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For Love of Nature

For Love of Nature: How does wildlife survive in winter?

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As temperatures dip into the teens and the wind howls, I wonder how birds, squirrels and other wildlife are staying warm.

Wild animals have a variety of strategies for surviving, including migration, torpor or hibernation and adaptation.

We know many birds, monarch butterflies and dragonflies migrate south for the winter, but what about the animals that stay?

Many endotherms, animals that generate their own heat, exhibit torpor or a state of decreased activity. They look like they are sleeping as they lower their body temperature, even below the freezing point. The result is a lower metabolic rate, reducing food demand. Hibernation is a prolonged version of torpor.

Torpor has energy conservation benefits for smaller-bodied wildlife in particular, including bats, songbirds and rodents; however, torpor makes animals vulnerable to predators.

Animals have evolved a variety of behaviors to help them beat the cold: herding, denning, burrowing and roosting in cavities are all good defenses. We suspect our bluebirds sleep together in their nesting boxes.

Other animals experience physiological changes as winter approaches, building fat reserves, growing thicker fur and trapping an insulating layer of air against the skin beneath the fur or feathers.

Nature has devised other neat tricks to help various animals deal with conditions that people would be unable to endure.

How, for example, can geese appear to stand comfortably on ice or squirrels in snow? The secret is the close proximity of the arteries and veins in their extremities that creates a gradient of warming and cooling to prevent tissue damage.

This system is used by many terrestrial and aquatic birds and mammals, and even explains how oxygen exchange occurs in the gills of fish.

Fish swim freely in not-quite-freezing temperatures below the solidified surface. In polar regions, fish and amphibians even have special “antifreeze proteins” that bind to ice crystals in their blood to keep them from freezing solid.

Bear, deer, squirrels and chipmunks feed on food like acorns and beechnuts in the fall to build up their fat stores for the winter.

Many scientists say bears are not true hibernators, even though they spend time in winter dens. They don’t eat, defecate or urinate, but warm weather or noise can wake them.

Other mammals, like woodchucks, are considered true hibernators, meaning they experience reduced body temperatures and decreased heart rates, which help them conserve energy as they use up their fat stores through the winter months.

Mice, voles and other small mammals can create tunnels through the snow, which help to insulate them from the cold, protect them from predators and allow them to feed on plants and seeds.

The fur of coyotes and raccoons grows thicker in winter, while deer and moose have hollow hairs in their winter coats, which provide insulation by trapping air.

Otter, beaver, mink and muskrat have a double layer of fur with dense, fine hairs near the body and longer guard hairs on top, which helps keep them warm.

These survival strategies are impressive, but I’m thankful for a warm house.

Shannon Brennan can be reached at


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