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Sorex monticolus Merriam
Dusky Shrew; Montane Shrew
Family: Soricidae

Species account author: David Nagorsen
Extracted from the Opossums, Shrews and Moles of British Columbia

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Distribution of Sorex monticolus in British Columbia.
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Source: Opossums, Shrews and Moles of British Columbia by David Nagorsen © Royal BC Museum

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Illustration Source: : Opossums, Shrews and Moles of British Columbia by David Nagorsen © Royal BC Museum

The Dusky Shrew is a medium-sized shrew with dorsal pelage that varies from greyish to dark brown in interior populations to nearly black in coastal populations; its ventral fur is brown to grey. The tail is bicoloured, but not distinctly. The second to fifth digits on the hind feet have more than four pairs of toe pads.

The skull is medium-sized. There are five upper unicuspid teeth, with the third smaller than the fourth. The medial edge of the first upper incisor appears straight in front view. The upper incisor has a large medial tine in the pigmented area on the upper incisor, but this trait is obscured in old animals with badly worn teeth. There appears to be geographic variation in both the intensity of pigmentation and the size of the medial tines of the upper incisor. Some coastal populations from central and northern British Columbia tend to have faint pigmentation of the face of the upper incisor, and the medial tines may be small.

Identification

The teeth of Trowbridge's Shrew (Sorex trowbridgii) and the Vagrant Shrew (Sorex vagrans) are similar to those of the Dusky Shrew: in both, the third unicuspid is smaller than the fourth. Trowbridge's Shrew can be distinguished from the Dusky Shrew by four traits: it has a distinctly bicoloured tail; the pigmented ridge on its upper unicuspid teeth is separated from the cingulum by a longitudinal groove; the medial tine on its first upper incisor is above the pigmented area on the tooth; and in front view the medial edge of the first upper incisors is curved. Discriminating the Dusky Shrew from the Vagrant Shrew is difficult, especially in coastal populations where pigmentation on the teeth is weak or in old animals with worn teeth. The Dusky Shrew averages larger in skull size and tail length, but the two species overlap substantially in most measurements. The Vagrant Shrew has no more than four pairs of toe pads on the second to fifth digits of the hind foot. The skull of the Vagrant Shrew can be distinguished by the medial tine on its first upper incisor, which is small and positioned above the pigmented region.

Dental Formula

incisors: 1/1
unicuspids: 5/1
premolars: 1/1
molars: 3/3

Measurements

total length: 117 (93-140) n = 404
tail vertebrae: 51 (35-68) n = 436
hind foot: 14 (10-18) n = 436
ear: 7 (4-9) n = 198
weight: 6.6 (3.7-13.0) n = 417

Reproduction

In southwestern British Columbia, males become sexually active in mid February and females in early April just after they complete their moult. The first young appear in the mid May. Females may produce two litters in the breeding season. Litter size ranges from two to seven, with four most common. Males and females do not breed in the summer of their birth. A nest containing seven young discovered under a log in Mount Rainier National Park, Washington, was constructed from dried grass.

Diet

Remarkably, there are very few studies of the Dusky Shrew's diet. The stomachs of a small sample taken from a population in the Cascade Mountains of Washington contained unidentified invertebrates, conifer seeds, fungi and lichens. Myrnal Hawes speculated that the Dusky Shrew has larger, more robust teeth than the Vagrant Shrew because it has adapted to feeding on small, hard-bodied invertebrates that would be expected to live in the dry acidic soils that characterize its habitats.

Natural History

Two shrews commonly associated with the Dusky Shrew in British Columbia are the Vagrant Shrew and Common Shrew. In the southern coastal region, the Dusky Shrew avoids open grassland habitats that are preferred by the Vagrant Shrew, but it appears to have a competitive advantage over the Vagrant Shrew in Western Hemlock forests with well-drained, acidic soils. The Dusky Shrew's relationship with the widespread Common Shrew is not clear. Walt Klenner found that the Common Shrew consistently outnumbered the Dusky Shrew in his study plots in the Thompson Plateau. In northern communities and high elevation habitats, the Dusky Shrew is often the dominant shrew species. On Vancouver Island, it coexists with the Vagrant Shrew and the Water Shrew, and on a few of the north coast islands it coexists with the Common Shrew; but on most British Columbian islands that have shrew populations, the Dusky Shrew is the only species.

In her forest plot near Maple Ridge, Myrnal Hawes estimated peak populations at 5 to 12 Dusky Shrews per hectare. The average home range was 1,227 square metres for non-breeding animals and 4,020 square metres for breeding animals. During the breeding season males occupied larger areas than females. In late summer, discrete territories are established and the daily movements of neighbouring animals show no overlap.

Only about four percent of the young will survive from their summer of birth to the following summer.

Found in all of the province's biogeoclimatic zones, the Dusky Shrew is associated with a broad range of habitats, from alpine tundra to boreal and coastal forests. At low elevations in southwestern British Columbia, the Dusky Shrew is primarily a forest species, and it is rare in open fields, wet meadows and grassland habitats. In the British Columbia interior, the Dusky Shrew inhabits closed forests of Douglas-fir and Ponderosa Pine, Engelmann Spruce and Subalpine Fir, and Tamarack and Black Spruce; it also lives on avalanche slopes and in willow thickets and meadows in alpine areas. The Dusky Shrew is also common in disturbed areas such as recent burns and logged forests. In coastal Western Hemlock forests and interior Engelmann Spruce forests, the Dusky Shrew is evenly distributed in all stands from recent clearcuts to old-growth.

The Dusky Shrew ranges across the Cordillera of northern Mexico and the western United States to western Canada and Alaska. It is the most widespread shrew in British Columbia inhabiting the entire mainland. It has been found on 87 islands in the province, including the highly isolated Queen Charlotte Islands. The smallest islands known to support populations of the Dusky Shrew are about one to four hectares in area.

In older literature, the Dusky Shrew was classified as Sorex obscurus, but the name Sorex monticolus has priority over Sorex obscurus. Because of its large distributional area and the occurrence of a number of isolated island populations, Sorex monticolus demonstrates considerable geographic variation in size and fur colour. In her recent taxonomic revision, Lois Alexander recognized 14 subspecies; 8 occur in British Columbia.

Sorex monticolus calvertensis Cowan is a weakly defined, insular subspecies confined to two widely separated islands on the north coast: Banks and Calvert. This race is larger than the mainland subspecies Sorex monticolus longicaudus, but it is very similar in skull size to S. m. elassodon and only differs in its paler colour. Genetic studies are required to evaluate the validity of this subspecies.

Sorex monticolus elassodon Osgood is an insular race associated with islands of the Alexander Archipelago in southeastern Alaska, the Queen Charlotte Islands (excluding Kunghit Island), and Porcher and Dewdney islands. It is smaller than S.m. prevostenis from Kunghit Island and the mainland race S.m. longicaudus.

Sorex monticolus insularis Cowan is an insular race confined to Reginald, Athlone (formerly Smythe) and Townsend islands in the Bardswell Group. Compared to S.m. longicaudus, the race found on the adjacent mainland, this subspecies is larger in some skull measurements but smaller in others.

Sorex monticolus isolatus Jackson is an insular race found on Vancouver Island, Denman Island, Cox Island in the Scott Islands, and a number of islands off the west coast of Vancouver Island. It is smaller than S.m. setosus, the subspecies found on the southern mainland and islands adjacent to the mainland.

Sorex monticolus longicaudus Merriam ranges from Taku, Alaska to Rivers Inlet on the coastal mainland. Eastern limits of the range are on the eastern slopes of the Coast Mountains. This subspecies also occurs on a number of islands: Campania, Campbell, Chatfield, Dufferin, Horsfall, the Hunter Group, West Kinahan, Pitt, Princess Royal, Spider, Swindle and Yeo. It is larger than S.m. setosus, the coastal subspecies to the south.

Sorex monticolus obscurus Merriam is a widespread subspecies that ranges from Alaska throughout the Cordillera of western Canada and the United States. In British Columbia, it is found across the entire mainland east of the Coast Mountains and Cascade Range.

Sorex monticolus prevostensis Osgood is confined to Kunghit Island (formerly Prevost) in the Queen Charlotte Islands. It is larger than S.m. elassodon, the other race found on the Queen Charlotte Islands.

Sorex monticolus setosus Elliot inhabits the southern coastal mainland from Rivers Inlet to Washington State. Eastern limits are the Coastal mountains and western slopes of the Cascade Range as far east as Manning Provincial Park. It also occurs on a number of coastal islands including Bowen, Cortes, Goose, Hecate, Marina, Maurelle, Quadra, Sonora, Stuart and Texada. The Texada Island population was originally classified as S.m. mixtus, but this taxon is no longer considered valid.

Although the Dusky Shrew is the most ubiquitous and one of the most abundant shrews in the province, it has been the subject of surprisingly little research. Its diet has not been described and information on habitat associations and breeding biology is limited to the southern coastal region. Comparative studies from representative biotic areas and isolated coastal islands would be revealing.

Origin Status	Provincial Status	BC List (Red Blue List)	COSEWIC
Native	S5	Yellow	Not Listed

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