People passionate about nature

The Weird & Wonderful Water Shrew

a tiny mammal with big skills

by: Deanna Dodgson

The semi-aquatic Water Shrew (Sorex palustris) is a mighty mammal in a tiny package.  Weighing in at 10-18 grams, about the equivalent of 2.5 teaspoons of salt, and reaching a maximum length of 6.7 inches (including tail), it is the world's smallest mammalian diver.  It is a voracious predator whose flexible snout and whiskers are used to detect prey.  It can follow scent trails when submerged, re-breathe air bubbles under the ice to increase stamina while foraging in winter, and is even known to run on water!

It has a blackish coat, sometimes tinged with white hairs, with a white underside.  It is less commonly entirely black.  In summer, the fur turns brown.  The eyes are small and the ears inconspicuous.  The Water Shrew's feet and toes are fringed with stiff white hairs and the toes are partially webbed.  The tail is long and bi-coloured.  The Water Shrew is distributed through boreal regions of the country, from Labrador and Nova Scotia through to Yukon.  It is also found in Alaska and in discontinuous areas of the United States south of the border.  It is found throughout Manitoba, north to Little Duck Lake (59th parallel). 

Mainly nocturnal and solitary, the Water Shrew is usually found along small flowing streams or on the edges of lakes and ponds, in areas with abundant riparian cover.  Areas near beaver dams are especially inviting.  These mammals may be largely dependent on aquatic insects but they also eat small fish, tadpoles, frogs, larval salamanders, terrestrial insects and spiders.  In bogs in southeastern Manitoba, where aquatic insects and fish are uncommon, Water Shrews will feed heavily on ground beetles.  They must eat their approximate weight in food every day to fuel their fast metabolism.  In winter, Water Shrews are believed to enter periods of torpor to minimize energy losses.  Hollow, globular nests made of leaves and other vegetation are located in sheltered sites such as under logs or in cavities in hollow logs, where the female raises two to three litters per year, giving birth to an average of 6 young per litter.  Predators include large fishes, mink, otter, weasel, snakes and predatory birds.  They may live a maximum of 18 months.

Water Shrews are well-adapted to foraging both on and in the water – the stiff hairs on the feet and toes trap air, enabling the shrew to travel on the water's surface for a distance of up to five feet.  All four feet are used to propel the shrew underwater, the main thrust made by the large hind feet.  Dives can last up to 47 seconds but most are shorter.  Unlike other diving mammals, the Water Shrew elevates its body temperature before diving into cold water and is thought to achieve this through shivering or by using brown fat to create heat.  This may help keep the sensory system in top working order, maximizing the chances of capturing prey.  Winter dives, though they may be frequent, are brief.  A layer of air in the fur helps to retain heat but also makes the shrew very buoyant, so that it must paddle constantly to remain submerged.  Swimming and diving sessions are followed by bouts of vigorous grooming. 

Below the surface of the water, the Water Shrew relies on senses other than vision to navigate and locate food sources.  Scent trails are followed by “underwater sniffing”, a technique also used by the Star-nosed Mole and River Otter.  This is achieved by exhaling air onto an object, or sometimes into open water when trailing sudden water movements made by fleeing prey, then re-inhaling the air bubbles to “smell” their quarry.  Sensitive whiskers detect water disturbances and investigate stationary objects to distinguish prey from non-prey, determining both the shape and texture of the subject.  Amazingly quick, it takes only 20 milliseconds for a Water Shrew to respond to a water movement glancing off their whiskers and only 50 milliseconds to advance to within 1-2 cms of the source of the disturbance, mouth agape!  Efficient hunting is necessary for survival – because Water Shrews are small, they have low oxygen storage capacity and these stores are quickly depleted. 

The Water Shrew's other senses are less well-developed.  Their hearing is acute up to ten feet but their vision is relatively poor, seeing well to only six or so inches away.  Recent studies suggest that echolocation and sonar do not play a part in foraging whether above or below water. 

I think you'll agree that Water Shrews are truly fascinating animals!