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A diving shrew: adaptations for underwater activity include water-repellent fur, sensitive whiskers, and hairs on the feet that increase surface area in the down stroke. 

Kenneth C. Catania

Do all shrews have this trick up their nostrils? It would be surprising indeed if terrestrial shrews did. I often catch terrestrial short-tailed shrews (Blarina brevicauda) while collecting star-nosed moles, and it was an easy task to train them to search for food in a shallow-water arena. As I anticipated, however, they were unable to sniff underwater.

In their natural habitats, water shrews and star-nosed moles probably use underwater sniffing to explore their surroundings and to identify food encountered right under their noses. I have tested both species by training them to follow an underwater scent trail (earthworm- or fish-scented for the moles, fish-scented for the shrews) leading to a food reward. Both star-nosed moles and water shrews are able to perform this task with great accuracy. But when the bubbles they exhale during underwater sniffing are blocked by a fine steel grid placed over the scent trail, neither shrews nor moles can follow the scent.

Underwater sniffing is not a water shrew’s only trick. In collaboration with my colleagues at the University of Manitoba, I’ve looked at other sensory abilities. Not surprisingly, we have found that, like most small mammals, water shrews also use their whiskers to detect prey. They have a dense array of whiskers geometrically arranged around their nostrils. Using those sensitive touch organs, they can detect prey shape and texture. To test those abilities, we made detailed model fish out of silicone. We then offered the shrews a series of silicone objects, both cylindrical and rectangular shapes, along with the fake model fish (all underwater and observed with infrared lighting to simulate night-foraging conditions).

The shrews investigated the different objects, rejecting the rectangular and cylindrical shapes but attacking the model fish. It was comical to watch them run back to their home cage with the model, uselessly chewing at the impervious silicone and then eventually stashing the model next to real food they had cached here and there in their cage. Although most of the shrews eventually stopped falling for this trick ( perhaps because the silicone did not smell like a fish), the experiment demonstrated the importance of shape and texture for identifying food.

Observations of the animals’ uncanny ability to detect and pursue fish, even in total darkness (evident in high-speed videos taken with infrared lighting), led to another experiment. We thought it likely that movement was an important cue that gave the fish away. To test that possibility, we prepared a small feeding chamber, which we equipped with four tiny water outlets connected to precisely controlled pumps. Once a shrew was accustomed to entering the chamber and finding a fish to catch and eat, we switched tactics by removing the prey. Instead, when the expectant shrew entered in search of a fish, an outlet pulsed water for less than a tenth of a second. Our goal was to imitate the disturbance caused by the tail-flick of a fleeing fish. In the absence of a fish, a shrew would attack the water movement as if pursuing a fish. In contrast, if the water was pumped in a continuous stream, the shrews ignored it.

Evidently water shrews can use their sense of touch to detect and pursue escaping animals. For such a strategy to work, an animal has to be fast, and that is certainly true of water shrews. By filming them at a thousand frames per second, we were able to precisely measure their response time. While foraging underwater, shrews began to turn toward a water movement in only twenty milliseconds (a fiftieth of a second), and in fifty milliseconds (a twentieth of a second) had moved as much as three-quarters of an inch while opening their jaws. That’s about ten times faster than the human eye can begin to move to follow a movement in the visual field.

Imagine the quandary that puts you in as a fish. You can either sit still and be detected by an underwater sniff or the touch of the shrew’s whiskers, or you can flee and give yourself away for sure by causing a disturbance in the water. Fleeing is probably the best option if you can get to open water. Fish are very difficult to catch when they have room to maneuver, and our observations suggest fish usually manage to escape in a large area. But a fish in a small space, in among rocks and vegetation, is apt to fall prey in only a fraction of a second.

Water shrews also feed on crayfish. That may seem a questionable strategy given the shrew’s small size and the daunting claws of a crayfish. But a crayfish doesn’t stand a chance. I suspect that is mainly because there is a fundamental difference between the nervous systems of mammals (vertebrates) and crayfish (invertebrates). All mammals, including water shrews, have nerve fibers covered with an insulating sheath called myelin. That greatly increases the speed with which nerve impulses are conducted, allowing for faster processing of sensory information and quicker reaction time. Invertebrate nerve fibers do not have myelin. The main invertebrate adaptation for speeding conduction and reaction time is to have large nerve fibers.