When the Going Gets Cold

Winter weather, new studies show, can trigger some amazing survival tactics in animals

12-01-1990 // Bruce Fellman

A desolate December landscape lies under a somber sky. Icebound lakes crack and moan, and bitter air rattles the trees. Life itself seems to grind to a standstill in the icy dead of winter.

Yet this picture of lifelessness is an illusion, for behind the frosted facade is a world teeming with vitality. Bird feeders bustle with finches, titmice and chickadees. Woodpeckers splinter dead wood in search of dozing insect larvae. Mice and shrews scurry through passageways under the snow. Frost rimes breathing holes near burrow entrances of hibernating animals. Turtles haunt lake shallows. And on nights too cold for most insects, furry owlet moths thud against windows aglow with the lights of menorahs and Christmas trees.

"Winter is so alive," declares Peter I. Marchand, author of the book Life in the Cold. "There's plenty going on in terms of animal activity." Marchand is among the growing army of scientists who brave blizzards and bone-numbing chill to study how nature's inhabitants cope with winter's bitter punishment. In recent years, these researchers have made some remarkable new discoveries about the abilities of animals to survive when the mercury takes a nose dive.

Of course, there are those creatures that simply head south and avoid the issue entirely. But many others linger and live to see spring. Some that stick it out stay warm by keeping their internal engines at a high rev, a process known as shivering. Others go underground or even underwater. A number of resourceful species flood their bodies with natural antifreeze or mysteriously stave off formation of ice in their tissues even when the thermometer says they should be frozen. And then there are the amphibians, reptiles and insects that simply freeze solid and wait out the winter as living Popsicles.

Like people, most animals cannot tolerate icy intrusions in their tissues. Normally, when the temperature drops low enough, ice forms in delicate blood vessels and either bursts them or stretches them beyond a point where they can rebound. The main problem with freezing, though, is dehydration: All that water bound up in ice becomes unavailable to cells, which can actually die of thirst.

A popular defensive mechanism among some creatures, particularly insects, is what scientists call supercooling. This is the seemingly impossible ability of the body to maintain a temperature below the freezing point without actually turning to ice. Some hardy insects in Alaska can resist freezing in temperatures as low as 70 degrees below zero Fahrenheit by ridding their bodies of "seed crystals" like dust and bacteria around which ice can grow.

Of all nature's supercoolers, only one, the arctic ground squirrel, is a mammal. Research by Brian M. Barnes, a zoologist at the University of Alaska, has revealed that, during hibernation, the creature's body temperature can drop several degrees below freezing without harm to its tissues. So far, however, the mechanism behind the trick remains a mystery.

Recently, scientists have discovered that some animals—including wood frogs, box turtles and garter snakes—perform even greater magic. "They freeze solid," says Richard E. Lee, Jr., a zoologist at Miami University in Ohio. "If you open them up, you can see chunks of ice interspersed among their organs."

Lee and others have found that when the temperature drops and ice begins to form in the wood frog's toes and skin, the animal responds by flooding its body with glucose, a form of sugar that somehow protects the cells from ice damage. Even if we humans were able to pump enough glucose into our tissues, the high sugar levels would trigger diabetic coma and death. In wood frogs, the sugar increase also brings on a kind of coma, as the excess glucose slows cellular metabolism to near zero. But mysteriously, these amphibian ice cubes are not harmed by excessive sweetness. When the warm rains of late winter arrive, the creatures thaw out and nonchalantly hop to their breeding ponds, using the sugar to fuel the trek.

En route, some of the de-iced amphibians likely will cross paths with frog-eating garter snakes, the last of North America's serpents to retire for the winter and the first to become active when the weather breaks. The reptiles spend the winter in rocky hibernation dens where the temperature stays around 40 degrees. In the den, their heart rate drops to a scant half-dozen beats per minute (it's ten times that on a balmy summer's day).

The demands of raising a family and packing in enough calories to make it through next winter's snooze put the snakes on a tight schedule. "They're trying to get a jump on things, and that means they often encounter severe conditions," explains biologist Jon P. Costanzo, also of Miami University. "I've seen garter snakes basking on snow banks."

Nighttime temperatures outside the den often drop below freezing, and that can turn the snakes to ice. Luckily, a day or two in the freezer is not necessarily fatal. Though no one is sure what protects the snakes from frost damage, as long as the mercury does not linger too long below 32 degrees, a warm sun can revive them.

Costanzo discovered that garter snakes can also escape the cold by hibernating underwater. About a decade ago, on an abandoned farm in central Wisconsin, the biologist stumbled onto a scene that would have made Indiana Jones apoplectic: several hundred slithering serpents coming to in the spring at the bottom of a cistern. "I was terrified of snakes," admits the scientist. Yet the sight fascinated him, so he began to monitor the den. The snakes returned the following autumn, and come winter the cistern filled with water. How did the animals survive?

Costanzo believes they were using their skin as a sort of lung, straining oxygen from the water around them. Most scientists consider this so-called transcutaneous respiration too inefficient to keep the reptiles alive. Yet evidently it supplies all the oxygen a snake needs in 35- to 40-degree water. "Its heart rate slows to one beat a minute, and its metabolism drops tremendously," says Costanzo.

As little as a garter snake "breathes" underwater, it is practically hyperventilating compared to the respiration rate of adult painted turtles. Once these common reptiles settle into the shallows for the winter, they may not take another breath for five months. "As long as they're underwater, they avoid freezing," says Brown University physiologist Donald C. Jackson. "But the turtles pay a price."

That price is lost access to almost all oxygen. Like garter snakes, painted turtles can strain tiny bits of oxygen from the water. For most of their energy needs, though, their cells rely on the chemical breakdown of carbohydrates. When there is no oxygen available, it's a rather inefficient process that also creates an unwanted byproduct: lactic acid, the same chemical that causes cramps in humans. Though lactic acid may not give turtles aches and pains, too much can cause a fatal chemical imbalance called acidosis.

Fortunately, the combination of cold water and low oxygen causes the turtles to shift into low gear. "They're very sluggish," says Jackson. "Their heart rate can be as low as one beat every 10 to 11 minutes; it's 30 to 40 beats per minute on a summer's day." And because the creatures' cells have little to do, they put out very little lactic acid.

Even with the slowdown, lactic acid accumulates in wintering turtles, yet few die from acidosis. That's because over the past 200 million years the creatures have developed the reptilian equivalent of Rolaids. Using calcium carbonate from their shells, they neutralize most of the excess acid. What they can't purge they tolerate.

Warming water temperatures let the turtles know when it's time to greet the spring. In much the same way, air temperatures appear to help regulate how long burrowing mammals stay underground. In a laboratory at the State University of New York in Binghamton, bats, ground squirrels, chipmunks, jumping mice and marmots snooze the winter away in cold, dark, artificial dens while ecologist Alan R. French scrutinizes their every move.

"These are the most boring experiments you'd ever want to look at," French admits. "The animals don't do much." Every so often, though, they do something surprising: Their body temperatures, which have dropped to near freezing, shoot back to normal, and they wake up for anywhere between a few hours and a day.

French has noticed correlations between an animal's size and how long it sleeps, how often it wakes and how much time it spends active in the den. Basically, the bigger the animal, the longer it spends underground at high body temperatures. The reason, he says, is that as animals increase in size, their ability to store fuel in the form of fat increases faster than the rate they use that energy.

French found another intriguing behavior among creatures like pocket mice that hibernate with caches of seeds. "How often they wake up depends on how much food they've stored," he says. "If I gave a pocket mouse 800 grams of seed, he'd wake up every day. But if I gave him 100 grams, he would remain torpid for five days at a time."

Then what's the point of waking up? After all, hibernation evolved as a way to conserve energy. And 80 to 90 percent of the energy these animals use in the winter is consumed when they are awake. There must be a good reason to burn up the precious fuel it takes to raise the body temperature. Perhaps, French suggests, getting up helps an animal keep track of time.

The researcher explains that when the Belding's ground squirrels (a species found in high mountain ranges) in his lab become active, they immediately scurry to the top of the burrow and touch the plug of dirt they've used to seal the den's entrance. The rodents make the trip once or twice in the roughly 12 to 16 hours they remain awake. "They're very sensitive to temperature changes in the soil," says French. To these mammals, warm dirt equals spring, he explains. So he experiments by wrapping the burrow "plug" with heat tape and raising the temperature. "When my squirrels feel that, they dig right out."

In deepest winter, the squirrels stay asleep for long stretches. Then, as good weather approaches, they rouse themselves with increasing frequency, stirred by some kind of internal alarm. Researchers have tried to isolate this biological clock for years, but French suspects the wake-up calls are coming from more than just a preset alarm. "Animals at cold body temperatures may be building up some sort of toxic waste product," he suggests. Turning up the thermostat periodically may help burn off these bodily poisons.

Virtually all cold-weather animals have some kind of natural defense to ward off the big chill. Many dress for the occasion. Downy feathers, fur coats, layers of blubber—all help insulate the wearers from the elements and minimize the work of the internal heating plant. A few members of the rodent, shrew and rabbit families produce a specialized high-energy substance called brown fat when the mercury drops. The fat is brown because it is packed with mitochondria, the microscopic engines in cells that convert food into energy, and its only function is to generate heat.

Other animals have the extra advantage of a system of veins and arteries called the rete mirabile, which delivers warm blood to areas exposed to frigid air or water. This "miraculous net" helps distribute warmth to whale flippers and duck feet, and it also helps enable a furry creature called the owlet moth to venture out when few other insects are traveling.

These chunky, inch-long bugs often seen mobbing houselights in winter are specially engineered for the cold. A heavy covering of what biologists call "pile" is a toasty moth parka. The insects have "air bags" to insulate their warm chests from their cold abdomens and, most amazing, they can create their own warmth.

At rest, the owlet's temperature may match that of the air. But when the moth needs to fly, it must crank up the thermostat to around 80 degrees for its flight muscles to work. How? It starts to shiver.

A shivering creature is like a car in neutral with its engine racing. The muscles are contracting all at once, yet the animal is going nowhere. It is, however, generating a lot of heat, and as long as the owlet moth can trap that warmth, its muscles can reach proper flight temperature. With its system of generating and retaining heat, the moth is more like a bird than an insect, though it cannot shiver for much more than an hour. And while this may be championship duration for an insect, it is minor league compared to the shivering prowess of a finch.

"Except when they're flying, finches basically shiver all winter," says Cynthia Carey, a University of Colorado biologist. Recently, Carey discovered that goldfinches have evolved a physiological trick to help them stoke their furnaces more efficiently. "In the winter, the birds switch from principally using carbohydrates to using fat," she explains. "This prolongs the amount of time they can shiver." The finches also are selective about the muscles they enlist for shiver duty. "When you're only moderately cold, you don't need the heat-producing power of every muscle," says Carey. "If you're very cold, you pull out all the stops."

Researchers are still busily chipping away at the secret defenses of winter's survivors. The more we learn, though, the clearer it becomes that, at least for now, the variety and complexity of nature's exquisite thermal dynamics still vastly exceed our ability to understand them.

Winter is never cold enough for Connecticut writer Bruce Fellman, whose chief natural defense is the fur on his face.

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