Next to the Pygmy Shrew (Sorex hoyi), the Common Shrew is our smallest shrew. Its fur colour is variable. Interior populations have a pale brown dorsal pelage with greyish-white undersides. Coastal populations are dark brown or grey. The tail is pale underneath. The winter pelage tends to be darker. The fur on the back does not contrast sharply with the sides.
The skull is small with a long narrow rostrum. There are five pairs of unicuspids; the third unicuspid is larger than the fourth. The pigmented ridge on the upper unicuspids extends to the cingulum and it is not separated by a longitudinal groove.
Only two other shrew species in British Columbia have five unicuspids with the third larger or equal to the fourth: Black-backed Shrew (Sorex arcticus) and Tundra Shrew (Sorex tundrensis). Both are larger than the Common Shrew (weights greater than 6.0 g, skull lengths greater than 18.5 mm, palatal lengths greater than 7.7 mm) and have a distinct tricoloured or saddle-backed pelage where the fur on the back contrasts sharply with the paler fur on the sides.
total length: 99 (76-119) n = 137 tail vertebrae: 42 (29-49) n = 137 hind foot: 12 (9-14) n = 138 ear: 7 (6-9) n = 74 weight: 4.1 (2.5-8.0) n = 137
The length of the breeding season in British Columbia is unknown. Data from museum specimens indicate that breeding females have been found from May to September. In eastern North America, the breeding season is generally from April to October; and in populations with abundant food resources, it may extend into November. Average embryo counts reported for various populations across North America range from five to eight. Females can produce at least two litters in a breeding season. Although uncommon, males and females may breed in their first summer. Newborn Common Shrews weigh 0.2 to 0.3 grams and are only 12 to 14 millimetres long. They grow quickly, attaining adult size in 20 to 27 days when they leave the nest.
Prey identified in the stomachs of the Common Shrew include: insect larvae, ants, beetles, crickets, grasshoppers, spiders, harvestmen, centipedes, slugs, snails and fungi. Seeds may also be consumed in winter. The Common Shrew's diet is very flexible, and because of its broad range this species demonstrates considerable geographic variation in diet. A study in Michigan revealed that ants were the major prey, representing 50 per cent of the food items; in New Brunswick, insect larvae were recorded as the predominant prey type. Common Shrews associated with Tamarack bogs in Manitoba fed almost entirely on Larch Sawfly larvae in late summer. A population living on a small island in Nova Scotia hunted mainly in the intertidal zone where they fed on kelp flies and marine amphipods.
Charles Buckner saw two Common Shrews hunting butterflies during the day. Darting quickly towards their prey, the shrews leaped into the air to pounce on the butterflies before they could take flight. They ate only the bodies of the butterflies and discarded the wings. An observation by Otto Horvath of Common Shrews raiding a Solitary Vireo nest 1.7 metres above ground suggests that some will hunt in shrubs and trees.
An abundant species, the Common Shrew is the dominant shrew species in many communities. In Manitoba, Charles Buckner estimated population densities of 5 to 22 per hectare in bog habitats. Nevertheless, abundance varies extensively among habitat types and pronounced population fluctuations occur from year to year that may be related to variations in the abundance of prey. The Common Shrew readily exploits habitats disturbed by fire or logging. Walt Klenner found this shrew in all his forest study plots in the Thompson Plateau, from uncut stands to recent clearcuts. The average home range is about 0.6 hectares.
The Common Shrew is active for very short periods (about two minutes) throughout a 24-hour period. Its peak activity is after dark, and activity is greatly increased when there is a night-time rainfall. Most activity is associated with feeding; captive Common Shrews, observed for a week, fed about every 13 minutes.
Less than half the young will survive beyond five months. The maximum life span is about 15 months, although a few individuals may reach two years of age.
The Common Shrew occurs in all of the province's biogeoclimatic zones, and its elevational range is from sea-level to 2,288 metres in the Rocky Mountains (Mount Assiniboine Provincial Park). It is associated with a variety of habitats: open and closed forests, open meadows, avalanche slopes, river banks and lake shores, Tamarack - Black Spruce bogs, and Dwarf Birch - willow thickets. The distribution of the Common Shrew correlates strongly with moisture: some of the highest populations occur in habitats with standing water.
The Common Shrew has the largest distributional area of any North American shrew, occurring across the northern United States, most of Canada and Alaska. In British Columbia, this species inhabits the entire mainland and a few islands on the central and north coast, but it is absent from Vancouver Island and the Queen Charlotte Islands.
The Sorex cinereus group includes several isolated island forms in the Bering Sea as well as a number of strongly differentiated continental subspecies. Their systematics has been problematic. The arctic (ugyunak) and prairie (haydenz) forms are now recognized as full species. Nine subspecies are recognized; two occur in the province.
Sorex cinereus cinereus Kerr is distributed across much of Canada and the entire province east of the coastal mountain ranges.
Sorex cinereus streatori Merriam inhabits the Pacific coast from Washington to Alaska. In British Columbia, it is found on the western slopes of the coastal mountain ranges, the coastal lowlands, and twelve islands on the central and north coast: Athlone (formerly Smythe) and Townsend in the Bardswell Group, Campbell, Hunter, Kaien, King, McCauley, Pitt, Princess Royal, Ruth, Spider, and Yeo. This subspecies is darker and larger than S.c. cinereus.
The Common Shrew may be an important predator of forest insect pests such as the Jack Pine Budworm and the Larch Sawfly. High population densities have been reported in forests where these insect species live. Charles Buckner demonstrated that captive and wild Common Shrews consume large number of Larch Sawfly cocoons and larvae, and a large Common Shrew population could have a significant impact on populations of this insect pest.
That few islands are occupied by the Common Shrew is interesting, given that this species is found throughout the entire coastal mainland. Ian McTaggart Cowan found the Common Shrew less abundant than the ubiquitous Dusky Shrew on the north coast islands. Low populations and a small body size may limit this shrew's ability to colonize and survive on these islands.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2021. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
2022-01-28 11:07:37 AM]
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