Shrew

Lore & Background

Shrews are tiny, terrestrial creatures that forage for seeds, insects, nuts, worms, and other foods in leaf litter and dense vegetation. They have small eyes and poor vision but excellent hearing and smell. Their metabolic rates are unusually high, requiring them to eat almost constantly—up to twice their body weight daily in captivity. They do not hibernate, but some species enter torpor, and many undergo dramatic morphological changes in winter, losing 30–50% of body weight, including shrinking bones and organs.

Shrews have sharp, spike-like teeth, unlike rodents' gnawing incisors. They lose their milk teeth before birth and have only one set of teeth for life. In some species, exposed tooth areas contain iron and are dark red, reinforcing surfaces under stress. Shrews are fiercely territorial, coming together only to mate. Females can have up to 10 litters per year, with gestation of 17–32 days, and often become pregnant within a day of giving birth. A characteristic behavior is 'caravanning,' where young form a line behind the mother, each gripping the fur at the base of the tail.

Some shrew species are venomous, delivering venom through grooves in their teeth. The venom of the American short-tailed shrew contains compounds studied for treating high blood pressure, neuromuscular diseases, migraines, and ovarian cancer. Along with bats and toothed whales, some shrews use echolocation—emitting ultrasonic squeaks for close-range spatial orientation, though not for pinpointing food. Unlike most mammals, shrews lack zygomatic bones, giving them incomplete zygomatic arches.

Reader's Guide

Shrews hold significance as one of the most species-diverse mammal families, occupying a wide range of terrestrial habitats across the globe. Their extremely high metabolic rates and voracious appetites make them important predators of insects and other invertebrates, influencing ecosystem dynamics. The family's 385 species, grouped into three living subfamilies (Crocidurinae, Myosoricinae, Soricinae), demonstrate remarkable adaptations: venom production in some species, echolocation in others, and the ability to shrink body size seasonally. The Etruscan shrew is the smallest known living terrestrial mammal, while the Asian house shrew is the largest. Shrews are unusual among mammals for their venom, echolocation, and lack of zygomatic bones. Their teeth, which wear down over a lifetime and are reinforced with iron in some species, contrast with rodents' ever-growing incisors. The study of shrew venom has yielded compounds with potential medical applications, including treatment for high blood pressure, neuromuscular diseases, migraines, and ovarian cancer. As a group, shrews illustrate the diversity of evolutionary solutions to small-body, high-metabolism life strategies.

Did You Know?

• The Etruscan shrew (Suncus etruscus) is the smallest known living terrestrial mammal, at about 3.5 cm and 1.8 grams.
• Some shrew species are venomous, delivering venom through grooves in their teeth rather than fangs.
• Shrews can lose between 30% and 50% of their body weight in winter, shrinking bones, skull, and internal organs.
• Along with bats and toothed whales, some shrews use echolocation, emitting ultrasonic squeaks for spatial orientation.
• Shrews are native only to northern South America (e.g., genus Cryptotis), having naturally colonized the region via the Great American Interchange; they are not native to New Guinea—only introduced species exist there.

Taxonomy & the Rodent Misconception

Shrews belong to the family Soricidae within the order Eulipotyphla, a grouping that also includes hedgehogs and moles. Despite their mouse-like appearance, they are not rodents at all. The only connection between shrews and rodents is their shared membership in the Boreoeutheria magnorder. Their teeth tell the story: where rodents possess ever-growing gnawing incisors, shrews have sharp, spike-like teeth that actually wear down over a lifetime. The name "shrew" itself is a source of confusion, as treeshrews, otter shrews, elephant shrews, West Indies shrews, marsupial shrews, and shrew opossums all belong to entirely different families or orders. With 385 known species spread across 26 genera and three living subfamilies—Crocidurinae (white-toothed shrews), Myosoricinae (African shrews), and Soricinae (red-toothed shrews)—shrews form the fourth-most species-diverse mammal family on Earth, trailing only the muroid rodent families Muridae and Cricetidae and the bat family Vespertilionidae.

Tiny Bodies, Enormous Metabolism

Shrews are extraordinarily small mammals, most no larger than a mouse. The Asian house shrew represents the upper end at roughly 15 centimeters and 100 grams, while the Etruscan shrew, at just 3.5 centimeters and 1.8 grams, holds the title of smallest known living terrestrial mammal. Their tiny frames demand an extraordinary metabolic engine. Shrews eat almost without pause, consuming anywhere from half to twice their own body weight in food each day. They do not hibernate, though some species can enter torpor, and in winter many dramatically shed between 30 and 50 percent of their body mass, actually shrinking their bones, skulls, and internal organs. Their dentition is a single set—milk teeth are lost before birth—and the enamel wears down permanently. In high-metabolism species, exposed tooth surfaces contain iron that appears dark red, reinforcing the areas under the greatest chewing stress. This iron-reinforced enamel is a rarity in the mammal world, shared only with rodent incisors.

Venom, Medicine, and Echolocation

Among the strangest traits of the shrew family are the presence of venom and the use of echolocation. Several species produce venom that is delivered not through fangs but through grooves in their teeth. The venom glands of the American short-tailed shrew contain enough toxin to kill two hundred mice if injected intravenously. Beyond raw toxicity, shrew saliva and venom have attracted medical interest: one compound may help treat high blood pressure, another may aid in managing neuromuscular disorders and migraines, and soricidin, a peptide found in the northern short-tailed shrew's saliva, has been studied as a potential treatment for ovarian cancer. On the sensory front, shrews in the genera Sorex and Blarina are the only terrestrial mammals known to echolocate, alongside tenrecs and solenodons. Their ultrasonic squeaks are low-amplitude, broadband, and frequency-modulated—quite different from the sharp clicks of bats. Rather than pinpointing individual prey, shrews appear to use the overall reverberation of their calls to gauge the type of habitat they are in, much as a human might distinguish a forest from a cellar by the echo of a shouted call.

Reproduction, Caravanning, and Global Range

Shrews are fiercely territorial animals that drive off rivals and gather only for mating. In the tropics, females breed year-round and can produce up to ten litters annually; in temperate regions, breeding pauses only through winter. Gestation is remarkably brief at 17 to 32 days, and a female can become pregnant again within a day of giving birth, lactating for the current litter while already carrying the next. Lifespan is modest, ranging from 12 to 30 months. One of the most endearing behaviors observed across many species is "caravanning," in which a line of young shrews follows the mother, each gripping the fur at the base of the tail of the individual ahead. Shrews are distributed almost worldwide, absent only from New Guinea, Australia, New Zealand, and South America—though the Great American Interchange has recently introduced a naturalised population into the northern Andes. They forage in leaf litter and dense vegetation, though some specialise in climbing, burrowing, living under snow, or hunting in water, all while relying on excellent hearing and smell to compensate for their small eyes and poor vision.