About Sorex trowbridgii Baird, 1857
Trowbridge's shrew, scientifically named Sorex trowbridgii Baird, 1857, is a medium-sized shrew with a long tail. Non-breeding individuals weigh approximately 3.8 g (0.13 oz), while breeding individuals average around 5 g (0.18 oz). Their fur color changes annually: they have a darker, more brownish coat in summer and a lighter gray coat in winter. They grow two coats per year via one summer molt and one winter molt. The winter coat is thick and gray, and the summer coat is lighter and dark brown. Young shrews have a hairy tail that becomes less hairy as they age. 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. Trowbridge's shrew has many long whiskers, and its ears are mostly covered in fur. They are pentadactyl, with feet colored whitish to light tan. The caps of their teeth have dark reddish-brown pigmentation, and their dental formula is 1.5.1.3 / 1.1.1.3 = 32. The skull is similar in size to the skulls of other long-tailed shrews. This species can be told apart from closely related shrews by its third unicuspid, which is smaller than the fourth. 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. 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. Trowbridge's shrew is found along the western coast of North America. Its range extends from extreme southwestern British Columbia, south of Burrard Inlet, through western Washington and western Oregon. 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. Different subspecies occupy different regions across this range. The species occurs from sea level up to an elevation of 1,820 m (5,970 ft). It inhabits forested areas where the ground is littered with debris that provides cover. It may stay in logged areas if sufficient ground cover remains. It can be found in dry forests, moist forests, and swampy woodlands. Populations on Destruction Island off the Washington coast live in deep rank grass near salmonberry patches. Trowbridge's shrew is less commonly found near streams. In the southernmost parts of its range, it may occur in chaparral. Trowbridge's shrew fills an important ecological niche. Known predators include raptors such as the barred owl (Stirix varia) and the Pacific giant salamander (Dicamptodon sp.). Domestic cats are known to kill Trowbridge's shrews, but usually do not eat them. Documented parasites include multiple species of ticks, mites, chiggers, fleas, worms, and single-celled organisms. 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. Their lifespan is approximately 1.5 years. They do not hibernate and remain active year-round. Their population is roughly twice as abundant in fall as it is in spring. Insects are their primary food source, but they also eat spiders, worms, and centipedes. 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. Food consumption is highest during the reproductive season. They also eat plants, and consume more vegetable matter than other shrews in the genus Sorex. In winter, they may feed on conifer seeds from Douglas-fir and pine, as well as other plant seeds. 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. Some information about the species' full range is extrapolated from studies of other species. When Trowbridge's shrews reach sexual maturity, they gain overall body mass. As noted, non-breeding shrews weigh around 3.8 g (0.13 oz) while breeding shrews average 5 g (0.18 oz). In males, testes size increases during breeding; in females, uterine horns widen. Both structures atrophy once the breeding season ends. The age when sexual maturity begins varies with local climate, maturing earlier in warmer areas. Males reach sexual maturity an average of two weeks earlier than females. 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 ranges from 3 to 5. In more northern parts of the range, pregnant females have fewer embryos, and the breeding season is shorter. The breeding season runs from March to May in Washington, and from February to June in California.
