What Is Hibernation?
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Hibernation is a controlled shutdown in which an animal drops its body temperature, heart rate and metabolism dramatically to survive a period when food is scarce. It is far more extreme than sleep, and one of the more surprising facts is that bears do not really qualify.
What hibernation does to the body
True hibernation involves a large drop in core body temperature, sometimes to just a few degrees above freezing, together with a collapse in heart rate and breathing.
An arctic ground squirrel can let its body temperature fall below zero without freezing. A hibernating ground squirrel's heart may slow from around three hundred beats per minute to fewer than ten, and its metabolic rate can fall to a small percentage of normal. This is not sleep, and a hibernating animal cannot simply wake up on demand.
Why bears are the exception
Bears enter a state called torpor rather than true hibernation. Their body temperature drops only a few degrees, and they can rouse relatively quickly if disturbed. Female bears even give birth and nurse cubs during this period.
What bears do achieve is remarkable in a different way. They spend months without eating, drinking, urinating or defecating, recycling waste products internally, and they lose relatively little muscle and bone despite the inactivity. Medical researchers study this closely for what it might teach us about muscle wasting.
How animals prepare
Preparation begins well before the cold arrives, typically triggered by shortening days:
- •Building fat reserves, in some species doubling body weight
- •Preparing a den, burrow or protected chamber
- •Some species cache food to eat during brief wake ups
- •Physiological changes to blood chemistry that prevent tissue damage
Waking up periodically
Most hibernators do not stay under continuously. They surface every week or two, warm up for a few hours, then drop back down. This is energetically expensive and consumes a large share of their winter fat budget.
Why they do it is not fully settled. Leading explanations include the need to sleep properly, since hibernation is not restorative sleep, and the need to restart immune function, which is largely suspended at low temperatures.
Other ways of waiting out bad conditions
Hibernation is one of several strategies. Aestivation is the summer equivalent, used by lungfish and some snails to survive heat and drought, sometimes sealed inside a mucus cocoon in dried mud.
Brumation describes the winter dormancy of reptiles, which differs because their body temperature already tracks the environment. Some frogs go further still and allow much of their body water to freeze solid, using glucose as an antifreeze to protect their cells, then thaw and hop away in spring.
Could humans hibernate
The question is taken seriously by researchers, particularly for spaceflight and emergency medicine. Induced hypothermia is already used clinically after cardiac arrest to reduce brain damage, which shows that lowering human body temperature deliberately is possible.
True hibernation is a different matter. Hibernators have specific adaptations preventing blood clotting, muscle wasting, bone loss and tissue damage at low temperatures, none of which humans possess. Researchers have identified some of the genetic switches involved, and the fact that some primates hibernate suggests the capacity may not be as distant from us as once assumed.
The takeaway
Hibernation is a deep metabolic shutdown with a large drop in body temperature and heart rate, quite different from the lighter torpor bears use, and it is an active physiological strategy rather than simply a long sleep.