The order Insectivora, which includes the shrews (family Soricidae), is the oldest living order of placental mammals. Shrews (about 290 species) are found worldwide, except for Australia, southern South America and the polar regions. Shrews are very small, with none being longer that 5 cm and 180 g; the lightest mammal in the world, at about 2 g (mass of a penny), is a shrew. Their small size leads to one of their chief problems, that of heat conservation and high metabolic rates. They need a continual source of energy (namely food) to keep warm and alive. They do not hibernate, and how they procure food during the cold parts of the winter is mostly a mystery. The seven species found in Saskatchewan are carnivores feeding chiefly on arthropods, but they will sometimes kill and eat other small mammals.
Several of the shrew species (the common shrew Sorex cinereus and the pygmy shrew Microsorex hoyi) are found in a variety of habitats throughout the province. The American water shrew (S. palustris) and the Arctic shrew (S. arcticus) live in boreal waterside habitats. The prairie shrew (S. haydeni) and dusky shrew (S. obscurus) live in marshy areas in the grasslands, while the short-tailed shrew (Blarina brevicauda) is limited to the streams of the aspen parkland in the southeast corner.
Shrews are both diurnal and nocturnal; yet they are rarely seen, even though in many places they are the most common species of terrestrial mammal. Because of this, their biology is not as well understood as that of larger species. Many patrol for insects and other arthropods beneath logs or fallen leaves, and in cavities beneath rocks. Rodent surface runways and burrows may also be followed. Shrews are most commonly associated with moist conditions; but some inhabit arid areas, and aquatic adaptations in the water shrews enable them to dive and swim to feed on aquatic arthropods.
They typically have long pointed noses, with small but functional eyes and visible external ears. Their teeth have high ridges, which break the chitinous exoskeletons of their prey. They are short-haired and dark- furred, often with a lighter belly. They usually have a long scaly tail covered in very short hairs. The feet are five-toed and generally unspecialized. Their litters range from two to ten young, with usually one litter per year; since they usually live only one season, this gives them only one chance to reproduce. They are preyed upon by weasels, snakes, small owls, and other shrews.
Except for the duck-billed platypus, shrews are the only mammals known to be venomous. The short-tailed shrew is an example of a species with venomous saliva. The neurotoxic venom is delivered into the wound along a channel in the first lower incisor teeth from specialized salivary glands located near the base of these teeth, and incapacitates prey. Short-tailed shrews can thus kill mice considerably larger than themselves.