Sorex palustris
(Water Shrew)

Range:
The water shrew is from southern Alaska and Yukon Territory, south through California, the Rocky Mountains, northern Great Lakes region, and New England. Disjunct population exists in Appalachians.

Description:
Of all the shrews in Idaho, the water shrew may be the most striking and certainly the largest. In fact it is the largest of all shrews in Idaho weighing 0.33 to 0.50 ounce (12 ­18 grams). Its total length is 5.5 to 6.7 inches (137-168 mm), tail length is 2.8 to 3.4 inches (70-85 mm). It is grayish-black on top and silvery white to grayish underneath, its fur has been described as iridescent. It has a fringe of stiff hairs on the edge of the hind foot and a partial webbing of skin between the third and fourth toes of the hind feet; adaptations for swimming, a common behavior which gives the water shrew its name. They are good swimmers and their dense fur traps air, which provides insulation and keeps their skin dry. Underwater, the trapped air forms bubbles that surround them and give them a striking , silvery appearance when they are swimming.

Habitat:
Most abundant along small, cold streams with thick overhanging riparian growth. Also found around lakes, ponds, and other aquatic habitats. Rarely found far from water. In Idaho, they are found in mountain and foothill streams, lakes, and wetlands (marshes, bogs, fens.) In Montana a study found them predominantly along streams where the banks were composed of large boulders and tree roots, which formed numerous crevices and overhanging ledges.

Diet:
They are primarily dependent upon aquatic insects, but will also eat various other invertebrates. May take small vertebrates (fishes or amphibians) when available.

Ecology:
Water shrews are generally active throughout the day in every season (two major activity periods have been reported: sunset to 4 hr after sunset; and just before sunrise). They hunt under and on top of water. May be seen running across water surface which is accommodated by their webbing and stiff fringe hairs on their hind feet, and their dense fur which traps air and gives they buoyancy. In a Manitoba study, home range for 2 individuals was 0.2-0.3 ha (0.5-0.75 ac). A Michigan study found 7 individuals along 20 m section of stream. In Idaho, predators include snakes, weasels, fish, owls, hawks, and large frogs. The species possesses highly odoriferous flank glands, acute hearing, and (possibly) echolocation.

Reproduction:
In Montana, it breeds from February to August. Gestation probably takes 3 weeks. litter size varies from 3 to 10 young (average 6). In Montana, females produce 2 to 3 litters per yr. Young become sexually mature in second calendar year (females before males), and live a maximum of 18 months.

Conservation:

Status:	Unprotected nongame species
Global Rank:	G5
State Rank:	S4

Important State References:
Medin, D.E. and W.P. Clary. 1990. Bird and small mammal populations in a grazed and ungrazed riparian habitat in Idaho. USDA Forest Service Inter. Res. Sta. Res. Paper INT-245.