Shrews, the small and frenetic mammals of the family Soricidae, rank among the most metabolically extreme animals on Earth. With roughly 400 recognized species spread across every continent except Australia and Antarctica, they are also one of the most species-rich mammalian families. Despite their superficial resemblance to mice, shrews are not rodents at all. They belong to the order Eulipotyphla, alongside moles and hedgehogs, and their biology is packed with surprises that set them apart from nearly every other mammal: venomous saliva, teeth tipped with iron, hearts that beat over a thousand times per minute, brains that shrink in winter and regrow in spring, and a rudimentary form of echolocation.
A Metabolism Running on Overdrive
The defining feature of shrew biology is speed. Everything about their physiology is cranked up to accommodate a metabolic rate far out of proportion to their size. A common shrew weighing about 10 grams burns calories at the same basal rate as a vole two and a half times its weight, and nearly twice the rate of similarly sized white-toothed shrews living in the same habitat.1PubMed Central. Metabolic rate in common shrews is unaffected by temperature, leading to lower energetic costs through seasonal size reduction The American water shrew pushes the envelope even further: its measured basal metabolic rate runs about three times the value predicted by body mass alone, and food passes through its gut in under an hour.2PubMed. Bioenergetics and thermal physiology of American water shrews (Sorex palustris)
This metabolic furnace demands constant refueling. Most shrews must eat every few hours or risk starving, and many consume more than their own body weight in food each day. Their diet is overwhelmingly invertebrate-based, consisting of insects, earthworms, larvae, spiders, and snails, though some species will take small vertebrates when the opportunity arises. The gut microbiome of shrews reflects this high-energy lifestyle. Compared to rodents, shrews show a markedly different ratio of key bacterial groups in their intestines, a pattern likely driven by the fat-rich, calorie-dense demands of digesting insects.3PubMed Central. Comparative metagenomics analysis reveals how the diet shapes the gut microbiota in several small mammals
The Heart That Barely Pauses
A shrew’s heart is an engineering marvel. Soricine shrew hearts beat at rates that dwarf those of any other mammal. Researchers studying the relationship between heart rate and muscle contraction in shrews found that the duration of each heartbeat contraction is inversely related to maximum heart rate, with the speed of contraction governed primarily by how fast calcium ions move in and out of heart muscle cells.4PubMed. Maximum heart rate of soricine shrews: correlation with contractile properties and myosin composition Some species sustain resting rates above 800 beats per minute and can exceed 1,200 during activity.
The anatomy of the shrew heart has been reshaped by these demands. The coronary arteries that feed the heart muscle are, proportionally, enormous. In a shrew, the left coronary artery is roughly half the diameter of the aorta. In a human, that same ratio would be closer to one-tenth. Both coronary arteries dive immediately into the ventricular wall rather than running along the surface, and the ventricular walls are mostly compact muscle with very few of the finger-like trabeculations seen in larger mammal hearts.5PubMed Central. Anatomy of the heart with the highest heart rate These proportionally oversized arteries likely exist to overcome the high resistance that blood encounters in narrow vessels, ensuring the heart muscle gets enough oxygen to keep up its relentless pace.
Dehnel’s Phenomenon and Seasonal Brain Shrinkage
One of the strangest adaptations in all of mammalian biology belongs to shrews. In species like the common shrew, individuals undergo a dramatic seasonal transformation known as Dehnel’s phenomenon: a substantial reduction in body mass, skull size, and brain volume from summer through winter, followed by partial regrowth the following spring.6PubMed Central. Geographic patterns in seasonal changes of body mass, skull, and brain size of common shrews The brain is the most metabolically expensive organ per gram of tissue, so shrinking it saves real energy during months when food is scarce.
Recent work has clarified how this happens at the cellular level. The shrinkage appears to occur through water loss from brain cells rather than cell death, making it a programmed and reversible process.7PubMed. Programmed seasonal brain shrinkage in the common shrew via water loss without cell death The skull bones themselves thin and remodel to accommodate the change. This is not trivial: the braincase can shrink by about 15 to 20 percent across its widest dimension. The fact that a mammal can lose a significant fraction of its brain and then rebuild it, without apparent lasting damage, has drawn attention from neuroscientists interested in regeneration and resilience.
Torpor and Other Energy-Saving Tricks
Not all shrews rely on brain shrinkage to survive lean times. White-toothed shrews in the genus Crocidura employ a different strategy: torpor. The greater white-toothed shrew in Mediterranean climates enters short bouts of torpor more frequently in winter than in summer, adjusting its daily activity levels and body temperature to match seasonal energy availability.8PubMed. Phenotypic flexibility in the energetic strategy of the greater white-toothed shrew, Crocidura russula Overwintering Crocidura can cut their total energy budget by up to about 30 percent through a combination of torpor, communal nest huddling, and selecting warmer resting sites.9Journal of Zoology. Ecological energetics of two European shrews: Crocidura russula and Sorex coronatus
The contrast between red-toothed shrews (subfamily Soricinae), which tend to stay active all winter and rely on Dehnel’s phenomenon, and white-toothed shrews (subfamily Crocidurinae), which use torpor, illustrates a recurring theme in shrew biology: the family has evolved multiple independent solutions to the same fundamental problem of being tiny and burning fuel at an extraordinary rate.
Venomous Saliva
Shrews are among the very few mammals that produce venom. The best-studied case is the North American short-tailed shrew, Blarina brevicauda, whose submaxillary glands secrete a toxin called blarina toxin, or BLTX. When injected into mice, BLTX caused irregular breathing, paralysis, and convulsions before death. The toxin acts as a serine protease with tissue kallikrein-like activity, meaning it cleaves proteins in the blood and triggers the release of compounds that dilate blood vessels and drop blood pressure.10PubMed Central. Blarina toxin, a mammalian lethal venom from the short-tailed shrew Blarina brevicauda: Isolation and characterization A genomic study of Blarina’s venom found that the overall composition is relatively simple, limited to about seven proteins from six gene families, but that synergism among the toxins likely targets vertebrate prey specifically.11PubMed Central. A Comprehensive Multi-Omic Approach Reveals a Relatively Simple Venom in a Diet Generalist, the Northern Short-Tailed Shrew, Blarina brevicauda
The Chinese gray shrew, Blarinella quadraticauda, uses a structurally distinct toxin called BQTX. This compound works differently from Blarina’s venom: it promotes blood clotting and raises blood pressure, while also producing strong painkilling effects by inhibiting an enzyme called elastase. Crucially, BQTX has a remarkably long half-life of about 16 hours in the bloodstream, meaning a bitten prey animal can remain immobilized for an extended period.12PubMed Central. Shrew’s venom quickly causes circulation disorder, analgesia and hypokinesia This extended paralysis appears to serve a food-hoarding function: the shrew can cache live but immobilized prey and return to eat it later, keeping the meal fresh without refrigeration. The toxin shares structural similarities with venoms found in snakes, wasps, and cone snails, hinting at deep convergent evolutionary patterns in how animals weaponize their saliva.
Iron-Tipped Teeth
If you ever get a close look at a soricine shrew’s teeth, the color alone tells you something unusual is happening. The tooth enamel in red-toothed shrews is conspicuously reddish-orange, and the cause is iron. Iron deposits around the nanometer-scale enamel crystals form a near-amorphous magnetite phase, and in some species the iron content reaches about 8 percent by weight.13PubMed. Structural and functional characterization of enamel pigmentation in shrews
The iron is not distributed evenly. Studies of Blarina brevicauda molars found that cusps responsible for crushing and grinding carry significantly more iron than those used for shearing, and the largest molar has a higher iron density than smaller posterior teeth.14Journal of Mammalogy. Elemental Analysis of Soricine Enamel: Pigmentation Variation and Distribution in Molars of Blarina brevicauda This pattern strongly suggests a functional purpose: reinforcing the surfaces under the greatest mechanical stress. Because shrew teeth do not grow continuously, unlike rodent incisors, resisting wear is critical. A shrew with worn-down molars cannot process enough food to sustain its metabolism, and tooth condition likely limits how long the animal can survive. Soricine shrews typically live only about 18 months in the wild, making them among the shortest-lived of all mammals.15PubMed Central. Muscle senescence in short-lived wild mammals, the soricine shrews Blarina brevicauda and Sorex palustris
Echolocation and Whisker-Guided Hunting
Several shrew species can echolocate, a trait otherwise associated almost exclusively with bats and toothed whales. The common shrew has been confirmed to possess echolocation based on behavioral experiments and the high-frequency twittering calls it produces.16PubMed Central. Evidence of Echolocation in the Common Shrew from Molecular Convergence with Other Echolocating Mammals The short-tailed shrew has also been shown to discriminate between open and blocked tunnels using ultrasonic clicks, even when other sensory cues were eliminated.17Journal of Mammalogy. Echolocation by the Short-Tailed Shrew Blarina brevicauda
Shrew echolocation is far simpler than what bats do. There is no evidence that shrews use it to track individual moving prey. Instead, their twittering calls appear to serve a habitat-assessment function, helping the animal distinguish between cluttered and open environments at close range, just beyond what their whiskers can reach.18PubMed Central. Why do shrews twitter? Communication or simple echo-based orientation
For actual prey capture, whiskers take over. The Etruscan shrew, the smallest terrestrial mammal at roughly two grams, is one of the fastest tactile hunters ever documented. It whisks its vibrissae at about 14 Hz and can react to prey contact with a latency as short as 25 to 30 milliseconds, fast enough that it likely identifies and classifies prey from a single touch.19PubMed Central. The neurobiology of Etruscan shrew active touch Removal experiments have shown that both the long macrovibrissae and the shorter microvibrissae on the snout are necessary for successful prey capture, and that tactile shape cues alone are sufficient to trigger an attack, even on an artificial cricket-shaped replica.20PubMed Central. Tactile guidance of prey capture in Etruscan shrews The system is remarkably robust: shrews recognize the general shape of prey regardless of the prey’s orientation or size, suggesting something like a Gestalt representation rather than a pixel-by-pixel tactile image.
Scent Marking and Chemical Communication
Vision is poor in most shrews, but their chemical world is rich. Many species maintain territories and communicate reproductive status through scent marking, using specialized glands on the flanks, perineum, and throat. In the musk shrew, males rely on testosterone to drive these marking behaviors: castration depressed all three types of marking, and testosterone replacement restored them. Females, interestingly, mark at equally high levels regardless of reproductive state, and ovariectomy had no effect. Only when the adrenal glands were also removed did marking decline, suggesting that female scent marking runs on adrenal hormones rather than ovarian ones.21PubMed. Scent marking in the musk shrew (Suncus murinus)
The semi-fossorial short-tailed shrew has a more developed olfactory organ than surface-dwelling species, and it is also more territorial, maintaining active scent-marking behaviors and depositing feces at territory boundaries.22PubMed. Comparative morphology and morphometry of the nasal fossae of four species of North American shrews This correlation between lifestyle, nasal anatomy, and chemical behavior suggests that underground-dwelling shrews lean even more heavily on smell to navigate social life in environments where other senses are limited.
Chromosomal Races in the Common Shrew
The common shrew, Sorex araneus, has become a model organism for studying how chromosome rearrangements drive population divergence. Across its range in Europe and Asia, the species is divided into dozens of “chromosomal races” that differ in which chromosome arms are fused together. When individuals from different races meet and hybridize, their offspring can have extraordinarily complex chromosome configurations during cell division, which reduces fertility.23PubMed. Natural hybridization between extremely divergent chromosomal races of the common shrew (Sorex araneus, Soricidae, Soricomorpha): hybrid zone in Siberia The degree of infertility in hybrids varies depending on how many chromosomes differ between the parent races.
This system of monobrachial homology, where different races share identical single chromosome arms but in different fusion combinations, is rare in mammals and has been studied extensively in the common shrew specifically because the variation is so dramatic within a single species.24PubMed Central. Notable homologous variation in chromosomal races of the common shrew In practical terms, these chromosomal barriers may be creating reproductively semi-isolated populations that could be on a slow path toward speciation, even though they still look identical and occupy the same habitats.
Ecological Roles and Cascading Effects
Shrews occupy an outsized role in soil food webs relative to their body size. In a field experiment in a temperate forest, the presence of the long-clawed shrew significantly reduced populations of earthworms, isopods, and spiders. But smaller invertebrates, including springtails and centipedes, actually increased in abundance when shrews were present, likely because shrews removed the predators and competitors that had been suppressing them.25Mammal Study. Top-Down Cascade Effects of the Long-Clawed Shrew (Sorex unguiculatus) on the Soil Invertebrate Community in a Cool-Temperate Forest These cascading effects illustrate how an abundant small predator can reshape the invertebrate community beneath the leaf litter.
Shrew introductions into new areas can also cause problems for native small mammals. In Ireland, the greater white-toothed shrew, Crocidura russula, arrived accidentally and has been expanding its range since. Surveys found that the native pygmy shrew, Sorex minutus, was completely absent from areas where Crocidura had become established and was only found at or beyond the invasion front.26PLOS ONE. Invading and Expanding: Range Dynamics and Ecological Consequences of the Greater White-Toothed Shrew (Crocidura russula) Invasion in Ireland The mechanism behind this displacement is still debated, but the pattern is clear: the invader is replacing the native species across a moving front, and follow-up visits to shared sites found the native shrews disappearing from locations where both had recently coexisted.
Shrews as Reservoirs for Zoonotic Viruses
From a public health perspective, shrews are increasingly recognized as hosts for viruses that may be relevant to humans. Hantaviruses, long associated primarily with rodents, have now been identified in at least 20 species of shrews and moles.27PubMed Central. Hantavirus reservoirs: current status with an emphasis on data from Brazil The list keeps growing. Genetically distinct hantaviruses have been found in Eurasian species like the flat-skulled shrew, where a virus named Kenkeme virus proved phylogenetically distinct from all previously known hantaviruses in both rodents and other shrews.28PubMed Central. Novel hantavirus in the flat-skulled shrew (Sorex roboratus)
In Sweden, researchers confirmed that the common shrew carries two distinct hantaviruses simultaneously, each with its own evolutionary and geographic history. The same study noted that shrews also harbor other zoonotic RNA viruses, including hepatitis B-like viruses, reinforcing the argument that insectivorous mammals deserve more surveillance as potential reservoirs for pathogens that could jump to humans.29Virus Evolution. Hantavirus co-circulation in common shrews (Sorex araneus) in Sweden The risk to people remains unclear for most shrew-borne viruses, but the sheer diversity of what these animals carry has prompted calls for broader sampling efforts.
Aquatic Shrews and Morphological Diversity
Water shrews represent one of the more dramatic lifestyle departures within Soricidae. Several species in the genus Neomys have adapted to semi-aquatic life, hunting aquatic invertebrates and small fish in streams and ponds. The Eurasian water shrew, Neomys fodiens, and the Mediterranean water shrew, Neomys anomalus, often live side by side but differ measurably in their swimming and diving mechanics. Neomys fodiens has a more streamlined body when diving, while the two species use their tails differently during surface swimming.30Journal of Ethology. Differences in swimming and diving abilities between two sympatric species of water shrews: Neomys anomalus and Neomys fodiens Water shrews have stiff hairs fringing their feet and tails that trap air bubbles for buoyancy and insulation, and their fur is dense enough to remain waterproof for short dives.
The family’s morphological range extends well beyond aquatic specialists. The hero shrew of central Africa has a bizarrely reinforced spine with interlocking vertebrae found in no other mammal. Elephant shrews, despite the name, are not part of Soricidae at all but belong to an entirely different order. Within the actual shrew family, evolutionary relationships have been reorganized repeatedly as molecular data accumulate. Two major subfamilies are well supported: the Soricinae (red-toothed shrews, mostly northern hemisphere) and the Crocidurinae (white-toothed shrews, mostly Old World tropics). A third, smaller group, the Myosoricinae, includes several African genera and has recently gained a surprising member from northeastern India, a new genus called Nagasorex, whose closest relatives appear to be a mix of extinct European and Asian forms along with extant African species.31Bulletin of the American Museum of Natural History. The Shrew of Nagaland: A Remarkable New Genus and Species from Northeast India Discoveries like this underscore how much basic taxonomy in Soricidae remains unsettled, even as the family continues to reveal biological feats that few other mammals can match.