Catherine Crépeau – The Rumor Detector
Agence Science-Presse (www.sciencepresse.qc.ca)
Each autumn, bears hibernate in their dens to sleep until spring arrives. But if we strictly follow the definition of the word “hibernate,” that’s not exactly what bears do. The Rumor Detector explains…
What is Hibernation?
To understand the activity – or rather, the inactivity – of bears, we have to know what it means to hibernate.
Hibernation is a state in which an animal slows down its metabolism – sometimes, by 98%. This translates into a slower heart rate, slower breathing and a lower body temperature. All this allows the animal to save energy and survive just on the reserves it has built up over its “active” months.
Hibernation will continue until food is once again plentiful. Animals will cool down their body temperature an average of 5 to 10 °C.
Their body temperature can get as low as 0 °C (3.5 °C for the ground squirrel – Spermophilus lateralis) and can even dip below the freezing point of water (−2.9 °C) for the abdomen of the arctic squirrel.
Rodents can lower their blood flow and their heart rate, which can drop from 350 beats a minute to 4 beats per minute for the chipmunk and from 500 to 5 for the garden dormouse. Their breathing is rare, and periods of apnea have been observed. These changes might be regulated by the brain chemical adenosine, indicates a literature review on hibernation among mammals published in 2007.
They Get Up
During this period of deep sluggishness, the only parts of the brain that remain active are those that manage the animal’s vital functions. Not all animals sleep right through the winter; many get up for a few hours at a time at regular intervals – 3 to 5 times for the golden hamster; and 15 times for the ground squirrel, in other words, once a week. They do this to eat the food they stored up in the fall, as well as to pee and to poop.
Getting up is hard work: hibernating animals expend 80 % of their energy reserves getting up and occasionally reheating themselves. Groundhogs can do this 12 to 20 times during the hibernation season.
On the list of “true” hibernators we include ground squirrels and groundhogs as well as skunks, hedgehogs, bats and hamsters.
And bears?
Why aren’t they on that list?
In part, because their behavior doesn’t entirely correspond to that of hibernating animals, at least as far as biologists are concerned. First of all, bears don’t lower their body temperature far enough. During their winter sleep they keep their body temperature relatively high, lowering it by a mere 5 degrees. That allows them to wake up faster in case of danger than normal hibernators can. A deeper sleeper can take hours or even a whole day to awaken.
This way, bears can move around to find food, or if they are threatened. And a mother can give milk to her cubs. A female bear can even awaken to give birth to cubs – then fall right back to sleep!
The list reveals that it’s the smaller animals, like hedgehogs, bats, hamsters and squirrels that hibernate. A bear is too big to shed all the body heat required to truly hibernate.
But a bear’s metabolic reactions are comparable to other hibernators. Their cardiac rhythm slows down to 4 beats per minute, and their oxygen consumption falls by 75%. A hibernating bear only breathes once or twice per minute. That suggests that the same physiological processes are at play for the bear as they are for other hibernating mammals.
Torpor or Hibernation
For these reasons, many researchers prefer the phrase “winter sleep,” or a state of light sleep they call “torpor.” The author of an article published in 2015 in the magazine Physiology talks about a continuous state of torpor over several months. Other researchers talk of winter sleep or winterization.
When in this state, the bear maintains a normal body temperature, breathing rate and heart rate during its active periods, but when it is inactive, it enters in a deeper sleep state that allows it to save its energy.
Verdict
Bears don’t completely hibernate during the winter: they slow down their metabolism. Unlike a true hibernator, they remain sufficiently alert to “wake up” quickly in case of danger. Hibernation is generally associated with cold climates, but some species hibernate to beat the heat or to survive food shortages. For example, in Australia, the echidnas – four mammal species that lay eggs – hibernate after fires while waiting for food resources to be re-established.
French version on the Reflet de Société website
This article is part of the Rumor Detector series. Click here to see other articles in the series.
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