Hibernation

Animals have many ways of surviving harsh weather when, because of heat and drought or low temperatures, the water and food supplies fall. Some simply move to where food is still abundant. The lives of some African herbivores are a continuous migration following food supplies. Many others animals go into a state of torpor, which is referred to as hibernation in cold conditions or aestivation in hot, dry climates.

Cold blooded animals, such as insects, amphibians and reptiles, hibernate without any great physical changes. They find a place where the temperature will not fall below freezing. As their surroundings cool, their bodily activities drop to a level where they are using little energy to stay alive.
No large mammals hibernate truly, but many small species, including some insectivores, bats and rodents, do. They must make enormous physiological changes in order to survive, perhaps for months, without water or food. To conserve energy, their body temperature drops by many degrees, their heart beat and blood circulation is reduced to a very low level and their breathing almost stops. Hummingbirds are able to drop into a torpid state when not in flight; nestling swifts may do so if food is not available, but the only bird known to hibernate fully is a poor will, and even it has periods of torpidity lasting days rather than months.