Overview
The American Water Shrew (Sorex palustris) is a marvel of semi-aquatic mammalian specialization, recognized as the largest long-tailed shrew in North America. Inhabiting cold, pristine mountain torrents, this hyperactive miniature predator is famous for its unique ability to literally run across the surface of calm water pools for short distances, supported by air bubbles trapped in specialized stiff fringe hairs on its wide hind feet.
Biology and Physical Characteristics
Sorex palustris possesses dense, water-repellent velvety fur that is jet-black to dark greyish-brown dorsally with a silvery-white belly. Its large hind feet are edged with stiff, comb-like swimming bristles that dramatically increase surface area and trap micro-bubbles. Its fur traps an insulating layer of atmospheric air when diving, giving the shrew a shimmering silvery glow underwater.
Behavior and Ecology
Operating with an astonishingly high metabolic rate that requires it to feed nearly constantly around the clock, the American Water Shrew dives into rushing, near-freezing mountain streams to hunt caddisfly nymphs, stonefly larvae, and sculpins. Its dives rarely exceed 15 to 30 seconds due to extreme buoyancy from its air-trapping coat. When diving finishes, it grooms frantically to restore air pockets in its fur.
Conservation and Human Impact
Assessed as Least Concern by the IUCN, Sorex palustris is an exceptional biological indicator of cold, unpolluted, high-oxygen aquatic ecosystems. It is vulnerable to acid mine drainage, heavy stream sedimentation from clearcutting, and rising stream temperatures caused by climate change.
Interesting Factoid
The American Water Shrew can literally run on water: stiff bristles on its wide hind feet trap tiny air bubbles that hold the tiny shrew up on the water’s surface tension for several steps without sinking.