Sorex cinereus, commonly called the Masked shrew or common shrew, is the most widely distributed shrew found in North American. Common shrews occur throughout the northern United States, most of Canada, and Alaska. They do not occur on Vancouver Island, the Queen Charlotte Islands, in tundra habitats, arctic islands, or in extreme northern Quebec. (Nagorsen, 1996; van Zyll de Jong, 1983).

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)) Alaska to Labrador/Newfoundland, south to Washington, Utah, New Mexico, the Northern Great Plains, southern Indiana and Ohio, through the Appalachian Mountains to northern Georgia and western South Carolina, and on the east coast to New Jersey and northern Maryland (Laerm et al. 1995, Brimleyana 22:15-21; Whitaker 2004).

Sorex cinereus is the second smallest shrew species. Sorex hoyi is slightly smaller. Although similar in size, their coloration is quite different. There is no significant sexual dimorphism in common shrews. Dorsal fur is brown, ventral fur is greyish-white. Pelage tends to be darker overall in winter. The tail is brown above and pale underneath, with a blackish tip. Average length of the tail is 39.9mm, comprising over 40% of the total length. Average length of adults is 99 mm. Young are born hairless and with fused eyelids, they weigh from 0.2 to 0.3 grams and are 15 to 17 mm long including a 3 mm long tail (Nagorsen, 1996; Wilson and Ruff, 1999; van Zyll de Jong, 1983).

Range mass: 2.5 to 4.0 g.

Average length: 99.0 mm.

Sexual Dimorphism: sexes alike

Average basal metabolic rate: 0.238 W.

Length: 10 cm

Weight: 5 grams

Generally paler and smaller than S. FUMEUS (95-129 mm total length) (Godin 1977). See Hall (1981) for a (somewhat outdated) key to North American species of Sorex. See Carraway (1995) for a key to western North American soricids based primarily on dentaries.

• Terrestrial

Common shrews occupy a diversity of habitats, most common are open and closed forests, meadows, river banks, lake shores, and willow thickets. Habitat suitability depends on the availability of water and the highest population densities can be found in moist environments. Common shrews also do well in disturbed habitats such as those disturbed by fire or logging. The average home range is 0.6 hectares (Nagorsen, 1996; Pagels, et al. 1994)).

Habitat Regions: temperate

Terrestrial Biomes: forest ; rainforest

Wetlands: marsh ; swamp

Comments: Occupies most terrestrial habitats excluding areas with very little or no vegetation. Thick leaf litter in damp forests may represent favored habitat, although appears adaptable to major successional disturbances. In Nova Scotia, diet indicated that much foraging was done among wrack on beaches (Stewart et al. 1989). Nest sites are typically in shallow burrows or above ground in logs and stumps.

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.

Because Sorex cinereus inhabits a wide range there is great geographic variation in diet. Ants represent 50% of the food source for common shrews in Michigan, whereas insect larvae are the dominant prey item in New Brunswick. Kelp flies and marine amphipods are major dietary items in Nova Scotia. They are also important predators of forest insect pests such as Jack Pine Budworms and Larch Sawflies. In general, common shrews consume a variety of invertebrates including insect larvae, ants, beetles, crickets, grasshoppers, spiders, harvestmen, centipedes, slugs, snails. Seeds and fungi are also consumed (Nagorsen, 1996).

Primary Diet: carnivore (Eats non-insect arthropods)

Comments: A generalist, opportunistic invertivore. Eats primarily insects and other invertebrates, carrion, small vertebrates, occasionally seeds. Echolocation may be used for detecting prey (Gould et al. 1964). Consumes daily its own weight in food.

Masked shrews can be very abundant in the communities in which they live. They can have a dramatic impact on insect communities because they have to consume such large quantities of insects. They are also important prey items for many small predators.

Masked shrews avoid being preyed upon by staying under cover and being active mostly at night, they are rarely seen.

Known Predators:

• weasels (Mustela)
• hawks (Accipitridae)
• falcons (Falconidae)
• owls (Strigiformes)
• domestic cats (Felis silvestris)
• red foxes (Vulpes vulpes)
• snakes (Serpentes)
• short-tailed shrews (Blarina brevicauda)

Large annual fluctuations in population size. Density estimates range from 1-12 shrews per acre (Buckner 1966). Home range about 0.10 acre. Usually in scattered, locally abundant populations. Rarely lives past second summer.

Little is known of communication in masked shrews. They have an excellent sense of smell and can see fairly well. They use their sensitive whiskers to find their way around and detect prey. Masked shrews also probably squeak and hiss as a way of communicating.

Comments: Active throughout the day (and the year) to secure enough food to maintain high metabolic rate. Peak active period 0100-0200 (van Zyll de Jong 1983). Cloudy, rainy nights increase nocturnal activity.

Masked shrews probably do not live much past 1 to 2 years old, most probably die before reaching adulthood.

Typical lifespan

Status: wild: 2.0 (high) years.

The breeding season for Sorex cinereus occurs from April to October in eastern North America but may extend into November if food is plentiful. Length of gestation is unknown. The average number of embryo produced by various populations ranges from five to eight across North America, with at least two litters produced by a female in a breeding season. Both males and females may breed in their first summer but this is not typical (Nagorsen, 1996).

Breeding season: Masked shrews breed from April to November.

Range number of offspring: 4.0 to 10.0.

Average weaning age: 20.0 days.

Range age at sexual or reproductive maturity (female): 5.0 to 11.0 months.

Range age at sexual or reproductive maturity (male): 5.0 to 11.0 months.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); viviparous

Average birth mass: 0.28 g.

Average gestation period: 19 days.

Average number of offspring: 6.22.

Young are cared for and nursed by their mother in her nest until they reach about 20 days old.

Parental Investment: altricial ; female parental care

Breeding season may last from March through September (there is evidence of mid-winter births in at least some years in Nova Scotia) (Stewart et al. 1989). Usually 2 litters, may be 3. Gestation lasts 18 days. Litter size is 2-10 (average around 7). Young are weaned in 3 weeks. Sexually mature in 20-26 weeks. Some young may breed in the year of their birth.

Common shrews are common and widespread and none of the Genus Sorex, including Sorex cinereus, are considered to be threatened or endangered species (Boyd et al., 1999; Nagorsen, 1996; Wilson and Ruff, 1999).

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

Comments: This and other generalist insectivores are not likely to be impacted negatively by selective insecticides such as Bacillus thuringiensis (Bellocq et al. 1992).

Negative impacts of S. cinereus are unknown. However, they may affect populations of some beneficial organisms or inhibit reproduction of some plants by consuming seeds (Nagorsen, 1996).

The extent to which common shrew populations affect humans is unknown. However, they have a significant impact on populations of insect pests and are important members of communities (Nagorsen, 1996).

Positive Impacts: controls pest population

Comments: Several formerly recognized subspecies recently have been regarded as distinct species (e.g., see van Zyll de Jong 1983). A population in western Washington and adjacent British Columbia was recognized as a distinct species (S. rohweri) by Rausch et al. (2007).

According to George (1988), electrophoretic data contradict morphometric evidence that S. fontinalis is conspecific with S. cinereus. However, results of a recent analysis of cranial morphology do not support the view that fontinalis is specifically distinct from S. cinereus; electrophoretic support for separation of the two as distinct species is based on a small sample size from one small geographic area (van Zyll de Jong 1989). Van Zyll de Jong (1999) and Demboski and Cook (2003) concluded that S. fontinalis should be regraded as a subspecies of S. cinereus, but the North American mammal checklist by Baker et al. (2003) kept S. fontinalis as a species, based on electrophoretic results of George (1988).

The same study (George 1988) supports separation of S. haydeni and S. cinereus as independent species. Despite distinct morphological and karyotypic (Volobouev and van Zyll de Jong 1994) differences between S. haydeni and S. cinereus, in Alberta these taxa were less genetically differentiated than were conspecific populations of S. cinereus across Canada; there were no fixed allele differences among the populations sampled, but there was a unique allele at a moderate frequency in S. haydeni; this may be indicative of no gene flow between these two taxa, and thus they may be valid species; evidently S. cinereus is a paraphyletic taxon (Stewart et al. 1993).

Based on evidence of bidirectional mtDNA introgression in Minnesota, Brunet et al. (2002) concluded that S. haydeni does not warrant specific status, but Demboski and Cook (2003) found that S. cinereus and S. haydeni do not appear to be sister species and regarded S. haydeni as a valid species (based on a mtDNA phylogeny). The North American mammal checklist by Baker et al. (2003) maintained S. haydeni as a distinct species.

MtDNA data are not concordant with currently accepted subspecific designations in S. cinereus (Stewart and Baker 1997).

Sorex milleri of Mexico apparently represents a relict population of S. cinereus and may not have attained specific status (van Zyll de Jong 1989).

See Stewart and Baker (1992) for information on genetic differentiation and biogeography of island and mainland populations in southeastern Canada, where certain island populations have diverged genetically to a level that may be indicative of incipient speciation or subspeciation.

Rausch and Rausch (1995) examined karyotypic and morphological characteristics of shrews on St. Lawrence Island and the Alaskan mainland and found no significant differences; they concluded that S. jacksoni should be regarded as a subspecies of S. cinereus. However, van Zyll de Jong (in Wilson and Ruff 1999) and Demboski and Cook (2003) continued to recognize S. jacksoni as a species. Based on the data of Demboski and Cook (2003), the North American mammal checklist by Baker et al. (2003) recognized S. jacksoni as a valid species.

Demboski and Cook (2003) used DNA sequence data to examine phylogenetic relationships among 8 members of the Sorex cinereus group (S. camtschatica, S. cinereus, S. haydeni, S. jacksoni, S. portenkoi, S. preblei, S. pribilofensis, and S. ugyunak) and S. longirostris. Phylogenetic analyses recovered 2 major clades within the species group: a northern clade that includes the Beringian species (S. camtschatica, S. jacksoni, S. portenkoi, S. pribilofensis, and S. ugyunak), S. haydeni