Last week I wrote about how birds survive cold weather. You may think the facts about how mammals survive winter are pretty cut and dried, but as always in the natural world – there may be a few surprises.
For the most part when talking about wildlife and how they survive winter there are three key words, "hibernate, migrate, and adapt." However, since there can be some confusion over exactly what is and is not hibernation let us skip that for a moment and look at migration.
Some animals may migrate (especially birds, of course) very far south into Mexico, Central and South Americas; while others may only migrate to the southern portions of our states.
This brings up another good point, rarely do large mammals need to migrate or hibernate, if anything they adapt. Large mammals have options that are available for winter survival such as migrating to warmer areas and growing long, dense winter coats with specialized hair. These mammals include, bear, deer, raccoons, beaver, otter, coyote, fox, and bobcat to name a few. Larger warm-blooded wildlife also eat heavily in the fall laying on a thick layer of fat to provide energy and warmth during leaner months.
Small animals already have a high metabolic rate with racing heart beats all to maintain their body temperature, so when outside temperatures continue to drop they cannot increase this further especially when food is scarce. So, how do they survive – they either hibernate or go into torpor. Torpor is also called, "daily torpor" and is thought of by most scientists as a short-term or even daily occurrence similar to hibernation, but not hibernation.
Torpor is driven by cold temperatures and food availability. While in torpor, wildlife will experience a fall in oxygen consumption and breathing; a reduction in heart and metabolic rates; a restriction of blood flow to the main organs and the ability to awake spontaneously when disturbed despite the outside temperature. Wildlife can go into torpor each night when temperatures drop and come out of torpor in the morning within 30 minutes.
The onset of hibernation is brought on by day length, hormone changes, and outside temperatures. Harris further explains that for wildlife in hibernation their "body temperatures drop a lot lower than in torpor, with metabolic rates falling to between 1 and 2 percent of the active animal." Also, if the outside temperature drops below freezing the metabolic rate increases to prevent the animal from freezing.
I know wildlife biologists that say that the difference between torpor and hibernation is that wildlife in hibernation develop a fecal plug and those in torpor do not. Many consider this to be the one definitive difference between the two. That is why many wildlife biologists state that bears do not hibernate – because they do not develop a fecal plug and, contrary to belief, they do wake several times throughout the winter and feed, when possible.
Page 2 of 2 - Both, torpor and hibernation are methods that wildlife have to reduce energy needs and survive cold temperatures. This is why you may be less likely to see small wildlife like chipmunks and mice during the cold Midwestern winters. If only our "furnaces" could be as energy efficient as theirs!
Lynn Youngblood is a Kansas City-based naturalist who writes for Gatehouse Media.