 |
The woodchuck settles down either in a grass bed at the end of his tunnel or in an unlined side chamber. He has an effective way of saying “Do Not Disturb” to the opossums, skunks, or rattlesnakes that would like to share his snug winter quarters. He buries himself alive by sealing off his sleeping chambers with dirt scraped from the far end of the room. Then he’s as safe from unwelcome visitors as a hotel guest who’s put a DO NOT DISTURB sign on his door. Once privacy has been ensured, the woodchuck rolls up in a ball, head between his hind legs.
Breathing slows down until it almost stops, and the pulse becomes faint. The animal gets colder and colder until finally his temperature drops to somewhere between 40 and 57 degrees Fahrenheit. Now the woodchuck is in true hibernation, insensible to touch or sound, his biological fires banked.
Ground squirrels, bats and badgers, snakes and turtles, frogs and salamanders, mosquitoes, spiders, crayfish, and even worms drop into this long-lasting and deathlike sleep, during which they live off accumulated fat. So anesthetized are they that they scarcely bleed, even if a limb is amputated. A ground squirrel dug out of a burrow remains as limp and inert as if its neck were broken. Insensitive as an Indian fakir on his bed of spikes, it can be shaken, dropped on a table, and even used as a pincushion. Revival is a slow process requiring prolonged exposure to heat.
The winter sleep of a bear, on the other hand, may be only fitful, and his body temperature doesn’t nose-dive when he beds down. It stays high enough to melt any snow drifting into his hideout—cave, windfall, or swampy thicket. Because of this and because his breathing stays at a normal rate (four or five times a minute), the bear’s big sleep is not considered true hibernation.
A bear is easily roused from his winter map. He may even wake up of his own accord and come out of his chosen haven to prowl around for a few hours, days, or even longer. Bears that do this winter-walking are often ones on the slim side. It’s the bruins bulging with fat who den up early and sleep the winter through. Hunters have learned through dressing fat bears killed at the beginning of winter that the stomach is empty and shrunken into a tight, hard knot, which prevents eating anything more. Apparently when a bear reaches a certain state of fatness, this contraction of the stomach acts as an automatic food shutoff, and he is ready for sleep.
Polar bears flout the conventional attitude toward the big sleep taken by blacks, browns, and grizzlies, all meat-eaters who like an occasional mess of greens, berries, or comb of wild honey. In a polar-bear family only the expectant mother hibernates. She settles down early in the winter, and during her dormant period gives birth to twin cubs. And since only expectant mothers hibernate, it looks as if the polar bears’ reactionary attitude toward hibernation contradicts two theories about it: one, that bears hibernate because of cold weather, the other that they hibernate because of food shortages.
Captive bears also contradict these theories. A New Hampshire farmer and amateur naturalist kept a pair of black bears in a well-built barn. Naturally the bears were sure of food and warmth throughout the seasons. But in the fall they buried themselves in straw, with only their ears sticking out. Paws were wrapped around their heads, which were dropped down and forward. If spoken to, the bears poked their heads out of the straw, looked momentarily interested, yawned, and then snuggled down again.
A certain amount of body conditioning goes into a bear’s preparation for the long winter nap. Guides on Kodiak Island, Alaska, home of the great Kodiak Brownies, say that just before the bears go into their kind of hibernation, they gorge themselves on wild cranberries, which act as a purge.
For the entrée of this prehibernation dinner, the bears eat fibrous roots. These form a tough plug, the so-called tappen, which remains in the bear’s system until the end of the big sleep the following spring. Black bears in Minnesota have plugs composed of pine needles and hair apparently licked from their own coats. According to some hunters, the completeness of this plug determines the completeness of the winter’s sleep.
Many vertebrates other than woodchucks and bears indulge in the big sleep, and the range of hibernation is great. Bats spice their lives with a varied hibernation pattern. The kind known as Leisler’s bat sleeps continuously, while the pipistrelle, pale yellowish brown and weighing less than an ounce, is an intermittent sleeper.
One of the most eccentric of the true hibernators ever watched was a common hedgehog, an insect-eater of the Old World. This particular hedgehog was Irish and as full of surprises as most sons of Erin. It roused from its deep winter sleep only on the coldest nights. These highly unorthodox appearances decreased progressively until February, when the hedgehog stayed put until spring.
The dormouse is notorious for its urge to sleep. In fact, the first syllable of its name seems to come from the French word dormir, “to sleep.” Lewis Carroll described it well in Alice In Wonderland, for all through the tea party, the dormouse yawned and rubbed his eyes, and dropped off to sleep from time to time. This habit so annoyed the Mad Hatter and the March hare that they tried to stuff the sleeper into the teapot to wake him up. By late September, the dormouse becomes exceedingly fat from his preferred diet of nuts, and in October and November, having finished building his winter nest and laying in a store of food (for a snack in case of a highly improbable wakeful interval), he settles down for a long winter nap, which may last six months. The breathing of a dormant dormouse can hardly be noticed, and he becomes so rigid that he can be rolled across a table like a ball. One can be aroused in about 20 minutes, but if the awakening is by too rapid exposure to heat, death follows almost at once. Certain African dormice only hibernate when brought to Europe.
A big sleep is the rule for reptiles in temperate climates. Land tortoises bury themselves in burrows or dens; water tortoises in the mud on the banks of streams and the bottoms of ponds. Many snakes and lizards go into hibernation by retiring underground, digging into the soil even though they may not ordinarily be given to burrowing. Some find seclusion in crevices in the rocks.
Most amphibians—frogs, toads, newts, and salamanders—seek moist places in which to retire, some of them underground, some of them, like the California arboreal salamander, in the cavities of trees. Common toads kept in a greenhouse will remain active throughout the winter but take time off for an occasional nap of a few weeks, showing that they need complete rest from time to time even when seasonal conditions do not demand it.
Some invertebrates hibernate, too. The forest, a noisy place in summer, is lacking in sound in winter because the noisemakers are deep in sleep. The young queen bumblebee may lie hidden in the mossy bank of a stream whose gurgle is silenced by a sheet of ice. During brief cold (or very hot spells), trap-door spiders seal their burrows with silk and mud, but it can scarcely he called true hibernation. Certain other spiders attach silken shelters to the under sides of rocks and remain in them as long as the cold prevents activity. Some mosquitoes in temperate regions pass the winter in caves like bats and live off the fat stored in their bodies. Hibernating females have lived more than two months without food in caves where temperatures were in the fifties.
Land snails like the big sleep, too. The common garden snail hides himself in some cozy cranny late in autumn and doesn’t come out until the spring sunshine has warmed things up a bit. Other snails batten down for winter under stones or dead leaves or bury themselves in fissures in rocks or in the earth. All shut the shell by a disc that fits as closely and tightly as a manhole cover and keeps out the cold.
Many fresh-water fishes hibernate in the temperate zone. Whole schools of carp, for example, retire to the muddy bottom and remain partially buried and dormant until the thaw. It has been found that the body temperature of a carp can go down to almost one degree centigrade below freezing before the fish is frozen to death. At the freezing point of water, the fish may appear dead, because its respiratory movement has ceased and it is in a state of torpor. But experiments have shown that the fish can be kept in this state many days and be revived quickly when the temperature is raised.
Among hibernators, of course, there are records. A mother bear and her three cubs stayed in a den for three and a half months. The all-time high in hibernation was chalked up by a young female ground squirrel. She slept for 33 weeks out of the year, thus outclassing all other mammals in the sleep marathon.
Not all animals hibernate. Instead of sleeping in winter, some animals estivate, or pass the summer in sleep. The tenrec, a Madagascar mammal with a fondness for earthworms, takes a long summer siesta during the hottest weeks of the year when earthworms are in short supply. Tenrecs in the London Zoo behaved just as if they were at home, though there was neither a heat wave nor lack of food.
Estivation is forced upon some fishes and reptiles by the drying up of water. Crocodiles and alligators bury themselves in the mud until heavy rains release them. The Iberian water tortoise retreats under a ledge of rocks during a dry spell and stays there—a torpid tortoise for months on end.
A fish that goes for summer sleep in a big way is the African lungfish, a swamp-dweller. As the swamps dry up, the lungfish takes itself out of circulation until the next rainy season. It dives down into the mud for about 18 inches. Then it bends its body around until its tail covers its head. A layer of mucus exuded from the skin forms a lining around the flaskshaped bottom of the burrow and envelops the fish’s body. Around the lips the mucus forms a tubular funnel leading to the mouth. This lets air pass to the lungs. Thus set, and with extra rolls of fat for nourishment, the lung-fish passes into a state of suspended animation. Other fishes that estivate are the Indian climbing perch, the gouramis of southeastern Asia and Malaya, the Indian "serpent heads," and some catfishes.
|
|
OW it’s the time of the big sleep for the bees, the bears, and even the buds of the plants that shed their leaves. And the intensity of this winter sleep, or hibernation, depends on who’s doing it.