A shrew or shrew mouse (family Soricidae) is a small mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different orders.

Although its external appearance is generally that of a long-nosed mouse, a shrew is not a rodent, as mice are, and not closely related to rodents. Shrew feet have five toes; rodent feet have four, and shrews have sharp, spike-like teeth, not the familiar gnawing front incisor teeth of rodents.

Shrews are distributed almost worldwide: of the major tropical and temperate land masses, only New Guinea, Australia, and New Zealand do not have native shrews at all; in South America, shrews are relatively recent immigrants and are present only in the far north. In terms of species diversity, the shrew family is the fourth most successful of the mammal families, being rivalled only by the rodent families Muridae and Cricetidae and the bat family Vespertilionidae. Shrews were among the first placental mammal, having originally evolved during the dawn age of mammals.

Characteristics

All shrews are comparatively 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 do not hibernate, but are capable of entering torpor. In winter, many species undergo morphological changes that drastically reduce the animal's body weight. Shrews can lose between 30% and 50% of their body weight, shrinking the size of bones, skull and internal organs.

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 molar 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. Shrew venom is not conducted into the wound by fangs, but grooves in the teeth. The venom contains various compounds and the contents of the venom glands of the American short-tailed shrew are sufficient to kill 200 mice by intravenous injection. One chemical extracted from shrew venom may be potentially useful in the treatment of blood pressure while another compound may be useful in the treatment of neuromuscular conditions and migraines.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. The nature of shrew sounds unlike those of bats are low amplitude, broadband, multi-harmonic and frequency modulated. They contain no ‘echolocation clicks’ with reverberations and would seem to be used for simple, close range spatial orientation. In contrast to bats, shrews use echolocation only to investigate their habitat rather than additionally to pinpoint food. Except for large and thus strongly reflecting objects, such as a big stone or tree trunk, theywill probably not be able to disentangle echo scenes, but rather derive information on habitat type from the overallcall reverberations. This might be comparable to human hearing whether one calls into a beech forest or into a reverberant wine cellar.

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).

• Family Soricidae
  • Subfamily Crocidurinae
    • Crocidura
    • Diplomeson
    • Feroculus
    • Paracrocidura
    • Ruwenzorisorex
    • Scutisorex
    • Solisorex
    • Suncus
    • Sylvisorex
  • Subfamily Myosoricinae
    • Congosorex
    • Myosorex
    • Surdisorex
  • Subfamily Soricinae
    • Tribe Anourosoricini
      • Anourosorex
    • Tribe Blarinellini
      • Blarinella
    • Tribe Blarinini
      • Blarina
      • Cryptotis
    • Tribe Nectogalini
      • Chimarrogale
      • Chodsigoa
      • Episoriculus
      • Nectogale
      • Neomys
      • Nesiotites
      • Soriculus
    • Tribe Notiosoricini
      • Megasorex
      • Notiosorex
    • Tribe Soricini
      • Sorex

References

1. http://books.google.com/books?id=z-XJ-UzdwIgC&pg=PA44&lpg=PA44&dq=seasonal+weight+changes+overwintering
2. Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
3. Brains of White Matter
4. Thomas E. Tomasi, "Echolocation by the Short-Tailed Shrew Blarina brevicauda", Journal of Mammalogy, Vol. 60, No. 4 (Nov., 1979), pp. 751–759.
5. Siemers BM, Schauermann G, Turni H, von Merten S. (2009). Why do shrews twitter? Communication or simple echo-based orientation. Biol Lett. 5(5):593-6. PMID 19535367

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• 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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