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 Insectivora. Shrews have feet with five clawed toes, unlike rodents, which have four. Shrews are also not to be confused with treeshrews, which belong to their own unrelated order, the Scandentia.
There are 368 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). 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 tropical part. In terms of species diversity, the shrew family is the fourth most successful mammal family of all, being rivalled only by the Muroidea families Muridae and Cricetidae and the bat family Vespertilionidae.
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, or even hunting in water. 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 Pygmy White-toothed Shrew (Suncus etruscus) which at about 3.5 cm and 2 grams is one of the smallest living mammals.
Shrews are unusual among mammals in a number of respects. Unlike most mammals, some species of shrew are venomous. The other venomous mammals are the platypus, solenodon, and slow loris. Also, along with the bats and toothed whales, some species of shrew use echolocation
Shrews have a high metabolic rate and so must feed almost constantly when not sleeping. Shrews are not able to hibernate.
Unlike most other mammals, shrews are born with permanent teeth and do not have a zygomatic bone.
Shrews hold nearly 10% of their mass in their brain, a relatively high brain to body mass ratio.