Pacific water shrew (Sorex bendirii) COSEWIC assessment and status report: chapter 3

Name and classification

Sorex bendirii Merriam 1884 belongs to the Order Insectivora, Family Soricidae. Jackson (1928) assigned S. bendirii to the subgenus Atophyrax (the sole member of this group), but most taxonomists (Findley 1955) now treat this species as a member of the subgenus Otiosorex, a group that includes a number of North American shrew species. Based on morphology, Findley (1955) considered S. bendirii and the common water shrew (Sorex palustris) to be sister species. Allozyme data (George 1988) and mitochondrial DNA (mtDNA) data (Demboski and Cook 2001; O’Neill et al. 2005) also support the recognition of S. bendirii and S. palustris as sister taxa. Although they differ in pelage colour and cranial/dental morphology, genetic divergence (cytochrome b gene) between these two species is about 3.1% within the range shown by most shrew species for intraspecific divergence (Fumagalli et al. 1999). A cladistic analysis (O’Neill et al. 2005) comparing S. bendirii with samples of S. palustris from Vancouver Island (S. p. brooksi), the Cordillera of Alaska, Yukon, British Columbia, and Utah (S. p. navigator), and northern Alberta (S. p. palustris), revealed that S. bendirii, S. p. brooksi, and S. p. navigator share a common ancestry, grouping together into a separate lineage from S. p. palustris. These phylogeographic patterns were attributed to vicariance and isolation in several refugia during the Pleistocene.

Traditionally three subspecies are recognized: S. b. albiventer (Olympic Peninsula of Washington), S. b. bendirii (California, Oregon, Washington, British Columbia), and S. b. palmeri (coastal Oregon and northern California). They demonstrate minor differences in ventral pelage colour and size (Jackson 1928; Hoffmann 1971). Data from mtDNA raise doubts about the validity of these subspecies. The genetic study of O’Neill et al. (2005) included samples of S. b. bendirii from British Columbia and S. b. palmeri from Oregon. Three phylogenetic trees calculated by O’Neill et al. (2005) showed no evidence of two clades or groups concordant with these taxa.

Other English common names are the Bendire shrew and marsh shrew. The French common name is musaraigne de Bendire.

Morphological description

The Pacific water shrew is the largest North American species of Sorex. It has a dorsal pelage that ranges from dark brown to black; the undersides are dark brown (Figure 1). The tail is a unicoloured dark brown. The large hind feet have a stiff fringe of hairs up to 1 mm in length that is most distinct in younger animals. According to Maser (1975), small fleshy projections are present along the outer edge of each nostril. The skull is robust with a rostrum that is curved ventrally in side view. The dental formula is: incisors 1/1, unicuspids 5/1, premolars 1/1, and molars 3/3. Distinctive dental traits include: a third upper unicuspid smaller in size than the fourth upper unicuspid, a large medial tine on the anterior face of the first upper incisor, and two ridges on the occlusal surface of the fourth lower premolar (Carraway 1995; Nagorsen 1996).

Figure 1. Pacific water shrew (Sorex bendirii). Photo by Ronn Altig.

Figure 1.  Pacific water shrew (Sorex bendirii).  Photo by Ronn Altig.

Body measurements (range in parentheses) for Canadian animals are total length 154 mm (137-176) n=95, tail vertebrae 70 mm (61-81) n=94, hind foot 19 mm (17-21) n=93, ear 8 mm (7-9) n=3, and body mass 10.6 g (10.0-17.2) n=13 (Nagorsen 1996).

Its large size (total length >130 mm; hind foot >18 mm; skull length >19.0 mm; palatal length >8.2 mm) and fringe of hairs on the hind feet distinguish S. bendirii from all other Canadian shrew species except for the common water shrew (S. palustris). S. palustris can be identified by: grey to black dorsal fur with a silver-grey belly, a bicoloured tail that has a paler underside, and a smaller skull with the rostrum not curved ventrally. Although the two species can be easily distinguished in the hand, reliable identification of live animals observed in the wild is difficult.

Genetic description

Nothing is known about the population structure of this shrew in Canada. In British Columbia, the only potential geographic barrier is the Fraser River, a major water body that would be expected to limit gene flow among Pacific water shrews inhabiting the north and south sides of the river. The only data on genetic variation are O’Neill’s et al. (2005) analyses of mtDNA variation (cytochrome b) using five individuals from British Columbia. Their genetic divergence values among these five individuals ranged from 0.3 to 1.0% suggesting some genetic variation. However, the variation showed no clear geographic structure and no support for genetic divergence on north and south sides of the Fraser River. Their cladogram revealed two groups of S. bendirii -- one with individuals from Sumas Mountain, the other with individuals from nearby Aldergrove and the Seymour River on the north side of the Fraser. A genetic study applying other genetic markers such as microsatellite DNA is needed to assess genetic structure and the possible isolating effect of the Fraser River.

Page details

Date modified: