Shrews are tiny, mole-like mammals with long, cylindrical bodies, flexible snouts, short limbs, and clawed digits. Although they may be mistaken for mice or other rodents, they actually belong to the family Soricidae under the order Eulipotyphla. Unlike rodents, shrews do not have ever-growing teeth with chisel-shaped gnawing incisors; instead, they possess spike-like teeth that wear out through their lifetime.
These mammals are distributed worldwide, except in New Guinea, Australia, New Zealand, and South America, where there are no native populations of shrews. Quite surprisingly, these animals echolocate by emitting a series of ultrasonic squeaks to sense prey in their immediate habitats. They typically feed on seeds, nuts, leaf litter, and small invertebrates, like insects, worms, snails, and spiders, among others.
Currently, there are 885 known species of shrews, making Soricidae the fourth most diverse family among mammals.
They are tiny, almost the size of mice. The common Eurasian shrew (Sorex araneus) represents the average size of most shrews, with a body length of 2 to 3 in (6 to 8 cm) and a body weight of 0.5 oz (14 gm). Its tail measures about 2 to 2.3 in (5 to 6 cm).
However, the smallest shrew, the Etruscan pygmy shrew (Suncus etruscus), measures 1.875 in (3.5 cm) in length. It weighs only about 0.06 oz (1.8 gm) and is also the smallest known extant terrestrial mammal. In contrast, the largest shrew, the Asian house shrew (Suncus murinus), is around 6 in (15 cm) long and weighs about 3.5 oz (100 gm).
These mammals have small cylindrical bodies with elongated heads, short limbs with five toes on each foot, and clawed digits. They have a long, flexible muzzle with whiskers to sense their surroundings.
Their bodies are covered in short, dense fur that ranges from gray to black or brown in color. The fur on their undersides is typically lighter or even completely white. In some species of the genus Sorex, the back is dark brown, while the sides and underside are gray, thus making them tricolored.
The ears are round and moderately large (except in water shrews and short-tailed shrews) with high auditory sensitivity. The eyes are extremely small and provide poor vision.
The length of their tails varies across species, with some being shorter than the body while others being longer. It is difficult to distinguish these animals by their sex because, in males, the testes are retained within the abdominal cavity and thus are not visible from outside.
They have long and narrow skulls lacking zygomatic arches, auditory bullae, and postorbital processes. The tympanic bone is distinctively ring-shaped, and the mandibular bones possess two bony projections called condyloid processes that enable double articulation.
The brain comprises small cerebral hemispheres; however, the olfactory lobes are large and prominent, indicating an enhanced olfactory sense.
Shrews have 26 to 32 sharp, spike-like teeth that wear down with time, unlike those in rodents that are ever-growing. They also lack the chisel-like gnawing incisors found in rodents.
Since they feed on insects and other invertebrates with hard exoskeletons, their incisors are modified into grasping pincers. The first incisor is particularly large with an elongated main cusp, followed by a smaller, posterior secondary cusp. The other incisors, canines, and premolars (when present) are small, peg-like, and have single cusps.
The chewing molars at the back of the mouth have sharp cusps for breaking through the hard parts of prey. The upper molars are dilambdodont, characterized by two lambda-shaped ridges.
Since shrews lose their milk teeth before birth and have only one set of teeth throughout their lives, tooth-wearing poses a problem, especially in older adults, which often die of starvation for lack of teeth.
Currently, 385 species of shrews are grouped under 26 genera, which in turn, are divided into 3 extant subfamilies: Crocidurinae (white-toothed shrews), Myosoricinae (African shrews), and Soricinae (red-toothed shrews).
Apart from the listed ones, some extinct subfamilies include Crocidosoricinae, Allosoricinae, and Limnoecinae.
Although shrews belong to the same magnorder (Boreoeutheria) as rodents, they are more closely related to hedgehogs and moles.
These mammals are distributed almost worldwide, though New Guinea, Australia, New Zealand, and South America originally lacked native shrews. Currently, South America has a naturalized population, especially in the northern Andes, owing to the Great American Exchange, which facilitated the entry of shrews into this continent from North America. They are also not found in polar regions.
Shrews are terrestrial and prefer inhabiting moist microhabitats, though some, like the desert shrew (Notiosorex crawfordi), live in deserts. A few species, like American and Eurasian water shrews, inhabit aquatic environments like streams, ponds, and marshes.
While some shrews live underground in abandoned burrows, others take shelter in tree cavities and even in man-made structures like huts.
Though different species have different dietary preferences, they generally feed on seeds, nuts, grass, leaf litter, and even fungi. They also prefer insects, worms, snails and slugs, spiders, centipedes, millipedes, and other small invertebrates as their food.
Larger species, like short-tailed shrews, also consume larger prey, such as frogs and salamanders, snakes, birds, mice, and other shrews.
Shrews are generally solitary and territorial, except in the breeding season when they must gather to mate. They move rapidly in quick, jerky movements. Constantly in motion, these animals have been recorded making 12 body movements per second.
When threatened, shrews bite their predators, injecting venom through the grooves of their teeth, along with saliva. This venom, produced in submandibular glands in their lower jaws, incapacitates the attacker by paralyzing it.
Being extremely tiny animals, shrews have extremely high metabolic rates. Thus, to satisfy their voracious appetite, they must eat frequently and be active both during the day and at night. Some shrews, however, are primarily nocturnal (for instance, the Merriam’s shrew), while others, like northern tree shrew, are diurnal (active during the day).
They grab the prey using their mouth and pin them down with their spike-like front teeth. The prey gets paralyzed by the injected venom and is often kept aside when resources are not so plentiful. Most of the time, they feed above the ground, though they may also do so in tunnels built among leaf litter or other debris.
Many shrews consume food equal to their own body weight each day, while those in captivity can eat up to twice their body weight, like hummingbirds.
Only three species of shrews: the common shrew, the American vagrant shrew, and the northern short-tailed shrew, are capable of echolocation, emitting a series of ultrasonic squeaks for communication. Unlike bats, their squeaks are low-amplitude, broadband, multi-harmonic, and frequency-modulated, and they use these sounds solely to investigate close-range habitats.
They also produce a wide variety of vocalizations, like clicks, chirps, twitters, whistles, and barks. These sounds are typically used to alarm their conspecifics, to defend themselves, or in courtship.
Shrews have an average lifespan of 12 to 30 months.
These rodents breed year-round in the tropics, while in temperate regions, breeding ceases during winter. Generally, the males and the females communicate using scents and high-pitched squeaks. Once sexual partners are selected and pairs are established, they mate.
The females give birth to up to 10 litters per year, following a gestation period of about 17 to 32 days. The young are mostly born in grass nests, hairless and blind. The female usually becomes pregnant within a day after giving birth and thus lactates through her pregnancy. She weans the litter after about 3 weeks, around when she is ready to produce her next litter.
Sometimes, the young form a chain behind the mother, trailing her by grasping the base of the tail. This behavior is called caravanning.
Their natural predators include foxes, raccoons, weasels, martens, skunks, cats, dogs, snakes, shrikes, owls, hawks, and other shrews. Moreover, a large number of parasites, like mites, ticks, fleas, flukes, and fly larvae live off their bodies.
Since shrews dig tunnels through litter and debris, they mix the soil nutrients and aerate them in the process. This also allows rainwater to penetrate deep into the ground, preventing surface runoff.
Moreover, they also decompose the organic debris of dead animals.
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