Sorex cinereus - Masked Shrew

Physical Description:

The masked shrew is brown on the back and has grayish-white under parts.  Its tail is slightly hairy, bicolored (dark brown above and pale below), and has with a black tip.  The chest and belly is black mixed with gray tipped hairs.  In the winter, the pelage of the masked shrew is darker (Kurta, 1995; Wilson, 1999).

The mean body weight of masked shrews ranges from 3.5 to 5.5 grams, and their body temperature is 38.8 degrees Celsius. (Forsyth, 1976; Churchfield, 1990; Kurta, 1995).  Masked shrews have relatively high metabolic rates (9.0 ml O2/g*h), high water requirements, and high respiration rates.  Masked shrew heartbeats were recorded at more than 1200 beats per minute (Burt, 1980; McCay, 1997).

Total length of masked shrews is 75-100 mm, tail length is 28-50 mm, hind foot length is 10-13 mm, skull length is 16-17.5 mm, and skull width is 7.4-8.4 mm. They have no significant sexual dimorphism.  The maximum life span for most species in the genus Sorex is 1.5 years.  The masked shrew has 32 teeth, and the dental formula is 3/1, 1/1, 3/1, 3/3 (Forsyth, 1976; Foresman, 1998; Kurta, 1995; Wilson, 1999). 

Masked shrews are distinguished from S. hadeni by having a bicolored and black tipped tail, and from S. vagrans and S.  monticolus by having a longer rostrum and a third unicuspid tooth that is not reduced.  S. hoyi has a shorter tail, shorter rostrum, and fewer visible upper unicuspid teeth (three in S. hoyi and four in S. cinereus)  (Kurta, 1995; Wilson, 1999).

Distribution:

The masked shrew occurs throughout much of North America, including all of Wisconsin, and is the most widely distributed and abundant member of the genus Sorex (Forsyth, 1976; French, 1984; Ryan, 1986; Wilson, 1999).

Ontogeny and Reproduction:

Newborn masked shrews weigh 0.28 grams, have transparent hairless skin, and are 15-17 mm long.  Their eyelids are fused at birth and open at 17 to 18 days.  Weaning in masked shrews occurs between 20 and 27 days.  Births occur mostly in May and June in the masked shrew, but they have been documented to breed into September and October in Manitoba.  Litter sizes vary from 4-10 young with an average of seven per female.  Females have 1-3 litters per year.  The young feed from their mother’s mammae (6) until weaning, and females do not regurgitate food to their young.  Adult male masked shrews are not found near the nest when litters are present, and they may be excluded from the nest after mating with the female (Forsyth, 1976; Foresman, 1998; Kurta, 1995; Wilson, 1999).

Testicular recrudescence, the growth of the testis that occurs when they are increasing in size prior to maturation, initiates in masked shrews when they are 7 to 10 months of age.  Spermatogenesis begins in the summer and lasts into autumn.  Juvenile shrews are sometimes capable of mature testicular development and spermatogenesis, but it is uncommon (Foresman, 1998).

Ecology and Behavior:

Masked shrews prey upon insects and other invertebrates.  Ants, spiders, fly larvae, adult beetles, and beetle larvae tend to be the main prey items for masked shrews.  S. cinereus is a generalist, and the most dominant prey item in masked shrew populations depends upon its geographic location.  Plant material and animal material have been reported as food items in masked shrews, but it is presumed that they become ingested accidentally while feeding on insects.  Masked shrews tend to stick to small prey (2-5 mm), and to pick prey based on size categories not taxonomic categories.  This is probably due to the small size of masked shrews.  They eat more than their own body weight each day.  A captive masked shrew was documented as eating three times its own body weight in one day (Bellocq, 1994; Burt, 1980; French, 1984; McCay, 1997; Ryan, 1986; Wilson, 1999).

Population density typically ranges from 1- 23 shrews per hectare, and home range size has been estimated at 5549 square meters.  Masked shrews rarely move far from their home site.  Masked shrews don’t like other conspecifics overlapping their territories.  When population densities of masked shrews are high, home ranges are reduced in size to avoid overlapping (Churchfield, 1990; Kurta, 1995).

The masked shrew is common in coniferous and northern deciduous forests; however, it is found in many other habitats including shrub thickets and prairies.  They are least common in dry upland areas and exposed fields.  Masked shrews prefer wet sites, and moisture may be the primary factor affecting local abundances of masked shrews.  Habitats with increased moisture may benefit shrews by increasing the relative humidity, which may help them maintain their water balance.  A high relative humidity could also benefit shrews by increasing the abundance and activity of their invertebrate prey (Churchfield, 1990; Kurta, 1995; McCay, 1997; Wilson, 1999).

Masked shrews tend to be loners.  They travel next to fallen logs, use obscure paths through the leaf litter, travel through mole tunnels, and sometimes utilize vole runways.  The masked shrew is active any time of the day or the night, although they are mostly nocturnal.  They tend to stay under cover when inactive, and are usually searching for food when active (Burt, 1980; Kurta, 1995;Wilson, 1999).

Masked shrews make nests of woven dry or fresh grass that are 4-6 cm in diameter on the outside and 1.5 cm on the inside.  The nests usually have two runway exits. (Forsyth, 1976).

Shrew sounds come from 3 ways: clicking of the tongue, sound from the larynx, or through exhalation or inhalation through the nose.  It appears that masked shrews have the capacity to echolocate.  They have been reported to emit short length (5-33 ms) high frequency pulses from the mouth in the magnitude of 30-60kHz when exposed to strange surroundings (Churchfield, 1990).

Remarks:

Common names of Sorex cinereus are the masked shrew, the common shrew, and the cinereus shrew.  The masked shrew has eight subspecies.  The subspecies Sorex cinereus lesueuri is found in Wisconsin (Wilson, 1999).

Literature Cited:

Bellocq, M.I., et. al. 1994. Diet of Sorex cinereus, the masked shrew, in relation to the abundance of Lepidoptera larvae in northern Ontario. The American Midland Naturalist. 132:68-73.

Burt, W. H., and R. P. Grossenheider. 1980. A Field Guide to the Mammals of North America. Third Edition. Houghton Mifflin Company, Boston and New York.

Churchfield, S.  1990. The Natural History of Shrews.  Comstock Publishing Associates, Ithaca, New York.

Foresman, K. R.; and R. D. Long. 1998. The reproductive cycles of the vagrant shrew (Sorex vagrans) and the masked shrew (Sorex cinereus) in Montana. The American Midland Naturalist. 139:108-113.

Forsyth, D.J. 1976. A field study of growth and development of nestling masked shrews (Sorex cinereus). Journal of Mammalogy. 57:708-721.

French, T.W. 1984. Dietary overlap of Sorex longirostris and S. cinereus in hardwood floodplain habitats in Vigo County, Indiana. The American Midland Naturalist. 111(1):41-46.

Kurta, A. 1995. Mammals of the Great Lakes Region. The University of Michigan Press, Ann Arbor.

McCay, T.S. 1997. Masked shrew (Sorex cinereus) abundance, diet and prey selection in an irrigated forest. The American Midland Naturalist. 138:268-275.

Ryan, J. M. 1986. Dietary overlap in sympatric populations of Pygmy Shrews, Sorex hoyi, and Masked Shrews, Sorex cinereus, in Michigan. Canadian Field-Naturalist 100(2): 225-228.

Wilson, D. E. and S. Ruff. 1999. Cinereus shrew. The Smithsonian Book of North American Mammals . Smithsonian Institution Press, Washington and London .

 
Reference written by Katie Berger, Biol 378: Edited by Chris Yahnke.
Page last updated 4-29-04.