What Is Hibernation?

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.

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.

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

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.

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.

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.

Waking from deep hibernation is a feat of heating. Many hibernating mammals rely on brown fat, a tissue packed with mitochondria that can release energy directly as heat instead of storing it as ATP. Deposits around the shoulders and chest warm the heart, lungs and brain first, and shivering joins in once the muscles are warm enough to work. Because rewarming from near freezing to normal body temperature is so costly, even a few extra wake ups over a winter can drain the fat reserves an animal needs to reach spring.

That energy budget explains the damage done by white-nose syndrome, a fungal disease that has killed millions of hibernating bats in North America since it was first recorded in New York State in the mid 2000s. The fungus grows on the bats' skin in the cold, humid caves where they hibernate, and infected bats wake far more often than normal. They burn through their fat before winter ends and many starve. Researchers track this with small temperature loggers attached to bats, which record every arousal and show how finely balanced hibernation really is.

For a long time scientists assumed birds could not hibernate. Then, in the winter of 1946, the naturalist Edmund Jaeger found a common poorwill, a small nightjar-like bird, tucked motionless in a rock crevice in a Californian desert. Its body was cold, it showed no sign of breathing, and it was found in the same spot in later winters. Jaeger's discovery confirmed that the poorwill can enter long periods of deep torpor, making it the only bird known to hibernate. The Hopi people had apparently noticed long before, as their name for the bird is often translated as 'the sleeping one'.

Hibernation is not limited to cold places either. The fat-tailed dwarf lemur of Madagascar hibernates for several months during the tropical dry season, when food is scarce, storing fat in its tail beforehand. Sheltering in tree holes, its body temperature rises and falls with the temperature of the hole around it. This shows that the real trigger for hibernation is a shortage of food and water, not cold itself, and that a primate, closely related to us, can survive the same deep metabolic shutdown as a squirrel.

Shortening days and falling temperatures help prompt hibernation, but some species also carry a calendar inside them. In the 1960s researchers kept golden-mantled ground squirrels in laboratories with a constant temperature, unchanging light and food always available. With no seasonal cues at all, the squirrels still fattened up, hibernated and woke again on a regular cycle, year after year. The telling detail was that the cycle was not exactly a year long. It typically ran somewhat shorter, so over time it drifted out of step with the real calendar.

That drift is the classic sign of a genuine internal rhythm, called a circannual clock, in the same way that people living in caves without daylight gradually drift away from a strict 24 hour day. In the wild, changing day length acts like a hand that nudges the clock back into line each year. An internal calendar is valuable for an animal that spends months curled up underground in darkness, where it cannot see the days lengthening. It allows the body to begin preparing for spring before conditions on the surface give any sign.

Hibernation has a shorter cousin called daily torpor. Hummingbirds burn energy so fast that a cold night could exhaust their reserves, so many species drop their body temperature and metabolism sharply while they roost, then rewarm before dawn. Some Andean hummingbirds have been recorded cooling to only a few degrees above freezing. Bouts of daily torpor last hours rather than the days or weeks of a hibernation bout, and the animal still feeds normally by day. Some small mammals, including certain mice and bats, use daily torpor too, which shows that hibernation is the extreme end of a range of energy-saving strategies.

In Britain, the hibernator you are most likely to meet is the hedgehog. Hedgehogs usually hibernate from around November to March, depending on the weather, in nests of leaves built under hedges, in compost heaps or in piles of garden debris. That habit puts them at risk around bonfire night, because a heap of wood and leaves gathered weeks in advance makes an inviting shelter. Wildlife charities advise building a bonfire on the day it is lit, or moving the material before lighting it. Young hedgehogs born late in the year may also be too light to survive a winter of hibernation without help.

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.