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A North American water shrew prepares to make a meal of a fish it has caught. The small mammals, which live in wetlands along streams and rivers, are expert divers.

Kenneth C. Catania

I’ve been trapping star-nosed moles for two decades to study their behavior and anatomy, so I am used to the disappointment of finding an empty trap. But one trap I set about twelve years ago disappointed me in another way. I had placed it in a wetland to catch a mole, but found that another small mammal had the nerve to get caught. Recognizing that it was a North American water shrew, I released it at the edge of a nearby stream, wondering just how good it could possibly be at swimming (it doesn’t look very different from its landlubber relatives). I was amazed to see it shoot straight down into a deep pool of water, swim across the stream, and disappear into submerged vegetation on the other side. It seemed more like a fish than a mammal. I never forgot the sight. Although my investigations of star-nosed moles continued to occupy me for years, eventually, in collaboration with Kevin L. Campbell and James F. Hare, biologists at the University of Manitoba, I set out to learn more about the North American water shrew, Sorex palustris. It is not only (at little more than half an ounce) the world’s smallest mammalian diver, but also a predator with remarkable abilities to sense and capture prey. To be fair, you’ll never have to worry about one latching onto your foot and dragging you into a lake. But I have come to think of water shrews as the great white sharks of their diminutive domain.

Shrews are members of the order Insectivora, which also includes moles and hedgehogs. Although they may look somewhat like mice, they are only distantly related to rodents. There are three subfamilies of shrews; the North American water shrew belongs to the Soricinae subfamily, the so called red-toothed shrews. This group gets its name from iron deposits in the animals’ teeth that are thought to provide added strength to the enamel (tooth wear is a major problem for shrews, because their teeth don’t grow continuously, as those of rodents do).

Water shrews by no means spend their entire lives in water. They live in wetlands and along streams and rivers, but pass much of their time on land, making their nests in dry areas and traveling through tunnels of grass, dirt, and mud. But as anyone who has flipped over a rock in a stream or pond can attest, there are plenty of creatures in the water to eat. Water shrews are well adapted to dive and swim for such prey. Their most obvious specialization is water-repellent fur. When diving, water shrews have a silvery, translucent appearance owing to a layer of air trapped by the fur. This keeps them dry and insulated—a critical ability, considering that they don’t hibernate during the winter and so must feed by diving into icy water.

Their feet are also adapted for swimming, but not in the most typical way. Instead of webbing, water shrews have a fringe of broad, stiff hairs along the sides of their feet and toes. These hairs rise up during the downstroke to increase the surface area of the foot, but fold down and out of the way during the upstroke. But perhaps most intriguing is how water shrews can find their food underwater—especially since their peak activity periods are at night, when vision is of limited use. It turns out they are able to use their sense of smell by sniffing air while submerged.

Logically that appears impossible: it is air that transports odorants to the olfactory receptors located in the nasal cavity, and there is no air underwater for a mammal to inhale. Water shrews, however, have evolved a simple and ingenious trick. While foraging underwater, they exhale air bubbles through their nostrils—often directly onto objects or prey they are investigating. They then inhale the same bubbles to collect odorants. This ability had been overlooked because the sniffing occurs so quickly it requires slow-motion video to observe, and not many shrews have been filmed underwater with high-speed cameras. I might have overlooked it, too, if I had not already discovered this trick in the star-nosed mole—another semiaquatic mammal that often forages underwater. That prompted me to test for the same ability in the water shrews, and I found it.