People do not often see the secretive, (mostly) nocturnal northern short-tailed shrew (Blarina brevicauda), a mammal that lives largely underground.
Though our largest shrew, these still weigh less than an ounce. They are a whopping 3 or 4 inches long, including the tail. These burrowing gray critters have soft, velvety fur, a short snout, small eyes and ears hidden by fur.
They are also one of the few venomous mammals. When a shrew bites another animal, a venom produced from a gland at the base of the lower incisors flows into the wound from between the two lower teeth. In one recorded instance, a shrew gnawed the base of a vole’s skull for 11 minutes until it died. The toxin paralyzes prey and leads to respiratory failure and convulsions in small mammals. Because of it, shrews can capture animals that are much larger than they are, such as salamanders, frogs, snakes, mice and birds. Luckily for us, humans are too big to be paralyzed by shrew toxin. If you were bitten by this shrew, the wound would swell and hurt for a few days (and you should probably have it checked out by a doctor), but it is likely no serious harm would occur.
Short-tailed shrews burrow more than other shrews and build round 8-inch diameter nests that they line with vegetation and sometimes fur. Outside of the nest entrance is a latrine area. Food is stored in other parts of the burrow.
During mating season, males and females mate 20 or more times per day (at least six matings per day are needed to induce ovulation). After a three-week pregnancy, females give birth to about six honeybee-sized pink babies, born hairless except for whiskers. The baby shrews sleep together in a pile, their eyes and ears closed, rarely staying in one position for more than a few minutes. They are squirming constantly to try to get to the bottom of the pile.
The young shrews leave the nest in about three weeks and are weaned soon thereafter, at about 25 days. Females are sexually mature in about six weeks; males at about 12 weeks. Because of how quickly they mature, these shrews become abundant in warm weather. They help keep down populations of insect pests as well as snails and mice. They are also an important part of the food chain for other animals, especially owls.
Northern short-tailed shrews do not see well. They can probably tell light from dark, but they cannot see distinct objects. Like bats and whales, they use echolocation to detect obstacles and prey. They send out a series of high-frequency clicks (too high for humans to hear) and listen for the returning echoes. Using this knowledge, the shrews can navigate through their environment and capture prey. They have a highly developed sense of touch, especially in their snout and whiskers. They also use their sense of smell to snuffle through undergrowth in search of prey.
These shrews are insectivores who eat three times their weight in food each day! To stay warm in winter, they must increase their metabolism to maintain their body temperature. Therefore, they eat 43 percent more food in winter than in summer. In summer, they eat mostly worms and millipedes. They also eat spiders, snails, insect cocoons and other insects. Like chipmunks, in the fall they store food for winter, including snails, beetles, nuts and seeds. During the winter, when insects are unavailable, they eat more mice. Winter takes a heavy toll, though and 90 percent may die during the coldest months.
Short-tailed shrews are an important part of the diet of many animals. Owls, hawks, snakes, foxes and coyotes all prey upon them. Shrews try to avoid these predators by staying hidden under leaves or soil or snow when out foraging – then becoming quite aggressive when cornered. They also emit a foul musky odor from glands on their belly and sides. Many mammals, unless they are desperate, refuse to eat short-tailed shrews because they taste so terrible.
Shrews come in two varieties: those with white teeth and those with red teeth. Northern short-tailed shrews are red-toothed shrews, meaning the tips of their teeth are red due to iron deposits there. The iron makes the enamel more resistant to wearing down. The tips of the teeth that are used for crushing and grinding have more iron than the tips used for shearing. Unlike humans, shrews are born with their adult teeth already in their mouth. (They shed their baby teeth before they are born.) Since their teeth need to last them their entire life and since they have a voracious appetite, the iron helps keep the teeth in good shape as they age. Red-toothed shrews have a higher metabolic rate than white-toothed shrews, and therefore need to eat more. Perhaps this is a reason for their tooth differences.
Although you may never see a northern short-tailed shrew, you can thank them for keeping insect pests in check and helping to control the mouse population. They are also an important food source for some of our favorite species, including owls and hawks.
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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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