A description of the changes in a mammal’s body from the start to the end.

Hibernation is a process by which animals enter a state of inactivity for long periods of time, usually during colder seasons, so as to conserve energy during times in which food may be unavailable.

In times when food sources become scarce, an animal may be searching for their next meal for most of their waking hours, and thus expending more energy than they would be able to take in from whatever food they might eventually find. Since this inefficient way of survival would ultimately lead to starvation, some mammals have evolved instead to sleep through the harsher periods, with their internal processes slowing to a minimum to reduce the energy demand.

Prior to settling down to hibernate, the animal will consume far more food than they would normally. This builds up a large store of fat which will fuel what little activity takes place during hibernation and prevent muscle tissue from being consumed.

When the animal finally begins to hibernate, certain physiological changes take place which vary between different mammals. For bears, the body is able to internally recycle it own waste products, so that the bear need not feed or excrete. Rodents, meanwhile, experience brief periods of a return to higher body temperature in between long stretches of dormancy.

The general pattern for hibernation is that, as the body shuts down, it adjusts so that the body temperature is far lower than normal (from around 310K to 277K). The advantage of running a lower core body temperature is that less energy is expended to produce heat, and thus the fat stores last for longer. Breathing and heartbeat then slow down to a minimum, leaving the body effectively ticking over so that, from outside, it may appear almost dead. Most mammals would remain in this state for months, but others – those which cannot store large amounts of fat – will awake periodically to feed on food stores in its nesting place before resting again.

The end of hibernation, if not automatic, is normally triggered by the return to a higher external temperature. When the environment becomes warmer, the hibernator senses the change and begins to re-animate. The core body temperature will rise to normal waking levels, and bodily functions will speed up again until the animal is fully awake. Mammals who engage in torpor, such as bears, can become active again relatively quickly, whereas “deep” hibernators may take days to fully regain consciousness. Re-activating the body burns a large portion of the fat reserves (to generate enough heat), so animals which are aroused too early may be unable to survive for the rest of the hibernation period.