Written by Melina Tiphticoglou
Translation: Vivienne Baillie Gerritsen
Winter is well on its way. And life is shifting from the outside to the inside. The trees have lost their leaves, nights are longer, temperatures are nearing zero degrees and there isn’t much food around. Times are becoming rough. Many animals are making their way to warmer climates, whilst others are getting ready to counter the cold. Small creatures like marmots, squirrels, hedgehogs, bats and even some birds drift into a state of total torpor without ever leaving their den. In other words: they are hibernating.
In spite of appearances, hibernating is not just a case of prolonged sleep but a particular physiological state which saves up to 90% of an animal’s energy! Naturally, hibernation demands certain arrangements. Towards the end of August, animals start putting aside important energy reserves – i.e. fat – that represent almost twice its weight. It then sets up its den where thermal variations are small, and lives in it in a position that loses the least heat, which is why they are generally rolled up in a ball. Their body temperature decreases drastically to reach the den’s temperature – between 2 and 3°C – whilst vital organs slow down. Their heartbeat drops from 200-300 beats a minute to barely 2-10, and they need 50 times less oxygen.
How does an organism adapt to these novel conditions? Squirrels may have the answer. Their acquired fat-rich diet happens to stimulate the massive production of one particular protein which plays an essential role in the process of hibernation. Indeed, pancreatic lipase digests the fats stocked before the winter, making them readily usable. Pancreatic lipase is an enzyme usually located in the intestines but, from September to March, it is found in huge quantities in the hearts of hibernating squirrels. Such a location is hardly surprising since the heart is such an essential organ, and it needs to be provided with fresh supplies constantly. Does this mean that pancreatic lipase is active despite very low temperatures? Yes. Whereas most enzymes are inactive below 5°C, pancreatic lipase still bears a little over 30% of its activity at 0°C. Why? No one knows yet.
How about humans? Could we also crawl into a den towards the end of the autumn and emerge when the spring tiptoes back? No, unfortunately. Below 35°C, we suffer from hypothermia and just lose consciousness below 33°C. In reality, our organs need a steady 37°C internal body temperature to work. However, a greater knowledge of biological adaptations that occur in the process of hibernation could help to understand how humans react to a sudden lack of oxygen, no food or an abrupt drop in body temperature.