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Why some mammals hibernate is pretty easy to grasp. It is mainly to do with the balance between a limited food supply and the increased energy needed to stay active in cold weather.

But the how involves significant changes in their metabolism. They have to prepare their bodies for an extended period of dormancy and make sure they can kick start their systems back up to normal 'running temperature' afterwards.

Science with Chris

Watch Chris Packham's hibernation explanation

During hibernation, breathing and heart rates drop massively. A hedgehog has been observed to hold its breath for 2½ hours.

Slowed respiration lowers energy consumption, but not to zero. The energy pathway for a hibernating mammal is called gluconeogenesis. Instead of relying on food sugars as a basic energy source, the animal's liver and kidneys produce sugar (glucose) from proteins and other body reserves.

A hedgehog shifts its body thermostat down to 4°C during hibernation but it is still controlling its temperature. If it falls too low, it will respond by raising its breathing and heart rate.

Waking up briefly

At intervals – once every week or so for hedgehogs – many hibernating mammals will also rouse from dormancy and may go in search of food or water. This could be prompted by variation in body temperature. These brief spells of activity let the animals excrete waste products that can't be allowed to accumulate in their bodies. A further function of the temporary rousing and activity is a chance for the animal to have a proper (conventional) sleep during which its brain and nervous system can relax and recover.

Implanted miniature sensors have let researchers study how activity rates pick up during these arousals. This graph shows typical breathing rate and body temperature as a European hedgehog rouses temporarily from hibernation one evening.

Graph of heart rate and body temperature against time of day (24hr clock) during arousal of a hibernating hedgehog.
(Sourced under Creative Commons Licence from the Open University's online course ).

Rousing from dormancy and warming up to active temperature consumes an enormous amount of energy. It relies on a type of fat that works differently from the normal reserves that all mammals possess.

Two types of fat

'Normal' fat is called white adipose tissue and is broken down by the body into chemicals that other cells then use as an energy source. It's WAT that the animal uses to keep its essential body systems ticking over whilst inactive.

Hibernating mammals (and many newborns, including human babies) also have brown adipose tissue distributed around their bodies. When the animal uses its BAT, much of the stored energy is directly converted into heat. That is one good reason why you should never disturb a hibernating animal. If it rouses too many times, its reserves of BAT can get exhausted and it will not then be able to get back to full steam once hibernation is over.