Eurasian Water Shrew

The Eurasian water shrew is a small, semi-aquatic shrew widespread across Europe and northern Asia. It has dense black fur above and pale underparts, with stiff hairs on its feet and tail that aid swimming. It hunts aquatic invertebrates and has a venomous bite to subdue prey.

The Eurasian water shrew (Neomys fodiens) exhibits a polyphasic activity pattern, alternating short bursts of intense activity with brief rest periods around the clock, though it shows peaks of activity during dawn, dusk, and nighttime hours. This high metabolic rate, typical of shrews, demands near-constant foraging, with individuals often active for only a few hours before requiring rest. As one of the largest and most aquatic of the Eurasian shrews, it has evolved a lifestyle deeply tied to freshwater habitats such as streams, ponds, and marshes.

Socially, this species is solitary and highly territorial outside of the brief breeding season. Individuals maintain and defend home ranges along waterways, using scent marking via specialized flank and ventral glands to signal territory boundaries and reproductive status to conspecifics. Communication also includes high-frequency vocalizations—squeaks and twitters—used in aggressive encounters or during rare social interactions, as well as some evidence of echolocation-like clicking used to navigate murky water and dense vegetation.

Home ranges are typically small, often confined to a few hundred meters of stream or pond bank, but can vary based on prey density and habitat quality. Adaptive behaviors include a dense, water-repellent fur coat trapping air for buoyancy and insulation, stiff fringe hairs on the hind feet and tail that act as paddles and rudders for swimming, and the remarkable ability to run briefly across the water’s surface due to surface tension and rapid limb movement, an escape and hunting adaptation rarely seen in mammals.

The Eurasian water shrew is a voracious insectivore with a diet centered on aquatic and semi-aquatic invertebrates, including caddisfly larvae, mayfly nymphs, freshwater shrimp (Gammarus), water beetles, and small crustaceans. It also consumes terrestrial prey such as earthworms, spiders, and insects found along waterway banks. Notably, this species is one of the few venomous mammals; its saliva contains a neurotoxin produced by submandibular glands that helps subdue larger or more resistant prey such as fish and amphibians, including small frogs and fish fry, which it occasionally hunts and caches for later consumption.

Foraging occurs primarily underwater, where the shrew uses tactile whiskers and its highly sensitive nose to detect prey in low-visibility conditions, since its eyesight is poor. Due to its extraordinarily high metabolism, it must consume food equivalent to roughly its own body weight daily to survive, foraging almost continuously. Seasonal shifts occur, with increased reliance on terrestrial invertebrates during colder months when aquatic invertebrate availability declines, though the species remains active year-round and does not hibernate. Water dependency is absolute—individuals rarely stray far from freshwater sources, as both foraging success and thermoregulation are closely tied to aquatic habitats.

Breeding in Neomys fodiens typically occurs between April and September, peaking in late spring and summer, with females capable of producing two to three litters per breeding season. The mating system is promiscuous, with males roaming across multiple female territories during the breeding period rather than forming pair bonds. Gestation lasts approximately 19 to 21 days, after which females give birth to litters averaging 3 to 8 pups, born blind, hairless, and highly altricial in nests constructed in burrows, under logs, or within dense bankside vegetation.

Maternal care is intensive and solely provided by the female, who nurses and protects the young until weaning occurs at approximately 21 to 24 days of age. Juveniles disperse shortly after weaning to establish their own territories, with sexual maturity typically reached the following spring after overwintering, though some individuals from early litters may breed within their first year. Lifespan in the wild is short, generally 14 to 19 months, reflecting the high-energy, high-risk lifestyle characteristic of shrews.

The primary ecological threats to the Eurasian water shrew stem from habitat degradation, particularly pollution and eutrophication of freshwater systems, agricultural runoff, and the drainage or canalization of wetlands, streams, and riparian zones. Because the species is highly sensitive to water quality—relying on healthy invertebrate populations as a food base—pesticide and heavy metal contamination in waterways can have outsized impacts on local populations. Habitat fragmentation from urban development and intensive agriculture further isolates populations, reducing genetic connectivity along river systems.

Natural predators include owls, herons, foxes, otters, and domestic cats, with predation pressure compounded by the species’ short lifespan and high reproductive turnover. Climate change poses an emerging concern through altered precipitation patterns affecting wetland hydrology and stream flow consistency. Despite these pressures, the species is classified as Least Concern by the IUCN due to its wide distribution across Europe and parts of Asia and relatively stable overall population trend. Conservation initiatives are largely indirect, focusing on broader wetland and riparian habitat protection, water quality legislation such as the EU Water Framework Directive, and general freshwater ecosystem restoration projects that benefit the species alongside other semi-aquatic wildlife.

• Venomous Mammal: The Eurasian water shrew is one of the rare venomous mammals worldwide, producing a neurotoxic saliva capable of immobilizing prey larger than itself, such as small fish and frogs.
• Walking on Water: Air bubbles trapped in its dense fur combined with rapid limb movement allow it to briefly run across the water’s surface, an adaptation aiding both escape from predators and ambush hunting.
• Extreme Metabolism: Its heart rate can exceed 800 beats per minute, requiring it to eat nearly its entire body weight in food daily to sustain its energy demands.
• Echolocation-like Clicking: Research suggests it emits ultrasonic clicks that may help it navigate dark or murky underwater environments, a rare trait among terrestrial mammals.
• Buoyant Fur Coat: Dense, water-repellent fur traps a layer of air, giving the shrew a silvery appearance underwater and providing both insulation and buoyancy in cold streams.
• Short but Intense Life: Despite living only about 14 to 19 months in the wild, individuals can produce multiple litters, ensuring rapid population turnover and resilience.
• Toxic Bite Discovery: Its venom was first scientifically documented in the 19th century, making it one of the earliest recognized cases of venom in mammals, predating modern research on solenodons and platypuses.