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By Phil Myers
The Primates are an ancient and diverse eutherian group, currently with around 233 living species placed in 13 families. Most dwell in tropical forests. The smallest living primate is the pygmy marmoset, which weighs around 70 g; the largest is the gorilla, weighing up to around 175 kg.
Primates radiated in arboreal habitats, and many of the characteristics by which we recognize them today (shortened rostrum and forwardly directed orbits, associated with stereoscopic vision; opposable hallux and pollex; unfused and highly mobile radius and ulna in the forelimb and tibia and fibula in the hind) probably arose as adaptations for life in the trees or are primitive traits that were retained for the same reason. Several species, including our own, have left the trees for life on the ground; nevertheless, we retain many of these features.
Primates are usually recognized based on a suite of primitive characteristics of the skull, teeth, and limbs. Some of these are listed above, including the separate and well-developed radius and ulna in the forearm and tibia and fibula in the hindleg. Others include pentadactyl feet and presence of a clavicle. Additional characteristics (not necessarily unique to primates) include first toe with a nail, while other digits bear either nails or claws, and stomach simple in most forms (sacculated in some leaf-eating cercopithecids). Within primates, there is a tendency towards reduction of the olfactory region of the brain and expansion of the cerebrum (especially the cerebral cortex), correlated with an increasing reliance on sight and increasingly complex social behavior.
The teeth of primates vary considerably. The dental formula for the order is 0-2/1-2, 0-1/0-1, 2-4/2-4, 2-3/2-3 = 18-36. The incisors are especially variable. In some forms, most incisors have been lost, although all primates retain at least 1 lower incisor. In others, they are intermediate in size and appear to function as pincers or nippers, as in other groups of mammals. In some primates, including most strepsirhines, the lower incisors form a toothcomb used in grooming and perhaps foraging. In the aye-aye (Daubentoniidae), the incisors are reduced to 1 in each jaw and are rodent-like in form and function. Canines are usually (but not always) present; they vary in size, including sometimes remarkable sexual dimorphism within species. Premolars are usually bicuspid (bilophodont), but sometimes canine-like or molar-like. The molars have 3-5 cusps, commonly 4. A hypocone was added early in primate history, and the paraconid was lost, leaving both upper and lower teeth with a basically quadrate pattern. Primitively, primate molars were brachydont and tuberculosectorial, but they have become bunodont and quadrate in a number of modern forms.
Living primates are divided into two great groups, the Stepsirhini and the Haplorhini. Strepsirhines include mostly arboreal species with many primitive characteristics, but at the same time, some extreme specializations for particular modes of life. Haplorhines are the so-called "higher" primates, an anthropocentric designation if ever there was one. They include many more species, are more widely distributed, and in most areas play a more important ecological role. Haplorhines are further divided into two major groups, the Platyrrhini and the Catarrhini. Platyrrhines have flat noses, outwardly directed nasal openings, 3 premolars in upper and lower jaws, anterior upper molars with 3 or 4 major cusps, and are found only in the New World (families Cebidae and Callitrichidae). Catarrhines have paired downwardly directed nasal openings, which are close together; usually 2 premolars in each jaw, anterior upper molars with 4 cusps, and are found only in the Old World (Cercopithecidae, Hylobatidae, Hominidae).
Most primate species live in the tropics or subtropics, although a few, most notably humans, also inhabit temperate regions. Except for a few terrestrial species, primates are arboreal. Some species eat leaves or fruit; others are insectivorous or carnivorous.
Here, we follow Anderson and Jones (1984) in dividing living primates into two suborders, the Strepsirhini and the Haplorhini. We differ, however, by following Wilson and Reeder (1993) in splitting Megaladapidae from Lemuridae, Galagonidae from Loridae (and in spelling the latter Loridae rather than Lorisidae), and by including humans with the great apes in a single family (Hominidae).
Literature and references cited
Thorington, R. W., Jr., and S. Anderson. 1984. Primates. Pp. 187-217 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
