Picture yourself at twice your body weight by the end of summer. Now find a comfortable spot which stays above freezing and curl up for the next four months. You drift off into a strange form of sleep. While in this sleep, your heart slows to two beats a minute and you take a breath once every four minutes. Your body temperature drops to an average of 40°F.
We would have to do just this if we responded to lessening food supplies, fewer hours of sunlight each day and falling temperatures as do some of our Northwoods mammals. Animals which accumulate body fat, reduce body functions and drift through the winter months in a deep sleep are called hibernators.
Woodchucks, chipmunks and ground squirrels feed heavily on the lush vegetation of late summer. They are storing, in the form of body fat, the nourishment they will need to survive the winter famine to come. The fattest and oldest individuals retreat first to their burrows with the young adults close behind. By late October, most will have found their way to the den many feet below the surface, which will be their home for the winter.
The sleep of hibernation overtakes woodchucks and ground squirrels as the temperature of their curled bodies hovers just a few degrees above freezing. Respiration occurs only a few times a minute and the heart is beating as little as 1 /40th of its normal rate. The appearance is that of death, but the spark of life continues to exist.
Unlike woodchucks and ground squirrels which are plant-eaters, bats feed primarily on small flying insects. In the fall, as this food source dwindles, some species of bats migrate to the south, but other species remain in our area to hibernate. Seeking shelter in a natural cavity, abandoned building or attic, they sleep suspended upside down by their hind legs. Hibernating bats may go up to eight minutes without breathing, followed by a few minutes of rapid respiration. They must wait for the warm temperature and rains of spring to supply the insects they feed upon.
In the spring, soil temperatures begin to rise as the sun’s intensity increases, stimulating the hibernators to awaken. Woodchucks and ground squirrels emerge from their burrows for short periods at first, as food is scarce, but as the environment takes on the green colors of spring, normal activities left behind many months before begin again.
Not all animals which store up a reserve of food and become inactive during the coldest days of winter are hibernators. Raccoons, opossums and skunks will emerge from their winter dens to forage for food during mild weather. They scurry back to the protection of their shelters with a return of colder temperatures. Woodchucks, ground squirrels and bats, however, are good examples of true hibernators.
But what term should we use for a hibernating woodchuck whose body temperature drops to around 36 degrees during the winter? Here’s an animal who can control its temperature independently of its surroundings by producing internal heat (the definition of a homeotherm), but who can sustain low body temperatures like an ectotherm without suffering hypothermia and dying. Hibernating mammals don’t fit within the cold- or warm-blooded definitional boxes.
They’re the political independents of the animal world, accepting some beliefs from both parties, but espousing an independent platform in the end.
Less independent, but still problematic in fitting into the classification drawers of either cold-blooded or warm-blooded, are those animals that have evolved a strategy called torpor. These animals have the ability to lower their body temperature by several degrees over a short period, usually long enough to get through a cold night.
All animals must be able to adapt to the seasonal changes which take place in their environment. Hibernation is only one of many responses used by the animals of the Northwoods to achieve their primary winter goal: survival.
Peter Dring is a retired natural biologist and phenologist who lives in the Land O’ Lakes area. To comment on this story, visit the “Outdoors” section of StarJournalNOW.com.
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