For bears, however, it is fatal if their body temperature falls below 30 degrees. A conversation with the biologist Roland Prinzinger about hibernation, hibernation and winter rigor – and why we humans somehow have to get through the cold season differently.
Mr Prinzinger, there was recently talk of the bears in northern Canada preparing for hibernation. You wake up to this topic, don’t you?
Of course, because bears don’t hibernate in the classic way!
But?
They hibernate, that’s different.
Where’s the difference?
I would like to go back a bit, if that’s right?
Go ahead!
There are three strategies in the animal kingdom to get through the cold season as energetically as possible. These are hibernation, hibernation and winter rigor. All non-vertebrate animals, i.e. insects, reptiles, amphibians, fish and so on, fall into freezing conditions. This means that when it gets cold outside, their body temperature falls passively and they can no longer move, they freeze. Before doing this, they look for a sheltered wintering place. Your body temperature can then drop well below zero. In spiders and other invertebrates, specimens have even been found that had cooled to minus ten degrees.
How is that possible?
They do this by storing water that might freeze and storing liquids that are frost-resistant instead. Similar to a car where you put antifreeze in the cooling system. It is different with warm-blooded animals like bears. There are big differences in terms of sibling types. Polar bears as pure carnivores have the hunting season in winter and they also give birth to their young during this time. For the omnivorous brown bears and grizzlies, on the other hand, who feed on berries and buds and therefore find little food in winter, it is better if they go into an energy-saving state of rest. Failure to do so could be dangerous for them.
Why this?
Because they wouldn’t find enough food. It takes a tremendous amount of energy to trudge through deep snow, looking for and finding food. This is an unbelievable effort even for the ungulates that live there, such as roe deer and deer, but they can also eat and digest bark, twigs and pieces of wood, which the bears cannot.
So the grizzly needs fresh salmon and other meat?
To person:
Well, they’ll eat that too, but more in late autumn. The fish roe in particular are good for the bears to eat winter fat, which is what makes survival possible. It can make up to a third of the body weight.
Is it true that bears don’t just sleep through the night?
Yes! It has to do with body temperature. Real winter sleepers don’t mind if their body temperature drops below 30 degrees. If you cool a groundhog down to two degrees, it doesn’t matter. A bat survives even if its body cools to zero degrees. But if the temperature of a bear or badger drops below 30 degrees, then it cannot survive. Because, unlike a winter sleeper, it cannot heat itself up again. That costs too much energy. The main reason for the bear because it is so big. It’s like braking a truck and then accelerating it again: it costs a lot more energy than just letting it go slowly. This can be represented very easily mathematically, but I’m afraid it leads too far here.
In that case, preferably in words instead of numbers.
Let me put it this way: animals only do what is necessary. The bear gets through the winter better if it only rests at a relatively high body temperature and does not have to wake up, whereas a marmot does not mind interrupting its hibernation 20 times or more. Why they do this is not yet fully scientifically clear. But they also do it to check every now and then: is everything still working?
Does that mean: large animals hibernate, small ones hibernate?
It’s not that simple either. This can be demonstrated well with the smallest mammals in the world, the mosquito bat and the Etruscan shrew, on which we have been researching for a long time. The mosquito bat is a typical winter sleeper, but the Etruscan shrew is wide awake in winter. This is because the shrew lives on the ground – and there is enough food there even in winter: cockroaches, worms, millipedes and so on.
The bat, on the other hand, cannot find any food because mosquitoes do not fly in the cold.
Exactly! In addition, the bat has a large body surface in relation to its body mass – that is only around four to five grams – because of its wings. It is about 300 times as large as humans, and it could not hold its body temperature for a minute at temperatures below zero degrees, then it would fall to the ground, cooled down. So the bat hibernates, in groups and in sheltered niches. And sometimes in summer too, when it is briefly cold and rainy. This is then called Torpor.
Sounds a bit gloomy …
But it is a very clever mechanism because it is a kind of very brief hibernation. With the difference that the body of the bat – unlike in winter, when it cools down to zero degrees – does not cool down below 18 degrees during torpor. This is another limit that has not yet been fully explored, but the Torpor is something of a daytime sleep lethargy. Hummingbirds do that too, when they are in such a lethargy, you can turn them upside down on the branch they are sitting on – and they stay that way.
If this is so practical for getting through uncomfortable weather conditions or seasons: Why don’t we humans hibernate? Or at least hibernation?
It’s not that easy to answer. First of all, we humans are omnivores and we are also very large. In addition, we do it like the European hamsters: we build up supplies. And they help us get through the winter. I can imagine that there are some who would say: “Wake me up when it’s spring again.” Apart from the fact that a hibernation – or should we say: a kind of daytime sleep lethargy that we all experience in the dark season know – is also dangerous because you are defenseless during this time: That would not work because of the physiological requirements, which are also reduced to a minimum level in hibernation. Our brains and hearts just need too much energy.
Interview: Boris Halva
