Cold Read online

  Certain Native Americans thought of den emergence as a form of rebirth. Males usually come out first. Females with cubs follow. Emergent bears might appear stiff, hobbling about like old men. They stretch. They sniff about. They yawn and scratch. They are thirsty but not immediately interested in food. When their appetites return, they dine first on roots and herbs, restarting a dormant digestive process. The mighty polar bear, often thought of as pure carnivore, has been seen pawing through the snow to feed on frozen salad just outside its den. Bears at emergence, at rebirth, may weigh as little as half their autumn weight.

  It is not just bears that hibernate. It is ground squirrels and chipmunks and groundhogs and raccoons and skunks and prairie dogs. Some, such as raccoons and skunks, hibernate softly, like bears, in a deep stupor but without the dramatic body temperature drops of ground squirrels and groundhogs. Some hibernate alone, others in groups. Groundhogs — variously known as woodchucks, marmots, and whistling pigs — sleep in family groups. All wake occasionally to warm themselves and to drift into a softer sleep, a nonhibernating sleep that allows them to dream.

  Birds, for the most part, migrate. Until the 1940s, scientists believed hibernation to be as unbirdlike as carrying an umbrella. It was then that biologist Edmund Jaeger, wandering in the Chuckwalla Mountains of the Colorado Desert, watched poorwills. Poorwills are nocturnal birds, seven inches long, that feed on flying insects. “Where do they go in the winter?” Jaeger asked a Navajo boy. “Up in the rocks,” the boy replied. The Hopi, too, knew the poorwill, and in their language called it “the sleeping one.” In winter, Jaeger found what at first seemed to be dead birds in the rocks, but when he picked them up, they flew away. He watched one that stayed in torpor for eighty-five days. He pushed a thermometer up the cloaca of a torpid poorwill and found its body temperature close to that of the air. Its pupils did not react to light. He found no heartbeat. The bird did not seem to breathe. But in spring it flew away. Its awakening, its rebirth, coincided almost perfectly with the reappearance of flying insects. In a 1949 paper, Jaeger wrote, “I take it as evidence that the bird was in an exceedingly low state of metabolism, akin, if not actually identical with hibernation, as seen in mammals.”

  Later, biologist Jon Steen watched titmice and finches. With enough food, the birds shivered through the night, their feathers puffed up, their heads tucked under their wings, their aura, to the extent that birds project an aura, pathetic. But hungry birds entered a nightly torpor. Their body temperature dropped. They have since been called “daily hibernators.” Biologist and author Bernd Heinrich has written, “Physiologically there is no distinction between hibernation and daily torpor.”

  Reptiles and amphibians are not exempt. Snapping turtles — air-breathing reptiles — can lie under the mud beneath the frozen surfaces of lakes for more than four months at a stretch. The Manitoba toad of Minnesota summers near flooded depressions left behind by Pleistocene glaciers, but in winter it hops upslope and burrows into gopher mounds. Two University of Minnesota biologists tagged hundreds of toads with radioactive chips and followed them through the winter. The toads, underground and without food, in amphibian semistupor, gradually burrow deeper through the winter, staying ahead of the ever-deepening frost line. Somewhere around four feet down, they stop digging. By spring, their body temperature is just above freezing, not much warmer than that of a hibernating ground squirrel. But unlike the ground squirrel, the toad will not shiver itself back to warmth. It will have to wait for its tunnel to warm, and then it will crawl out of its burrow, looking for sun.

  The wood frog is to the Manitoba toad as Maine is to Florida. The wood frog’s habit of overwintering frozen, with ice in its veins and between its cells, was not understood until 1982. Naturalist John Burroughs, walking in the woods of New York in December 1884 — nearly a hundred years earlier and nearly seven decades after the Year Without Summer — heard a frog calling from beneath the leaves. He lifted the leaves. “This, then,” he wrote, “was its hibernaculum — here it was prepared to pass the winter, with only a coverlid of wet matted leaves between it and zero weather.” Burroughs at first believed this to be a predictor of an easy winter. “Forthwith,” he wrote, “I set up as a prophet of warm weather, and among other things predicted a failure of the ice crop on the river…. Surely, I thought, this frog knows what it is about, here is the wisdom of nature.” The frog, he believed, would burrow into the ground if a cold winter was ahead. But he was wrong. Two feet of ice formed on the nearby Hudson River, and it was still bitterly cold when Burroughs went back to look for his frog in March. The leaves of the hibernaculum were frozen. He peeled them back, and beneath he found frozen ground. Between the frozen leaves and the frozen ground lay his frog. “This incident convinced me of two things,” he wrote, “namely, that frogs know no more about the coming weather than we do, and that they do not retreat as deep into the ground to pass the winter as has been supposed.”

  When handled, Burroughs reported, the frog blinked. In this he was mistaken, for frozen frogs do not blink. The idea that a frog could spend the winter frozen was so outlandish that Burroughs appears not to have considered it. He saw what he believed because he could not believe what he saw.

  Flash forward one hundred years to when a physiologist named William Schmid found a wood frog under the winter leaves in Minnesota. The frog was frozen. Schmid thawed it out and watched it come to life. In 1982, he published “Survival of Frogs in Low Temperature.” So far, four frog species are known to overwinter in a frozen state. To be clear, these are not frogs that are cold, but frogs that are literally frozen. Pick them up, and they are as hard as ice. They are, in fact, largely ice. Almost two-thirds of their body water may be frozen. Ice crystals form between their cells and throughout their body cavities, but the cells themselves, protected by high concentrations of glucose, do not freeze. In this state, the frogs can survive body temperatures as low as eighteen degrees.

  Bugs are stranger than frogs. Tent caterpillar eggs are full of glycerol, a form of alcohol that acts as an antifreeze. Caterpillars — my Fram and Bedford — simply freeze solid. Take an African desert fly, dry it out, throw it in liquid helium at temperatures below minus 450 degrees, warm it up, and pour some water on it, and it will demonstrate what it is to be a survivor.

  And there is the trick of supercooling. Water, it turns out, has a sharp and consistent melting point. Warmer than thirty-two degrees, ice becomes liquid water. But chill water below thirty-two degrees, and it may still be liquid water. In this supercooled state, the liquid is unstable. Add a speck of dust or a snowflake — nucleation sites for ice formation — and ice crystals will grow. That, though, understates the process. Supercooled water, once it goes, goes quickly. It flash freezes. The ice crystals blossom. They explode into being. The trick to survival through supercooling is to avoid anything that might trigger flash freezing. To flash freeze is to die. Yellow jacket wasp queens latch onto the underside of a leaf or a piece of bark and then hang suspended, their body temperature dropping as low as four degrees. Supercooled, they do not freeze. But tap them, or let a snowflake hit them from above, or a drop of water, and they turn to ice.

  Bear biologist Lynn Rogers was walking one day in January. Quite possibly, he had passed the burrows of toads earlier that day. He likely had trod past frozen wood frogs. He certainly would have been in the company of insects with antifreeze in their tissues, of frozen insects, and of supercooled insects. Then he crawled into a bear’s den. “On January 8, 1972,” he wrote,

  I tried to hear the heartbeat of a soundly sleeping five-year-old female by pressing my ear against her chest. I could hear nothing. Either the heart was beating so weakly that I could not hear it, or it was beating so slowly I didn’t recognize it. After about two minutes, though, I suddenly heard a strong, rapid heartbeat. The bear was waking up. Within a few seconds she lifted her head as I tried to squeeze backward through the den entrance. Outside, I could still hear the heartbeat, which I timed (after checking
to make sure it wasn’t my own) at approximately 175 beats per minute.

  It is October thirteenth and thirty-six degrees in central Pennsylvania. A Friday the thirteenth cold front, invading from the northwest, jumping the Canadian border without hesitation, decimated yesterday’s T-shirt weather. The front dumped two feet of snow near Buffalo, New York, 150 miles from here. The Washington Post ran a photograph of a Labrador retriever bulldozing through the snow, its legs invisible beneath the white stuff, its nose iced over, the look in its eyes one of joyful confusion. The New York Times ran a front-page color photograph of cars buried in snow and people in winter coats, shoulders hunched, walking away from the camera, subliminally headed south. In Minnesota, the same weather system dropped temperatures to twelve degrees. It is not supposed to be twelve degrees in October, not even in Minnesota. “Don’t forget the School Children’s Blizzard,” the weather is saying. “Remember Greely. Remember who is calling the shots.”

  Officially, I am here for a seminar on underwater sound, the sort of educational opportunity that only a large university can offer. Unofficially, I am far more interested in learning through experience and observation. I want to learn more about cold and to see firsthand how temperate zone students respond to it. I drive across the Pennsylvania State University campus. The students are slimmer than those in Fairbanks, and clean-shaven: more primped, less insulated. Their warm clothes are formfitting, lacking the space underneath for three sweaters. In general, the students here are less prepared for an ice age, but at least one of them, graduated now and moved on, was a cryophile, a lover of cold. While here, he learned of a place just outside town known variously as the Barren Valley, the Scotia Barrens, and just the Barrens. With his roommate, he nailed a thermometer to a pine tree in the heart of the Barrens. The two of them drove the four miles out from campus once a month. “It is amazingly quiet, amazingly clear, and amazingly cold,” he once wrote. When it was thirty degrees on campus, it was eight degrees in the Barrens.

  Below-zero dips in temperature occur thirty times more often in the Barrens than on campus, and below-freezing temperatures occur twice as often. In the daytime, the Barrens warm up. By midafternoon, the Barrens are as warm as the campus. But at night, temperatures plummet.

  The area, a narrow valley between steep hills, was called the Barrens because the soil was too sandy to support productive farming, a common naming practice. It was mined for iron ore late in the nineteenth century. The forest that had stood there was cut down and the wood turned into charcoal. The sandy soil was exposed. Scrubby plants grew into sparse patches of trees. Today houses encroach on the Barrens, but the heart of the place is owned by the state and managed for game. Heat escapes from the dry, sandy soil as soon as the sun dips below the hills. Cold air tumbles downhill into the valley and settles in for the night. A temperature of forty below was once measured in the Barrens — forty below zero in central Pennsylvania.

  I leave campus at seven thirty in the morning. It is thirty-six degrees. Ten minutes later, I am in the Barrens. A possum rests peacefully in the road, its gray fur dusted with frost. I open my windows, enjoying the cold. At a sign that says grouse hunting area, it is thirty-four degrees. At a shooting range a few minutes down the road, it is thirty degrees. The forest is still recovering from the work of woodcutters and charcoal production. The trees are uniformly young and scattered. My left ear, exposed to the full blast of wind coming through the window, is now comfortingly numb. In the heart of the valley, still fewer than ten miles from campus, it is twenty-six degrees, a full ten-degree drop from campus. My hands are stiff on the steering wheel. My left ear is beginning to hurt. I turn the car around, my curiosity satisfied, ready to migrate back to the warmth of campus.

  The poorwill — to the Hopi people, “the sleeping one” — hibernates through the winter, but it is an exception among North American birds. Another hundred or so of the continent’s birds tough it out through the winter, feeding frantically to supply the calories needed to stay warm. The other 550 species that breed north of Mexico migrate.

  A migrating bird might fly and fly and fly some more, past cities, above browning cornfields and forests, over vacant Gulf of Mexico waters, a night and a day and a night passing in the air, a few ounces of feathers and flesh and aerodynamics, finally landing, wasted, almost nothing left. Or if it is, say, a Clark’s nutcracker — a gray jay with a white face — it might go no more than a few miles, like a moose moving from a mountainside to a neighboring valley. If it is a mallard, it might hop south, moving from pond to pond one step ahead of the freeze line. Many birds stop in the southern United States. More than two hundred species go on to the beaches and coastal forests of Mexico. Others head for the Caribbean. A few dozen make it as far as the Amazon, overwintering with parrot friends. A few — the barn swallow, the upland sandpiper of the American grasslands, the Swainson’s hawk — make it to the Pampas of Argentina. Some change their migratory habits from year to year, wintering wherever they find the right balance between availability of food and the lessening of calorie-sapping cold. Great gray owls, very much northern birds, head south only if they get hungry enough. They showed up south of their normal range in the winter of 1978 and again in 1983. One made it as far south as Long Island. There, well outside their normal range, they were sometimes seen feeding in broad daylight, not behaving as good owls do. They were hungry.

  Aristotle believed that swallows hid underwater during the winter. He also believed that worms came from horsehair, and he thought that the European redstart transmogrified into the old world robin. For centuries, others saw the world through glasses tinted by Aristotle’s errors. As late as 1555, Olaus Magnus, archbishop of Uppsala, wrote of swallows, “They cling beak to beak, wing to wing, foot to foot, having bound themselves together in the first days of autumn in order to hide among the canes and reeds.” But Aristotle knew more than his widely publicized mistakes might suggest. “Others migrate,” he wrote, “as in the case of the crane; for these birds migrate from the steppes of Scythia to the marshlands south of Egypt where the Nile has its source…. Pelicans also migrate, and fly from the Strymon to the Ister, and breed on the banks of this river.” Other ancients, too, knew of migration. Homer said that cranes “flee from the coming of winter and sudden rain and fly with clamor toward the streams of the ocean.” The Old Testament reports “the stork in the heaven knoweth her appointed times; and the turtle [dove] and the crane and the swallow observe the time of their coming.”

  We continue to learn about migration. For example, each year the spectacled eider, one of the nation’s most beautiful ducks, disappears from the summering grounds of the Arctic and reappears each spring. Where does it go in the winter? Prior to 1994, no one was sure. For all anyone knew, it could have overwintered underwater, bill to bill and wing to wing. In 1994, biologists followed the signal of a transmitter implanted beneath the skin of a spectacled eider. They found the eider, and more than a hundred thousand of its cousins, secure in the pack ice, jammed together in a pond of open water surrounded by thick ice and bathed in Arctic winter darkness, with air temperatures of thirty below. The eiders’ collective motion and body temperature, it seemed, contributed to the maintenance of the open hole that was their home. They overwintered by feeding and paddling about in that.

  There is more to be learned. There are, for example, physiological adaptations. Not unexpectedly, birds put on fat, but in some cases nonessential organs shrink. Just before migration, the bar-tailed godwit becomes fifty-five percent fat, but its kidneys, liver, and intestines shrink. Then it flies nonstop at something like 45 miles per hour for days on end. The speed and exact route of many birds are not known. Migrating sea ducks tracked by radar in the Arctic fly at more than 50 miles per hour. A dunlin — a long-beaked shorebird — was once clocked at 110 miles per hour, passing a small plane. Other unknowns: How do they cope with man-made obstructions? How do they respond to the lights and noises of cities and ships and smokestacks? Do flashing light
s warn them away or just confuse them? In 1998, migrating Lapland longspurs came upon an antenna tower in the Kansas fog. Apparently confused by the tower’s blinking lights, they circled it, again and again and again. They ran into guy wires, into the tower itself, into one another. Before it was over, ten thousand were dead on the ground.

  Bird banding has been to avian biologists what the telescope has been to astronomers. A band — a metal or plastic tag — is clipped to a bird’s leg, pinned to a wing, or placed as a collar around a neck. Each band has a number and an address. The bird might be found dead, or shot, or captured by other banders, giving up information on its movements. Henry IV of England banded falcons at the beginning of the Little Ice Age. Duke Ferdinand banded a heron in 1669. John James Audubon is said to have been the first bander in the United States. Today a single banding station, staffed for the most part by volunteers, might band a thousand birds in a week. The U.S. government issues one and a half million bands each year. Something like sixty-three million birds have been banded, and just under four million bands have been recovered. A letter from China accompanying the return of a pintail duck band reads, “I feel very glad to wrote you. I did not know you and you did not know me. Who introduced I to you? It is your pigeon. She Flew To China. What a far way she flew! It is marvelous.” Less helpfully, other bands have been returned with notes asking for recipes.

  A returned band will say something about where a bird went but not how it got there. Bird navigation is no simple matter. Navigation skills are both learned and instinctive. Birds follow rivers and shorelines. They use the sun and the stars. They hear breaking surf. They may detect differences in air pressure. Some have magnetite in their nasal cavities — built-in magnetic compasses. Experimenters have done odd things. One looked at bird behavior in a planetarium, confusing his birds by turning off stars and star groups and entire portions of the sky. Another put electromagnets in birdcages. After all of this, nothing is completely clear. The truth behind migratory navigation defies generalizations.