Winter showcases nature's cold-coping strategies

Inside the Outdoors

“These are the times that try men’s souls,” wrote American Revolutionary War era patriot and author Thomas Paine, in offering encouragement to the Continental Army and colonial militias to persevere in the struggle for American independence.  On a much different level of seriousness, Minnesotans were in need of a little encouragement as we recently faced the trying time of this winter’s first assault of extreme cold.  Lulled into complacent comfort by one of the warmest Decembers on record, it was a shock to experience the plunge down to air temperatures of 20 degrees and more below zero and wind chills that approached 50 degrees below zero. 

This recent event should not be a great surprise to anyone who has spent even a few winters in Minnesota, a place that has become famous for testing new car designs and automotive batteries for their winter-worthiness.  But, just like the Peanuts comic strip character Charlie Brown, who every season forgets that Lucy will jerk the football away as he tries to kick it, some of us conveniently forget until we have to deal with this climatic extreme each year. 

Somehow, we manage to cope.  Some migrate to warmer climates, like the ducks and geese, the warblers, the wrens and the hummingbirds.  The rest get out the long underwear, snowmobile suits, chopper mittens, stocking caps and down jackets.  We create little micro-climates around our bodies that help keep us from frostbite, or worse.  Or we stay indoors and turn up the thermostat.

For wild creatures that don’t migrate as many bird species do, it’s a time when the margin between life and death is greatly narrowed.  But Nature has equipped these hardy wild Minnesotans with mechanisms to survive, and the odds may not be as long as we who can hide in climate-controlled conditions might think.

When I opened the back door for a quick trip to the garage Sunday at midday, I was struck by the confident nonchalance of a squirrel perched statue-like in one of our crabapple trees.  I had to look twice to realize that it wasn’t a gray squirrel, but instead, its smaller red squirrel cousin.  It required a second look because its fur was so fluffed out that seemed almost the size of its larger relative.  It had probably been attempting to raid one of the bird feeders and was either satisfied or frustrated; just which, only he knew. 

Birds and furred animals often in winter will be seen with their feathers or fur greatly puffed out or fluffed up, their feet barely visible, their apparent size noticeably larger than they typically appear when they’re moving about.  This expansion of their body covering traps air in the same way that a goose down garment worn by a human traps air to create an insulating layer to hold in our body heat.  Survival in winter’s cold depends on a delicate balance between the caloric value of food consumed, and the loss of body heat to the environment.  Reducing that loss of body heat by fluffing when not active gives the squirrel or the songbird a little extra survival margin.

Humans and animals both respond to extreme cold by shivering.  We’re likely to think of it as undesirable, but it actually is just the opposite.  Shivering is an involuntary movement of muscles which, in and of itself, generates body heat.  We know from experience that being active, as in snowshoeing or cross-country skiing, generates enough heat that we need less clothing to keep warm.  Shivering is a more economical way of generating body heat in extreme cold and is especially common to those songbirds that stay with us throughout the winter. 

There is in biology a principle called “Bergmann’s rule.”  This principle states that among closely-related creatures, the farther north one travels the greater the probability that relative will be larger.  Bergmann’s rule is thought to explain, for example, why a whitetail deer in Minnesota will average a greater weight than a whitetail in Texas.  The heavier Minnesota deer will have less body surface area per pound of body weight, which enhances its retention of body heat.  It is the same principle that explains why the polar bear is much larger and heavier than bears of southern climates.  The polar bear actually has less surface area per pound of body weight, and therefore retains heat more efficiently, an efficiency that is relatively unimportant to southern bear subspecies. 

There are a number of other defense mechanisms against winter’s extreme temperatures. Ruffed grouse will roost under an insulating layer of snow, if it’s deep enough, during the long intervals when they’re not out feeding.  Both grouse and deer experience a “shift” in digestive enzymes that allows them to digest woody stems and buds, rather than the warm seasons’ greens.  This is one reason why winter feeding of deer with such agricultural staples as corn is not a good idea.

Muskrats and beavers build shelters that offer substantial protection.  The pelts of such furbearers as otter and mink grow denser and more protective, as any trapper knows.  They also, like many wild creatures, accumulate a layer of fat that supplements their energy intake during the winter months. 

An animal’s appendages, such as their ears, tend to be smaller in similar species the farther north they are found.  The smaller the ears, to continue the example, the less they contribute to the loss of vital body heat.  The ears of snowshoe hares found in our climate are far smaller proportionately than the ears of jackrabbits, which are found in more southerly climes.   

Ducks and geese, some of which remain near open water through our northern winters, have a special arrangement of adjacent arteries and veins in their legs and feet.  In Latin it is called rete mirabile, which translates roughly as “miracle net.”  Warmth from the blood in outgoing arteries is absorbed by adjacent veins sending blood back to the heart, thereby economizing on body heat that could otherwise be lost in legs and feet paddling in very cold water, or standing on ice or frozen ground.

These and other adaptations enhance the ability of many wild creatures to survive our northern winters.  Humans have learned to alter our environments, ether on a grand scale in the buildings we inhabit, or by the garments we put on or take off, as needed.  Wild creatures, which are not so equipped, have been endowed by Nature with some similarly amazing survival gifts.

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