There are over 300 species of shrews , making them the most diverse group in the order Insectivora (shrews, moles, and relatives). They are found throughout the world, except for Australia, southern South America, and the polar regions.
Most shrews are very small, only between 2 and 3 grams in weight as adults (about the weight of a nickel). Because of their size, shrews must eat almost constantly, they also cannot hibernate. As a result they are active at almost all times of day and throughout the year. Shrews do not live long, probably only for about one year at the longest. Some shrews secrete venomous saliva, which helps them to subdue and eat prey that is as large as them. Humans that are bitten by these shrews may have allergic reactions or experience pain similar to that of a bee sting.
Shrews have small eyes and sense the world mainly through smell, hearing, and touch. Some shrews use a form of echolocation, emitting ultrasonic clicks and interpreting the echoes that return. Most shrews live in and under soil and leaf litter in moist habitats, though some are found in more arid conditions. Some shrews have developed well-haired toes that allow them to swim well, or even to run across the water!