Bats seek a suitable hibernaculum (place to hibernate) in response to changes in the weather. In the UK this is usually from October or November. With such tiny bodies energy conservation is crucial. The reduced supply of insects in winter months means it is not possible to balance this increased energy need in the colder months with the food supply. Even in hibernation bats might cluster together to reduce energy loss. They are not asleep throughout the whole hibernation period. Period of wakefulness might be filled with drinking, some foraging for food, urination, defecation and occasionally in some species mating. However bats are in a deep sleep for periods as long as two months. Hibernation ends in Spring around March or April. Bats choose their hibernaculum for its constant temperature and humidity and so it is thought their awakening is governed by some sort of internal mechanism more than changes in external temperatures.
Every species of bat has a preference for a particular temperature in hibernation. They choose the site, called a hibernaculum, based on this need. All bats need a fairly humid environment to prevent them from drying out during the long winter hibernation. Often this is obvious from drops of water which accumulate on the skin and fur of hibernating bats. There are three main groups with similar hibernation choices. Horseshoe bats are typical of those species that like to hang freely from the ceiling or a wall by their hind feet. Daubenton's bats will select a similar environment but require the presence of crevices into which they can crawl if the main environment starts to become less suitable for their needs. The pipistrelle is typical of the third group which likes small enclosed spaces, preferably allowing their body to be in contact with a wall on all sides. Caves, cellars and tunnels are the most likely places to find a combination of high humidity and small crevices. Roof spaces which are happily used for breeding and roosting are not so suitable for hibernation.
In winter, some bats will hibernate for several months, while others migrate to areas where insects are still to be found. Hibernating bats do not suffer in low temperatures even though, of all mammals, bats undergo the most dramatic change when they go into hibernation. Their body temperatures may drop below the freezing point of water.
In North America, red bats have been found alive and perfectly healthy with body temperatures of -5C; they do not freeze solid because blood freezes at a lower temperature than water. Such bats often become covered with dew, which does freeze, and so they may spend several weeks encased in ice.
When in hibernation, bats use only a tiny fraction of the energy they require when active. Whatever energy they do need is supplied from the fat reserves laid down at the end of summer. The fat is stored between the shoulders, and it is here that the highest temperature can be measured when the animal begins to emerge from its hibernating state.
I don't think bats hibernate because we have lots of bats around are house and in the winter they go in are upstairs.then when we make noise up there they come out and are cat has been killing them and ate one of there heads off and it was still flying.
