shrew

shrew

[shroo]
shrew, common name for the small, insectivorous mammals of the family Soricidae, related to the moles. Shrews include the smallest mammals; the smallest shrews are under 2 in. (5.1 cm) long, excluding the tail, and the largest are about 6 in. (15 cm) long. Light-boned and fragile, shrews have mouselike bodies and long, pointed snouts with tiny, sharp teeth. They are terrestrial and nocturnal, mostly living under vegetation; some occupy the burrows of other small animals. Their musky odor, produced by a pair of glands on their flanks, deters some of their potential predators. Extremely active and nervous, they have a higher metabolic rate than any other animal. The heart of the masked shrew, Sorex cinereus, beats 800 times a minute, considerably faster than that of the hummingbird. Shrews must eat incessantly in order to stay alive; most will starve to death if deprived of food for half a day. They eat anything available, but prefer small animals; they are economically important as destroyers of insects and slugs that harm crops. Shrews are easily startled and will jump, faint, or drop dead at a sudden noise. They are vicious fighters, killing and eating larger animals, such as mice, as well as other shrews. A belief that the shrew's bite is poisonous was dismissed for years as a folk tale, but has since been substantiated: the saliva of at least one species of shrew is lethal to mice and can cause considerable pain to humans. Shrews live about 15 months and reproduce rapidly, bearing up to four litters a year, with up to eight young in a litter. Shrews are found in Europe, Asia, North and Central America, and N South America. There are over 100 species, all rather similar, classified in about 20 genera. The subfamily of red-toothed shrews, with orange- or red-tipped teeth, includes both Old and New World species; the white-toothed shrews are confined to the Old World. The common shrews of the Northern Hemisphere belong to the red-toothed genus Sorex, with many species in North America and a few in Europe and Asia. The water shrew of Canada and N United States, Sorex palustris, is adapted to aquatic living and can actually walk on the surface of water for a short distance. There are other aquatic shrews, of several genera, in Europe and Asia. The giant water shrew of Africa is not a true shrew but an insectivore related to the tenrec. The elephant, or jumping, shrews of Africa are insectivores of the family Macroscelididae; they resemble miniature kangaroos with trunks. The Oriental tree shrew is an insectivorelike primate. True shrews are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Insectivora, family Soricidae.

Any of more than 300 species of small insectivores constituting the family Soricidae. About 40percnt of these species live in Africa, but shrews are also found throughout the Northern Hemisphere. Shrews are absent from Australia and most of South America. They have tiny eyes and ears, a movable snout, and long, hook-tipped incisors. Typically 2 to 3 in. (6 to 8 cm) long, with a shorter tail, many shrews weigh only about 0.5 oz (14 g). Some are considered the smallest mammals, weighing only a few grams, with bodies less than 2 in. long. Most species live in ground litter, but some live in burrows or trees and a few are semiaquatic. Because they are so small, shrews have the highest metabolic rates of any mammals (with pulses as high as 800 beats per minute). They spend most of their time searching for food, as they can survive only a few hours without eating. Their normal prey is invertebrates such as worms, though some will eat other small animals as well. Some species have toxic saliva (painful to humans). Raptors and snakes eat shrews, but mammals avoid them. Tree shrews (family Tupaiidae) belong to a separate mammalian order (Scandentia) unrelated to true shrews.

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Shrews are small, superficially mouse-like mammals of the family Soricidae. Although their external appearance is generally that of a long-nosed mouse, the shrews are not rodents and not closely related: the shrew family is part of the order Soricomorpha. Shrews have feet with five clawed toes, unlike rodents, which have four. Shrews are also not to be confused with either treeshrews or elephant shrews, which belong to different orders.

Shrews are distributed almost worldwide: of the major temperate land masses, only New Guinea, Australia, and New Zealand do not have native shrews at all; South America has shrews only in the far-northern tropics, including Colombia. In terms of species diversity, the shrew family is the fourth most successful of the mammal families, being rivalled only by the muroid families Muridae and Cricetidae and the bat family Vespertilionidae.

Characteristics

All shrews are small, most no more than mouse size. The largest species is the House Shrew (Suncus murinus) of tropical Asia which is about 15 cm long and weighs around 100 grams; several are very small, notably the Etruscan Shrew (Suncus etruscus) which at about 3.5 cm and 2 grams is the smallest living terrestrial mammal.

In general, shrews are terrestrial creatures that forage for seeds, insects, nuts, worms and a variety of other foods in leaf litter and dense vegetation, but some specialise in climbing trees, living underground, in the subniveal layer or even hunting in water. They have small eyes, and generally poor vision, but have excellent senses of hearing and smell. They are very active animals, with voracious appetites and unusually high metabolic rates. Shrews must eat 80-90 % of their own body weight in food daily. They are not able to hibernate.

Whereas rodents have gnawing incisors that grow throughout life, the teeth of shrews wear down throughout life, a problem made more extreme by the fact that they lose their milk teeth before birth, and therefore have only one set of teeth throughout their lifetime. Apart from the first pair of incisors, which are long and sharp, and the chewing molars at the back of the mouth, the teeth of shrews are small and peg-like, and may be reduced in number. The dental formula of shrews is:

Shrews are fiercely territorial, driving off rivals, and only coming together to mate. Many species dig burrows for caching food and hiding from predators, although this is not universal..

Female shrews can have up to ten litters a year, and the animals only stop breeding in the winter in temperate zones, and breed all year round in the tropics. Shrews have a gestation period of 17–32 days. The female often becomes pregnant within a day or so of giving birth, and lactates during her pregnancy, weaning one litter as the next is born. Shrews live for between 12 and 30 months.

Shrews are unusual among mammals in a number of respects. Unlike most mammals, some species of shrew are venomous. Also, along with the bats and toothed whales, some species of shrew use echolocation. Unlike most other mammals, shrews lack a zygomatic bone (also called the jugal), and therefore have an incomplete zygomatic arch.

Shrews hold nearly 10% of their mass in their brain, a relatively high brain to body mass ratio.

Echolocation

The only terrestrial mammals known to echolocate are two genera (Sorex and Blarina) of shrews and the tenrecs of Madagascar. These include the Vagrant Shrew (Sorex vagrans), the Common or Eurasian Shrew (Sorex araneus), and the Northern Short-tailed Shrew (Blarina brevicauda). The shrews emit series of ultrasonic squeaks. In contrast to bats, shrews probably use echolocation to investigate their habitat rather than to pinpoint food.

Classification

There are 376 species of shrew in 26 genera, which are grouped into three living subfamilies: Crocidurinae (white-toothed shrews), Myosoricinae (African white-toothed shrews) and Soricinae (red-toothed shrews). In addition, the family contains the extinct subfamilies Limnoecinae, Crocidosoricinae, Allosoricinae and Heterosoricinae (although Heterosoricinae is also commonly considered a separate family).

Metaphorical usage

In the English language, the word shrew is also used to describe a woman with a violent, scolding, or nagging temperament, as in Shakespeare's play The Taming of the Shrew. The animals were believed historically to behave aggressively and with cruelty, and to have a poisonous bite; the term "shrew" was then applied to a person thought to have a similar disposition.

Shrew was one of the names originally proposed for the British Fighter Airplane which finally became known as the Spitfire and played a key role in the Second World War.

References

  • Buchler, E.R. 1973. The use of echolocation by the wandering shrew, Sorex vagrans Baird. Diss. Abstr. Int. B. Sci. Eng. 33(7): 3380-3381.
  • Buchler, E.R. 1976. Experimental demonstration of echolocation by the wandering shrew (Sorex vagrans). Anim. Behav. 24(4): 858-873.
  • Busnel, R.-G. (Ed.). 1963. Acoustic Behaviour of Animals. Amsterdam: Elsevier Publishing Company.
  • Forsman, K.A., Malmquist, M.G. 1988. Evidence for echolocation in the common shrew, Sorex araneus. J. Zool., Lond. 216 (4): 655-663. .
  • Gould, E. 1962. Evidence for echolocation in shrews.Ph.D. Thesis, Tulane University.
  • Gould, E., Negus, N., Novick, A. 1964. Evidence for echolocation in shrews. J. Exp. Zool. 156: 19-38.
  • Hutterer, R. 1976. Deskriptive und vergleichende Verhaltensstudien an der Zwergspitzmaus, Sorex minutus L., und der Waldspitzmaus, Sorex araneus L. (Soricidae - Insectivora - Mammalia). Unpublished Ph.D. Thesis, Univ. Wien.
  • Hutterer, R., Vogel., P. 1977. Abwehrlaute afrikanischer Spitzmäuse der Gattung Crocidura Wagler, 1832 und ihre systematische Bedeutung. Bonn. Zool. Beitr. 28(3/4): 218-227.
  • Hutterer, R., Vogel, P., Frey, H., Genoud, M. 1979. Vocalization of the shrews Suncus etruscus and Crocidura russula during normothermia and torpor. Acta Theriol. 24(21): 267-271.
  • Irwin, D.V., Baxter, R.M. 1980. Evidence against the use of echolocation by Crocidura f. flavescens (Soricidae). Säugetierk. Mitt. 28(4): 323.
  • Kahmann, H., Ostermann, K. 1951. Wahrnehmen und Hervorbringen hoher Töne bei kleinen Säugetieren. Experientia 7(7): 268-269.
  • Köhler, D., Wallschläger, D. 1987. Über die Lautäußerungen der Wasserspitzmaus, Neomys fodiens (Insectivora: Soricidae). Zool. Jb. Physiol. 91: 89-99.
  • Sales, G., Pye, D. 1974. Ultrasonic communication by animals. London.
  • Tomasi, T.E. 1979. Echolocation by the short-tailed shrew Blarina brevicauda. J. Mammalogy 60(4): 751-759.

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