At least one species of shrew has poisonous saliva, a rare example of toxicity in mammals. Delivered with a bite, the toxin slows down or kills prey, and can cause soreness and swelling in humans. Shrews are short-lived. They die from floods, starvation, rapid temperature changes, accidents, fights with other shrews and even from shock due to fright. Many predators catch and kill them, perhaps in mistake for mice, but since shrews secrete a repelling musky odor, predators often do not eat them.
Shrews raise several litters each year.
Gestation is about three weeks. Newborn young are helpless and unfurred, but grow rapidly and reach adult size when 4-6 weeks old. For more specific life history information, see listings under each species. The masked shrew is the most widely distributed shrew in North America, ranging over almost all of the continent's northern half. It occurs throughout Pennsylvania.