Summers in the far north can be a time of lush and lavish living for the animals—plenty of fresh new plant growth for the vegetarians, lots of insects and nectar for birds, berries and salmon for bears, and for wolves there's often an abundance of voles, mice, and snowshoe hares. The endless days of nonstop eating allow many animals the luxury of putting on fat.
But summers are fleeting and winters long—up to 8 months—and brutally cold. Temperatures can plunge to minus 70 degrees. Gales make it feel even colder, and deep or drifted snow can make simply moving around an energy-draining ordeal.
Of course, if you can fly, one choice is to opt-out of winter all together.
Alaska's migratory birds do just that. Some birds, like loons whose summer lakes are frozen in winter, only go as far as the coasts of Alaska, Washington, and Oregon. Robins only need to reach places with frost-free ground, so they can hunt for worms and insects. Other birds go for the relative warmth of the southern US, and some take advantage of the tropics and subtropics—enjoying two summers.
Arctic terns take migration to the ultimate extreme—after a summer in the far north they spend their "winter" in Antarctic waters, so these amazing birds live most of their lives in perpetual daylight.
But what about the animals who stay?
Some birds, like ptarmigan, stick it out through the long and unimaginably tough northern winters. And most other animals—the walkers and crawlers and swimmers—simply don't have the option of leaving.
To stay and tough it out—to survive at 40…50…60 degrees below and colder—animals have evolved some ingenious and fascinating adaptations.
At the opposite extreme from the arctic terns that span the globe in pursuit of an endless summer, is an inconspicuous little amphibian called the wood frog. When leaves drift on the autumn wind, the wood frog finds a suitable place to spend a quiet winter, digs down into the forest duff, and as the surrounding temperature drops he proceeds to freeze solid. No heart beat, no breathing, no signs of life—a tiny node of suspended existence, a perfectly inert frogsickle—buried under moss and snow for up to 8 months.
Scientists are studying this miraculous little creature, not only because it can freeze solid but perhaps especially because it thaws out and emerges completely unscathed in spring. We don't completely understand the wood frog's ability to survive the seemingly unsurvivable, but we do know that their livers produce a flush of glucose which protects the vital inner parts of their cells from freezing, which would cause lethal and irreversible damage.
In the spring, as the wood frog begins to thaw, its glazed frosty eyes transform to gold and ebony, its tiny heart spontaneously begins to beat, its throat pumps and its lungs take in the first breath of a resurrected life. Once the wood frog is fully warmed, it's ready for all that summer has to offer—hopping to a nearby pond to sing and mate and lay eggs, then moving back into the wet, mossy forest to feed on insects and enjoy the warmth of the endlessly circling sun.
A little less extreme, but equally impressive, are animals that get through the winter by hibernating. The arctic ground squirrel, which huddles in an underground burrow for 7 to 8 months of the year, is a hibernator extraordinaire. When an animal truly hibernates, its body temperature drops and all metabolic functions slow way down. What's unique about arctic ground squirrels is that their body temperature falls below freezing—the only mammal on earth to do this and survive.
Researchers have measured artic ground squirrel body temperatures as low as 26 degrees Fahrenheit. But it's not as simple as chilling down in autumn and warming again next spring. As the cold months pass, an arctic ground squirrel goes through cycles of deep cooling that last for about 24 days. Then it spontaneously begins to shiver, which raises its body temperature back to the normal 98 degrees. After about half-a-day, it slowly cools down into another long cycle of bone-cold torpor that lasts until the next brief rewarming.
We usually think of grizzly bears and black bears spending the winter months "hibernating in their dens", as if they were in a coma. Denning bears experience lowered body temperature, heart rate, and respiration, but they are not true hibernators. Their body temperatures don't drop as low as animals that hibernate, like the "super cooled" arctic ground squirrel. A wintering bear is in a deeply lethargic state, not the deep sleep of a true hibernator; and a seriously disturbed bear can become active even in the middle of winter.
Both arctic ground squirrels and bears live off their stored body fat in the winter.
Some animals spend the winter fully awake and active, but they rarely venture out into the open air and sky. The key to their winter lifestyle is a cozy home and a very busy summer. During the fleeting time of endless daylight and lush growth, these animals store enough food to see them through 8 or 9 months of winter.
The hard working, plan-ahead pika furtively darts about the rocky summer slopes, cutting grass and other vegetation, and then scurries back to its storage places with big mouthfuls of loot for the stockpile. Food is stacked in crevices and under rock overhangs in such a way that air can get in and "cure" it.
Pikas look like miniature rabbits with short round ears, wrapped in thick gray fur and scampering around on fur-covered feet. But there's not much fat on their 5-ounce frames, so to keep their metabolic furnaces stoked, they need to eat well all winter.
Pika "haystacks" can build up to a two-foot square pile by the end of summer. They might look innocent, but these little hyperactive leprechauns are not above stealing from each other's hay piles and they get quite defensive later in the season. With good reason—a pika depends on its haystacks to survive the months of bitter cold and mountain gales.
Another animal that stores a whole winter's worth of food is the beaver. This highly successful northerner could not survive the frigid outside temperatures for long —imagine coming out of the water, soaking wet, into sub-zero air! But beavers have a warm and lazy winter because they prepare well for it. All summer, they cut branches of willow, aspen, poplar, alder and birch and store them in their underwater feed pile near the den or lodge. When the pond or river freezes, they swim under the ice from their home to the stockpile, grab some food and head back, without ever being exposed to the lethally cold air.
Like the pika, the beaver labors all summer, even getting skinny from overexertion, making use of the endless arctic days to ensure enough food for the long winter ahead. So these animals exemplify one of the most important qualities for success in the far north—foresight.
For animals that spend the winter outside, fully exposed to the deep cold and blizzards, sleeping through the whole thing is not an option and stockpiling enough food may be impossible.
The survival strategy for these animals has much to do with conserving body heat—which usually means having specialized fur…and lots of it.
Moose and caribou have very dense fur, and the outer or "guard" hairs are hollow, so each one traps a tiny pocket of air—adding up to millions of them on every animal—providing exceptional insulation. These animals also have soft, wooly underfur that fills the spaces between the guard hairs to add more warmth. Without its dense coat of beautifully designed fur, an animal could not survive exposed to the pervasive and often wind-driven cold of the far north.
As far as food goes …picky eaters need not apply.
The lush green growth of summer is a faint memory and many plants are buried under three to six feet of snow. Moose can live by browsing the twigs, branches, and bark of trees and shrubs; and they paw down through snow to find smaller plants. Caribou also dig through the snow for small dried plants, twigs and sticks, but they most prefer lichen, which they also find under the drifts. Sprawling areas of snow covered with small craters are a sure sign that caribou have been digging for food.
Moose and most caribou winter in the shelter of the boreal forest. Although temperatures are actually colder than on the open tundra or mountains, the trees offer protection from fierce winds and blizzards. Powdery snow can be very deep in the forest, however, and moose sometimes struggle to move around. Their long legs post-hole into the snow, just as a human sinks down without the benefit of snowshoes. During snowy winters, moose tend to stay in one area, restricted to their own compacted trails. Less movement helps to conserve energy, but at these times moose are highly vulnerable to predators like wolves.
Caribou are specially equipped for snow travel because they have wide, splaying hooves, which help them to stay on the upper layers of snow. Their hooves also make good snow shovels for digging around for lichen and other plants.
While moose and most caribou spend their winters in the boreal forest, another animal—the extraordinary muskox—stays out on the tundra fully exposed to the arctic gales, seemingly impervious to cold. How do they do it?
Like caribou and moose, muskox also have two layers of fur. The outer hairs are very long and give them a shaggy appearance, and the underfur, called quiviut in Eskimo, is actually many times warmer than sheep's wool. Muskox are almost totally covered in fur—only their lips and nostrils are exposed. They have very compact bodies, which helps to conserve body heat, and they lie down or find sheltering ravines when blinding storms rage across the tundra.
For access to food, muskox actually depend on those arctic gales, which keep the ground free from deep snow. They use their cloven hooves to dig down to the dried tundra plants. They can also use their heads—a bit like a mallet—to break up hard, crusted snow.
And what about the ptarmigan, one of the few birds that stays for the winter while most of the feathered ones high tail it southward? These uniquely adapted birds have feathers on their legs and feet, which not only helps to keep them warm but also gives them the advantage of built-in snowshoes for walking on top of the snow. When it's really cold, ptarmigan dive or tunnel down into the snow and make a snug cave where the temperature can be more than fifty degrees higher than the frigid air above. Fresh snow is a great insulator—typically it's 90 to 95 percent air, so it acts something like the caribou's hollow hairs. And dinner for ptarmigan? Seeds, twigs, buds of trees and whatever they can scratch out of the snow.
There's one other problem of winter…everything is covered in white. If you are a prey species, the last thing you want is to stand out like a bull's eye against a pure white background. So for ptarmigan, artic fox, and snowshoe hares, the best strategy is to blend in by growing white fur for the winter.
The disguises aren't quite perfect though. For example, snowshoe hares and arctic foxes have dark eyes. And the ptarmigan? A Koyukon riddle tells the tale:
Wait, I see something: Tiny bits of charcoal scattered in the snow.
Answer: The black bills of ptarmigan.
Whatever the means or method… animals that live through the extreme cold of far northern winters show us marvelous and fascinating examples of evolutionary adaptation. And for those who make it through the long, deep cold—the lush and lavish summer awaits.