When the weather turns so frightfully cold that we don parkas and boots before venturing outside, how do plants and animals survive? They can’t add layers of clothing. They can’t toast their tootsies before a fire. They’ve never even heard of hot chocolate.
Scientists focus on two coldweather survival strategies that are used by living organisms, including man: avoid the cold if you can, and if you can’t, hunker down and endure it. Each plant and each animal devises its own variation on those themes.
Among the most obvious ways to avoid winter’s chill are death, migration, and hibernation. Death seems a bit extreme, yet many life-forms do die when cold weather arrives. Annual wildflowers squeeze their entire life cycles of sprouting, growing, flowering, and setting seeds into the months between spring and autumn. Literally billions of plants die when winter comes, but not before they have insured the continuation of their species.
Many insects follow this same pattern. After wintering as eggs, they hatch, grow, mature, mate, and lay eggs before cold weather resumes; then, they die – fortunately. If those insects survived into a second spring, their appetites, added to that of the newly hatched young, would soon overrun the world food supply.
If an organism is to complete the cycle of seasons, it needs a more sophisticated strategy than that of dying off to leave room for the next generation. Consider the tactics of broad-leaved trees. They can’t grow to full size in a single season, so they cope with winter by dropping their leaves and entering a form of botanical hibernation. Metabolic activity continues (as it must to preserve life), but it operates at a reduced level and only in the protected core of roots that are hidden safely underground.
Because they can move around, animals have an advantage over plants. Some birds fly south, perhaps traveling thousands of miles. Other birds and many mammals migrate from the highlands to the valleys during the winter, returning only when insects hatch or when grasses and leaves sprout the following spring.
Cold-blooded animals and some mammals endure rather than escape the cold. They hibernate, drowsing through the winter in sheltered dens. As they become inactive, their body temperatures drop, enabling such animals to conserve energy. This lowering of body temperature would be dangerous if it went too far, but these creatures are equipped with a physiological warning device that awakens them before they reach a danger point. At that point, they rouse and move around enough to raise their body temperatures again.
Reptiles – ectothermic animals whose body temperatures depend upon external sources – have no such alarm device, so their only hope of survival is to hibernate deep enough to escape the frost. Bears, in contrast, rely upon fat and fur as insulation and select dens that offer little more than protection from the wind.
For decades, black bears were considered not to be true hibernators, because they maintain a relatively high body temperature, even during the winter. Now, scientists recognize these bears as highly efficient hibernators. They sleep for months without eating, drinking, or eliminating wastes. Hibernators with lower body temperatures, such as ground squirrels and chipmunks, awaken every few days in order to move about and raise their body temperatures.
Black bears living in the north country commonly go for seven months without eating (mid-September to mid-April), but in order to survive the winter, they must gorge themselves during the summer. Bears that don’t find sufficient food before going into hibernation will lose weight and starve. A diminishing food supply, not the onset of cold weather, drives bears into their burrows. A bruin selects a protected site, drags in grasses or moss to provide a cozy mat, and then curls up for a long winter’s nap.
Not all warm-blooded animals hibernate, of course; a variety of other mechanisms allow animals to retain enough body heat to survive through the winter. Some birds respond to cold by restricting the flow of blood to their extremities, thereby retaining the blood’s heat within their bodies’ inner core. To cite one example, many species of waterfowl can cut off virtually all blood flow to their legs, which minimizes heat loss to either the water or the ground.
Shivering, an automatic vibration of body muscles, is another way in which warm-blooded birds and mammals outsmart the cold. One Minnesota researcher observed that American goldfinches that winter in his area literally shiver all season long. Shivering burns prodigious amounts of energy, thereby requiring these little birds to eat almost constantly. However, several other methods of adapting to the cold conserve rather than produce heat, and, as such, are considered passive methods of insulation.
Fat, when deployed in even layers beneath the skin, has a high insulation value. That’s one reason bears and other mammals put on pounds before winter comes. Polar bears and husky dogs have such an efficient covering of fat that they can sleep comfortably on ice.
Fur and feathers provide insulation, as anyone who owns a down parka or a fur coat will attest. An animal’s winter coat must be both thicker and more fluffy than its summer coat in order for it to trap a layer of heated air next to its skin.
Nests also shelter animals from the cold. By first selecting a protected location and then lining the nest with feathers or fur, the architect produces a snug little home. Small birds that live in cold climates face special problems dealing with the cold, but the pygmy nuthatch has found the ultimate solution. While traveling in northern Arizona, we met a graduate student who was researching the nesting habits of this tiny bird. He told us that he’d counted more than a dozen pygmy nuthatches piled atop each other in a single nest. How those on the bottom failed to smother, he couldn’t say, but in spite of subfreezing temperatures, the birds appeared to be warm.