the aid of symbiotic bacteria. There is also a consistent and immediately obvious difference in skull morphology, in that the dis-

A grivet (Chlorocebus aethiops) probes for insects in bark. (Photo by David M. Maylen, III. Reproduced by permission.)
A Guinea baboon (Papio papio) nurses her baby. (Photo by Tom McHugh/ Photo Researchers, Inc. Reproduced by permission.)

tance between the eye sockets (interorbital distance) is small in cheek-pouched monkeys and large in leaf-monkeys.

Two tribes can be recognized among the cercopithecine monkeys: the Cercopithecini (guenons) and the Papionini (baboons, geladas, mangabeys, drill, mandrill, and macaques). The guenons are generally smaller and more arboreal than the baboons and their relatives, which tend to be large-bodied and at least partially terrestrial; but there is some degree of overlap in these features. For instance, the patas monkey (Ery-throcebus patas) is a member of the guenon tribe but is quite large-bodied and predominantly terrestrial.

Old World monkeys commonly show some degree of sexual dimorphism, in which males and females differ in features other than those directly related to reproduction. Males and females of a species can differ markedly in fur coloration, in overall body size and in the size of the canine teeth, although these features can vary to some extent independently. The most striking example of sexual dimorphism in all three aspects is provided by the mandrill (Mandrillus sphinx), in which males weigh more than twice as much as females, have strikingly large canine teeth and are more brightly colored.

Chromosomal and molecular evidence indicates that the two tribes Cercopithecini and Papioni are both monophyletic, each being derived from a separate common ancestor after the cheek-pouched monkeys diverged from the leaf-monkeys. Within the Cercopithecini, there appear to be two main clusters, one formed by most of the forest-living guenons (Cerco-pithecus species) and the other containing the talapoins (Miopithecus), the vervets or grivets (Chlorocebus), l'Hoest's guenon (Cercopithecus lhoesti), and the patas monkey (Erythro-cebus patas). Within the Papionini, there is a basic division between the baboons, macaques, and geladas on one branch and the mandrills on another. Unexpectedly, the Old World monkeys known as mangabeys, which were all originally classified as species of the genus Cercocebus, turned out to belong to two distinct lineages. The genus name Cercocebus is now reserved for more terrestrial species related to the mandrill, whereas the genus name Lophocebus is used for more arboreal species related to baboons.

The early fossil history of the Old World monkeys is still poorly documented. Two early Miocene forms from Africa, from deposits dated at about 20 million years ago (mya) are

The Celebes macaque (Macaca nigra) is native to Sulawesi, part of Indonesia. (Photo by Art Wolfe, Inc./Photo Researchers, Inc. Reproduced by permission.)
A mandrill (Mandrillus sphinx) baring its teeth as a threat. (Photo by C. K. Lorenz/Photo Researchers, Inc. Reproduced by permission.)

Prohylobates and Victoriapithecus, both of which have bilophodont (two-ridged) molar teeth. These early fossil forms were originally known only from isolated teeth and jaw fragments, and this is still the case for Prohylobates. However, a fairly complete skull and parts of the postcranial skeleton have been reported for Victoriapithecus, and as a result it is known that this genus was characterized by a short interorbital distance and by possession of ischial tuberosities on the pelvis. But it is unclear whether Victoriapithecus is specifically related to modern cheek-pouched monkeys, as the small interorbital distance suggests. It is not until the late Miocene and the Pliocene, less than 10 mya, that fossil remains of Old World monkeys become relatively well documented, and by that stage it is certainly possible to distinguish between cercopithecines (relatives of cheek-pouched monkeys) and colobines (relatives of leaf-monkeys). Cercopithecine monkeys are comparatively common in Pliocene deposits of Africa. Many of them resemble modern baboons (e.g. Dinopithecus, Dolichopithecus and Gorgopithecus), and there are also relatives of the modern gelada, some of them almost as big as a female gorilla, which are placed in the same genus Theropithecus. During warm interglacial periods of the middle Pleistocene, macaques related to the modern Barbary macaque (Macaca sylvanus) were present in central Europe, but they subsequently became restricted to North Africa.

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