| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature Whiskers! A Feel For The Dark
On the face of it, they look ordinary enough—overgrown hairs adorning animal jowls. But find out what they can do, and whiskers really begin to stand out. The versatility and acute detection abilities of these bristly appendages have made whiskers critically important for a wide range of mammals. From those that haunt the night to the flippered residents of the oceans, many creatures that live much of their lives in the dark, rely on whiskers. Over the eons they have been tailored into a custom fit for each species, and can be applied to more jobs than a Swiss Army knife. A whisker, like any other hair, sprouts from a follicle embedded in the skin, and is supplied with a basic survival kit that includes a blood vessel, a few nerve endings, and the muscles responsible for goosebumps. But that’s where the similarity ends. Long, stiff whisker hairs, known as vibrissae, are far more sophisticated than plain old pelage fur. Like regular hairs, vibrissae are really just stout pillars of dead skin cells. It’s the fancy accessories hidden below the surface of the skin that make vibrissae special. All vibrissae are sunk in a follicle sealed by a capsule of blood, known as a blood sinus. Just as the air inside a balloon will shift when the balloon is squeezed, when a ibrissa is touched it bends and pushes blood against the opposite side of the sinus. The sloshing blood amplifies the message of movement, jangling nerves that allow an animal to detect the softest touch. “I think of vibrissae as an extension of the brain, much the way eyes are,” says Chrisopher Marshall, who studies marine mammal vibrissae at Texas A&M University in Galveston. The majority of mammals rely most heavily on the hairs growing from their faces. Most noticeable are the mystacial whiskers arranged in orderly rows on animal cheeks. Cats use them to help find and keep what they catch. While on the prowl, a cat holds these vibrissae out to the side to detect the faintest drafts stirred up by prey. As it pounces, it pulls its whiskers hard forward to form a living net capable of sensing the escape attempts of its victims. Cheeks aren’t the only fertile fields for whisker growth. Like eyebrow hairs on steroids, supraorbital whiskers help animals measure whether they can squeeze their bodies into tight spaces without getting stuck. Noses are often graced by rhinal whiskers that may help seals sense when their noses clear the surface of the water and it’s safe to take a quick breath. Whiskers can run absurdly long. For example, the vibrissae of chinchillas (Chinchilla lanigera), rodents from the Andes Mountains, can extend for a good third of the length of their bodies. That kind of reach is particularly helpful when a small animal is in the water. Swimming rats have been observed to extend their whiskers sideways like a tightrope-walker’s bar to stay balanced in turbulent water. In the water, these organs can spell the difference between life and death. One researcher found that rats which had paddled along swimming in turbulent water drowned after their vibrissae had been removed. Without whiskers, the animals were unable to keep their noses above the waterline. Rodents make the most of their whiskers as they creep around at night trying to avoid predators. Most are cursed with poor vision, and get around by reading the information gleaned from their whiskers. Rodents use the same technique as blind humans who swing a white cane, when they whisk their vibrissae across objects in the environment. Each whisker may hit the same object several times in different places, allowing the animal to draw a clearer picture with each pass. It turns out that rodent brains may be exquisitely attuned to how fast those whiskers whisk. Rats typically make about eight sweeps per second, while mice squeeze in twelve. Part of that difference may be due to physics; the shorter the whisker, the faster it can scan. But rodent brains seem to snap to attention at a preferred whisking rate. As the whisking pace approaches a frequency of eight cycles per second, “the brain dynamically sharpens up the representation of the whisker. That may be a sweet spot for the brain,” says neuroscientist Chris Moore, who studies rat vibrissae at the Massachusetts Institute of Technology in Cambridge. He has found that wiggling one whisker faster or slower than this rate changes the message it sends to the brain, and tells the rat the direction the whisker was wiggled. In new research, Moore takes the idea of whisker frequencies one step further. A rat’s whiskers range from heavy-duty fibers several centimeters in length near the cheeks to finer hairs of just a few millimeters at the mouth. His work suggests that rats use this range to extract even more details from their feelers. Early sensory hairs likely weren’t as sophisticated. About 250 million years ago, when the ancestors of mammals began shuffling through the forests, all the most advanced creatures were still wearing scales. Because a little hair is as useless for insulation as a few feathers are for flight, scientists say the first hairs to project from their armored hides may well have been used as tactile detectors—low-tech versions of today’s vibrissae. The acute sensitivity of modern whiskers can be a lifesaver for animals who frequent perilous places. High-flying squirrels wear vibrissae on their feet and legs, the better to ensure a safe landing between tree leaps. Bats that bunk in rock and tree crevices may wiggle the vibrissae on their bottoms around to find good roosts. And subterranean pocket gophers and golden moles feed information from their tail vibrissae into an internal GPS system; catching the breezes wafting from surface exits helps them pinpoint their position in an underground maze of tunnels.
Yet the undisputed champions of vibrissa verve are the marine mammals. Seals, walruses, and even the placid sea cows (manatees and dugongs) can use their vibrissae to accomplish feats eclipsing the best efforts of any bewhiskered land animal. Even whales and dolphins have held on to their hair follicles despite sacrificing many other reminders of their terrestrial origins. The dark waters of rivers and oceans are precisely where whisker sense excels. Water is a dense medium in which evidence of a passing animal lingers in the form of a wake for minutes at a time. Water alters colors and extinguishes light so that the clearest crystal springs quickly darken to black at depth. Finding food and keeping tabs on other animals in these conditions is almost impossible to do solely with sight. So mammals turned to their sensitive whiskers to meet those needs. Vibrissa researcher Guido Dehnhardt of the University of Bochüm in Germany has shown that seals can detect water waves just a millionth of a meter in size—the size of the wake left behind by a goldfish. Later, he trained a blindfolded harbor seal (Phoca vitulina) to search for a remote-controlled toy submarine released in its pool moments earlier. Swimming with whiskers pointed forward like antennae at the ready, the seal found the sub in 256 out of 326 total attempts. The seal’s sub-finding behaviors left no doubts about its methods. “You could track on the monitor exactly the trail of the sub and, afterwards, the trail the seal followed. These were absolutely the same every time,” Dehnhardt says. When its whiskers were covered by a stocking mask, the seal swam about at random, no longer able to detect tiny changes in water turbulence. “They are chasing their prey with their whiskers; there is no other explanation,” he says. As a rule, the whiskers of aquatic mammals are wired with far more neural fibers than those of their landlubber cousins. For example, the Australian water rat (Xeromys myoides), which dives into river bottoms for its dinner, has 500 axons per vibrissa to the 100 to 150 of the Norway rat (Rattus norvegicus). In sea lions, that increases to an astonishing 1,000 to 1,500 axons per whisker. “For active touch, the number of nerve fibers in a rat whisker should be sufficient. So there must be something more to it,” Dehnhardt says. “In the aquatic environment, there’s a whole new tactile world not available for terrestrial mammals. This could be why they put more innervation into the system.” Most whales and dolphins lost their snout hairs along with the rest of their body fur, but beneath the surface of their skins, some still nurture their vibrissae follicles. Toothless cetaceans such as northern right whales (Eubalaena glacialis) have prominent vibrissa follicles around their blowholes and jaws. “Possibly they’re picking up the density of plankton washing over their bodies and fins,” Marshall says. Keeping all of that sensory machinery warmed up and ready to go is costly for marine mammals, which tend to live in chilly waters. As a rule, whales, dolphins, seals, and sea lions drop the temperature of their surface skin to match that of the surrounding waters to stay warm. But just as cold numbs human hands into clumsy mitts, dropping temperatures probably have a similar effect on the sensitivity of vibrissae. Yet when Dehnhardt photographed the face of a river dolphin (Sotalia fluviatilis) with heat-sensitive film, its toasty-warm chin vibrissae lit up the picture like Christmas lights. In the wild, these dolphins hunt fish and crustaceans in rivers the color of coffee, and chin whiskers probably help them feel around for their prey. Finding a fishy meal is well worth the cost of keeping their vibrasse folicles warm. Dehnhardt suspected that seals, too, chased fish with their lengthy vibrissae—and might rely on them even more than dolphins do. When he photographed a harbor seal with heat-sensitive film, he got similar results showing warm, ready-to-work facial vibrissae. But seals, unlike dolphins, can’t supplement their search for prey with sonar. Nor is vision likely to be much of a help. Many species live in the Arctic and Antarctic, where it stays dark for much of the year. And there are many known instances of wild seals remaining healthy and well-fed despite being totally blind. Other pinnipeds, such as Pacific walruses (Odobenus rosmarus divergens), use their vibrissae to catch different kinds of seafood snacks. With chunky bodies ill-suited for chasing down fleet and agile fish, walruses stick to sedentary prey such as clams and other mollusks. At roughly the size of a Volkswagen bug, an adult walrus must find, open, and slurp up a whole restaurant supply of clams and geoducks per day just to stay healthy—54 kilograms, about the weight of a small woman. “You’d think that for a big animal like that, which can weigh 1,800 kilograms, it would be a waste of time to bother with such small foods,” says marine mammal researcher Ron Kastelein of the Hardervijk Dolphinarium in the Netherlands. “It would be like you and me trying to eat a wild grain. You would spend more energy finding, peeling, and eating it than you would get out of the grain.” But walruses are so efficient at finding prey that tracking down even small game is worthwhile. When foraging, walruses dive to the bottom and comb through the silt and sand for shellfish with flippers pointed skyward. Eyes closed tight against the billowing clouds of silt they kick up while feeding, walruses root about with some 480-odd vibrissae roughly the texture of uncooked spaghetti to locate shellfish as small as a couple of centimeters long. Kastelein found that blindfolded walruses could tell the difference between flat circles and triangles half the size of an M&M. “With muscle fibers crisscrossed in all directions like the beams of the Eiffel tower, they can control each individual vibrissa. It’s amazing to see what they can do,” Kastelein says. Drop a piece of squid on the edge of a walrus’s vibrissae pads, and it will broom the morsel mouthward with a few quick jerks of its whisker hairs. Bearded seals also feed on bottom-dwellers such as shellfish, but vary that diet with crabs, sea cucumbers, and other creatures. Marshall predicts that the abilities of their vibrissae, which are more luxurious than in any other pinniped, will lie somewhere in-between the specialized clam-digging whiskers of walruses and the fish-tracking vibrissae of harbor seals. Dehnhardt has found harbor seals to be just as talented as walruses when using their vibrissae for size discrimination. Their ability to tell apart objects of the same shape but slightly different sizes “comes close to what primates can do with their hands,” Dehnhardt says. But overall, slow-moving manatees may have the most phenomenal set of vibrissae in the animal kingdom. In Florida, these strict vegetarians spend most of their time paddling about in often murky coastal waters, browsing on sea grasses growing from the bottom. Setback eyes and a bizarrely flattened snout make it impossible for a manatee (Trichechus manatus latirostris) to see anything right in front of its nose. Unable to inspect objects closely by sight, manatees explore the world with fleshy, prehensile lips covered with some 600 vibrissae. Manatees mouthing mooring line knots have sent more than a few researchers swimming madly downstream after loosened equipment. Now Marshall and Roger Reep of the University of Florida have discovered how seemingly clumsy manatees manage that trick. The researchers set up underwater video cameras to film captive manatees eating vegetables wedged into plastic pegboards. To the astonishment of the scientists, the manatees used their nearly prehensile lips and special vibrissae like fingers to explore and grasp food. “You see what looks like two little hands pop out, come together at the midline to grasp things, and pull vegetation into the mouth,” Marshall says. Like rats, manatees have dedicated a sizable chunk of their brains to deciphering data from their bristles. When Reep and Marshall looked more closely at these neural connections, they found that many lines connected not just to the face, but elsewhere on the body as well. Curious, they removed dozens of hair follicles from the bodies of several manatee cadavers and examined the tissues under the microscope. “Every single hair we found was a vibrissae. That’s unique. No other animal has only vibrissae hairs and no pelage hairs,” Reep says. Manatees use such an all-over sensory array the same way that fish use their lateral lines. Aranged in a stripe that stretches the length of a fish’s body, the line houses thousands of receptors capable of detecting minute changes in water pressure. The line helps individual fish dart and dive in perfect unison with thousands of schoolmates. In the manatee’s case, waves lapping against underwater formations will reflect back in patterns that indicate large objects such as limestone rock formations and pier pilings lie ahead. The hairs could also help manatees determine when to ride ebb tides out to a river mouth, and when to coast inland again on high tides. A whole-body vibrissa network could also explain one of the more uncanny behaviors of manatees— synchronized swimming. “Groups of animals will sink to the bottom and rest, motionless and with their eyes closed, for ten minutes or more before coming up. When an animal starts to come up for breath, all the other animals will also rise in unison with it, even though they never open their eyes,” Reep says. The same sense may help manatees keep their distance from unwanted intruders—including nosy whisker scientists.
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