Hair, Glorious Hair
Mammal fur furnishes rich territory for scientific exploration
Richard and Joyce Wolkomir
AT THE AGE Of 38, Karl Kranz has a vigorous head of curly dark hair. Nothing very unusual in that: The one place on our bodies where we humans grow a profusion of hair is our heads. And, like Kranz (whether he thinks about it consciously or not), we use our tresses for social signaling. The messages are rich with information: I'm handsome, beautiful, rebellious, tidy. Also like most people, for the sake of modesty and protection, Kranz wears clothes—in his case jeans, a cotton shirt and running shoes. But unlike the rest of us, Kranz, general curator of the Philadelphia Zoo, might add that his clothes are a substitute for the pelt that once warmed our distant ancestors, and that the hair on his head is a remnant patch of fur. For Kranz has been fascinated with hair and its diverse functions since he was a young mammologist in the early 1980s at the National Zoo in Washington, D.C. "Hair probably evolved initially so that the early mammals could maintain their body temperatures," he says. "Then hair started evolving for all kinds of other purposes."
Our own social signaling, despite the annual $20 billion we spend worldwide on coiffing our hair, is certainly no more important than the warning to predators of African crested porcupines rattling their rigid tail hairs.
Or the floating of newborn sea otters due to air trapped in their natal fur. Or the camouflaging of sloths in tropical foliage by the green algae growing on their brown hairs. Not to mention the running on water by water shrews, which exploit surface tension with foot hairs.
Hair, along with milk, is a key mammal characteristic. Some mammals, like whales, elephants and rhinoceroses, are limited to scattered bristles. Others, such as tigers and leopards, are completely covered. Scientists in a range of different fields are now studying hair for everything from animal behavior to cancer research (human growth-promoting proteins, which play a role in hair growth. also trigger the multiplication of cancer cells). Textile researchers analyze hair hoping to manipulate sheep genetic material into producing super-wool. Dermatologists study stump-tailed macaques—monkeys that get bald just like human males—to probe the cause of hereditary baldness. Engineers developing solar energy systems research polar bear hair, which absorbs solar radiation. And studies of our mammal cousins also reveal something of our own hairy past.
Paleontologists believe mammals began evolving about 160 million years ago, during the Jurassic Period, when dinosaurs roamed. Therapsids, members of the reptile family, began developing mammalian features such as temperature regulation, nursing of the young—and hair. Like dinosaurs, therapsids were unable to sense their environment through their armorlike plates.
Hair seems to have begun as tiny sensory cones between scales. The cones brushed against objects and sent messages to reptilian brains. Some modern mammals have remnant scales—rats' tails, armadillo shells, the backs of pangolins. And hair grows between the scales.
Animal whiskers—hairs that are long, thick and stiff—still are sense organs. Each whisker has sensory receptacles, alerting the animal when it touches something. Kranz cites the fresh-water dolphin of the Amazon. "It has so many whiskers on its snout it looks like Willie Nelson," he says. Each lance-shaped whisker has tiny nubbins sprouting from it. "They use the whiskers to sense prey—crabs, shrimp, fish—because the water gets too muddy for them to see," says Kranz.
Under an electron microscope, a hair—human or animal—looks, literally, like an asparagus stalk. It is, in fact, a form of skin. Its chief constitu ent, the protein keratin, also makes up skin's epidermis, or upper layers, as well as feathers, horns, hooves, claws and fingernails. Each shaft of hair contains water, oily fats called lipids and the pigment melanin.
Much of what we know about hair comes from the beauty industry. At the Gillette Corporation, where hair is paramount, chemist Frank Girard holds up a model epidermis. "Hair is harder than fingernails," he says. The model contains a larger-than-life hair follicle. A follicle is a pit in the skin, extending a sixteenth of an inch down through the epidermis into the underlying dermis. At the bottom is the dermal papilla, a knob of blood vessels and hormones. From the papilla, hair cells multiply upward through the follicle, forming a hair shaft.
All hair, from fur to whiskers, is similar in basic structure. Tiny keratin fibers bind together to form larger fibers. They then entwine into still larger strands, which twist together into a sort of rope.
A hair shaft resembles a tree trunk. Around the outside, analogous to bark, is a protective sheath, the cuticle. "The cuticle cells look like fish scales," says molecular biologist Peter Steinert, an expert on hair morphology at the U.S. National Institutes of Health. Just under the cuticle is the cortex, where cells are densely packed. At the hair's core is the loosely packed medulla. The medulla is porous, with huge air spaces that make up as much as 80 percent of a hair's total width. The trapped air helps retain the owner's body heat.
Steven Webster, director of education at the Monterey Bay Aquarium in California, cites the sea otter, which not only uses its hair for buoyancy, but insulates itself in frigid seas with the trapped air. Sea otters roll on the ocean's surface to fill their fur with tiny air bubbles. They also float on their backs, pawing their fur to spread waterproofing oils from their skin to the hairs. The critical role of their fur is one important reason oil spills are such a threat to the creatures; the oil "fouls their hair so they can't trap air," says Webster.
Sea otters, like most mammals, have three types of hair. Snout whiskers are sensitive to touch. Long, rough guard hairs protect a thick undercoat of down. This "underfur," as it's called, is extremely fine and thick. We humans have 100,000 or so hairs on our heads. Sea otter underfur contains from 170,000 to 1,000,000 hairs per square inch, more than any other animal on Earth.
A sea otter's two-layer fur coat is better insulation than fat. Sea otters, in fact, are skinny. However, for deep divers like whales and dolphins, blubber is better than hair because the pressure of the ocean depths would flatten a pelt against the body, neutralizing the hair's insulating qualities.
From a filing cabinet in his Philadelphia Zoo office, Karl Kranz pulls a bag of white hair from a tiny African forest antelope, the klipspringer. "Feel it," says Kranz. The hair is so springy it is like a thick sponge. While sea otters trap insulating air between hairs, klipspringers trap air inside each hair, in the virtually hollow core.
"They live on cliffs and mountains, where they're exposed to a lot of wind chill," Kranz says. "I also suspect that they fall sometimes, jumping from rock to rock, and their springy hair protects them against getting hurt."
Platypuses, small aquatic mammals of Australia and Tasmania, keep warm the sea otter way—trapping air bubbles between hairs. Kranz displays a close-up photograph of platypus fur, showing the thick layer of down with sparser guard hairs sticking up. "It's like a forest in a storm—the treetops move back and forth, but at the bottom it's relatively still," he says. "The guard hairs lie on top of the down hairs and prevent them from being ruffled and losing air."
Platypuses swim in warmer waters than sea otters: They average 837 hairs per square millimeter to the otters' average of 1,253 hairs per square millimeter. Surprisingly, the polar bear, which lives in the planet's freezer, has only 9 to 16 hairs per square millimeter. But the polar bear has a secret.
"A polar bear is a walking greenhouse," says Northeastern University electrical engineer Charles DiMarzio, who has been studying the creature's fur. Northeastern engineers developing solar energy systems sidestepped into zoology when they noticed a Canadian government study showing that polar bears are excellent absorbers of solar radiation.
It turned out that polar bear hairs have hollow cores. The cores scatter light, making the hairs seem white. Actually, they are colorless. Sunlight passes through the bear's long guard hairs to its black skin, which absorbs the radiation as heat. Then the bear's insulating underfur stops the heat from radiating back into the air. That keeps Arctic polar bears toasty. For zoo polar bears, however, the system is a grooming problem:
Algae sometimes grow in the hairs' cores, turning the bears green. The zoos' warmer climates, combined with the bear's hollow hair shafts, provide a natural breeding ground for algal growth.
Mammals have all sorts of hairy heating systems. Koalas' back fur is darker, to absorb solar heat. Their belly fur is spar-set, and they can erect it at will to adjust their heat retention. Erect, the hair is thicker and retains heat. When the koala gets hot, it flattens its belly hair to let heat escape. We humans once had such erectable pelts. Now the tiny muscles produce only goosebumps.
The South American guanaco, a llama relative, has thickly matted back fur, but nearly nude belly patches. By simply changing its stance to expose or protect its hairless belly patches, the guanaco can raise or lower its insulating capacity fivefold in still air, and even more in the wind.
White-tailed deer typify another mammalian trick for keeping warm, according to Aaron Moen and C. W. Severinghaus of Cornell University: The deer grow winter coats as much as four times deeper than their summer coats. Meanwhile, their fawns' coats are spotted for camouflage as the infant deer hides in the forest.
From the human point of view, deer camouflage is nothing compared to some of the more exotic coats. Jentink's duiker, an African antelope, for example, has a black head and shoulders, white collar and gray back. But it is born drab brown to blend with its surroundings. Baby tapirs look like watermelons; as adults, they lose their camouflage stripes.
Yellow-backed duikers are jet black, with a patch of bright yellow hair on their rumps. This is reverse camouflage: Biologists believe that the patch throws off the timing of pursuing predators. "The idea is that you have some coloration that causes a predator to attack prematurely," explains Kranz. "In the duiker, the skin around the patch is thicker, and so a bit more defensible—and it focuses the predator on the rear end, perhaps giving the duiker more of a chance to take off and run."
Sometimes the defense is more direct, such as the intimidating quills (actually modified hairs) of the American porcupine. Normally, hair scales point up, away from the root. But quill scales point down, toward the porcupine's own skin. That way, quills easily penetrate an attacker. And once imbedded, a quill is hard to pull out. As the victim's muscles contract, the quill works deeper and deeper into the flesh.
A rhinoceros also protects itself with hair—its horn, actually compressed hair. Other rhino hair grows mainly on the tail and ear tips. Its nudity is not an evolutionary accident: "When you're that massive, keeping cool is the issue," says Kranz.
For other mammals, hair may be part of a game of "let's pretend." The coats of baby cheetahs, for example, seem to resemble the coat of the fierce African ratel, a wolverine-like carnivore. And at the Philadelphia Zoo, Kranz points to an aardwolf, asleep in its cage. A relative of the hyena, the creature is only the size of a basset hound. But when it is attacked, the aardwolf snaps up its enormous mane, suddenly "growing" by a third, looking formidable. Says Kranz: "Actually, they're little things." Nearby, a Siberian tiger lolls. It is striped, with pale fur to match the snowy woods where it hunts. "That's camouflage because they stalk; they like to get right to the edge of cover and then rush," says Kranz.
Another mystery of fur coloration is zebra stripes. British zoologist Jonathan Kingdon has theorized that stripes enable zebras to identify each other. He notes that members of the horse family—like zebras—are normally antisocial. Yet they may group under a tree offering the only shade for miles or at a lush grazing spot. Familiar stripes may ease the usual tensions among the animals. Nerve cells in many animals' eyes and brains, scientists have found, are programmed to respond to particular patterns. Zebra foals may learn to associate the stripe patterns they see on their mothers with rewarding social interaction. "It's like a uniform they wear to counteract their natural mutual intolerance," says Kranz. " 'We can march together even though we don't always get along.' "
Other mammals send even more offbeat hair messages. Kranz has studied the capybara, a South American rodent the size of a large dog. Specialized hairs under the capybara's tail become coated with odoriferous secretions. The capybara drops these hairs as it walks through the forest, marking its trail.
Moose have hairs between their "toes" that turn green. Researcher David Chapman of Canada's Dalhousie University has discovered that the hairs absorb gland secretions, apparently for chemical signaling during rut. Dietland Muller Schwarze, a State University of New York expert on animal chemicals, has found that black-tailed deer have "ankle" glands that ooze secretions onto specialized hairs for other deer to sniff.
Then there are the extremes of the human use, social signaling, though exactly what we mean with our extremes of hair styling is not always clear. Take the Guiness-record-book case of Rachel Marete, who in 1989 styled the hair of Colinda Sirls into an 8-foot-high freestanding "flagpole" in California. Or the prisoner serving a life sentence in New Delhi, India, who in 38 years grew a 94-inch mustache before his death in 1987. Then again, perhaps the point in these instances was a common, purely human motive: to be remembered.
Writers Joyce and Richard Wolkomir keep warm in their Vermont environs with a variety of artificial pelts.