Prosimians are the most primitive of the living primates, which also include the monkeys and apes. The name prosimian means pre-monkey. The living prosimians are placed in the suborder Prosimii, which includes four families of lemurs, (the Lemuridae, the Cheirogaleidae, the Indriidae, and the Daubentoniidae), the bush babies, lorises and pottos (family Lorisidae), and the tarsiers (family Tarsiidae). Some authorities also include the tree shrews, though others separate the treeshrews into an order of their own.
Prosimian are primarily tree-dwellers. They have a longer snout than the monkeys and apes, and the prosimian snout usually ends in a moist nose, indicating a well-developed sense of smell. A larger proportion of the brain of prosimians is devoted to the sense of smell than the sense of vision. Prosimians actively scent-mark their territories to warn other animals of their occupancy. The scent-marks are made with a strong-smelling fluid produced by special glands, or with urine or feces.
Prosimian eyes are large and are adapted for night vision, with a tapetal layer in the retina of the eye that reflects and reuses light. Prosimian eyes are not as well positioned for stereoscopic vision as are the eyes of other primates.
Like all primates, prosimians have hands and feet that are capable of grasping tree limbs. The second toe of the hind foot of prosimians has a long claw which they use for grooming. The other toes, on both the hands and the feet, have flattened nails instead of curved claws. Lemurs, which walk along branches on all fours, have longer hind legs than front ones. Tarsiers, which are adapted for leaping between vertical tree trunks and then clinging to them, have short legs whose bones are fused together for strength.
Prosimians have inflexible faces compared to those of monkeys and apes. Most prosimians have 36 teeth, while west simians generally have 32 teeth. The lower front teeth of prosimians lie horizontally and protrude, forming a grooming structure called a dental comb. The dental comb is used to comb fur and scrape nourishing gum from trees, after which it is cleaned with a hard structure located beneath the tongue.
Prosimians spend much less time in infancy than simians do, perhaps only about 15% of their lifespan as opposed to 25-30% for monkeys and apes.
The early primates were distributed throughout most of the world. Today, however, the majority of the living prosimians, the ones collectively called lemurs, live only on the large island of Madagascar, off Africa. After human beings arrived on Madagascar about 1,500 years ago, at least 14 species of lemurs have became extinct.
The smallest living Madagascar prosimian are the mouse lemurs in genus Microcebus, while the largest lemur is the indri (Indri indri). Other prosimians, often described as those that do not live in Madagascar, fall into groups—the lorises, pottos, and galagos or bushbabies of Africa, India, and Southeast Asia, and the tarsiers of Southeast Asia.
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Napier, J.R., and P.H. Napier. The Natural History of the Primates. Cambridge, MA: The MIT Press, 1985.
Preston-Mafham, Rod, and Ken Preston-Mafham. Primates of the World. New York: Facts on File, 1992.
Jean F. Blashfield