Sorex palustris

Water Shrews are almost invariably found near streams or other bodies of water, where they find food and also escape from predators. These shrews readily dive to stream bottoms, paddling furiously to keep from bobbing to the surface—their fur, full of trapped air, makes them buoyant. They feed on aquatic invertebrates, insect larvae, and even small fish. In the water they are susceptible to predation from larger fish and snakes. On land, Water Shrews have a more typical shrew diet, feeding on a variety of invertebrates, including earthworms, snails, and insects. They also eat fungi and green vegetation.

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Water shrews are found throughout Alaska and Canada to the northern mountain regions of the United States.

Biogeographic Regions: nearctic (Native )

This species occurs in boreal and montane regions of Labrador, Nova Scotia, and New England across Canada to east-central Alaska (Cook et al. 1997), south to the northern Great Lakes region and in the western mountains to mid-California, Nevada, Utah, and New Mexico in the United States. A small, disjunct population in Arizona's White Mountains has not been observed in recent years (Hoffmeister 1986); there is another apparently disjunct series of populations in the Appalachians ranges from southwestern Pennsylvania to North Carolina, Tennessee, and Georgia.

Pagels (1990, 1991) noted that in the southern Appalachians the water shrew is limited to prime habitat: "high elevation situations where moist, cool, shaded situations have prevailed throughout historic time…elevations ranging from about 760 m (2,500 ft.) in Pennsylvania to 1150 m (3,800 ft.) and above in North Carolina and Tennessee."

Water shrews, Sorex palustris, are found throughout Alaska and Canada to the northern mountain regions of the United States.

Biogeographic Regions: nearctic (Native )

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) Boreal and montane regions of Labrador, Nova Scotia, and New England across Canada to east-central Alaska (Cook et al. 1997), south to the northern Great Lakes region and in the western mountains to mid-California, Nevada, Utah, and New Mexico; a small, disjunct population in Arizona's White Mountains has not been observed in recent years (Hoffmeister 1986); another apparently disjunct series of populations in the Appalachians ranges from southwestern Pennsylvania to North Carolina, Tennessee, and Georgia (Laerm et al. 1995, Brimleyana 22:47-51). Hall (1981) mapped the ranges of the subspecies.

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.

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 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

Length: 16 cm

Weight: 17 grams

Sexual Dimorphism: Males average slightly heavier and longer than females.

Length:
Average: 151.4 mm
Range: 130-170 mm

Weight:
Average: 13.8 g
Range: 8-18 g

Stiff hairs along the sides of the hind feet are found only in the water shrew and the Pacific water shrew (SOREX BENDIRII). The latter, a Pacific Northwest species, differs in being slightly larger (8.9-9.7 cm, 3.5-3.8 inches, body length) and dark brown rather than blackish-gray (Burt and Grossenheider 1976). See Carraway (1995) for a key to western North American soricids based primarily on dentaries.

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.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: forest

Aquatic Biomes: lakes and ponds; rivers and streams

Other Habitat Features: riparian

Habitat and Ecology

Systems

• Terrestrial

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

Comments: Most abundant along small cold streams with thick overhanging riparian growth. Also around lakes, ponds, marshes, bogs, and other lentic habitats. Rarely far from water. Nest sites are near water in underground burrows, rafted logs, beaver lodges, and other areas providing shelter.

Van Zyll de Jong (1983) described the habitat in Canada: "Lakes, ponds, swift and sluggish small streams, all provide suitable habitat if adequate cover is available in the form of overhanging banks, boulders, tree roots, logs, etc. In the mountains, the species occurs along cold fast mountain streams. On the other extreme, it is also found in stagnant water of marshes or bogs, and a few have been caught in places with very little water. The water shrew appears to have some flexibility in adapting to habitats with little water or even to habitats where water is present only seasonally. However, the species appears to be most abundant along cold mountain streams with abundant cover."

Pagels (1990, 1991) noted that in the southern Appalachians the water shrew is limited to prime habitat: "high elevation situations where moist, cool, shaded situations have prevailed throughout historic time...elevations ranging from about 762 m (2500 ft.) in Pennsylvania to 1158 m (3800 ft.) and above in North Carolina and Tennessee. In addition to close proximity to water, habitat features often include moss-covered rocks, crevices, fallen trees, and boulder-strewn and/or overhanging stream banks. Understory vegetation is generally dense with mountain laurel and rhododendron being the most abundant species."

In Montana, Conaway (1952) examined 101 capture sites and found typical habitat to be "along fast cold mountain streams having banks which offered favorable cover." All these shrews were trapped within 18 cm (7 inches) of water.

Though normally associated with water, water shrews "have been found more than 100 m (328 ft.) from streams in mature northern hardwood stands in northern New Hampshire" (DeGraaf and Rudis 1986). This and other captures far from water probably represent dispersing individuals.

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

Water shrews are, for the most part, insectivores. Diving to the bottoms of streams or other water habitats, they forage for aquatic insecta, especially for the larvae and nymphs of Trichoptera, Tipulidae, Ephemeroptera, and Plecoptera and occasionally for small actinopterygii . Besides aquatic animals, they will also feed on land for diptera, Oligochaeta, gastropoda, 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.

Animal Foods: fish; insects; mollusks; terrestrial worms

Other Foods: fungus

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 )

Comments: Primarily dependent upon aquatic insects; also eats various other invertebrates and ENDOGONE. May take small vertebrates (fishes, amphibians) when available. Hunts under and on top of water. May even be seen running across the water surface.

Like other shrews, the water shrew seems to be an opportunistic predator, and the diet varies greatly with geographic area and probably with season. In North Carolina, immature aquatic insects constituted 83% of stomach contents, the remaining 17% being harvestmen (Arachnida: Opiliones). In Minnesota, Whitaker and Schmeltz (1973) found that slugs, earthworms, and spiders totaled 60% of stomach contents. Terrestrial insects, small fishes, salamander larvae, fish eggs, planarians, and snails are also eaten. Zevelof (1988) and others have noted plants and fungi as significant food items, but Conaway (1952) thought such items were accidentally ingested. Other shrews are known to eat small amounts of leaves and seeds.

Water shrews, with their high metabolic rates, need to consume approximately their weight in food every day (Conaway 1952, Sorenson 1962). In the wild they seem unable to store significant body fat and can die of starvation within a few hours. When a surplus of food is available, it is often horded, the shrew sometimes defecating on it to keep other shrews away.

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 dive and swim to escape from predators like thamnophis, accipitridae, strigidae and mustela.

Known Predators:

• garter snakes (Thamnophis)
• hawks (Accipitridae)
• owls (Strigidae)
• weasels (Mustela)

Anti-predator Adaptations: cryptic

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 dive and swim to escape from predators like garter snakes, hawks, owls and weasels.

Known Predators:

• garter snakes (Thamnophis)
• hawks (Accipitridae)
• owls (Strigidae)
• weasels (Mustela)

Anti-predator Adaptations: cryptic

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: > 300

Comments: Thousands of occurrences.

>1,000,000 individuals

Comments: Generally common, especially in the northern portion of the range (Christian, pers. comm., 1994), though some (e.g., Kirkland, pers. comm., 1994) have described abundance as uncommon to rare throughout the entire range. Certain subspecies are rare.

There are almost no data on home range size. Home range was 0.2-0.3 ha for two live-trapped Manitoba individuals (Buckner 1966, Buckner and Ray 1968). Churchfield (1990) reported that "aquatic species such as NEOMYS FODIENS and SOREX PALUSTRIS also have linear home ranges based on the banks of streams and rivers. They occupy a length of riverbank plus the adjacent area of water and, often, a small area of hinterland. N. FODIENS typically has a home range length of 20-93 m (65-305 ft.) alongside a stream." After 14 months of observing S. PALUSTRIS in captivity under near-natural conditions, Sorenson (1962) concluded that "a dominance hierarchy and territoriality did not exist." He noted that even nests were not defended.

Information on population density likelwise is scant. In Michigan, seven individuals were found along a 200-ft section of stream in one night with 30 Sherman traps (Master 1978). Population densities in the Appalachians appear to be quite low, though numbers are not available (Pagels 1990, 1991). In Canada, Banfield (1974) reported that this shrew is usually uncommon but sizeable populations sometimes occur in favorable locations. Generally numbers are highest in summer when young are born and drop off sharply in autumn, with very little decrease through winter. Population densities for shrews in general depend on species, season, habitat, and probably prey density (Churchfield 1990).

Common predators include fishes such as trout, bass, and pickerel, minks, otters, weasels, snakes, and, occasionally, hawks and owls (Merritt 1987, Godin 1977). Internal and external parasites commonly include nematodes, tapeworms, fleas, ticks, and mites (Conaway 1952, Webster 1987, Whitaker and Schmeltz 1973), but there is no evidence that they cause significant harm (Churchfield 1990).

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.

Communication Channels: acoustic ; chemical

Perception Channels: acoustic

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: acoustic

Comments: Two major activity-periods reported: sunset to 4 hours after, and just before sunrise (van Zyll de Jong 1983). On average, thirty-minute periods of activity alternate with hour-long naps (Sorenson 1962). Generally active throughout the day and in every season, but secretive and seldom seen.

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 short-lived. The typical life span of a water shrew is about 18 months.

Average lifespan
Status: wild: 18 months.

Observations: The typical lifespan of these animals is 1.5 years. Without further longevity studies, however, their maximum longevity remains unknown.

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.

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); 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 mammalia, 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 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); 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)

Breeds February-August in Montana (Conaway 1952). Gestation lasts probably 3 weeks. Litter size is 3-10, average 6; 2-3 litters/year (Montana). Optimum conditions in captivity, or abundant food supply in nature, may increase the number of litters (Churchfield 1990, Godfrey 1978). Sexually mature in second calendar year. Maximum lifespan is about 18 months (Beneski and Stinson 1987).

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Specimens with Barcodes: 14
Species With Barcodes: 1

Water shrews are widespread but rarely captured.

IUCN Red List of Threatened Species: least concern

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

Year Assessed

Assessor/s

Reviewer/s

Contributor/s

Justification

History

• 1996
  Lower Risk/least concern
  (Baillie and Groombridge 1996)

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

Canada

Rounded National Status Rank: N5 - Secure

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

Reasons: Large range in the boreal and montane regions of North America; secure, if not abundant, throughout the northern part of the range; in the south, habitat has been fragmented since the retreat of the last glaciers, making isolated populations vulnerable to extirpation; some subspecies are rare enough to be of concern.

Population

Population Trend

Global Short Term Trend: Relatively stable to decline of 30%

Comments: Stable throughout the northern part of the range (Christian, pers. comm., 1994). Southern range may be shrinking through habitat fragmentation and local extirpation, but current information is too inadequate to allow an evaluation of this hypothesis.

Major Threats

Degree of Threat: C : Not very threatened throughout its range, communities often provide natural resources that when exploited alter the composition and structure over the short-term, or communities are self-protecting because they are unsuitable for other uses

Comments: Churchfield (1992) stated that water shrews (both S. PALUSTRIS and NEOMYS sp.) "are particularily vulnerable to the destruction of their aquatic habitats through pollution and drainage." Activities such as logging, agriculture, road building, and surface mining contribute to the loss of habitat. Logging may not be a serious threat if water quality is protected and buffer strips are maintained at the site (Christian, pers. comm., 1994). Regarding S. P. PUNCTULATUS in the east, Pagels et al. (1991) stated: "Perhaps the greatest continuing threats to the water shrew are the loss of additional habitat resulting from infestation of introduced insect species, such as the gypsy moth (LYMANTRIA DISPAR), or the use of insecticides to control such infestations. The effects of acid rain, particularily on the shrew's microhabitat and food supply could be particularily devastating." Shrews feeding on invertebrates accumulate and concentrate pesticides and heavy metals in their tissues. Northern short-tailed shrews (BLARINA BREVICAUDA) were found to contain DDT nine years after application of the pesticide to a forest habitat (Dimond and Sherburne 1969). "Contamination of habitats, and the resultant effects on the food chain, by mining and refining of metalliferous ores, by motor exhausts and even by the application of sewage sludge to land, is becoming more widespread. Shrews from contaminated grasslands have been found to accumulate some of the highest concentrations of metals recorded in wildlife" (Churchfield 1992). Southern populations are limited to the mountains in both the east and west and consequently tend to be isolated. When an isolated population disappears for any reason, it is unlikely to be restored by natural dispersal. Hence these isolated populations may be especially vulnerable to extirpation from human activities or long-term climate change.

Conservation Actions

Preserve Selection and Design Considerations: Preservation of water quality and suitable riparian habitat conditions through protection of the watershed is a primary consideration.

Management Requirements: Protection of habitat, especially water quality, is the primary management consideration. Guidelines should be developed for activities with potentially adverse impacts such as logging. During timber harvest, buffer strips should be maintained along potential water shrew habitat (Christian, pers. comm., 1994). Pesticide use that might impact aquatic/riparian invertebrate populations should be avoided whenever possible. In some cases it may be desirable to reintroduce shrews from nearby populations to restored habitat.

Management Research Needs: Research is needed on geographic variation in habitat requirements. Further research should be conducted on dispersion pattern and dispersal. Research is needed on the effects of land and stream pollution on water shrew survival. The minimum population size needed to maintain genetic viability in a fragmented habitat should be investigated.

Global Protection: Many to very many (13 to >40) occurrences appropriately protected and managed

Comments: There are many areas (e.g., state and national parks) in which habitat is adequately protected.

The water shrew has no known negative effects on humans.

The water shrew has no known negative effects on humans.

Stewardship Overview: The water shrew is a boreal species, also inhabiting relict habitat in southern mountains. It requires high quality water, preferably mountain streams, and abundant cover such as rocks, logs, or overhanging streambank. Suitable management consists primarily of maintaining these conditions.