Lily Whiteman, Washington DC
Bar-headed geese (Anser Indicus) migrate over Mount Everest, where oxygen is scarce and life is rare. How do they survive in such conditions?
At 29,028 feet, Mount Everest is tall enough to poke into the jet stream, a high-altitude river of wind that blows at speeds of more than 200 miles an hour. Temperatures on the mountain can plummet low enough to freeze exposed flesh instantly. Its upper reaches offer only a third of the oxygen available at sea level--so little that if you could be transported instantly from sea level to Everest's summit, without time to acclimatize, you would probably lose consciousness within minutes. Kerosene cannot burn here; helicopters cannot fly here. Yet every spring, flocks of bar-headed geese--the world's highest-altitude migrants--fly from their winter feeding grounds in the lowlands of India through the Himalayan range, sometimes even directly above Everest, on their way to their nesting grounds in Tibet. Then every fall these birds retrace their route to India. With a little help from tailwinds, they may be able to cover the one-way trip--more than 1,000 miles--in a single day.
Considering the bar-headed goose's stamina and tolerance for thin air, one might expect this bird to take the form of a giant lung. But it doesn't. Rather, its robust gray body, long neck (the most notable characteristic that distinguish geese from ducks, in the same family, Anatidae), and short, tapered beak create an elegant S-shaped silhouette. Adults weigh about five pounds and stand about two feet high. Two horizontal black stripes on the back of the bird's white head give the species its name.
The yearly migration of these geese is apparently triggered by a biological alarm that rings early enough in the spring for them to miss the summer monsoon season and early enough in the fall for them to miss the worst of winter's storms. Still, they cast their fates to the wind only with due consideration. Geese poised to take off, for example, may delay their flight if strong headwinds kick up. And when airborne birds get tossed about, they may turn back and land or change altitude in search of better conditions.
Moreover, by using tailwinds, the geese capitalize on weather that could pulverize lesser creatures. "These birds are powerful flappers, not soarers that just glide with the wind," says M.R. Fedde, an emeritus professor of anatomy and physiology at Kansas State University's School of Veterinary Medicine, who has conducted laboratory studies of the bar-headed goose's respiratory system. Partly because their wings are huge, have a disproportionately large surface area for their weight, and are pointed to reduce wind resistance, "they can fly over 50 miles an hour on their own power," Fedde says. "Add the thrust of tailwinds of perhaps 100 miles an hour if they are lucky, and these birds really move." Able to gauge and correct for drift, bar-headed geese can even fly in crosswinds without being blown off course. The same powerful and unremitting flapping that helps propel them over the mountains also generates body heat, which is retained by their down feathers. This heat, in turn, helps keep ice from building up on their wings.
What's the secret to the bar-headed goose's aerobic success? "First of all, bar-headed geese are birds," says S. Marsh Tenney, an emeritus professor of physiology at Dartmouth Medical School, whose research on respiratory adaptations to oxygen deprivation includes studies of these highfliers. "And all birds are built for particularly efficient oxygen uptake." The avian breathing system is uniquely structured. Among its special features are air sacs that temporarily store inhaled air that has passed through the lungs and then send it back through their lungs before it is exhaled. Thus, birds circulate inhaled air through their lungs twice--once more than earthbound mammals do--increasing their opportunities for capturing oxygen.
Birds can also pant for prolonged periods without constricting the blood vessels in their brains. So even when physically taxed, they keep their wits about them. By contrast, prolonged panting in people reduces blood flow to the brain, which primes them for bad decision making--hence the occasional unfortunate climber who blithely strolls off a cliff.
Bar-headed geese are "super birds," says Tenney. "They do everything even better than other birds." They have a special type of hemoglobin (open hemoglobin; higher oxygen affinity than “closed” hemoglobin in humans; even though avian RBCs have nuclei, lesser surface area than mammalian RBCs) that absorbs oxygen very quickly, combining with high RBC density per blood volume and the unique lungs and “air sacs”, one of the trademark of avian as described in last two paragraphs, that enable bar-headed geese to take in oxygen twice in only single breathing cycle, making absorption of oxygen even quicker at high altitudes. As a result, they can extract more oxygen from each breath of rarefied air than other birds can. Once their blood is stoked with oxygen, it rushes through capillaries that penetrate particularly deep into their (flight; pectoral) muscles, which is very rich in “red” fibers that is biologically designed for sustained contractions and relaxations with lesser fatigue in long exercising durations, one of the trademarks of “avian migrants”, with thanks from aerobic respiration. Thus energized, their wings flap with seemingly inexhaustible vigor. Other migratory birds, without the superior flapping, respiratory, and circulatory power of the bar-headed goose, fly closer to the ground. Most songbirds, for example, fly at between 500 and 2,000 feet, and most waterfowl stay between 200 and 4,000 feet.
The awesome engineering of the bar-headed goose's body aside, the logic behind its migratory route remains baffling. After all, the cold, high desert of Tibet, which offers few productive ponds, hardly seems to be a breeding ground worthy of death-defying journeys, even by such tough birds. Moreover, the geese reject much lower routes between India and Tibet that slice through the Himalayas only several miles from their high-altitude flyways. One explanation for this behavior was advanced by the late Lawrence Swan, an explorer and a biology professor at San Francisco State University. One evening during a Himalayan expedition, he was startled by the honking of night-flying bar-headed geese passing directly over Mount Makalu's 27,824-foot summit. This experience caused Swan to speculate that the species had originally settled in India before collisions between the tectonic plates under the Hima-layas had pushed the range up.
Watered by melting ice-age glaciers and without the shadow later cast by the Himalayas, ancient Tibet may have been a land of green valleys and summer lakes and streams, according to Swan's theory. Such a landscape could have drawn bar-headed geese for seasonal visits. The mountains presumably rose slowly enough to accommodate evolution, so these geese could have continued to fly their customary routes while adapting to the altitude. Each new generation probably learned the route by traveling in flocks with its elders during its first year and then taught it to its own offspring in later years. The species would have thus perpetuated its flyways through the ages. If, indeed, bar-headed geese are "older than the hills below them," as Swan believed, their current migrations remain a "behavioral fossil" from early Himalayan Asia.
Swan also speculated that the summits provided bar-headed geese with more obvious landmarks. In addition, only at high altitudes can the birds catch the fastest tailwinds, which blow far above the earth's obstacles. Other explanations suggest that high flyways actually save the goose both time and energy, compared with longer routes that circumvent the mountains.
Although no one knows for sure how migrating birds navigate, most scientists suspect that they follow a combination of cues--tall peaks and other geographic features, such as rivers; the position of the sun and the stars; geomagnetic signals; and changes in barometric pressure. The honking of bar-headed geese during flight may convey messages that help flocks stay together, and may also create orienting echoes.
Despite an abundance of theories about the behavior and the biology of bar-headed geese, their true strength has yet to be measured, let alone fully understood. Since their remote habitat has precluded the comprehensive study of the geese, most current information about their physiology has been gleaned from individuals that were monitored while resting or waddling on treadmills. Though such research offers insights, bar-headed geese have still never been studied while flying, says Kansas State's Fedde. "We could analyze bar-headed geese while they fly in wind tunnels," he says. "Or, like the geese portrayed in the movie Fly Away Home, they could be imprinted to follow an ultralight airplane while we measured their responses with portable monitoring equipment. But so far, funding for those sorts of studies just hasn't been there."
Scientists believe that better data on the physiology of bar-headed geese could ultimately help people cope with altitude and respiratory diseases. Frank Powell, a professor of medicine and the director of the University of California's White Mountain Research Station, a high-altitude field laboratory in Bishop, California, says, "We will never be able to engineer a human lung to work like a bird lung. But with more information we might be able to develop drugs that would help duplicate some of their cellular responses."
Other High-Altitude Creatures
Bar-headed geese are not alone at the top of the world. Several kinds of birds and one spider also frequent this rare-fied atmosphere. Living at 22,000 feet on Mount Makalu are jumping spiders, the highest known full-time land dwellers, says John Edwards, a professor of zoology at the University of Washington and an expert in high-altitude insects and spiders. Remarkably, these denizens of the heights are hardly different from the varieties of jumping spiders that populate backyard gardens at sea level. In fact, the spiders of Makalu lack "any obvious adaptations for life under such rigorous conditions," according to a report by F.R. Wanless, an invertebrate researcher at the British Museum and one of the few scientists to ever study these remote creatures.
Several other bird species regularly brave extreme altitudes. Among them are whooper swans, which were once observed by a pilot at 27,000 feet over the Atlantic Ocean between Iceland and the European continent, and bar-tailed godwits, which have been spotted at almost 20,000 feet. And then there's the occasional hardy individual that makes a high-altitude cameo.
The highest-flying bird ever recorded was a Ruppell's griffon, a vulture with a wingspan of about 10 feet; on November 29, 1975, a Ruppell's griffon was sucked into a jet engine 37,900 feet above the Ivory Coast--more than a mile and a half higher than the summit of Mount Everest. The plane was damaged, though it landed safely.
In 1924 a yellow-billed chough, a crowlike bird that's among the highest-nesting species, followed a climbing expedition's food scraps to 26,500 feet on Everest. The avian altitude record in North America is held by a mallard, which collided with an airplane on July 9, 1963, at 21,000 feet above Elko, Nevada.
Question
Explain briefly why some bar-headed geese are able to be very metabolically active at altitudes over 35,000 feet without any assisting device, even though they have red blood cells that have less surface area than mammalian red blood cells?
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