Luke Lenard
Sorex palustris
The Northern Water Shrew
The hardy water shrew is active day and night and can swim under the ice. © Moose Peterson
Description
Sorex palustris belongs to the Order Insectivora, Family Soricidae, and the
Subfamily Soricinae. It is of the genus Sorex, and the subgenus Otisorex. 9 or 10
subspecies of Sorex palustris are extant, and include; Sorex palustris albibarbis,
S.p.brooksi, S.p. hydrobadistes, S.p. gloveralleni, S.p. navigator, S.p. labradorensis,
S.p. palustris, S.p. punctulatus, and S.p. turneri. (Beneski Jr. and Stinson 1987)
The water shrew is a large shrew, adults range from about 130-170mm in total
length. Tail lengths range from 55-90mm in adults. Males typically are larger than
females, especially sexually active males. They show significantly longer total lengths
and total body masses. Fur is darker dorsally than ventrally as in most
shrews. Typically the dorsal fur is black or dark grey, and the ventral pelage is white,
with the chin region being the lightest color on the body. They also have stiff hairs called
fibrillae on their feet, which provide added thrust for swimming during dives (Beneski Jr.
and Stinson 1987).
Sorex palustris has the dental formula i 3/1, c 1/1, p 3/1, m 3/3 = 32. Compared
to other shrews, S. palustris has a relatively large skull. The upper jaw has 10 unicuspid
teeth (5/side), with the fourth being larger than the third. The zygomatic arch is absent
in all shrew species.
Distribution in Wisconsin
The distribution of the water shrew has been debated and studied for years. The
northern limit of the range in particular is not very well known, as specimens turn up
from regions further north than previously suspected. In general, the shrew thrives in
cool areas of North America, including Canada, parts of Alaska, and cool regions of the
U.S. It has been found as far north as the Yukon Territory. (Jarrell 1986)
The water shrew is abundant in the northern two-thirds of Wisconsin, but rare in
the southern third. Specimens have been found in Douglas, Vilas, and Marinette
counties, and 4 more were found in the vicinity of Rhinelander, Oneida county (Cory,
1912). They are confined to the northern part of the state, mostly in the Canadian life
zone and upper Transition zone, south in the central part of their range to northern Clark
county (Jackson 1961). The black dots on the map represent areas where a specimen of
Sorex palustris was previously collected.
Ontogeny and Reproduction
Sorex palustis' maximum life span is about 18 months. Water shrews mature
earlier than most other species of shrews, but most females don't reproduce until after
their first full winter. Ovulation is induced by copulation, and most ovarian activity takes
place starting in January, and ends in August. A typical female will have three litters per
year, with an average of 6 young per litter. The gestation period is not known, but for
most shrews it is about 21 days (van Zyll de Jong, 1983).
Ecology
Water shrews, are usually found near water, hence the name. Most commonly,
they are near water with a high number of invertebrate prey. This includes streams and
small rivers, springs, or sometimes near lakes or ponds. They often travel on small
runways near stream banks.
Predators of the water shrew include many species of hawks, fish, and owls, as
well as mink and weasels. Water shrews are insectivorous. A large portion of their diet is
slugs and earthworms, and in many cases, aquatic invertebrates, but they are also known
to eat terrestrial invertebrates such as grasshoppers and crickets. (National Geographic
Society 1981) A small percentage of their diet may be small or young fish, fish eggs,
leeches, small crustaceans, and even plantlife. (Dunstone and Gorman 1998)
Behavior
Water shrews can dive into water or run across the surface. They have been seen
running as much as five feet across the waters surface. They dive into the water to escape
predators or catch their prey, and they continue to use the water in this manner
throughout winter. They walk across the surface by using air bubbles trapped in the
fibrillae of their hind feet (Grosvenor 1979).
Sorex palustris often feeds in the substrate, but its dive time is limited because
the shrew is covered in a layer of air which causes it to float if it stops paddling. If prey
is abundant, water shrews will often cache their food in logs or crevaces for later
consumption. They kill larger prey items by biting them in the head, and often use their
forefeet to hold the prey down while tearing off pieces to eat.
Remarks
The water shrew needs to be studied in more detail in Wisconsin. There is a
drastic lack of material on water shrews in Wisconsin, but a plethora of material from
Canada and Alaska especially, and also a number of articles from western states such as
Montana, Utah, and Wyoming. There are also more articles from the east coast states
such as Virginia and Maine. Our general area near the great lakes also has a lot of
articles on the water shrew, but most are from Michigan and Minnesota.
According to the Wisconsin department of natural resources the water shrew is
not threatened or endangered, but is one of the many animals that are of special concern.
If their habitat isn’t preserved, and the environment continues to degrade, the water shrew
will soon be on the threatened or endangered species list as well. (WDNR, 2003)
Literature Cited
Beneski Jr., John T. and Stinson, Derek W. 1987. Sorex palustris. The American
Society of Mammologists.
Burt, William H. 1977. Mammals of the Great Lakes Region. The University of
Michigan Press.
Cory, Charles B. 1912. The Mammals of Illinois and Wisconsin vol. 11. Field museum
of Natural history zoological series.
DNR, Wisconsin. 2003.
http://www.dnr.state.wi.us/org/gmu/upsis/imp/appendix%203.pdf
Dunn, Ann Bailey. 2003. http://www.wnrmag.com/stories/2003/oct03/shrew.htm
Dunstone, Nigel and Gorman, Martyn. 1998. Behavior and Ecology of Riparian
Mammals. The Press Syndicate of the University of Cambridge.
Grosvenor, Gilbert M. 1979. Wild Animals of North America. National Geographic
Society.
Jackson, Hartley H. T. 1961. Mammals of Wisconsin. The University of Wisconsin
Press.
Jarrell, Gordon H. 1986. A Northern Record of the Water Shrew, Sorex palustris, from
the Klondike River, Yukon Territory. Canadian Field-Naturalist 100(3): 391.
National Geographic Society. 1981. Book of Mammals vol. 2. Special Publications
Division of the National Geographic Society.
Wojcik, Jan M. 1998. Evolution of Shrews. Mammal Research Institute, Polish
Academy of Sciences.
Reference written by Luke Lenard, Biol 378: Edited by Chris Yahnke. Page last updated