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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

The hardy water shrew is active day and night and can swim under the ice. © Moose Peterson
The hardy water shrew is active day and night and can swim under the ice. © Moose Peterson

October 2003

Bobbing, bustling and busy

Water shrews work the surface, shore and streambed in all seasons.

Ann Bailey Dunn

Among the unique creatures you may spy streamside, keep an eye out for water shrews, but you'll have to watch closely. They spend more of their time under the water than on it.

This five- to six-inch shrew is mostly aquatic and is active throughout the year. It swims, dives, floats, pops to the surface, runs along the surface (up to five feet, thanks to webbed feet) and will run along a stream bottom. In winter, this hardy shrew swims under the ice.

The water shrew (Sorex palustris) has large hind feet. The third and fourth toes are partially webbed and a fringe of stiff whitish hairs mat between the other toes to assist the shrew in swimming and provide warmth in winter. Guard hairs keep the shrew warm and dry trapping air bubbles in its fur.

The guard hairs have another interesting adaptation. A microscopic view of the guard in cross-section shows the hair is shaped like the capital letter "H." This unusual shape provides a lot more surface are for air to adhere to each hair. So many air bubbles are trapped that the swimming shrew appears silvery under water like a speeding little bullet. On the other hand, all those trapped bubbles make the shrew extremely buoyant, and it can only stay submerged for about 48 seconds. When finished swimming and foraging, the water shrew stops, and trapped air bobs it back to the surface.

Water shrews dive and forage in cool, well-oxygenated streams for aquatic insects, especially the nymphs of stoneflies, mayflies, caddisflies, crane flies and occasionally small fish. Such delicacies are found by probing the bottom mud and crevices of submerged rocks with its nose. It also feeds on land for snails, earthworms and flies. The water shrew has to escape predators like weasels, hawks, owls, snakes and fish to survive.

Shrews live hard and fast. Like all shrews, their heartbeats race quickly, their metabolisms are fast and a "full life" lasts about 18 months. Water shrews may feed every 10 minutes, although they can survive up to three hours without eating. A more typical pattern is 30 minutes of foraging followed by 60 minutes of resting. Shrews are active day and night in an activity pattern referred to as arrhythmic.

Water shrews are mainly found in the northern two-thirds of Wisconsin, north of the tension zone near streams, lakes and wetlands. In winter they may hide out in the tangle of sticks that form beaver or muskrat lodges. They are also at home in the jumbled rocks, logs and undercut banks along forested streams. The shrew's dense velvety fur keeps it warm. Also air trapped between sticks and the water is often much warmer than the surrounding winter weather.

Water shrews are early breeders. Mating activity starts in late January and continues through late summer. A typical litter includes five to seven offspring and adults may have one to three litters in their 18-month life span. Nests are usually constructed in tunnels and in or under hollow logs. The nests are composed of shredded grasses and leaves. Young are weaned and independent at four weeks of age.

So if you see a black mouse-like creature walking on water, bobbing or swimming, your eyes are not playing tricks on you. You are one of the fortunate few to catch a glimpse of the secretive water shrew at work on the water.

Ann Bailey Dunn writes from Campton, Kentucky.