The marsh shrew (Sorex bendirii), also known as the Pacific water shrew, Bendire's water shrew, Bendire's shrew and Jesus shrew, is the largest North American member of the genus Sorex (long-tailed shrews). Primarily covered in dark-brown fur, it is found near aquatic habitats along the Pacific coast from southern British Columbia to northern California. With air trapped in its fur for buoyancy, marsh shrews can run for three to five seconds on top of the water. It measures about 16 cm (6.3 in) in length, including a 7-centimetre (2.8 in)-long tail, and weighs an average of 14.5–16 g (0.51–0.56 oz). The marsh shrew's diet consists mainly of invertebrates, which it hunts on land and in the water. They are rare; their populations are thought to be in decline, and they are considered endangered in parts of their range.
Description
The marsh shrew is the largest member of the genus Sorex in North America, and mammalogist David Nagorsen described it as "an attractive mammal". Its fur is primarily dark brown, and it has a long tail. Although the marsh shrew's fur is usually uniformly dark on its back and abdomen, the abdominal fur of populations (S. b. albiventer) on the Olympic Peninsula may be white. The marsh shrew is about 16 cm (6.3 in) in length, including a 7-centimetre (2.8 in)-long tail, and weighs about 14.5–16 g (0.51–0.56 oz). Its hind feet, slightly fringed with coarse hairs on the toes, measure about 19.2 mm (0.76 in). Although in some areas the marsh shrew is sympatric with other members of the genus Sorex, no other large, velvety, gray-black shrew shares this geographic range. Its size distinguishes it from all but the American water shrew (Sorex palustris). Although the marsh shrew and the American water shrew (the two largest shrews in North America) share some features, the American water shrew has more dark-grey-to-black fur on its back, a silver-grey belly and a bi-colored tail and the marsh shrew's fringed hairs are more distinct. The American water shrew has a smaller skull, without the marsh shrew's characteristic curvature, and its upper incisors have less-distinct medial tines. The marsh shrew typically has a longer snout than that of the American water shrew, which is more streamlined when viewed from the side. The marsh shrew's skull is relatively large, and its condylobasal skull length is usually greater than 19.3 mm (0.76 in). There is a distinctive, downward-sloping curve along the snout (rostrum). Its dental formula is incisors: 1/1; unicuspids: 5/1; premolars: 1/1; molars: 3/3. Of the five upper unicuspids the third is distinctly smaller than the fourth, and they have a pigmented ridge extending to the cingulum. There is a large medial tine on the large upper incisor, in the anterior pigmented region. The reddish pigmentation of the enamel, the result of iron deposits, is thought to be an adaptation strengthening the enamel. There are no known fossil remains. The marsh shrew's karyotype somatic number is 2n = 54, and its fundamental number = 70.
Taxonomy and naming It was first described in the scientific literature in 1884 by Clinton Hart Merriam with its original name, Atophyrax bendirii (a monotypic taxon at the time). The first specimen was obtained 18 mi (29 km) southeast of Fort Klamath in Klamath County, Oregon, at a location 1 mi (1.6 km) from the Williamson River. Merriam obtained the specimen from Charles Bendire, an ornithologist and army captain stationed at Fort Klamath. The shrew was caught in late July or early August by one of the dogs in the camp, and a soldier gave it to Bendire. Merriam named it Bendire's shrew (Atophyrax bendirii) in appreciation of Bendire's contribution. Merriam reported that the animal represented a new genus, Atophyrax, deriving from the Greek and meaning "anomalous sorex". The marsh shrew was later reclassified in the genus Sorex.
The marsh shrew and the American water shrew (Sorex palustris) share many physical characteristics. The former is found in a narrower area from the northwest coast to the lower slopes of the inland mountains. The American water shrew is more widely distributed across the western mountains and through the subarctic regions of Canada and the eastern U.S. The species' ranges are primarily allopatric; although they may overlap (sympatry) in coastal regions of the Pacific Northwest, differences in elevation tend to separate them. Early taxonomists placed these mammals into separate subgenera; Merriam assigned the marsh shrew to Atophyrax, and Jackson (1928) assigned the Pacific water shrew to Neosorex. A closer, dentition-based relationship was assigned by Findley (1955), with both species assigned to the subgenus Otisorex. Findley hypothesized that in the early Pleistocene, the ancestors of masked and vagrant shrews (Sorex cinereus and Sorex vagrans, respectively) diverged; during the Yarmouth interglacial, the American water shrew and marsh shrew diverged from their vagrant-shrew ancestors. Three other Sorex species evolved during the Sangamonian Stage: the dwarf shrew (S. nanus), the southeastern shrew (Sorex longirostris) and the ornate shrew (Sorex ornatus). Findley's assignment of the marsh shrew and the American water shrew to Otisorex was later reinforced by biochemical and genetic studies. In 2005, findings were published (based on mitochondrial DNA analysis) which better defined the nature of the relationships between marsh shrews, Pacific water shrews and their respective subspecies. Variations in the sequencing of cytochrome b mitochondrial DNA were assessed, and the results of the 2005 phylogeny for the marsh shrew are shown below in detail.
Subspecies The marsh shrew has three subspecies:
S. bendirii albiventer (Merriam, 1895) S. bendirii bendirii (Merriam, 1884) S. bendirii palmeri (Merriam, 1895) S. b. albiventer is found on the Olympic Peninsula of Washington, S . b . bendirii in the Cascades and southwestern British Columbia and S. b. palmeri in coastal Oregon. These specifications have historically been based on fur markings, skull shape and dental details of unclear significance, and their validity is uncertain.
Distribution and habitat
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