How plants and animals handle the stress of winter

One holiday down and several more to go. Are you stressed yet?

Animals and plants face stress at this time of year, too. Unlike the psychological stress we bring upon ourselves, their stress is largely physiological. Thousands of winters have honed effective adaptations for surviving the challenges of winter. Consider some of the stress that wild creatures face, and yours will pale in comparison.

Take trees, for example. To understand the danger they face, think back to the growing season and in your mind's eye picture all those leaves. Leaves contain lots of water in their cells. There's also water in the pipeline of tree trunks or, more technically, in the xylem and phloem where plant sugars produced by the leaves and nutrients taken up by roots are in solution. If there's so much water inside a tree, how can it dehydrate?

The crux of the problem is the potential for water in between plant cells to form ice crystals at freezing temperatures. If that happens, a process called osmosis kicks in, whereby water from inside the cell is drawn out of the cell. This sounds the death knell for a tree. If water moves out of the cell, the cell dehydrates - and the plant dies.

The first line of defense for a tree is to abandon its leaves. This is called the "deciduous habit" - which is scientific jargon for "fall." It means that broad-leaved trees in our climate drop their leaves all in one season to avoid stress - and death - in the winter. The leaves' job is done. They have sent their goods in the form of plant sugars into the plumbing of the tree. Come frost, they become a risk factor and, serving no further purpose, they're discarded.

But what about the "pipes?" What if they freeze? This is where cryobiology comes in. Cryobiology is the branch of science devoted to the study of life below freezing temperatures. Scientists have found that plants in our climate have evolved two responses to this stress. One is supercooling, which is the super cool phenomenon of lowering the temperature of water below its freezing point without turning to ice. (Some amazing physics here.) The second is the presence of solutes - or other chemicals floating around in the plant's pipes. These solutes are plant sugars. They form a slushy antifreeze, and this reduces the freezing point of the water.

Thus northern trees handle winter stress quite effectively. With the advent of warm sunshine and the spring thaw, the sugary solutions in the pipes will start flowing again. Spring trees will benefit from the pumping of the stored sugars and nutrients to new buds up in the branches - we will benefit from the sap run in maple trees in the form of delicious maple syrup.

So much for trees - how about critters? Many animals use the antifreeze approach to surviving winter. Insects, for example, produce an organic compound called glycerol in their hemolymph, the rough equivalent of our blood. With glycerol in suspension, the freezing temperature of hemolymph is lowered.

Supercooling also takes place. Amphibians use tactics similar to insects'. A frog's liver can crank out lots of sugar in the form of glucose. The resulting glucose solution turns their insides into a nice, thick antifreeze.

Not only are they equipped with their own antifreeze, creatures like insects and frogs can go into a state of suspended animation during the winter. This is called diapause. In diapause, adult frogs burrow underground and just chill for a while. Depending on the species, insects enter diapause in different stages.

Closer to our kin, many mammals deal with the stress of winter by sleeping. Wild mammals, however, do not snooze on the couch in front of the TV with a bag of chips. They have a sophisticated system of sleeping through cold spells. It's called dormancy, and there are several different varieties of the theme.

The most extreme form of dormancy is hibernation. Only ten species of mammals in Illinois are true hibernators, and in northern Illinois hibernators are limited to the groundhog and the thirteen-lined ground squirrel. In hibernation, metabolism, respiration and heart rate drop drastically.

"While a normal body temperature (of a given mammal) may be 95°F," explains the Illinois Department of Natural Resource's publication Wild Mammals of Illinois, "during hibernation it can drop as low as 36°F. A normal heart rate of over 100 beats per minute may drop to only four or five per minute. Breathing may slow to less than one breath per minute."

Hibernators can and do wake up, albeit groggily, if the temperature drops below a critical point. They may shiver a bit to warm up and find their way to a deeper, warmer den.

Some mammals are lighter sleepers. They sleep away the stress of cold in a type of dormancy called torpor. Torpid animals experience a drop in metabolism and heart rate, but not to the extent that hibernators do. Like hibernators, an animal in torpor can wake up if necessary. They may stumble around as if they're due for a sobriety test - but they're just going to grab a bite to eat and look for warmer digs.

During holiday stress many humans indulge in the national pastime of overeating. This, along with lack of activity, leads to the dreaded accumulation of body fat. In the wild world, however, building up fat reserves before months of inactivity is a good thing. Before going down for the big sleep, many animals pig out on a variety of high-fat diets. Packing in the calories, they build up a special, highly vascularized fat called brown fat. This special type of fat is the main source of energy for hibernators throughout the winter.

When not dormant in cold winter weather, animals face tremendous stress in their quest for calories. That coyote prowling the edge of the subdivision, the red-tail hawk soaring above the snow-covered field, and the goldfinch at your bird feeder have to eat if they're going to make it through the winter. It's imperative that they not miss a meal. The smaller the animal, the higher the metabolism and the greater the need to eat continuously.

Donning luxurious fur coats is a wonderful adaptation for winter stress. From the smallest shrew to the largest bobcat, mammals grow a thick coat to fight the biting cold of winter. Guard hairs and underfur are specialized types of hair that add to insulating and waterproofing of fur. If you're lucky, you may catch a glimpse of a red fox in its prime winter coat - a beautiful sight to see.

Last but not least, some animals respond to the stress of impending winter by packing up and moving for a while. Humans, of course, do this in droves.

Wild plants and animals deal successfully - and healthfully - with the stress of winter during our holidays. They need no visits to therapists or pharmacies, no memberships in gyms or health clubs. They just do what comes naturally to survive.

If you're stressed by the holidays, head to a nearby forest preserve, walk off the excesses of the season, take a look around you and find inspiration in the wild plants and animals of the winter woods. Here's to happy and healthy holidays!

Valerie Blaine is the Nature Programs Manager for the Forest Preserve District of Kane County. You can reach her at blainevalerie@kaneforest.com.