Some say bears don’t actually hibernate in the winter.
Don't worry though, if you thought for your whole life bears hibernate you're not completely wrong. This issue is more about technicalities among researchers than it is an absolute truth. The notion of bears hibernating through our six months of frigid, hard-hitting winters isn't completely false.
“I believe bears are true hibernators even though not all researchers agree,” said Michael Joyce, a graduate student at the University of Minnesota Duluth, who studies mammalian carnivores. “They lower their body temperature like all hibernators but just not as low as some smaller mammals."
Everybody deals with winter in different ways. Some prefer curling up under a blanket with a cup of hot chocolate to watch their favorite holiday film. Others like to get out and enjoy winter by snowboarding or gliding around on ice rinks.
You may notice – I know I do – that it is a little more difficult to get out of bed in the morning in order to tend to a frigid car, waiting to be cleared of last night’s snowfall. You might be a little more sluggish.
Imagine if you could simply curl up into a ball and lower your body temperature to just above freezing and snooze the winter off, waiting for better days. How does that sound? Some mammals do just that, with bears landing somewhere in the middle.
Dr. Matthew Andrews is a professor and researcher at the University of Minnesota Duluth, and he agrees with Joyce in regards to bears and hibernation. Andrews studies the molecular and genetic hardware involved in the hibernation of thirteen-lined ground squirrels, which is similar among all hibernators. This ground squirrel is the same that is familiar to most Minnesotans, also known as Goldy the Gopher.
Hibernators, big and small, undergo a mechanism of slowing their metabolism, switching their energy consumption to built up fat storages, and lowering their body temperature, which is called torpor.
“Bears will reduce their basal metabolic rate to about 20 percent. They will also lower their body temperature to about 30 degrees Celsius (about 80 degrees Fahrenheit),” Andrews said. Keep in mind if a humans body were to drop to 30 degrees Celsius from 37 degrees Celsius most people would begin to undergo hypothermia.
At this temperature, Andrews explained, bears still are able to maintain relatively high neuronal function, meaning that they can feel and sense their surroundings quite well. So, in essence, you may not want to poke around a hibernating bear, unless you want to risk staring into the eyes of a grumpy ball of fur.
Bears only lower their temperature to about 80 degrees Fahrenheit making them a “shallow hibernator.” This is why some don’t consider them true hibernators. You may think that is quite an incredible feat, but it’s nothing compared to what smaller mammals can achieve.
The species Andrews studies, the thirteen-lined ground squirrel, goes through an even more fascinating stasis of torpor by dropping their temperature to about five degrees Celsius (40 degrees Fahrenheit). “They curl up into a ball and basically become little balls of ice,” said Andrews.
These animals, in contrast to bears, take about 90 minutes to wake from hibernation. If you were to pick up one of these animals while they are hibernating it would take them almost an hour and half before they could sense your touch and react to it. Andrews explained that the difference in temperatures has a lot to do with the size of the animal. Bears have a very large surface and internal area, making it incredibly difficult to lower their body temperature to close to freezing and still survive.
But wait, there’s more.
A common misconception is that hibernators sleep continuously throughout the entire winter. “All hibernators go through a cycle where they awake for a short period of time by raising their temperature back to normal and then re-enter torpor. This awakening time is called interbout arousals (IBA). No one is really sure why this happens,” said Andrews.
One speculation is that these IBA’s are used to replenish and refresh cellular functions. This cycle occurs every few days and is currently being researched.
So, why do animals hibernate?
Andrews and Joyce explained the main reason for hibernation is food availability. Hibernators, like black bears and thirteen-lined ground squirrels, begin to lose access to their food sources when it begins to get cold. Throughout the summer these animals eat a lot of food while the getting is good, building up their fat storage. Then, when the days begin to shorten and the temperatures drop, like it is right now, hibernators enter their sleeping chambers, which will house them for the next six months
However, not all animals hibernate.
Joyce said that animals have physiological, morphological, and behavioral adaptations to dealing with the cold winter. For example, the pine marten, an animal that Joyce studies, does not hibernate but reduces its daily activity to about four hours a day as a behavioral adaptation to conserve energy.
“Pine martens have a long and skinny body type, which is not good for conserving body heat because of their high surface area to volume ratio. They don’t have body fat storages to use for insulation or energy reserves.
“But when pine martens are resting they do go into torpor to reduce energy costs.” Joyce said.
So what is your strategy for the winter? Are you a sleeper or do you embrace the cold?