Explained: The mysteries of hibernation
Many animals survive extreme conditions by taking shelter and shutting down for months on end. But how does hibernation protect them from the elements?
When winter is on its way in the Northern Hemisphere, many of us would like to put our heads down for the night and wake up only when the snow has melted and the days are getting longer again. But sleep is the closest experience we have to hibernating, which differs in more ways than just the period of time we spend doing it. Many different animals hibernate, but they all do so for similar reasons: in the winter, when food is in short supply and the temperature can become dangerously cold, animals must adapt by slowing down their metabolic rate and entering a long period of inactivity, keeping energy expenditure to a bare minimum. Hedgehogs are considered ‘true hibernators’ because they’re not actually asleep; their bodies are simply running at such a low metabolic rate that they are unable to move. However, when temperatures hit extreme lows, they will sometimes withdraw slightly from hibernating to shiver and warm themselves up. They might even wake up and leave their hedgerow nest for another, in exceptional circumstances. Hedgehogs will hibernate from October or November (depending on how early the cold weather arrives) through to March or April. During the summer months, they will feed continuously to build up a fat reserve that will see them through this period. This is vital, as a hedgehog loses a third of its body weight during winter; if it weighs less than around 600 grams (21 ounces) when it enters hibernation, it runs a real risk of starving to death.
How hibernation works
Animals have evolved many forms of hibernation, each suited to their own biology and feeding habits, but none of them are simply a long nap. Although this state of inactivity shares some similar characteristics with human sleep, they are far more pronounced in the hibernator: metabolism can slow to 25 per cent of the normal metabolic rate, breathing becomes incredibly shallow, heart rate can drop to a tenth of what it was and as a result, body temperature will also drop. Because vital organs like the heart and brain still need to maintain a certain temperature, brown fat around these organs (as opposed to white fat stored under the skin and around the skeletal muscles) also increases to help insulate them. True hibernators, including most rodents, experience such extreme drops in temperature and metabolism that part of their body can fall to below the freezing point of water, making them take on a deathly appearance. There is a different form of hibernation called aestivation, where animals become dormant to avoid high temperatures and dry climates, such as those found in hot deserts that might desiccate them or cause heatstroke. This process isn’t as deep as hibernation, because the goal isn’t so much about conserving energy as retaining water and maintaining a sufficiently low body temperature, but in principle, hibernation and aestivation are very similar.
To the untrained eye, a human sleeping can look as if it’s in the same state as an animal in hibernation. It shares some less extreme physiological characteristics, like the reduced heart and breathing rate, plus the dip in body temperature (that’s why you need a duvet while sleeping, especially in winter). But sleep is mostly a mental – rather than physical – change of state that is fairly easy to break out of in minutes, even if you’re a very deep sleeper. Mammals and birds all sleep regularly.
Torpor is a period of reduced activity in animals that doesn’t necessarily mean unconsciousness, but usually involves immobility and a reduced heart rate, temperature and metabolism. This state can be entered as often as every day, and can last just a few hours. Ectotherms (cold-blooded creatures) rely on their environment to maintain their body temperature, and often enter a state of torpor during periods of cold.
The physical changes in hibernation are much more pronounced; body temperature and metabolism are significantly reduced, the state lasts for an extended period of time, and the animal takes a while to return to a normal state. In Arctic squirrels, body temperature can drop to -2.9 degrees Celsius (26.8 degrees Fahrenheit), below the freezing point of water. Their bodies are in a coma-like state and they aren’t awoken from it easily.
Bears are not true hibernators
Like many other animals, bears will escape the harsh existence of a long, cold and hungry winter by denning in a cave or similar shelter and waking up only when the snows begin to melt. However, they don’t experience a very deep hibernating state; their temperature only drops by around ten degrees Celsius (50 degrees Fahrenheit) and their metabolism, heart rate and breathing are only depressed by a relatively small amount. This does allow the bear to perform some amazing biological feats, however. Unlike true hibernators, who must wake up once a week or so to defecate and sleep before returning to hibernation, bears can go for 100 days or more without waking up. Yet because of their shallow hibernation, they can wake up and quickly react to a threat, if they need to. Bears are able to regenerate muscle tissue lost through atrophy during hibernation by breaking down the waste urea in their blood into proteins – the building block of muscle. A pregnant female can also absorb embryos back into her body, should the fat reserves she built up over the summer prove insufficient for both her and her cubs. Some polar bears will even give birth to their young while hibernating.
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