Having recently moved to Prague, I have spent many of my weekends in the Czech countryside. Life there is slow, with a usual day involving a long walk in the forest as the main event. I am often reminded by my grandmother to keep my eyes and ears peeled (my words, not hers) for wild boars – especially young ones. Now might be a good time to look at this image of these delightful beasts.
I had hoped that by the time winter rolled around, my fellow forest dwellers would have entered hibernation. However, wild boars do not hibernate in the winter, much to my dismay. This got me thinking about hibernation more generally, and I realized how little I knew about this very well-known phenomenon. Did you know that bears don’t actually hibernate? Or that animals can hibernate in the summer? If not, you have come to the right place to learn what hibernation is, what it isn’t, and to learn new terms you can whip out the next time hibernation becomes a topic of conversation.
Setting the Record Straight
Far too often, individuals incorrectly refer to all forms of extended animal dormancy as “hibernation”. Three terms often fall under the hibernation-term-umbrella: hibernation, torpor, and estivation. Their definitions are as follows:
Hibernation: a voluntary state animals enter to conserve energy when food is scarce, typically in the winter months
Torpor: an involuntary state that animals enter, usually due to the same environmental conditions as hibernation, but this type of dormancy lasts shorter periods of time.
Estivation: a form of animal dormancy that occurs in response to high temperatures. This process is far less common than torpor or hibernation.
The main difference between hibernation and torpor is that during the process of hibernation, animals will typically go into extreme hypothermia. Their internal body temperature matches that of the outside environment, with some animals reducing their internal temperature to 0ºC! Animals that go into torpor also experience induced hypothermia; however, it is typically not as drastic as true hibernators. For example, during torpor, bears reduce their internal temperature by about 5ºC. Due to the significant change in metabolic rate and physiological state experienced by animals in hibernation, it may take between several hours and a day for an animal to wake up. During torpor, however, animals can wake up quickly in the face of danger, or in the case of bears, if a female bear needs to give birth.
The debate over whether bears are “true hibernators” stems from the fact that, while their torpor lasts as long as typical hibernation, their metabolism and internal temperatures do not decrease as much as true hibernators. While the duration of the period of unconsciousness is relevant, the more important aspect is what the animals’ body is actually doing when in this state. Therefore, it is generally agreed that bears enter a state of prolonged torpor in the winter, rather than hibernation.
Often, the size of the animal is a good determiner of whether they will enter hibernation or torpor. Generally, larger animals will enter torpor, while smaller animals will enter hibernation during the winter months. Larger animals do not go into deep hibernation because it is ultimately not energy-efficient for them. If they were to reduce their body temperature by the same margin that most true hibernators do – nearly a tenth of their normal level - the energy required to reheat the large mammal to get it back to its “normal” temperature to wake up would be nearly impossible.
As I mentioned earlier, this prolonged “sleep” does not only happen in response to the winter months. Certain insects and vertebrates enter periods of estivation due to the environmental pressures that come with dry and arid conditions. Like in hibernation and torpor, organisms reduce their metabolic activity to conserve energy and limit water loss. Estivation is a far more “light” physical state than hibernation or torpor, with organisms returning to their natural state more quickly.
Torpor in Humans?
Human torpor is a process that has been considered by scientists and researchers as a novel way to heal traumatic injury – and more recently has been deemed relevant for space travel.
Recently, therapeutic hypothermia has been used to reduce brain damage among patients following cardiac arrest. This procedure typically lasts no more than 24 hours, and the patient’s body temperature is lowered to 32ºC-36ºC. While the full mechanism of action is not fully understood, experts find that inducing hypothermia slows both the chemical reactions and metabolism within the body. Inducing hypothermia reduces the amount of oxygen these cells need, which aids their preservation when the body is unable to deliver oxygen. Additionally, lower temperatures help reduce brain inflammation. Both effects have been shown to minimize brain damage in patients.
Artificial torpor in humans has also been discussed within the context of space travel. Not only would inducing this physical state allow astronauts to avoid the lengthy (to say the least) journey to their final destination in space, but it would cut down on the amount of food and water required for such a journey. However, a major roadblock in this emerging field is the lack of knowledge on how animals “know” when it is time to go into and wake up from torpor. Moreover, they can do this safely. For humans, to enter such a state would require around-the-clock monitoring and numerous drugs to override our natural thermoregulation.
For me, I think I would rather dream about human torpor and space than actually experience it. And for that, all I need is to get cozy under my duvet and get some good old-fashioned shuteye.
