Should We Let Them In?
by Kurt Buermann
It’s been especially cold this winter. As I look out the window, I empathize with the wild creatures who visit our yard. Sometimes I wonder if I shouldn’t let them all into the house for a while to warm up. I can’t see that it would be much trouble. The raccoons, who have surprising manual dexterity, could open the cupboards and refrigerator, and help feed the rest.
But before rolling out the welcome mat, I decided to do a bit of research and find out just how animals do manage to cope in the snowy world outside. As it turns out, animals have a good assortment of winter survival tools.
Surviving cold is all about conserving energy. An animal will make use of any available insulation and heat source. Special layers of fur or feathers grow, triggered by cooler weather and shorter daylight hours. Some animals dig dens. Squirrels fashion dreys—large enclosed nests secured in tree branches. (They give birth in these as spring approaches.) Deer huddle together in deer yards, depressions they form in the snow. In winter their digestive systems change to allow them to eat what winter offers—bark and dead leaves. Sometimes well-meaning folks put out food such as corn or fruit. This can be fatal to deer, whose digestive systems are attuned to winter forage. They cannot suddenly switch back and digest summer foods. Thus a deer may in fact starve with a full stomach!
HibernationAnimals huddle together in their refuges to exchange mutual warmth. Chipmunks have been known to den with rattlesnakes, taking advantage of the reptiles’ seasonal dormant condition as well as their slight body heat.
Another way to cope with winter is to enter a state of dormancy: hibernation, torpor, diapause, or brumation. All involve slowing down the metabolism, the difference being one of degree. Respiration and heart rates drop, and the body cools down.
For example, bears will lower their body temperature, slow their life processes (metabolism), and enter a sleeplike state. Interestingly, bears sleeping for long periods do not have to wake up to drink water or excrete wastes. They actually have body processes that reuse waste products. Water is extracted from the stored body fat, which also nourishes them. Bears do not sleep as deeply and continuously as some other mammals do. Like raccoons and opossums, bears may rouse during the winter if conditions are right.
Groundhogs (woodchucks), though, are super hibernators. While a bear’s temperature drops about 12 percent from its normal level in preparation for dormancy, a groundhog’s can drop 40 percent, to near freezing. Perhaps this is because the groundhog relies on an all-plant diet, unavailable in winter. Animals whose body temperature does not drop so low, such as raccoons and opossums, may sometimes rouse to scrounge something to eat if an occasional warmer day occurs.
Birds practice torpor. They lower heart rate and temperature. The period of torpor is usually limited to hours rather than days or weeks. Birds may exhibit torpor only in the dark hours, rousing to feed when the sun warms things up a bit. Birds also resort to shivering to maintain warmth. And don’t worry about birds getting cold feet in the snow or as they swim in chilly water. The scales on their feet are especially designed to prevent heat loss, and a special arrangement of blood vessels allows the legs and feet to function at lower temperatures than the body.
Reptiles are brumators, who must rouse for intermittent drinks of water.
Aquatic animals like amphibians not only decrease the need for fuel, living on fat reserves, but also decrease their need for oxygen, allowing them to winter underwater, often in the mud on pond bottoms.
Of note is the wood frog. When the temperature drops to freezing and below, the wood frog tucks in under leaf litter and is able to freeze solid! This tactic in triggered as soon as the frog senses the presence of ice in the environment. Its amazing ability is made possible by the body’s production of glucose. An ingredient of auto antifreeze, glucose prevents the water in cells from freezing and causing the cells to burst.
Insects hibernate in a state called diapause; for them, everything may grind to a halt. Development is suspended, sometimes in a particular stage of metamorphosis. Diapause can help them survive winter cold, but it may also take place if temperatures are too high or other conditions—water, food sources—become too unfavorable.
So I guess the critters are doing all right out there. Much to my wife’s relief, our house will not have to serve as a hibernaculum just yet.