Birth and death
Shrews are small, mouse-like mammals of the family Soricidae, class Insectivora. They have large cutting, or incisor teeth, similar to those of a mouse. But unlike a mouse (which is a rodent and thus has teeth that continually grow), the teeth of shrews must last a lifetime. Also, their snout is narrower and more pointed than that of a mouse.
There are more than 260 species of shrews. They vary upward in size from the pygmy white-toothed, or Etruscan shrew (Suncus etruscus), weighing only 0.07 oz (2 g) and 1.3 in (3.5 cm) long, and probably the smallest mammal in the world. The largest species are the ratsized musk shrew (Sorex murinus) and the African forest shrew (Crocidura odorata), which may reach a weight of more than 3.7 oz (106 g). There are some genera of shrews that have been examined so rarely by biologists that little is known about them.
Shrews live everywhere but the southern half of South America, Australia, and Antarctica. Some of them even live in Arctic regions. The tiny pygmy shrew (Microsorex hoyi), for example, has a range that extends from the tundra of northern Alaska and Canada southward to New England. It is just a millimeter longer than the Etruscan shrew. The pygmy shrew is so small that it has been known to use burrows created by beetles.
One characteristic indicating that shrews are more primitive (i.e., with an older evolutionary lineage) than most mammals is the presence of a cloaca in many species. This is an external opening into which both the genital and urinary tracts empty. Reptiles, from which mammals evolved, also have a cloaca.
Shrews digest their food very rapidly, so quickly, in fact, that much of it is not fully digested. Consequently, some shrews re-eat their feces, to capture the undigested nutrients. Having a large surface: volume ratio, and a very high metabolic rate, shrews must eat almost continuously to get enough food energy to support themselves. This is particularly the case of the smallest species.
The shrew family is divided into two subfamilies, the red-toothed shrews, which get their name from the fact that the tips of their teeth are colored, usually reddish, and the white-toothed shrews, which do not have that coloration. All shrews have a long snout, which gives their head a triangular shape when seen from above. Their snout is mobile and continually moves so that their vibrissae (long, sensory hairs) can do their job. The snout ends in a moist pad. Most shrews have dark-brown fur, though some tend toward yellow, reddish, or gray.
The eyes and ears of shrews are clearly visible on their head (as opposed to the related moles, which have these organs covered with fur). Shrews do not see very well, relying more on smell, touch, and hearing, especially to avoid their primary enemies. The latter are mostly birds of prey and small predatory mammals, such as weasels.
Sound is very important in the life of shrews. Squeaks, squeals, and high-pitched clicks are made on various occasions. Female shrews looking for a mate make a small peeping sound. For the most part, though, shrews of the same species avoid each other, except at mating time. Their territories rarely overlap, and if they meet, they chitter loudly at each other until one gives way. Some shrews can apparently use their high-pitches squeaks as a kind of sonar; the noises echo back from objects, helping the shrews to define their local environment. Many shrew sounds are so high pitched that they cannot be detected by humans.
Shrews prefer moist, well-vegetated habitats. They prey on various invertebrates, such as earthworms and insect larvae, though some shrews will also eat seeds and nuts. A group called the water shrews feeds on aquatic life in ponds, lakes, and streams. Unlike moles, shrews do not burrow much, tending to spend their time on the surface or just under loose cover of plants and litter.
They will, however, take up residence in burrows abandoned by other digging animals. Shrew territories are marked by a musky odor. A few species of shrews will climb shrubs and trees in search of prey.
Several genera of water shrews dig burrows in the banks of rivers and lakes, with the entrances underwater. They feed on aquatic worms, snails, and insect larvae. Their long, narrow toes have an edging of stiff hairs that works as a substitute for webbed toes. Only one species, Nectogale elegans, has webbed feet.
Some shrews, such as the American short-tailed shrew (Blarina brevicauda), have poison in their salivary glands that allows them to prey on animals much larger than themselves. Some water shrews with poisonous bites can kill large fish. The poison, which acts on the prey's nervous system, has been known to cause pain in bitten humans for several days.
Within a colony of shrews, the breeding season may last seven or eight months. The female weaves an enclosed, dome-shaped nest of grasses and moss, often hidden beneath a log or in a burrow. After a gestation period of 25-30 days, she produces 5-11 blind and hairless young. The young make loud squeals that sound almost like barks. By the time the female stops nursing the young, they are almost as large as she is. Some mother shrews take their young on exploration adventures in which each one links to the sibling before by grasping its fur in the mouth, making a living chain of shrews. They reach sexual maturity at less than a year and begin to breed in late spring.
The common shrew (Sorex araneus) of Europe averages about 2.3 in (6 cm) long plus a tail about half that length, and weighs about 0.35 oz (10 g). It often lives near human dwellings, liking compost heaps and hedgerows. It has the ability to become pregnant with a new litter immediately after giving birth to the previous litter. Thus a female may be nursing and gestating at the same time. Both events last only about two weeks.
Most shrews die before a new winter sets in, giving them a life span of little more than a year. Only the most recent generation survives the winter. They also molt twice a year, growing summer fur in the springtime, and winter fur in autumn. Because shrews are extremely nervous little mammals with a high metabolic rate, they can die of starvation after just a few hours without food. They can also die of fright.
See also Tree shrews.
Bailey, Jill. Discovering Shrews, Moles & Voles. New York: The Bookwright Press, 1989.
Caras, Roger A. North American Mammals: Fur-Bearing Animals of the United States and Canada. New York: Meredith Press, 1967.
Kerrod, Robin. Mammals: Primates, Insect-Eaters and Baleen Whales. Encyclopedia of the Animal World Series. New York: Facts on File, 1988.
Nicoll, Martin E., and Galen Rathbun. African Insectivora and Elephant-Shrews: An Action Plan for Their Conservation. Island Press, 1991.
Jean F. Blashfield
Science EncyclopediaScience & Philosophy: Semiotics to Smelting