Sorex palustris J. Richardson 1828

77. American Water Shrew Sorex palustris French: Musaraigne palustre / German: Amerikanische Wasserspitzmaus / Spanish: Musgano americano Other common names: Vater Shrew Taxonomy. Sorex palustris J. Richardson, 1828, “ Marshy places, from Hudson's Bay to the Rocky Mountains,” Canada. Sorex palustris is in the S. palustris group and subgenus Otisorex along with S. albibarbis, S. alaskanus, S. navigator, and S. bendiri. It previously included S. albibarbis and S. navigator as subspecies, but genetic and morphological data support both as distinct species, although additional studies are needed to morphologically distinguish S. albibarbis. Sorex palustris is sister to S. albibarbis, and S. navigator is sister to S. bendi- rii. The S. palustris group is closely related to the S. montanus group and the S. sonomae group, but their exact relationships are uncertain. Distributional limits between S. palustris, S. navigator, and S. albibarbis are still very uncertain, and because of this, much of the ecological data cannot be reliably attributed to any of the three species. Thus, all ecological data for S. albibarbis, S. navigator, and S. palustris are presented under S. palustris here. Two subspecies recognized. Subspecies and Distribution. S.p.palustris|.Richardson,1828—fromEBritishColumbiaandSNorthwestTerritoriesEtoCOntario(Canada)andStoNorthDakotaandMinnesota(NCUSA). S. p. hydrobadistes H. H. T.Jackson, 1926 — from E South Dakota E to Michigan (NC USA). Descriptive notes. Head-body 62-98 mm, tail 58-71 mm, hindfoot 17-21 mm; weight 8:7-20 g. The American Water Shrew is a large semi-aquatic shrew. Pelage is highly variable; dorsum is generally blackish brown and occasionally flecked with white, giving grizzled appearance. Venter is silvery white to buffy brown. Occasionally, there is a nearly all blackish form, with lighter throat and chest, although this is rare. Feet are large and broad, and hindfeet are partially webbed and have small white hairs protruding on their sides. Tail is long, somewhat laterally flattened, and bicolored, dark blackish brown above and lighter below, with small tip of fur at end. Skull hasrelatively shorter unicuspid toothrow, largerfirst unicuspid, and shorter rostrum compared with the Western Water Shrew (S. navigator). Teeth are pigmented dark red. There are five unicuspids, third is smaller than fourth, and fifth is much smaller. Habitat. Around streams, ponds, lakes, swamps, marshes, and bogs. American Water Shrews are most common around small streams with dense overhanging riparian vegetation. Although they are most commonly associated with water, individuals have been captured far from water sources, probably dispersing individuals. They are often associated with areas with at least 75% ground cover, providing protection while foraging. American Water Shrews are often associated with North American Beavers (Castor canadensis) because their dams create more marsh edge habitat with moving water and moist humus. Food and Feeding. American Water Shrews eat a large variety ofterrestrial and aquatic prey. Slugs and earthworms make up large proportions of their diets, along with plant material, Endogone fungi, aquatic insect larvae (stoneflies, mayflies, caddisflies, etc.), and otherterrestrial insects. They are also known to eat carrion of dead mice and will take small fish, tadpoles, and larval California giant salamanders (Dicamptodon ensatus). In hatchery ponds, they occasionally eat fish eggs. When food is plentiful, they will cache it in their tunnels, sometimes defecating on it to stop others from taking it. Breeding. Testicular enlargement in male American Water Shrews occurs in December or January; ovarian activity has been recorded as early as January. Reproduction occurs from February until about August in Montana, although breeding season probably varies by region. Females generally have 2-3 litters/year, and few individuals breed in the same year they are born, although it does occur in some individuals born earlier in the season. Litters have 3-10 young based on embryo counts. Females have postpartum estrus. Activity patterns. American Water Shrews are semi-aquatic. They are active day and night, although activity is highest during sunset and sunrise. Throughout the day, they have 1-5hour rhythms during which they spend 30 minutes active and 57 minutes sleeping. Active periods are spent foraging for food and exploring surroundings on land and in water. They are very capable swimmers and use their hindlegs to propel themselves through the water. They are also able to walk on water for short periods of time using the air bubbles trapped under hairs on their hindfeet, although mechanisms behind this need to be better studied. They forage underwater by poking and probing rocks and crevices on bottoms of streams. American Water Shrews seem to use their sense of touch and smell to find prey, but sight might also be important. Because a layer of air surrounds them while swimming, they float easily, which makes it difficult for them to stay underwater. Their fur also starts to get wet after a few minutes, and because of this, they come ashore and dry themselves off after each dive. They dry their fur with their hindfeet for ¢.10-30 seconds before entering the water again. On land, American Water Shrews use regular trails and tunnels to get around and get into the water and might sometimes block entrances to tunnels with sticks or dirt. Tunnels of deermice (Peromyscus sp.) are often used instead of creating their own. American Water Shrews build nests near banks of streams or ponds. Nests are constructed in tunnels, in logs, or under logs, and are formed by collected grasses and other vegetation, forming a depression, and stitching walls together with the mouth to form a somewhat dome-shaped nest. Movements, Home range and Social organization. American Water Shrews are largely solitary and protect elongated territories along stream or pond edges from conspecifics. Home ranges are relatively small and have been estimated at 0-2-0-3 ha. When two American Water Shrews meet each other, a brief and sometimes intense fight occurs. Generally, however, one shrew backs down after the initial threat, although when neither shrew backs down, fighting is inevitable. Fights generally begin with both shrews emitting high-frequency squeaks c¢.3-5 cm apart. If neither backs down, they will rise onto their hindlegs and lash out with forefeet. If they continue to fight, they will end up in a tight ball, scratching and biting at each other until one gives up and runs away. The winner chases the loser away and is more likely to attack another shrew it meets; the loser is generally wearier about picking fights thereafter. Status and Conservation. Classified as Least Concern on The IUCN Red List. American Water Shrews are overall not threatened, but localized water pollution and habitat destruction are detrimental because they rely so heavily on aquatic environments. Bibliography. Beneski & Stinson (1987), Catania (2006), George (1988), Gusztak & Campbell (2004), Himes & Kenagy (2010), Hope et al. (2014), Mycroft et al. (2011), Nagorsen et al. (2017), Sorenson (1962), Whitaker & Schmeltz (1973).

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