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.