Water shrews are common inhabitants of northern forests. As the name would suggest, water shrews are often found around streams and other aquatic habitats. Areas with high humidity surrounded by heavy vegetation, logs and rocks are preferred. (Beneski and Stinson, 1987)
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 (Wilson and Ruff,1999). Although the colour of the pelage may be variable, it is generally black or grey-black dorsally and a silvery-grey ventrally, but appears more black in the winter and becomes more brown in the summer. Water shrews, as a member of the long tailed shrews, can have tails varying from 57 to 89 mm in length (Wilson and Ruff, 1999). The tail is bicoloured, dark above and white or grey below or occasionally concoloured (Beneski and Stinson, 1987). The hind feet (18 to 21 mm) are larger than the fore feet and have a trim of 1 mm long stiff hairs (fibrillae) on the toes and the inner and outer sides of the feet (Peterson, 1966). A fringe of smaller stiff hairs is also found on the fore feet. The skull of the water shrew is large (21 to 23 mm and width 10 to 11 mm) with a dental formula of 1/1 5/1 1/1 3/3 = 32; the fourth upper unicuspid is characteristically smaller than the third. (Beneski and Stinson, 1987; Wilson and Ruff, 1999)
The breeding season is usually from December to September (Nagorsen, 1996). In one breeding season, two to three litters may be produced, each litter ranging from 3 to 10 offspring (Wilson and Ruff, 1999). Three weeks are devoted to gestation and then birth takes place in spring or summer (van Zyll de Jong, 1983; Nagorsen, 1996). Males reach sexual maturity in the winter following birth. During this time, their body weight increases and their testes become enlarged. The testes of sexually mature males can weigh more than 110 mg (Conaway, 1952). Most females, like males, attain sexual maturity in winter and breed in late winter or early spring, but there have been reports that some become reproductively active during their first summer. (Beneski and Stinson, 1987; Conaway, 1952; Nagorsen, 1996; Wilson and Ruff, 1999)
Like all female mammals, water shrew mothers provide their young with milk after they are born.
Water shrews are solitary creatures, active throughout the day and night. Their activity patterns are characterized by two periods; one between sunset and 2300 h, the second occurs one hour prior to sunrise (Sorenson, 1962). For every 30 minutes of activity, the shrew spends the next hour resting (van Zyll de Jong, 1983).
When active, water shrews dive and swim in water to forage for food. Water shrews can control their own metabolic demands so that they can dive year-round, even in winter-cold bodies of water (Boernke, 1977). Each dive can last from 31.1 to 47.7 seconds (Beneski and Stinson, 1987). In water, the fur is lined with a layer of air that reduces their heat loss by 50% (Calder, 1969) as well as make them buoyant. Therefore, when water shrews swim or dive, they must paddle vigorously to keep from floating 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 (Jackson, 1928). It has been suggested that water shrews can walk on water because they can trap air bubbles in the stiff hairs of their feet (Jackson, 1928).
Nests of water shrews are usually about 8 cm in diameter and are either new nests or reconstructions of old nests built from dried vegetation 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 muzzle (Nagorsen, 1996).
Water shrews are aggressive and fighting is common between conspecifics. Males and females are equally likely to fight (Sorenson, 1962). Most encounters are short but may be intense. Encounters between two individuals usually start off with each emitting high pitched squeaks followed by standing on their hind legs to expose their light-coloured 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 (Sorenson, 1962). These fights have not been proven to be of a territorial nature. (Beneski and Stinson, 1987; Boernke, 1977; Calder, 1969; Jackson, 1928; Nagorsen, 1996; Sorenson, 1962; Wilson and Ruff, 1999)
Sensory abilities of water shrews are not well understood. The vibrissae and the muzzle are thought to serve the purpose of locating prey (Sorenson, 1962). During explorations they release continuous high pitched sounds. This has led people to believe that water shrews echolocate (Sorenson, 1962). Distinguished by the strong, sometimes nauseating odor they emit, water shrews are believed to have a well-developed sense of smell. These odors have been proposed to serve to attract mates or for species recognition (Hamilton, 1940). (Sorenson, 1962)
Water shrews are predominantly 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 (van Zyll de Jong, 1983). Besides aquatic animals, they will also feed on land for flies, earthworms, snails, fungi and green vegetation (Wilson and Ruff, 1999). Once in possession, the food is held by the fore feet and torn to pieces using the teeth through upward thrusting of the head (Sorenson, 1962). Water shrews can live without food for up to 3 hours, but captive shrews have been found to feed almost every 10 minutes (Nagorsen, 1996). The amount of food required by a water shrew has been estimated to be 0.95 g/day. (Nagorsen, 1996; Sorenson, 1962)
Water shrews dive and swim to escape from predators like garter snakes, hawks, owls and weasels. (Wilson and Ruff, 1999)
Water shrews are important predators of the insects on which they feed, and they are an important food source for the predators listed above. (Whitaker and Hamilton, 1998)
The water shrew has no known negative effects on humans.
Water shrews are widespread but rarely captured. (Wilson and Ruff, 1999)
Allison Poor (editor), University of Michigan-Ann Arbor.
Ma Carmen (author), University of Toronto.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
An animal that eats mainly insects or spiders.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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.