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Sorex trowbridgii Baird
Trowbridge's Shrew
Family: Soricidae
Species account author: David Nagorsen
Extracted from the Opossums, Shrews and Moles of British Columbia
Introduction to the Mammals of BC

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Map


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

Illustration

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

Species Information

Trowbridge's Shrew is a medium-sized shrew with a dark or sooty dorsal pelage and paler underparts. Its long tail is distinctly bicoloured, with a dark grey dorsal surface and nearly white underside. The winter pelage tends to be more grey than the summer pelage.

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 curved in front view; the upper incisor has a small medial tine at the upper edge or slightly above the pigmented region on the face of the tooth. The pigmented ridge on the upper unicuspid teeth does not extend to the cingulum and is separated from the cingulum by a longitudinal groove.

Identification

In British Columbia, this species could be confused with the Common Shrew (Sorex cinereus), Dusky Shrew (Sorex monticolus) and Vagrant Shrew (Sorex vagrans). The bicoloured tail and sooty-grey fur colour distinguish Trowbridge's Shrew from these three species. The Common Shrew is also smaller, and its third upper unicuspid tooth is larger than the fourth. The Dusky and Vagrant shrews have similar upper unicuspid teeth to Trowbridge's Shrew, but their skulls can be identified by two other dental traits: the pigmented ridge on their upper unicuspid teeth extends to the cingulum and is not separated by a longitudinal groove, and the medial edge of their first upper incisors appears straight in front view.

Dental Formula

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

Measurements

total length: 118 (101-128) n = 198
tail vertebrae: 55 (41-62) n = 198
hind foot: 13 (10-15) n = 193
ear: 7 (5-8) n = 5
weight: 5.6 (3.8-8.0) n = 23

Biology

Reproduction

In the United States, Trowbridge's Shrew has an earlier breeding season than other shrew species. Reproductive activity begins in February, with females reaching sexual maturity several weeks after males. Pregnant females have been observed from February to late May, with most reproductive activity finished by June. In western Oregon, 97 per cent of the females captured in May were either pregnant or nursing, and few breeding females were found after June. The number of embryos ranges from three to six, with four or five most common. Females are capable of producing at least two litters during a breeding season. The meagre data available for British Columbia suggests a similar breeding season. Dates for pregnant females range from 24 March to early May, and nursing females have been found in May and early June.
Diet

The diet of Trowbridge's shrew consists of a diverse assortment of soil-dwelling and surface-dwelling invertebrates. Major prey types include centipedes, spiders, slugs, snails, beetles, and other adult and larval insects. This shrew also consumes plant material: fungi, including underground fungi or truffles, have been found in the stomachs of a few individuals, and several researchers have noted this shrew's tendency to feed on seeds from coniferous trees such as Douglas-fir. Captive animals readily consumed seeds from various herbs, shrubs and trees. Hoarding behaviour was common among captives; they transported seeds a considerable distance from the feeding trays and then buried them for later consumption.
Natural History

Although there are no estimates of absolute numbers, there is evidence to suggest that Trowbridge's Shrew is the most common shrew in many coastal habitats. Results from recent United States Forest Service surveys in the Cascade Range of Oregon and Washington revealed that this shrew was the most common small mammal in Douglas-fir forests. Similarly, it is the most abundant shrew in the lower Fraser River valley of British Columbia.

Captive Trowbridge's Shrews demonstrate short periods of activity at regular intervals over a 24-hour period; they are active about 39 per cent of the time, mostly after dark. Animals in breeding condition are more active than young animals or nonbreeding adults.

Owls, especially the Barn Owl, are probably the major predators of Trowbridge's Shrew in British Columbia.

Habitat


In coastal regions, Trowbridge's Shrew is most common in dry mixed and coniferous forests with rich soil and abundant decaying wood and litter on the forest floor. Nevertheless, this species has broad habitat requirements: it can be found in wet forests, riparian habitats and ravines. It generally avoids damp marshy areas with saturated soil. Trowbridge's Shrew constructs tunnels in the humus layer of the forest litter, which may explain why it prefers habitats with dry, loose soil and deep litter, and avoids wet areas with a high water table. Several studies in coastal habitats of the United States have shown that this species occupies a wide range of forest stands from recent clearcuts to old-growth forests. In British Columbia, Trowbridge's Shrew has been found at elevations up to 640 metres, but most populations occur at lower elevations. In a recent field study, Gustavo Zuleta and Carlos Galindo-Leal analysed habitat requirements of Trowbridge's Shrew. They found this species widespread and common in various habitats south of the Fraser River, with the highest captures in dry, mixed forests of Red Alder, Western Hemlock, Western Red-cedar and Bigleaf Maple. It was found in forests of various ages, but in young forests only if they had extensive canopy cover.

Distribution


Trowbridge's Shrew ranges from California through western Oregon and Washington to extreme southwestern British Columbia, where it is restricted to the lower Fraser River valley. The eastern limits of its range are Chilliwack Lake and Hope; there is a historical record, taken in 1905, from the "second Summit, Skagit River". The Fraser River generally delimits the northern extent of the range, although there are three records north of it (Point Grey, Maple Ridge and Harrison Lake).

Taxonomy


Five subspecies are recognized. The British Columbian population is classified in the subspecies Sorex trowbridgii trowbridgii Baird, a race characterized by dark fur and a narrow skull, that ranges from northwestern California to British Columbia.

Remarks


Trowbridge's Shrew is on the provincial Blue List, but results from recent field surveys indicate that it is probably more common in British Columbia than originally thought. Nevertheless, local populations may be at long-term risk because much of its range coincides with an area that is undergoing rapid urban growth and habitat loss.

Status Information

Origin StatusProvincial StatusBC List
(Red Blue List)		COSEWIC
NativeS3BlueNot Listed



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