Sorex trowbridgii Baird, 1857

Scientific Naming and Body Shape

Trowbridge's shrew, scientifically named Sorex trowbridgii Baird, 1857, is a medium-sized shrew with a long tail.

Weight

Non-breeding individuals weigh approximately 3.8 g (0.13 oz), while breeding individuals average around 5 g (0.18 oz).

Annual Fur Color Variation

Their fur color changes annually: they have a darker, more brownish coat in summer and a lighter gray coat in winter.

Molting Pattern

They grow two coats per year via one summer molt and one winter molt.

Coat Characteristics

The winter coat is thick and gray, and the summer coat is lighter and dark brown.

Tail Hair Age Variation

Young shrews have a hairy tail that becomes less hairy as they age.

Tail Coloration and Identification Trait

The tail is bi-colored, darker on the top than the bottom, with a sharp line separating the two colors; this trait helps distinguish Trowbridge's shrew from similar species.

Whiskers and Ear Features

Trowbridge's shrew has many long whiskers, and its ears are mostly covered in fur.

Foot Structure and Color

They are pentadactyl, with feet colored whitish to light tan.

Dental Pigmentation and Formula

The caps of their teeth have dark reddish-brown pigmentation, and their dental formula is 1.5.1.3 / 1.1.1.3 = 32.

Skull Size Comparison

The skull is similar in size to the skulls of other long-tailed shrews.

Unicuspid Identification Feature

This species can be told apart from closely related shrews by its third unicuspid, which is smaller than the fourth.

Additional Skull Distinguishing Features

Additional distinguishing skull features include a post-mandibular foramen, a more posteriorly positioned orbit, and specific positioning of the lacrimal and infraorbital foramina relative to the molars.

Dusky Shrew Differentiation Traits

Four traits distinguish Trowbridge's shrew from the dusky shrew: a curved first upper incisor, a bicolored tail, a pigmented ridge on upper unicuspid teeth separated from the cingulum by a longitudinal groove, and a medial tine on the first upper incisor that sits above the tooth's pigmented area.

General Distribution

Trowbridge's shrew is found along the western coast of North America.

Northern Range Extent

Its range extends from extreme southwestern British Columbia, south of Burrard Inlet, through western Washington and western Oregon.

California Distribution Split

In northern California, the distribution splits: the western population continues south through the Coast Ranges to Santa Barbara County, while the eastern population extends south through the Warner Mountains and Sierra Nevada to Kern County.

Subspecies Distribution

Different subspecies occupy different regions across this range.

Elevation Range

The species occurs from sea level up to an elevation of 1,820 m (5,970 ft).

General Habitat Requirements

It inhabits forested areas where the ground is littered with debris that provides cover.

Logged Area Habitat Use

It may stay in logged areas if sufficient ground cover remains.

Forest Habitat Types

It can be found in dry forests, moist forests, and swampy woodlands.

Destruction Island Habitat

Populations on Destruction Island off the Washington coast live in deep rank grass near salmonberry patches.

Streamside Occurrence

Trowbridge's shrew is less commonly found near streams.

Southern Range Habitat

In the southernmost parts of its range, it may occur in chaparral.

Ecological Niche Role

Trowbridge's shrew fills an important ecological niche.

Known Predators

Known predators include raptors such as the barred owl (Stirix varia) and the Pacific giant salamander (Dicamptodon sp.).

Domestic Cat Interaction

Domestic cats are known to kill Trowbridge's shrews, but usually do not eat them.

Documented Parasites

Documented parasites include multiple species of ticks, mites, chiggers, fleas, worms, and single-celled organisms.

Population Abundance Assumption

Because Trowbridge's shrew is the most frequently captured shrew within its range, it is assumed to be the most abundant shrew species in this area.

Lifespan

Their lifespan is approximately 1.5 years.

Annual Activity Pattern

They do not hibernate and remain active year-round.

Seasonal Population Variation

Their population is roughly twice as abundant in fall as it is in spring.

Primary Diet Components

Insects are their primary food source, but they also eat spiders, worms, and centipedes.

Common Stomach Content Items

Stomach content analysis shows the most commonly consumed items are centipedes (Chilopoda), spiders (Araneae), invertebrate internal organs, slugs and snails (Gastropoda), and unidentified invertebrate material.

Reproductive Season Food Consumption

Food consumption is highest during the reproductive season.

Vegetable Matter Consumption

They also eat plants, and consume more vegetable matter than other shrews in the genus Sorex.

Winter Plant Food Sources

In winter, they may feed on conifer seeds from Douglas-fir and pine, as well as other plant seeds.

Dispersal Barrier Hypothesis

It is unclear whether Trowbridge's shrew is a strong swimmer like some other shrew species, so rivers may act as a barrier to dispersal for this species.

Range Data Limitation

Some information about the species' full range is extrapolated from studies of other species.

Sexual Maturity Body Mass Change

When Trowbridge's shrews reach sexual maturity, they gain overall body mass.

Breeding vs Non-Breeding Weight

As noted, non-breeding shrews weigh around 3.8 g (0.13 oz) while breeding shrews average 5 g (0.18 oz).

Breeding Season Reproductive Organ Changes

In males, testes size increases during breeding; in females, uterine horns widen.

Post-Breeding Reproductive Organ Atrophy

Both structures atrophy once the breeding season ends.

Sexual Maturity Climate Variation

The age when sexual maturity begins varies with local climate, maturing earlier in warmer areas.

Sexual Maturity Gender Difference

Males reach sexual maturity an average of two weeks earlier than females.

Concurrent Pregnancy and Lactation

Pregnant lactating females have been recorded, so it is suspected that females can become pregnant while nursing young from an earlier brood.

Average Brood Size

Average brood size ranges from 3 to 5.

Northern Range Reproductive Variation

In more northern parts of the range, pregnant females have fewer embryos, and the breeding season is shorter.

Regional Breeding Season Timing

The breeding season runs from March to May in Washington, and from February to June in California.