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The challenge of winter

With a return to cold weather this week, I thought about wild animals and their ability to survive in subzero temperatures. Humans have furnaces and insulated homes. Before we step outside, we put on heavy coats and layers of high-tech fabrics th...

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Flying squirrels huddle together for warmth in their nest. (Photo by Trudy Vrieze, taken at Wildwoods in March 2014.)

With a return to cold weather this week, I thought about wild animals and their ability to survive in subzero temperatures. Humans have furnaces and insulated homes. Before we step outside, we put on heavy coats and layers of high-tech fabrics that keep us warm.

Duluthians know that winter weather can be lethal, so we take steps to be safe and warm. But how do wild animals stay warm? They must adapt or die.

The cold can freeze flesh and bodily fluids into solid ice. Cold is intensified by rain and wind which increase the rate of heat-loss. Wet fur and feathers lose their ability to insulate and wind increases evaporation, carrying heat away.

Another winter challenge for wild animals is that food becomes more difficult to find. Snow covers the ground and hides food sources.

It requires high levels of energy just to stay warm. Small birds can lose 10 percent of their body weight on a single cold night. A hard frost before the snow falls or a series of subzero days can be dangerous for wild animals.

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What are the solutions? Some migrate to more survivable temperatures and abundant food supplies. Classic migration is the robin that heads south in the fall and returns to raise its young in the spring. In mountainous terrain, elk and other animals may migrate to lower elevations for milder weather and plentiful food sources. Even earthworms migrate, going 6 feet under, below the frost.

Another solution is hibernation. Examples include groundhogs, bears, skunks, chipmunks and others. They prepare by overeating in the fall, storing body fat they need for energy while hibernating. Some animals hoard food in their nest to eat in winter when they briefly awaken.

Hibernation involves a decrease in the animal's temperature, heart rate and breathing. This conserves energy during a period of cold and food shortage. Hibernating turtles don't breathe for weeks, yet when their brains detect an increase in light, they awaken and surface to breath.

Cold-blooded animals like snakes find a hole where they spend winter in a dormant state similar to hibernation.

Other animals don't migrate or hibernate. They stay and tough it out, seeking shelter in hollow trees, under rocks and leaves, or even under the snow. Mice build tunnels under the snow where they are insulated from cold similarly to an igloo.

Pigeons find roosts out of the wind. Crows perch on a warm chimney. Deer absorb sunshine during the day and then gather in the woods at night out of the wind in snow depressions.

Many animals become more social in winter and huddle with others of their species to share body heat. Voles and deer mice huddle. Raccoons may share a den. Up to 60 wrens have been seen in one nesting box.

Animals may grow a thicker fur coat. Deer increase the length and density of their guard hairs and undercoat. Fox and porcupine do the same. Birds fluff and may even grow additional feathers. The goldfinch has 50 percent more feathers in winter.

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For wild animals, winter fat is a good thing even if they don't hibernate. Fat provides insulation from the cold as well as a source of energy. Deer live on stored body fat when food is scarce.

Body chemistry may change in winter. Goldfinches metabolize fat more quickly. Birds have faster heart rates in winter to circulate warmth. At night, chickadees huddle and shiver, allowing their body temperature to drop 10 degrees to conserve energy.

Some behaviors adjust as well. Long-legged moose seem designed for breaking trails through deep snow. Deer and smaller animals follow those trails to save the energy needed to make a fresh path.

Some animals freeze solid in winter. The wood frog spends winter as a "popsicle," then thaws and awakens in the spring. Higher-than-normal blood levels of sugar and urea serve to protect them from damage while frozen.

Many insects, like the mosquito, simply die. Their life cycle ends with the cold weather. Eggs are laid in the fall. New adults emerge in the spring.

Each fall and winter, Wildwoods receives animals injured by the cold. If you discover an animal that appears to be suffering, call Wildwoods for advice.

John Jordan is a volunteer at Wildwoods. He is also an RN who lives in Duluth with the photographer Trudy Vrieze and a clowder of rescue cats.

Wildwoods is a 501(c)(3) wildlife rehabilitation organization in Duluth. The writers are volunteers at Wildwoods and/or experts in their fields. For information on how you can help wildlife, including volunteer opportunities, visit wildwoodsrehab.org or call (218) 491-3604.

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