Water shrew  (Sorex palustris)

The water shrew is classified as Least Concern (LC) on the IUCN Red List (1).

The water shrew (Sorex palustrus) is the world’s smallest warm-blooded diving species (3). It is a relatively large shrew, with a slender body, a long tail (4) (5) (6), and a long, pointed, highly moveable snout, which has conspicuous vibrissae (specialised whiskers). The snout is almost incessantly rotated (3) (6). The feet of the water shrew have a fringe of stiff hairs, called fibrillae, which increase the surface area to help propel the water shrew as it swims (2) (3) (7) (8).

The water shrew has dark brown-black fur covers the upperparts of the body, sometimes with grey frosting, while the fur underneath is white, often tinged with grey or brown (2) (4) (5) (7). The chin is considerably lighter than the rest of the body, and the tail is distinctly dark above and light below (2) (7). The male and female water shrew have a similar appearance, but the male is typically larger and heavier than the female (2).

The genus name Sorex is derived from the Latin word ‘soric’, which means ‘shrew mouse’, while this species’ specific name palustris means ‘dwelling in marshes’. Up to ten different subspecies of the water shrew have been recognised (7).

The water shrew occurs in the cool, boreal, montane regions of North America, Canada and Alaska (1) (5) (7) (9).

It ranges from Labrador, Nova Scotia and New England, across Canada to east-central Alaska, and south to the northern Great Lakes region, and is found south along the western mountain ranges of mid-California, Nevada, Utah and New Mexico in the U.S (1). Water shrew populations are also known from the Appalachian Mountains, ranging from south-west Pennslyvania to North Carolina, Tennessee and Georgia (1). The water shrew has been recorded as far north as the Yukon Territory (2).

In Wisconsin, the water shrew is mostly confined to the northern part of the state (2).

The water shrew occurs along small, usually fast-flowing, cold-water streams with thick, overhanging riparian growth (1) (6). It is also found around lake, ponds, bogs and marshes (1) (4) (6). Logs, rocks and crevices are all common features of water shrew habitat (6) (7).

Rarely found far from water, the water shrew is extremely well adapted to its aquatic lifestyle (7). Although the eyes of the water shrew are small and vision is poor, it has acute hearing and an excellent sense of smell. In addition, the hairs on the feet trap air, allowing the water shrew to run across the surface of water (2) (5) (8).

The water shrew regularly dives under the water to capture prey or to avoid danger, sometimes remaining below the surface for over 30 seconds at a time (7). During dives, air bubbles trapped in the fur reduces heat loss, although the bubbles also make the water shrew fairly buoyant, meaning that it must paddle vigorously in a walking motion to propel itself forwards (5) (7) (10).

Mostly active during the day, the water shrew consumes a large amount of food given its small size (1) (8). Due to its high energetic demands, the water shrew can only survive for about three hours without food (5). Aquatic insects form the bulk of the water shrew’s diet (1) (5) (10). Slugs, snails, earthworms, small fish, amphibians and spiders may also be taken (2) (7), and the water shrew may occasionally feed on terrestrial invertebrates such as grasshoppers and crickets (2) (5) (10).

The water shrew forages using its sensitive nose, which has specialised hairs or whiskers, called ‘vibrissae’. During times when prey is abundant, the water shrew may cache food in hollow logs or crevices, returning when food is scarce (2) (5) (7).

Breeding occurs from February to August in North America (1). The nest is typically made from grass and plant material, with a depression in the centre (8), and is placed close to water, usually in an underground burrow, on a raft of logs or in a beaver lodge (1). The water shrew may also construct tunnel systems, or it may use those of other species (7) (8).

The water shrew may have up to three litters a year, each with between three and ten young (1). The gestation period lasts for around three weeks and, following birth, the young water shrews develop rapidly. The water shrew does not reproduce until after the first full winter, and it has a short lifespan, usually only living to about 18 months (1) (2) (5).

The destruction of aquatic habitats through pollution and drainage are the major threats to the water shrew. Additionally, activities such as logging, agriculture, road building and mining are further driving habitat loss in many areas (1).

Introduced insect species, such as the gypsy moth (Lymantria dispar), may affect some subspecies populations by contributing to habitat loss (1). Similarly, insecticides used to control outbreaks of pest species may have a negative impact on water shrew populations (1) (6), especially where they enter and contaminate the food chain (1).

Isolated populations of the water shrew are extremely vulnerable to extinction resulting from human activities or long-term climate change (1). Species with small, fragmented distributions are particularly at risk, as often they are unable to adapt to the rapidly changing climate or disperse to more suitable habitat (11).

There are no known specific conservation measures currently targeted at the water shrew, although the presence of this species in numerous state and national parks means that many populations receive adequate protection (1). 

Recommended conservation measures for this species include maintaining and protecting the mountain stream habitats where it is found, including ensuring that there is abundant cover such as rocks, logs and overhanging stream banks (1).

Find out more about the water shrew:

• Beneski, Jr, J.T. and Stinson, D.W. (1987) Sorex palustris. Mammalian Species, 296: 1-6. Available at:
  http://www.science.smith.edu/departments/Biology/VHAYSSEN/msi/default.html
• Mammals of Wisconsin Online - Water shrew:
  http://www.uwsp.edu/biology/facilities/vertebrates/Mammals%20of%20Wisconsin/Sorex%20palustris/Sorex%20palustris%20page.htm