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.
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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.
[pagebreak]Because escape responses are so critical, the nerve fibers that control them in many invertebrates tend to be especially large. The tail-flick escape response of crayfish, which is often successful, is mediated by such “giant” nerve fibers. But even those giant fibers are no match for a shrew’s myelinated fibers. And shrews have a second advantage as well: they are warm-blooded, and thus their nervous system is always at the optimum temperature for peak performance. The combination of those two attributes makes shrews formidable predators, at least from the perspective of a crayfish. If escape fails and a battle ensues, a shrew quickly prevails.
The shrew’s brain is ultimately responsible for its sensory abilities, so we have sought to understand how the animal’s brain is organized and how that might contribute to the shrew’s skill as a predator. In all mammals, an outer six-layered sheet of tissue called the neocortex is the final processing station for visual, tactile, and auditory information. To investigate how the cortex is organized into different subdivisions for each of those functions, we can flatten it out, section it on a microtome, and stain it for anatomical markers that reveal the different areas. Along with recordings of brain activity, this technique enables us to map the size, shape, and location of brain regions devoted to the different senses and body parts.
In water shrews, a remarkable 85 percent of sensory cortex is devoted to processing information from touch. Vision and hearing take up only 8.5 percent and 6.5 percent of sensory cortex, respectively. Within the touch region of cortex, most of the area (about 70 percent) is devoted to processing sensory information from the whiskers, leaving only 30 percent for the trunk and limbs. That is an astounding mismatch between the size of body parts and the size of their representation in the neocortex—a phenomenon called cortical magnification. But it makes sense if one considers the importance of the whiskers, rather than their relative size. A similar “rule of thumb” governs body maps in human brains, where much of the touch region is devoted to the hands and lips, leaving only a meager area representing the trunk and legs.
The mammalian brain does not develop in isolation; rather, it is shaped by information from the body. A number of studies in different species suggest that inputs from the different senses compete for brain territory during development. We can get a clue to this process in shrews by peeking into the nest and observing the young. At early stages of development, just when the maps in the neocortex are being laid down, the skin housing the whiskers is swollen and vascular—standing out from the rest of the face. This reflects the enormous metabolic resources being devoted to whisker development. Thousands of touch receptors have been generated in the skin surrounding the nascent whiskers, and a massive cable of nerve fibers is already connecting them to the brain and sending signals to the developing neocortex. In developing water shrews, important inputs from the whiskers essentially carve out their large share of space in the neocortex. When the shrews finally emerge from the nest, at the age of three weeks, they are well-equipped with a keen sense of touch, and a week later they start diving for food on their own.