Hibernation: It’s Not for Everyone

Hibernation: It’s Not for Everyone

By Marilyn Shy, Kalkaska Conservation District

Some days, in the dead of winter, it feels like the whole world is hibernating.

But then just a couple of days before Christmas, on an evening in December, I saw an opossum foraging for food in my driveway. And I got to thinking: Which of of our resident critters hibernate, and which do not?

During a warm spell, you may see a raccoon or a skunk out and about. Although these animals, along with opossums, hole up in their dens on the coldest days, they do not actually hibernate. Skunks even have the ability to lower their body temperature by 10 degrees for short periods of time.

So who are the true hibernators in our area? Up until recently there was some debate about which creatures were true hibernators. Scientists now believe the main hibernators are: bats, bears, ground squirrels, woodchucks, certain frogs, snails, box turtles, some butterflies and moths, ladybird beetles, and bumblebee queens.

What is hibernation, anyway? Hibernation is a state of inactivity characterized by low body temperatures, slow breathing and heart rate, and low metabolic rate. Historically reserved for deep hibernators like rodents, the term now includes animals such as bears, and is now based on active metabolic suppression, rather than absolute body temperature decline.

In the past, there was disagreement over whether or not bears truly hibernate, since they experience only a modest lowering of body temperature. Many researchers thought that their deep sleep was not comparable with true, deep hibernation. But this theory was refuted by a study done in 2011 by researchers at the University of Alaska Fairbanks.

Black bears hibernate for 5-7 months per year, and during this time do not eat, drink, urinate or defecate. They generally begin hibernating in September or October, and emerge again in April the following year. During this time their heart rate drops from 40-55 beats per minute, to 8-19 beats per minute. They suppress their overall metabolism by about 25 percent.

In spring, bears emerge from their dens as the temperature warms up and food becomes more available. Having not eaten for months, these newly awoken bears are hungry. Since they are desperate for food both before and after they hibernate, fall and spring are the times when bears are most likely to get into conflicts with humans over what they see as easy food sources such as bird feeders and pet food left outdoors.

Black bear hibernation has been called torpor, winter sleep, dormancy, and carnivorean lethargy. But the leading animal physiologists now simply call it hibernation.

Bear hibernation is different from other animals. Their greater level of metabolic activity, as compared to smaller mammals like bats and ground squirrels, is a function of the fact that their large bodies need more energy to operate. As a result, their systems have to be kept on a very low simmer instead of going into a near total shutdown.

What about chipmunks? A true hibernator spends late summer and early fall seeking out as much food as possible and consuming it, but the chipmunk is a food hoarder. While true hibernators try to put on as much fat as possible before the big sleep hits, the chipmunk stores its food instead of eating it. Deep in their tunnels (sometimes 30 feet long) the chipmunks store nuts and seeds in many chambers. They wake up periodically to munch on these stored provisions. However, mild days in February and March can bring chipmunks out of their cozy dens to forage for additional food.

Although different animals have different strategies for surviving the cold months of winter, all (including us!) are eager for the longer, warmer days of spring ahead.