Keeping Warm

By Spike Knuth © 1992
Information Officer
Virginia Department of Game and Inland Fisheries

Issue: February, 1992

Up to now it hasn't been a particularly cold winter here in Virginia. Of course, being a native of Wisconsin, I always kiddingly say that Virginians are lucky, they only get about six days of winter in Virginia instead of three or four months. However, it was a cold, frosty morning when I let our cat, "Tuna Breath," in after being out all night. He chewed me out a little, but hey, he missed his chance at 11:30 PM the night before. He wasn't around and I wasn't going to wait up for him. Who does he think is running this house anyway?

It never ceases to amaze me how animals can spend a night in the cold even in severe cold with freezing rain or sleet - and still survive none the worse for wear. Of course, one way they do so is by finding shelter. They burrow into the ground or under insulating snow. They find den holes in trees or crevices. Here they may even huddle up as a group to preserve heat. They make nests of leaves or other vegetation. Birds will find old tree cavities that were nests for woodpeckers, or they'll find shelter in bird houses, under the eaves of houses or in conifers as they sidle up close to the trunk of a pine or cedar, especially at night.

On severely cold nights small birds may go into a type of overnight hibernation with body temperatures and heartbeats dropping, reducing its need for food. One study showed small birds can survive 15 hours without food in temperatures around five degrees. However, at 20 to 30 below zero, that drops off to ten then seven hours or only about half a winter's night. So food and shelter are the most important factors concerning survival.

Another good reason warm blooded animals can survive and stay warm in their shelters in the cold is because their bodies have the ability to maintain their temperatures. Some smaller birds will eat their own weight daily which doubles or triples heat production and energy levels. Some northern species like the redpoll have a little storage pouch where seeds are stored to be used at night or during storms when the bird can't venture out. In addition to calorie or heat producing food, animals have a special fat, called "brown fat." Biologists tell us that this type of fat seems to generate heat. Many young of some mammals have it and hibernating mammals such as groundhogs have it. Another way animals generate heat is by shivering. However, this uses energy quickly. It's my understanding that people shiver for the same reason, but I can remember many a duck hunt where it didn't work too long!

The most obvious way animals keep warm is by their fur and feather coverings. Birds like the little chickadee or titmouse stay warm by fluffing up their feathers. This and the layer of down underneath traps warm insulating air to protect them and keep them warm. It's kind of like when people layer their clothing to keep warm. Warm air is trapped in each of the layers providing a buffer to the cold. Birds will further reduce heat loss while breathing by tucking their bills into their back or breast. The worst enemy of birds in winter is moisture in the form or rain, sleet or freezing rain. Birds have an oil gland on the top base of their tail enabling them to waterproof their feathers to some extent by preening and spreading the protective covering. However, if a bird gets wet, its heating system can't work and it will not survive the night, even in moderate weather in some cases. This is true of certain mammals as well.

Mammals have underfur that does somewhat the same thing, traps warm air to provide an insulating layer. Mammals associated with water such as otters and muskrats have a dense underfur that traps air in and keeps cold water from penetrating and coming in contact with their skin. However, even they need to find shelter during severe wind and cold. Finding a muskrat dead on the ice because it was somehow shut out of its watery entrance back to its den in not unusual. White-tail deer have a thick winter coat made up of long, hollow hairs. In this case warm air is trapped in the core of these hairs to keep its body heat in and the cold out.

How do these critters walk around in the snow or on ice and not freeze their feet? Birds have warm blood circulating to the feet, then back to the body through arteries and veins that run up and down the legs side by side, which slowly warms it up again as the process starts all over. Of course, the muscles that move the feet are buried under a warm covering of feathers. Some animals, especially those that live in the Arctic, have a special kind of fat that stays soft and pliable even in sub-zero weather. If their foot pads weren't resilient it would be extremely difficult for them to move or jump or grasp.

What baffles me is not Tuna Breath's ability to withstand cold, but how can he lay in the sun pouring through the window and get so hot you can hardly touch him? I guess that's another story.