Pacific water shrew, Bendire's water shrew, Bendire's shrew, Jesus shrew
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.
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.
The geographic range of the marsh shrew extends from southwest British Columbia, along the western regions of the U.S. states of Washington and Oregon and through northwestern California to the area north of San Francisco. The subspecies S. b. albiventer is found on the Olympic Peninsula. S. b. palmeri is found from western Oregon (south of the Columbia River) to extrema northwestern California. S. b. bendirii is found from the northern parts of the range (except for the Olympic Peninsula) south along the eastern range to the remaining range in California. In British Columbia the eastern limits are the Chilliwack River and Agassiz, and the northern limits are the low elevations on the north shore of Burrard Inlet.
Marsh shrews typically live in wetlands (such as marshes), and their habitat includes extensive forest canopy and ground cover from shrubs, logs, and debris; they may also be found in riparian environments. During cold, rainy seasons, they may travel as much as a kilometer from wet areas to more sheltered habitats; these generally include mixed deciduous or coniferous forest with downed logs and surface cover. Marsh shrews have been collected from near sea level to as high as 4,356 ft (1,328 m) in the Cascades. They may inhabit forests of red alder, bigleaf maple, western hemlock or redcedar, often near marshes with western skunk cabbage. In British Columbia the marsh shrew is generally found below 600 metres (2,000 ft), but it has been collected at 850 metres (2,790 ft) in Mount Seymour Provincial Park. Environmental officials in British Columbia believe that marsh shrews are one of the rarest small mammals in the province. In 1992, Carlos Galindo-Leal and Gustavo Zuleta trapped 1,000 small mammals at 55 locations in a large area of southwestern British Columbia; only three were Pacific water shrews.
During spring and summer 1983, biologists in western Oregon studied small-mammal distribution (including marsh shrews) near streams and along the riparian fringes of coniferous forests. "Riparian fringe" was defined as at least 15–20 m from a stream. The North American deermouse was caught in greater numbers than any other mammal, representing around 80% of all captures. Sixteen species were trapped, with the marsh shrew representing less than two percent of mammals captured in this study. All the marsh shrews were trapped at streamside, and were found in all three ages of coniferous forests: old-growth, mature and young growth.
The marsh shrew eats invertebrates, including spiders, earthworms, sowbugs, centipedes, termites and other terrestrial and aquatic arthropods. It was observed in captivity pursuing and killing a goldfish, but not eating it. In contrast, the American water shrew has been observed killing and eating fish. A study of the gastric contents of marsh shrews in Oregon indicated that at least 25% of their diet is aquatic, including insect larvae, slugs and snails, mayfly naiads and other, unidentified invertebrates. Other researchers have reported that their diet may also include aquatic insects (water beetles and stonefly nymphs), craneflies, ground beetles, spiders, harvestmen, centipedes, earthworms, slugs and small terrestrial snails. The marsh shrew swims, making short dives in search of food; its mobile snout, whiskers and lips are used to find and capture underwater prey. Before eating, it returns to land. Air trapped in its fur provides buoyancy, and marsh shrews can run for as long as 3 to 5 seconds along the top of the water. According to mammalogist Donald Pattie, they can "scull on the surface like whirligig beetles". The air trapped in its fur gives it a silvery sheen. On land, its foreleg and opposite hind leg move at the same time.
It is active throughout the year, primarily at night. Gestation is about three weeks, and the female has a litter of three or four young. Nests in the wild, built from shredded bark, are in a tunnel or under a log. Marsh shrews typically live about 18 months, and males are not thought to reach sexual maturity during their first summer. Since their lifespan is short, they apparently breed for only one season. Although no breeding data exist for British Columbia, the breeding season elsewhere is from late January to late August; most young are born in March. The number of litters a female rears is unknown.
The strong odor associated with marsh shrews (in common with other long-tailed shrews) may be a means of communication. Similar to other shrews, they have poor eyesight. If a marsh shrew is placed in a foreign environment (such as along an edge of a raised surface), it will run off the edge and continue to run after landing on the surface below. Landing in water, it dives beneath the surface. Marsh shrews are easily trapped in sunken cans, possibly due to their inability to see where the edges of surfaces drop. In captivity they vocalize when they are displaced or scuffle with other animals in their cage, twittering shrilly if disturbed while eating or in a confrontation over food (such as a worm). Although it is uncertain whether marsh shrews cache (or hoard) food in the wild, in captivity they set aside nightcrawlers in a corner of the cage for later consumption; no other food items were set aside in this fashion.
Acarine parasites include the Glycyphagidae (Glycyphagus hypudaei and Orycteroxenus soricis ); the Laelapidae (Androlaelaps fahrenholzi, Echinonyssus obsoletus, Haemogamasus occidentalis and Haemogamasus reidi ); the Listrophoridae (Listrophorus mexicanus ); the Myobiidae (Amorphacarus hengererorum, Amorphacarus soricis, Protomyobia atophyracis and Protomyobia brevisetosa ), and the Pygmephoridae (Pygmephorus horridus and Pygmephorus whitakeri ). Probable predators include owls, fish and the Pacific giant salamander.
Marsh shrews are listed as "Endangered" by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC), their habitat in British Columbia limited to the lower Fraser Valley. Their available habitat continues to degrade as a result of economic activity in the area; with little chance of the trend reversing, they are rare in that part of Canada. COSEWIC designated the marsh shrew as "Threatened" from April 1994 until May 2000, updating its status in April 2006 to "Endangered."According to the IUCN, marsh shrews are of "Least Concern" in terms of conservation. Although the shrew is considered a rare mammal and its numbers are thought to be in decline, no population estimates are currently provided and its rate of decline is not considered fast enough to warrant placing it in a more-threatened category. Suitable wetland habitat is declining, due primarily to urbanization and the conversion of habitat to agriculture, and areas of protected habitat are expected to be provided in the shrew's broad geographic range.
