SHREWS: Soricidae

PHYSICAL CHARACTERISTICS

Sometimes confused with mice, the typical shrew has a long, pointy snout with sensitive whiskers, a long and thin tail, tiny eyes that are sometimes hidden under their fur, noticeable ears, and fairly short legs with five clawed toes on each foot. Most have short, brown or gray fur, and many of them have red-tinged teeth. The vast majority of shrews are no bigger than a house mouse, but a few species, like the water shrews, can top 5 inches (12.5 centimeters) in head and body length. Overall, shrews range from 1.4 to 5.3 inches (3.6 to 13.5 centimeters) in head and body length and 0.06 to 1.5 ounces (2 to 40 grams) in body weight. Tails are typically from half the length to the same length as the head and body measurement. The smallest shrew, and indeed one of the tiniest living mammals, is Savi's pygmy shrew with a body that is just 1.4 to 2.1 inches (3.6 to 5.3 centimeters) long. The tail is about half that size. The tiny shrew weighs 0.4 to 0.1 ounces (1.2 to 2.7 grams).

GEOGRAPHIC RANGE

Shrews can be found nearly worldwide, including North America, Central America, northern South America, Africa, Europe, and Asia.

HABITAT

This is a very wide-ranging family, but most species tend to prefer areas with at least some moisture. Many scuttle along the damp earth under leaf litter, but a few will climb trees in search of food. The aquatic species naturally seek out water sources that may range from bogs and swamps to streams and rivers. A few species survive well in the desert.

DIET

Shrews are not picky eaters. While insects and other invertebrates (animals without backbones) make up the bulk of their diet, they will also eat fruit and seeds, as well as small mammals, lizards, frogs, and even other shrews if food is scarce. They burn energy very quickly, so many shrews spend just about every waking moment either eating or looking for their next meal. Many species eat at least their body weight, and sometimes up to four times that amount in food every day.

BEHAVIOR AND REPRODUCTION

Most shrews are active at night and rest during the day. A few, however, like the long-tailed shrew, stay awake for much of the day trying to feed their hefty appetites. When they can't find enough food, some species may spend a few hours in an inactive state called torpor that decreases their energy needs. Unlike most other mammals, some shrews actually produce venom to immobilize their prey, and then either kill the prey immediately or save it for a later meal. European water shrews, for instance, have a deep groove in the lower front tooth to help direct the venom from a duct at the base of the tooth into the prey.

Shrews are well-known for being aggressive toward members of their own species and sometimes other species. By making and marking small territories with scents, they typically avoid one another and thus sidestep fights. However, when two shrews, like the short-tailed shrews of North America, encounter one another in a confined space, they will commonly attack quickly and continuously, often until one dies. Despite their reputation as fighters, a few species tolerate other shrews quite well. Adult small-eared shrews will even share a nest.

SCARED TO DEATH

Shrews are very active little animals, dashing from place to place with noses almost always twitching. A typical heart rate for a shrew is in the hundreds, five or more times higher than a human heart rate, and can nearly double if the animal is frightened. In fact, a shrew can actually die of fright if it is startled by a loud noise, like a clap of thunder.

Most shrews spend their whole lives on land, usually running from place to place. A few species are good swimmers. These aquatic shrews typically have stiff, fringed hairs on their feet that serve to enlarge the surface area of their feet and help them paddle through the water. The elegant water shrew has actual webbing on its feet to aid in swimming.

Shrews generally breed two or more times a year, giving off specific odors or making characteristic movements, such as tail-wagging in house musk shrews, to announce that they are ready to give up fighting long enough to mate. Females may mate with several males during each breeding period, so the offspring in one female's litter may have several different fathers. Many species build nests. The short-tailed shrew, for example, makes a small nest of leaves and grass in a hidden spot, often under a rock or inside a tunnel. Pregnancies last only three to four weeks for most species, and the babies are small and quite helpless. The number of offspring varies, but three to seven is a common litter (young born at the same time) size for shrews. Babies grow very rapidly and are ready to face the world on their own at just three to four weeks old. Before they do so, however, some species of the group, known as white-toothed shrews because they lack the reddish tinge seen in other shrews, take part in an odd behavior. The mother leads them around in a row, with each shrew using its teeth to grasp the hair on the rump of the one in front of it. This line-up of shrews is called a caravan, or chain behavior. Scientists now believe that families of some red-toothed shrews may use this peculiar but effective method of travel, as well.

As noted, shrews develop quickly and they begin having young of their own before they reach their first birthday. Shrews rarely live much past fourteen to eighteen months of age.

SHREWS AND PEOPLE

Since they are small, usually active only at night, and like to hide, shrews avoid human attention most of the time. They do, however, play an important role for farmers and gardeners, who have fewer destructive insects in the crops, thanks to the shrews' appetites. Shrews have cultural significance, as well. For example, Shakespeare's The Taming of the Shrew is a classic tale, and people in Taiwan consider a shrew to be a symbol of good luck. On very rare occasions, shrews have bitten people. If the shrew is venomous, this can be quite painful.

CONSERVATION STATUS

According to the Red List of the World Conservation Union (IUCN), twenty-eight species are Critically Endangered, facing an extremely high risk of extinction; twenty-eight are Endangered, facing a very high risk of extinction; fifty-five are Vulnerable, facing a high risk of extinction; and four are Near Threatened, not currently threatened, but could become so. In other words, more than one-third of all shrew species are at some risk. The U.S. Fish and Wildlife Service names one species, the Buena Vista Lake ornate shrew, as Endangered. Many of the at-risk shrews live in limited areas and have very small known populations. This combination puts them in danger, because a single natural disaster, like a flood or one human disruption of their habitat, such as a mining operation, could destroy the entire population.

AMERICAN LEAST SHREW (Cryptotis parva): SPECIES ACCOUNTS

Physical characteristics: Just 2.2 to 3.1 inches (5.5 to 7.8 cm) in head to body length, this small shrew has a brownish gray back and whitish belly, a long snout, red-tinged teeth, and a tail that is no more than a third of the length of its head and body. It weighs from 0.1 to 0.3 ounces (4 to 8 grams). Its eyes are small and its ears are unnoticeable.

Geographic range: United States, extreme southeastern Canada, Mexico, and much of Central American to Panama.

Habitat: American least shrews are common in open, grassy fields, sometimes near a stream, but may also live in damp forests. This species spends much of its time in shallow tunnels it either makes itself or borrows from other animals.

Diet: Active day and night, year-round, they spend most of their time running about in search of food, which can include caterpillars, worms and other invertebrates (animals without backbones), small frogs and lizards, or bits of already-dead animals they find.

Behavior and reproduction: Least shrews are skittish animals that are mainly active at night, although they will also warily venture about during the day. Their brownish gray coloration, small size, and tendency to hide among grasses or underground helps them avoid their numerous predators, animals that hunt them for food, which include owls, skunks, snakes, and a variety of other animals. They make a variety of sounds, some of which may be used to help them find their next meal. Just as bats make high-pitched noises and listen as the noises bounce off objects and back to them, American least shrews may make clicking noises, and then listen for the bounced clicks to detect objects, like prey, in their surroundings. This ability to "see" objects with reflected sound waves is called echolocation (eck-oh-loh-KAY-shun).

Unlike most other shrews that like to live alone, several to sometimes even more than two dozen adult American least shrews may share a burrow, where they click at one another to communicate. They mate all year long in warmer areas of their range, but limit mating to spring, summer, and fall in cooler areas. The female has her young after a pregnancy of about three weeks in grassy and/or leafy nests built in the burrow. The litter size is usually three to seven, but may be as small as one or as large as nine. The babies stay with the mother for almost three weeks.

American least shrews and people: This shrew usually remains out of sight, but it can assist gardeners and farmers by eating crop-destroying insects.

Conservation status: Neither the IUCN nor the U.S. Fish and Wildlife Service lists them as endangered, but some states consider them to be threatened. Connecticut, for example, lists American least shrews as endangered because their habitat is rapidly disappearing. ∎

AMERICAN WATER SHREW (Sorex palustris):

Physical characteristics: The American water shrew ranges from 2.5 to 3.2 inches (6.3 to 8.1 centimeters) in head and body length with a similar-sized tail, and weighs 0.3 to 0.6 ounces (8 to 18 grams). They have dark brownish gray backs and whitish bellies, a likewise two-toned tail, red-tinged front teeth, and hind feet that are larger than the forefeet. Like many other water-loving shrews, they have stiff, fringed hairs on their feet that aid in swimming.

Geographic range: United States and Canada.

Habitat: Usually found in or near water, these shrews prefer damp, forested areas with many places on land where they can hide, such as fallen logs, a thick understory, and/or rock piles. They readily take to the water, where they can make good use of their specially designed feet and swim underwater or run across the water surface like some water insects do.

Diet: Active mainly at night, they eat caterpillars, grubs, worms, and an occasional fish. Unlike many shrews that have to eat their body weight in food every day, this species can survive on just a tenth of its body weight or less in food per day. Compared to humans, however, that is still a considerable amount.

Behavior and reproduction: A variety of land animals find the American water shrew to be a tasty treat, but the shrews are quite adept at escaping into the water. Unfortunately, they must also be wary of several fish species, including trout, which also eat shrews. The shrews float well, so they must paddle with their hindfeet furiously to stay underwater. This species also makes chirping noises that may be used to find food through echolocation. Adults keep to themselves most of the time and will fight other adults that come too close. Mating occurs in the spring and summer. Pregnancies last about three weeks, and mothers retreat to tunnel nests to have their young. She may have two or three litters each year with three to ten babies at a time. Although the babies are helpless when they are born, they grow quickly and leave their mothers in about a month. The young can start their own families a few months later. Those that survive to adulthood usually only live to be about eighteen months old.

American water shrews and people: Other than a fleeting glimpse, people rarely have any contact with this shrew.

Conservation status: American water shrews are not considered threatened. ∎

FOR MORE INFORMATION

Books:

Nowak, Ronald M. Walker's Mammals of the World, 6th ed. Baltimore: Johns Hopkins University Press, 1999.

Stone, David, and the IUCN/SSC Insectivore, Tree Shrew and Elephant Shrew Specialist Group. Eurasian Insectivores and Tree Shrews-Status Survey and Conservation Action Plan. Gland, Switzerland: IUCN, 1995.

Wilson, D. E., and S. Ruff, eds. The Smithsonian Book of North American Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Periodicals:

Matsuzaki O. "The Force Driving Mating Behavior in the House Musk Shrew (Suncus murinus)." Zoological Sciences 19, no. 8 (2002): 851–69.

Web sites:

"American Water Shrew." BBC. http://www.bbc.co.uk/nature/wildfacts/factfiles/642.shtml (accessed July 1, 2004).

Endangered Species Program, U.S. Fish & Wildlife Service. http://endangered.fws.gov/ (accessed on July 1, 2004).

IUCN Red List of Threatened Species—Species Information. http://www.redlist.org (accessed on July 1, 2004).

"Least Shrew." All Taxa Biodiversity Inventory. http://www.dlia.org/atbi/species/animals/vertebrates/mammals/soricidae/Cryptotis_parva. html (accessed July 1, 2004).

"Savi's pygmy shrew, Etruscan shrew." America Zoo. http://www.americazoo.com/goto/index/mammals/48.htm (accessed July 1, 2004).

"Soricidae—Shrews." All Taxa Biodiversity Inventory. http://www.dlia.org/atbi/species/animals/vertebrates/mammals/soricidae/ (accessed July 1, 2004).

"Water Shrew." All Taxa Biodiversity Inventory. http://www.dlia.org/atbi/species/animals/vertebrates/mammals/soricidae/Sorex_palustris.html (accessed July 1, 2004).