Water shrews are relatively large shrews with males tending to be longer and heavier than females. The total length of a water shrew can range between 130 and 170 mm, and the weight ranges from 8 to 18 grams. Although the color of the fur may be variable, it is generally black or grey-black on the back and a silvery-grey on the belly, but appears more black in the winter and becomes more brown in the summer. Water shrews can have tails varying from 57 to 89 mm in length. The tail is dark above and white or grey below. The hind feet (18 to 21 mm) are larger than the fore feet and have a trim of 1 mm long stiff hairs on the toes and the inner and outer sides of the feet. A fringe of smaller stiff hairs is also found on the fore feet.
Water shrews are found throughout Alaska and Canada to the northern mountain regions of the United States.
Water shrews are common inhabitants of northern forests. As the name would suggest, water shrews are often found around streams and other aquatic habitats. Moist areas surrounded by heavy vegetation, logs and rocks are preferred.
The breeding season is usually from December to September. In one breeding season, two to three litters may be produced, each litter ranging from 3 to 10 offspring. Females are pregnant for three weeks and then birth takes place in spring or summer. Males reach sexual maturity in the winter following birth. Most females, like males, become sexually mature in winter and breed in late winter or early spring, but there have been reports that some reproduce during their first summer.
Like all female mammals, water shrew mothers provide their young with milk after they are born.
Water shrews are short-lived. The typical life span of a water shrew is about 18 months.
Water shrews are solitary creatures, active throughout the day and night. When active, water shrews dive and swim in water to forage for food. Water shrews can dive year-round, even in cold bodies of water in the winter. Each dive can last from 31.1 to 47.7 seconds. In water, the fur is lined with a layer of air that reduces their heat loss by 50% and makes them float. Therefore, when water shrews swim or dive, they must paddle hard to keep from drifting to the surface. The hind feet, and the stiff hairs on them, propel them through the water. Immediately after swimming, water shrews dry off their fur using the hind feet. Besides swimming, some water shrews have been seen walking on the surface of water. It has been suggested that water shrews can walk on water because they can trap air bubbles in the stiff hairs of their feet.
Nests of water shrews are usually about 8 cm in diameter and are either new nests or reconstructions of old nests built from dried plant material in tunnels or under hollow logs. Water shrews dig their own tunnels by digging with the fore feet and throwing out soil with their hind feet. New nests are built using their feet and legs to form a depression and the walls of the nest shaped with the snout.
Water shrews are aggressive and fighting is common between them. Males and females are equally likely to fight. Most encounters are short but may be intense. Fights between two individuals usually start off with each giving off high pitched squeaks followed by standing on their hind legs to expose their light-colored bellies. If neither shrew retreats after these displays, they will begin to slash each other with their teeth as they wrap up into a tight ball. Head and tail injuries often occur. These fights have not been proven to be territorial.
Sensory abilities of water shrews are not well understood. The whiskers and the snout are thought to serve the purpose of locating prey. During explorations water shrews release continuous high pitched sounds. This has led people to believe that water shrews echolocate. Water shrews give off a strong, sometimes nauseating odor, suggesting that they have a well-developed sense of smell. These odors may help them attract mates or to recognize other water shrews.
Water shrews are, for the most part, insectivores. Diving to the bottoms of streams or other water habitats, they forage for aquatic insects, especially for the larvae and nymphs of caddisflies, crane flies, mayflies, and stoneflies and occasionally for small fish . Besides aquatic animals, they will also feed on land for flies, earthworms, snails, fungi and green vegetation. Once it's caught, the food is held by the fore feet and torn to pieces using the teeth through upward thrusting of the head. Water shrews can live without food for up to 3 hours, but captive shrews have been found to feed almost every 10 minutes. The amount of food required by a water shrew has been estimated to be 0.95 g/day.
Water shrews are important predators of the insects on which they feed, and they are an important food source for the predators listed above.
The water shrew has no known negative effects on humans.
Water shrews are widespread but rarely captured.
Allison Poor (editor), University of Michigan-Ann Arbor.
Ma Carmen (author), University of Toronto.
Beneski, J., D. Stinson. 1987. Sorex palustris. Mammalian Species, 296: 1-6.
Boernke, W. 1977. A comparison of arginase maximum velocities from several poikilotherms and homeotherms. Comp. Biochem. Physiol., 56B: 113-116.
Calder, W. 1969. Temperature relations and under water endurance of the smallest homeothermic diver, the water shrew. Comp. Biochem. Physiol., 30A: 1075-1082.
Conaway, C. 1952. Life history of the water shrew (Sorex palustris). Amer. Midland Nat., 48: 219-248.
Jackson, H. 1928. A taxonomic review of the American long tailed shrews. N. Amer. Fauna, 51: 1-238.
Nagorsen, D. 1996. Opossums, Shrews of British Columbia. British Columbia: Royal British Columbia Museum.
Sorenson, M. 1962. Some aspects of water shrew behavior. Amer. Midland Nat., 68: 445-462.
Whitaker, J., W. Hamilton. 1998. Mammals of the Eastern United States. New York: Cornell University Press.
Wilson, D., S. Ruff. 1999. Water shrew, Sorex palustris. Pp. 38-39 in The Smithsonian Book of North American Mammals. Vancouver: UBC Press.