Each animal has its own way to overwinter


Carol Saseen of Wheeling, W.Va. writes:

“I really enjoyed your column in the News-Register last week describing how birds survive cold weather.

Could you do another article focusing on other vertebrates such as opossums, raccoons, squirrels and deer?”

That’s a great suggestion, Carol. I always appreciate hearing from readers about topics they would enjoy.

Water temperature

Let’s start with fish. Water beneath icy ponds and lakes is very cold, but it’s always above the freezing point, 32 F. Fish are ectothermic, that is their body temperature is determined by the environment.

In very cold water, fish slow down and become lethargic. So, essentially, fish chill for the winter until water temperatures rise in April.

Reptiles and amphibians become dormant during the winter. Snakes seek refuge in burrows beneath the frost line, often sleeping together in tangled masses.

Frogs and aquatic turtles burrow into the muck on pond and stream bottoms. Their body temperatures and pulse drops.

And they breathe through their skin. My favorite winter survivor is the wood frog. It burrows deep into the leaf litter on the forest floor. Miraculously, wood frogs survive even the coldest winter temperatures.

When living tissue freezes, ice crystals form, cells rupture, and death usually results. But right now, wood frogs are frozen solid. The body is rigid, breathing ceases, and the heart stops beating. They are literally “frogsicles.”

Wood frogs have perfected the cryogenic freezing process. Almost half of the frog’s body may freeze and turn to ice. Freezing is made possible by specialized proteins and glucose, which prevent intracellular freezing and dehydration.

But don’t take my word for it. See for yourself:

Mammals also use a variety of strategies to survive cold winters.

Denser coats

Some grow denser coats of fur to reduce heat loss, some store food to get them through the winter, and some hibernate. Squirrels, raccoons, skunks, opossums, and foxes remain active all winter long, but during extremely cold weather, they sometimes curl up in a hollow log, den tree, or burrow for days.

Mice, chipmunks, and squirrels rely on food they collected in the fall.

Foxes capture and cache extra food on nice days, so they don’t have to spend a lot of time outside on cold ones. Opossums are particularly vulnerable to cold temperatures.

Their paper thin ears and naked prehensile tails sometimes freeze. If you see a ‘possum in the backyard with ragged ears and a stubby tail, you’ll know it experienced at least one severe winter.

Black bears go dormant for four to five months each year, but their body temperature drops only a few degrees. Biologists know that black bear body temperatures drop just a few degrees because they have actually visited dens and taken rectal temperatures.

It’s a risky way to make a living, but someone has to do it.

Beavers and muskrats survive in relatively cozy lodges from which they can swim to foods stored under the ice of frozen ponds.

Winter coat

White-tailed deer and other ungulates grow a special winter coat of fur. Longer, courser outer hair covers a shorter, denser pelage to form a warm insulating coat.

True hibernators go dormant in the fall and do not stir until they emerge in the spring. Furthermore, body temperature, heart rate and respiration rate plunge.

Here in the east the list of true mammalian hibernators is short: ground hogs and two species of jumping mice. A ground hog’s body temperature drops about 57 degrees to 44 F.

Their breathing slows, and their heart rate drops from about 100 beats per minute to about four beats per minute. Ground hogs lose about half their body weight during the winter.

Tucked in

Meadow and woodland jumping mice overwinter in a chamber about the size of a grapefruit up to three feet under ground. They tuck their nose between their hind legs and wrap their long tail around the body.

Some years more than half die in the hibernaculum. To us, winter seems long, cold and dark. Just be glad you don’t live in a hollow tree or a hole in the ground.


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Scott Shalaway, who holds a Ph.D. in wildlife ecology from Michigan State University, writes from his home in rural West Virginia. A former faculty member at Oklahoma State University and the University of Oklahoma Biological Station, he has been writing a weekly nature column for newspapers and freelancing for magazines since 1986. He can be heard on Birds & Nature from 3-4 p.m. Sunday afternoons on 620 KHB Radio, Pittsburgh, or live online anywhere at www.khbradio.com, or on the Tune-In radio app. Visit his website at www.drshalaway.com or contact him directly at sshalaway@aol.com or 2222 Fish Ridge Road, Cameron, WV 26033.



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