The Eurasian water shrew, Neomys fodiens, is a unique semi-aquatic mammal inhabiting various freshwater environments across Europe and parts of Asia. Often called the water shrew in the UK, it represents an unusual adaptation within its family, thriving both on land and in water. It is one of the few mammals known to have venomous saliva. Its physical traits are tailored for its watery domain.
Physical Characteristics and Aquatic Adaptations
Its physical traits facilitate its semi-aquatic lifestyle. Its dense, water-repellent fur maintains body temperature and buoyancy in cold water. The fur traps air bubbles, creating an insulating layer. While aiding buoyancy, it must actively anchor itself to remain submerged.
Stiff hair fringes on its feet act like paddles for propulsion. Its flattened tail, about three-quarters its body length, has a keel of short white hairs, serving as a rudder. It measures up to 10 cm (4 inches) in body length and weighs 15-19 grams (0.5-0.6 ounces). Its fur is grayish-black on the head, back, and sides, with sharply demarcated dirty white underparts.
Its venomous saliva is a distinct feature. Produced by glands, this venom subdues prey, particularly larger invertebrates and small vertebrates. Potent enough to incapacitate prey, its bite is generally not strong enough to pierce human skin, though it can cause irritation. Its sharp teeth are often red-tipped from iron deposits that strengthen the enamel, especially in the molars.
Habitat and Geographical Range
Eurasian water shrews inhabit areas near freshwater sources. They are found in and around slow-moving water bodies like ditches, streams, ponds, and damp meadows. Riverbanks and bushy ground adjoining water also provide suitable environments. Clean, unpolluted water is important for their survival, as their diet includes aquatic organisms sensitive to environmental degradation.
Its geographical distribution spans the Palearctic region. Its range includes most of Europe (excluding Iceland, Ireland, certain Mediterranean islands, and the Balkans). In Asia, it extends from western Siberia and Asia Minor eastward to North Korea and the Pacific coast. They rarely venture far from aquatic habitats, often burrowing in banks.
Diet and Predatory Behavior
The Eurasian water shrew is a carnivore, primarily eating aquatic invertebrates. This includes insect larvae, crustaceans, and mollusks. They also prey on small vertebrates like fish and amphibians. Occasionally, their diet expands to include small rodents or spiders found near water.
Their hunting techniques adapt to both aquatic and terrestrial environments. They are adept divers, remaining submerged for up to 20 seconds while searching for prey. Underwater, sensitive whiskers detect vibrations and locate prey, as their eyesight is poor. Once prey is apprehended, venomous saliva helps immobilize larger or more resistant food items; for instance, it can stun a field vole. They also cache excess prey for later consumption.
Reproduction and Life Cycle
Eurasian water shrews have a short lifespan, averaging 18 months in the wild, though some live up to three years. They are generally solitary, avoiding contact outside the breeding season. Males venture into female territories for mating.
The breeding season occurs between April and September, with peak activity in June. After a 24-day gestation, females give birth to litters of four to eight young; some litters can be as large as 15. Young are born helpless; their eyes open 15-18 days after birth. They are fully weaned at seven weeks, after which juveniles disperse to establish territories.
Females can produce two or three litters annually. Shrews reach sexual maturity at six to eight months old, allowing quick reproduction within their short lifespan.
Conservation and Threats
The IUCN lists the Eurasian water shrew as “Least Concern.” Despite this, regional populations can face specific threats. Primary challenges include habitat loss and degradation. This often manifests as waterway pollution, directly impacting food sources and health.
Destruction of riverbanks and other natural habitats near water sources also poses a threat. Changes to water flow, drainage, and development can diminish suitable environments. Predation from foxes, large predatory fish, raptors, and domestic cats also contributes to population pressures. Maintaining healthy, unpolluted aquatic ecosystems is important for their continued well-being across their extensive range.