The name Sorex palustris comes from the Latin word soric meaning "shrew-mouse" and paluster for "marshy".
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).
Communication Channels: acoustic ; chemical
Perception Channels: tactile ; acoustic ; chemical
Water shrews are widespread but rarely captured.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
IUCN Red List of Threatened Species: least concern
The water shrew has no known negative effects on humans.
Water shrews are important predators of the insects on which they feed, and they are an important food source for the predators listed above.
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.
Animal Foods: fish; insects; mollusks; terrestrial worms
Other Foods: fungus
Primary Diet: carnivore (Insectivore )
Water shrews, Sorex palustris, are found throughout Alaska and Canada to the northern mountain regions of the United States.
Biogeographic Regions: nearctic (Native )
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.
Habitat Regions: temperate ; terrestrial
Terrestrial Biomes: forest
Aquatic Biomes: lakes and ponds; rivers and streams
Other Habitat Features: riparian
Water shrews are short-lived. The typical life span of a water shrew is about 18 months.
Average lifespan
Status: wild: 18 months.
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.
Range mass: 8 to 18 g.
Range length: 130 to 170 mm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger
Water shrews dive and swim to escape from predators like garter snakes, hawks, owls and weasels.
Known Predators:
Anti-predator Adaptations: cryptic
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.
Breeding interval: Water shrews produce two to three litters per breeding season.
Breeding season: The breeding season is usually from December to September.
Range number of offspring: 3 to 10.
Average gestation period: 3 weeks.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Average gestation period: 23 days.
Average number of offspring: 5.
Average age at sexual or reproductive maturity (female)
Sex: female: 90 days.
Like all female mammals, water shrew mothers provide their young with milk after they are born.
Parental Investment: pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)
The American water shrew or northern water shrew (Sorex palustris) is a shrew found in the nearctic faunal region located throughout the mountain ranges of the northern United States and in Canada and Alaska.[3] The organism resides in semi-aquatic habitats,[4] and is known for being the smallest mammalian diver.[5]
The American water shrew is a sexually dimorphic species in which the males are generally larger and heavier than the females. The size of the shrew is 130–170 mm (5.1–6.7 in) and weight is 8–18 g (0.28–0.63 oz). Their tail length is 57–89 mm (2.2–3.5 in). The shrew exhibits a black and brown pelage which varies in shade depending on the season.[4] When underwater, the animal appears to have a silver veneer on account of its water repellent fur trapping air bubbles. The snout features vibrissae which in the case of water shrews are specialized for aquatic hunting. Like other small mammals who spend part of their time in water, American water shrews have short hairs covering their hind limbs to bolster each paddle with increased surface area, an adaptation not dissimilar to flippers.[5] Externally the American water shrew is indistinguishable from the western water shrew, a genetically distinct species whose morphology differs from the American water shrew strictly over subtle cranial and dental aspects.[6] The American water shrew has a large skull ranging from 21–23 mm (0.83–0.91 in) and width 10–11 mm (0.39–0.43 in) and has a dental formula of 1.5.1.31.1.1.3 × 2 = 32 teeth.[3]
American water shrew populations span the central and eastern regions of Canada and the northern United States, as well as a small isolated section of the Appalachian mountain range. A separate distribution of the population can be found in the western United States and Canada.[4] Because much of the range water shrews occupy today was historically a landscape of continental ice sheets, they considerably expanded their range in the time since the Last Glacial Maximum.[7]
American water shrews prefer to inhabit streams of higher inclines, although they have been observed in a variety of water bodies including more gently sloping streams, springs, mud flats, and even beaver dens. Hence their name, they are rarely found far from water since their diet consists almost entirely of small aquatic invertebrates and fishes.[4]
Shrews are known to hunt land prey depending on the ease of the attack, but mainly target aquatic prey by diving from rocks or elevated banks of streams, making them the smallest of the mammalian divers. Most remarkable about this behavior is that water shrews are mainly nocturnal hunters, meaning they don’t use eyesight while locating their prey underwater. While it has been suggested that water shrews may use sonar or electroreception for this purpose, all investigations into these claims have produced no supporting evidence. According to Kenneth C. Catania’s research, nocturnally diving shrews manage to locate their prey in the obscured stream by detecting movement along their whiskers or by “underwater sniffing,” a strategy in which they exhale air bubbles onto a perceived target and then re-inhale their own air bubbles to confirm the presence of prey.[5]
Unlike most species of shrews which breed later in the year, American water shrews have been found to breed between the months of February and August based on analysis of wild specimen whose ovarian and testicular activity was at a height during these months.[4] Females usually have a three-week gestation period, and offspring are born in the spring and summer. They usually produce two to three litters during that time. These litters can contain three to 10 offspring.[3]
The American water shrew has nine subspecies:[1]
The American water shrew or northern water shrew (Sorex palustris) is a shrew found in the nearctic faunal region located throughout the mountain ranges of the northern United States and in Canada and Alaska. The organism resides in semi-aquatic habitats, and is known for being the smallest mammalian diver.