There is at least a foot of snow on the ground, the temperature at nights are in the low 20s, and most of the food for wildlife is already eaten or inaccessible.

Now I know why field crickets squeezed under my garage door a month ago, seeking shelter within, and why a field mouse found a hole somewhere in the foundation and came up through the walls.

A squirrel or raccoon would be in the attic right now if there was a place for one to enter. Even the Asian ladybugs saw the cold coming and came in.

To wildlife, our homes are like caves, places with stable temperatures and shelter from storms that make them ideal for riding out the extremes of the weather and seasons.

Few animals actually succeed in sharing our homes. The animals are part of our adaptation to the environment, but our nests do not work well if we share them with too many wild creatures.

So the squirrels have gone into dormancy, curled up together in their biggest leaf-and-stick nest or, better yet, in a tree hollow furnished with insulating leaves. Opossums hole up too, hoping their fat reserves outlast the cold snap.

The snow is of some service to the field mice, which can now create tunnels and even ground-level nests out of sight of their many predators.

But the long-tailed weasel, the most widespread carnivore on this continent, goes right after its favorite prey in those tunnels, too. If the snow continues like this through much of the winter, weasels would turn white for camouflage.

Raccoons resort to another kind of manmade cave - the storm sewers buried in the ground in all urbanized areas.

The chipmunk and the groundhog are among the few creatures that actually hibernate.

Curled up in its winter burrow, the groundhog's body temperature falls from 97 degrees to less than 40 degrees. It draws a breath only once every six minutes, and its normally hyper heart rate of 100 beats per minute drops to just four. The slowed metabolism allows its stored fat to last until about Groundhog Day on Feb. 2.

Chipmunks wake every couple of weeks to eat from their extensive underground food cache filled with the nuts and seeds we saw them stuff into their cheeks in the fall.

All of these animals have evolved to thrive in a world that varies quite a bit from hot to cold. If the seasons disappeared, a different set of creatures would prevail, ones best adapted to an unchanging world.

Which makes me think, on this first full day of winter, about the kinds of variations that have determined our strengths and ensured our continued success.

Having evolved as other animals have, maybe we, too, need the occasional period of deprivation. Maybe eons of repeated famines have optimized us for lives that include at least a few lean times.

Economists only recently started developing an understanding of how psychology drives financial behavior and contributes to economic cycles.

I wonder, as we head into what will likely be the most difficult two months of the recession, whether to some degree we need booms and busts because we have evolved to thrive through them.

Contact Kevin Post:

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