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By Deborah Ciszek and Phil Myers
With over 300 species in 23 genera, Soricidae is by far the most speciose family in the order Insectivora. Its members can be found throughout the world, with the exceptions of the polar regions, Australia, and southern South America.
The 
skulls of shrews are long and narrow, usually with a flat profile. They lack zygomatic arches, auditory bullae, and postorbital processes. The tympanic bone is distinctively annular (ring-shaped). The mandibles have distinctive doubled condyloid processes, forming a double articulation. The dental formula of this family is 3/1-2, 1/0-1, 1-3/1, 3/3 = 26-32. The first incisor is large and made up of an elongate, projecting main cusp and smaller, posterior secondary cusp. The second cusp is sometimes mistaken for an additional tooth. The first lower incisors are long and forward-projecting. The rest of the incisors, canines, and premolars 1-3 (if present) are small and peglike. These unicuspid teeth are useful for identifying some kinds of shrews. The upper molars are dilambdodont, with a strongly developed W-shaped ectoloph. Shrews lose their milk teeth before birth. Tooth wear can therefore become a problem, and older adults may starve to death when their teeth become too worn to function.
Most shrews are very small, with the smallest only 2-3 grams adult weight. Their eyes are tiny and their main senses are probably touch, hearing, and smell (some species are believed to use echolocation). Shrews have a high metabolic rate and consequently a voracious appetite. Shrews must eat very frequently, and so are active throughout the day and night, feeding primarily on invertebrates. The pointed shape of their snout probably helps them burrow into the ground as they look for prey items such as grubs. Some shrews are poisonous, allowing them to hunt and kill larger prey (including small vertebrates). Most shrews seem to prefer moist microhabitats, although a few species are found in deserts. A few kinds of shrews are aquatic, well adapted to swimming and catching aquatic invertebrates and small fish. Many of these have long, bristly hairs between their toes and along the sides of their feet, which aid them in swimming by increasing the surface area of the feet. These hairs also can hold air bubbles, allowing the shrew to actually run across the surface of the water!
Shrews do not fossilize well because their bones are small and delicate, but some European fossils are known from as early as the late Eocene.
References and literature cited:
Churchfield, S. 1990. The Natural History of Shrews. Ithaca, New York, Comstock Publishing Associates.
Macdonald, D., ed. 1984. The Encyclopedia of Mammals. New York, Facts On File Publications.
Nowak, R.M., and J.L. Paradiso. 1983. Walker's Mammals of the World, 4th ed., Vol. I. Baltimore, Johns Hopkins University Press.
Vaughan, T.A. 1972. Mammalogy. Philadelphia, W.B. Saunders Co.
Yates, T. L. 1984. Insectivores, elephant shrews, tree shrews, and dermopterans. Pp. 117-144 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.
Contributors
Deborah Ciszek (author), University of Michigan. Phil Myers (author), Museum of Zoology, University of Michigan.
