Thursday, February 07, 2008

Adapting to winter

Adapting to winter

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One of the great pleasures of coming to know the natural world is coming to see human strategies for coping with living on earth in a broader context.

Take surviving winter.

Sure, we’ve got central heat and funny hats, and some of us even avoid the cold by heading south. But in the natural world there are innumerable variations of those strategies, as well as some others that simply aren’t available to Homo sapiens.

Think of how complex the phenomenon of migration is. More people than ever, for example, are now aware that the monarch butterflies we see in August and September are on their way to spend the winter in the mountains of central Mexico to take advantage of the warmer climate there. But fewer people remember that the monarch butterflies that migrate through Illinois have never before been to Mexico before. They are generations removed from the monarchs that set out on the journey north the previous spring, and scientists have yet to discover how they know where to go. Imagine traveling south without directions and being able to find where your great, great, great grandparents spent the winter!

Of course the many Illinois creatures that do not travel to warmer places for the winter must adapt to the cold in one way or another. Most interesting among these are the select group of reptiles and amphibians that can actually tolerate being frozen. The young of our state reptile, the painted turtle, for example, hatch from eggs in the fall but remain in the nest where they are born until spring. In this situation they are exposed to temperatures so cold that all of the fluids in their bodies freeze. A complex set of physiological adaptations allows them to endure this freezing without damage to their cells or organs.

After their first winter, painted turtles lose the ability to recover from freezing, but as adults they possess another striking adaptation for living through winter. They have the greatest tolerance for oxygen-deprivation of any vertebrate animal, which means they can hibernate underwater for months on end.

Even some of our closer relatives, other mammals, employ strategies for surviving Illinois winters that we might envy from time to time. I’m thinking here of the true hibernators, mostly rodents and bats, which do everything but check out altogether for the coldest months of the year. (Although the animals most often associated with hibernation, bears, can sleep all winter without eating they do not experience the other physiological changes that true hibernators do.) During true hibernation a mammal’s body temperature drops, its metabolism slows dramatically, and its heart beats just fast enough to sustain life and prevent freezing. True hibernators are entirely undisturbed by winter storm warnings, and they have no use for the concept of the wind chill factor.

Of course you need not look past the birds and squirrels at your feeder to remember there are plenty of creatures that deal with winter in much the same way we do—a heavier coat, a little more to eat, a little less activity when the weather is at its worst.

Fortunately, winter doesn’t last forever.