Evolution and systematics

Higher primates (suborder Anthropoidea) include the broad-nosed monkeys of the New World (infraorder Platyrrhini) and the narrow-nosed monkeys and apes of the Old World (infraorder Catarrhini). Old World monkeys and apes, which are widely distributed in Africa, Asia, and Southeast Asia, are uniformly characterized by a dental formula of I2/2 C1/1 P2/2 M3/3. They hence differ from all New World monkeys by reduction in the number of premolars from 3 to 2 in each tooth row. All Old World monkeys and apes have trichromatic color vision comparable to that of humans. Old World monkeys (superfamily Cercopithecoidea) differ from apes (superfamily Hominoidea) in possessing in both upper and lower jaws four-cusped molars with their cusps linked in pairs to form transverse cutting ridges (bilophodonty). Moreover, all Old World monkeys possess prominent hardened sitting pads (ischial callosities) on the buttocks, which are supported by broad, roughened bony flanges (ischial tuberosities) on the pelvis. Apart from gibbons, such a development is lacking in apes. Old World monkeys (family Cercopithecidae) are divided into two main groups, leaf-monkeys (subfamily Colobinae) and cheek-pouched monkeys (subfamily Cercopithecinae). Defining features of these two subfamilies reflect feeding habits. Whereas all cercopithecine monkeys are characterized by possession of cheek pouches for temporary storage of food, all leaf-monkeys have a complex stomach. The complex stomach, which is unique among primates, is subdivided into 4 distinct compartments (cardiac pouch, gastric sac, gastric tube and pyloric chamber). The complex stomach represents an adaptation for housing symbiotic bacteria to permit digestion of plant cell walls in a typically leaf-rich diet. Available evidence indicates that colobine monkeys have somewhat lower basal metabolic rates than cercop-ithecine monkeys, and this may be connected with their leaf-eating specialization. Leaf-monkeys also differ consistently and obviously from cheek-pouched monkeys in skull morphology: the distance between the eye sockets (interorbital distance) is large in colobines and small in cercopithecines.

Although there seems to be a fairly clear distinction between leaf-monkeys living in Asia and Southeast Asia and those living in sub-Saharan Africa, this is not recognized in any formal subdivision (e.g., as tribes) in current classifications.

Numerous Old World monkeys show some form of sexual dimorphism, in which males and females differ in features not directly related to reproduction. Males and females of a species can differ markedly in general appearance, in overall body size and/or in the size of the canine teeth, although these features can vary somewhat independently. As a general rule, sexual dimorphism is less pronounced in leaf-monkeys than in cheek-pouched monkeys. Nevertheless, there are some quite striking examples of dimorphism in leaf-monkeys as well. The most outstanding example of sexual dimorphism in all three aspects is provided by the proboscis monkey (Nasalis larvatus), in which males weigh more than twice as much as females, have significantly bigger canine teeth and exhibit

The proboscis monkey (Nasalis larvatus) is found in mangrove and lowland forests in Borneo. (Photo by Aaron Ferster/Photo Researchers, Inc. Reproduced by permission.)

more extreme development of the prominent nose that characterizes this species.

Although morphological evidence is equivocal, chromosomal and molecular evidence indicates that the African and Asian groups of leaf-monkeys are both monophyletic, each being derived from a separate common ancestor after the leaf-monkeys diverged from the cheek-pouched monkeys. The African leaf-monkeys, which can be referred to collectively as colobus monkeys, include 15 species belonging to 3 genera (Colobus, Piliocolobus, and Procolobus). All of these monkeys were originally included in the single genus Colobus, but it is now recognized that their diversity merits separation at the generic level. Nevertheless, the Asian leaf-monkeys are undoubtedly more diverse both numerically and morphologically, and the 44 species are allocated to 7 different genera. Some of the Asian leaf-monkeys are generally labeled langurs and can be allocated to 3 genera (Presbytis, Semnopithecus, and Trachyp-ithecus). The remaining Asian leaf-monkeys all show some kind of special modification of the nose and can be collectively labeled "odd-nosed leaf-monkeys." They can be allocated to four different genera: Nasalis, Pygathrix, Rhinopithecus and Simias.

The early fossil history of the Old World monkeys is poorly known. Early Miocene deposits of Africa, dated at about 20 million years ago (mya), have yielded Prohylobates and Victoriapithecus, both possessing bilophodont molar teeth. These early fossil forms were originally known exclusively from isolated teeth and jaw fragments. This is still the case for Prohylobates, but a fairly complete skull and parts of the postcranial skeleton are known for Victoriapithecus. As a result, it is known that this genus was characterized by possession of ischial tuberosities on the pelvis and by a short interorbital distance. It is unclear whether Victoriapithecus is specifically related to modern cheek-pouched monkeys, as might be suggested by the small interorbital distance, but there is certainly no trace as yet of an early relative of leaf-monkeys. It is not until the late Miocene and the Pliocene (less than 10 mya) that Old World monkeys become relatively well documented in the fossil record. By that stage, it is certainly possible to distinguish between colobines (relatives of leaf-monkeys) and cercopithecines (relatives of cheek-pouched monkeys). Skulls of colobine monkeys are comparatively common in Pliocene and Pleistocene deposits of northern and sub-Saharan Africa (e.g., Libypithecus, Paracolobus, and Cercopithecoides). In southern Europe the late Miocene leaf monkey Mesopithecus is documented by several skulls and almost all elements of the skeleton, and the Pliocene genus Dolichopithecus is also well documented. Furthermore, partial jaws and isolated teeth from late Miocene deposits in Pakistan have been allocated to the modern genus Presbytis as the species Presbytis sivalensis.

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