Adapting to Winter’s Challenge

Vol. 8 Issue 4, Fall 2003

By Cliff Fairweather

Winters in Loudoun County are pretty mild compared to more northerly regions, last winter notwithstanding.  Nonetheless, winter here still poses a challenge to the survival of wildlife.  For most animals, energy is harder to obtain and easier to lose in winter than in any other season.  Energy conservation to maintain a balance between energy gains and losses during winter becomes the key to survival.

Solar radiation, the ultimate source of energy for most life on earth, is much less abundant during winter.  This is because the northern half of the earth is tilted away from the sun in winter, causing solar radiation to diffuse over a larger area.

To see how this works, shine a flashlight straight at a wall from a few feet away and notice how much of the wall the beam covers.  If your flashlight beam were a sunbeam, its radiation would be concentrated on this relatively small area.  Now, tilt the flashlight up a bit to give the effect of the earth tilting away from the sun.  Note how much more of the wall is being hit by the same beam of light.  If this were radiation from sunlight, it would be spread over a wider area.  With a lower concentration of solar radiation, photosynthesis stops or slows and most plant growth ceases.  Since plants form the base of the food web, this means less energy available for animals ― from insects to birds and mammals.

Not only is less energy available for wildlife in winter, but conserving energy becomes more difficult.  Cold, wind, snow, rain, ice, and longer nights can all tax an animal’s energy reserves at a time when replenishing that energy becomes more difficult.  Animals have three basic strategies for coping with the rigors of winter:  migration, dormancy, or resistance.  Of course, things are never so cut and dry in nature, and many animals use some combination of these strategies.

Migratory animals move to where they can find a better food supply.  Flight is a much more efficient means of long-distance travel than walking.  Not surprisingly, then, our local migratory animals all fly, including many birds, some bats, and a few insects.  While a lot of migratory species leave Loudoun and head south for the winter, this area is the “south” for others.  The season brings with it such winter resident birds as winter wrens, white-throated sparrows, and kinglets.

Hibernators reduce energy demands by lowering their metabolism.  For example, woodchucks reduce their heart rate to a few beats a minute and let their body temperature drop to only several degrees above freezing.  Black bears, on the other hand, maintain a relatively high metabolism during hibernation.  Other winter-dormant animals allow their body temperature to match the surrounding air.  Many reptiles and amphibians choose an over wintering site protected from sub-freezing temperatures, such as an old mammal burrow or the mud at the bottom of a pond.

Stored fat fuels hibernating animals during their dormancy.  Smaller hibernating mammals, such as chipmunks, cannot store enough fat to get them through an entire winter.  They must regularly replenish their fat reserves from food cached in their burrows.  Some animals, however, can actually tolerate sub-freezing temperatures.  Ice crystal formation in cells is the killer at such temperatures, but a few frogs, reptiles, and insects produce their own antifreeze.  Wood frogs can allow up to 40% of their body to freeze.  Glucose released from their livers prevents the formation of ice inside their cells.  Within hours of thawing, a wood frog is back to normal and even ready to mate!

Many insects and some other animals can avoid freezing by super cooling.  Super cooling involves eliminating as much water from their bodies as possible, breaking what water remains into tiny droplets, and removing impurities around which ice crystals might form.  A super cooled animal must also remain absolutely still.  Any disturbance while in a super cooled state will cause flash freezing and immediate death, making this a sometimes risky strategy.

Animals that remain active throughout the winter have evolved a variety of adaptations to resist winter’s impact on their energy balance.  Adaptations to conserve energy become particularly critical to their survival.  Storing fat gives many animals greater ability to resist winter by providing them with an energy reserve when food is scarce.  Fat storage begins in summer when food is more abundant, and most animals can find enough to meet both their daily needs and to store some as fat.

Larger animals, such as white-tail deer, have an advantage over smaller ones because they can store fat faster than they use it.  As a result, they can go longer without eating.  Smaller animals need to eat more frequently and many cache a surplus for winter.  Red squirrels, for example, cache nuts and pine cones.  Finding their mid-dens of nutshells or chewed pine cones in winter is a good sign that red squirrels are nearby.

Some mammals, especially rodents, have a kind of fat called brown fat that they can use to produce heat quickly.  Normal fat cells need to fuel some activity, such as shivering, to produce heat.  Brown fat can be turned directly into heat energy without any physical activity.

Nothing beats a good coat for getting through the winter and mammal fur is about the best natural coat around.  Fur consists of two layers: long, stiff, coarse outer guard-hairs and finer, shorter, often wavy under fur.  Under fur traps a layer of air within it which warms up from the animal’s body heat.  The trapped, warmed air reduces conductive heat loss, that heat loss to the surrounding cold air.  Mammals grow additional under fur from secondary follicles in preparation for winter, giving them a denser coat.  Sebaceous glands next to the follicles produce oil for waterproofing.

The avian equivalent of fur is feathers.  Soft down feathers concealed beneath a bird’s sleek outer feathers trap a layer of insulating air.  To increase their insulation, birds puff out their feathers in the cold.  They also grow additional feathers in winter.  However, birds have very high metabolisms, and feathers alone are not enough to maintain their body temperatures.  Most birds shiver continuously in cold weather to keep warm when not in flight.  Also, chickadees and other small birds can allow their body temperature to drop several degrees below normal during the night.  By lowering their body temperature they use less energy.  Short bursts of shivering keep their temperature from dropping too far.

Animals lose heat through their body surface, and small animals are particularly vulnerable to heat loss due to their greater surface-area to body-volume ratio ― the greater the surface area relative to body volume, the greater the rate of heat loss.  Some small mammals and birds overcome this problem by huddling.  Huddling effectively makes several small animals into a single, larger animal with a more favorable ratio of surface area to body volume.  Bluebirds, brown creepers, and nuthatches huddle in tree cavities or nest boxes on cold nights.  Even normally non-social animals, such as least shrews and white-footed mice, tolerate close contact and huddle in the winter.

Animals also lose significant heat through their extremities.  Blood circulating through legs and tails cools off, and an animal must expend additional energy to warm it back up when it returns to the body.  Cold-weather adapted animals often have counter-current heat exchangers in their legs or tails to reduce cooling.  A counter-current heat exchanger has arteries with warm blood from within the body next to veins carrying cooler blood from the extremities.  This arrangement allows arterial blood to warm venous blood before it re-enters the body.  The animal saves considerable energy that would otherwise be required to re-warm the returning blood.

Birds have well developed heat exchangers in their thin, poorly insulated legs.  They further reduce heat loss by tucking one leg up into their feathers while standing on the other.  A beaver’s tail contains an especially, well-developed network of heat exchanges that reduces heat loss to a small fraction of the heat that the animal produces.

In summer, this system also helps a beaver cool down.  Even insects take advantage of counter-current heat exchangers.  A few species of the adult tiger moth are active during the winter and use heat exchangers to warm cooled blood returning from the abdomen to the thorax.  Although not “warm-blooded” in the sense of mammals or birds, these insects produce heat with their flight muscles.  A counter-current heat exchanger is critical to maintaining the high muscle temperature they need to fly.