Their winter quarters may be up to 40 miles from their summer breeding- grounds; but once a female has chosen a site, she sticks to it for life. Hanging upside down, she will hibernate in a torpor for a week or 10 days at a time, but then emerge at night to fly about and get a drink. Only if really cold weather sets in after Christmas will she stay put for longer periods; and if need be her store of body fat will carry her right through until early May.
On warm evenings a few bats may still be seen fluttering through the dusk round old buildings and trees; but most have already moved to the caves or disused mine-shafts in which they will spend the winter, and the rest will depart any night now. The mechanics of bat hibernation are still poorly understood. The shortening of daylight hours seems to trigger changes in the creatures' metabolism. In the last few weeks female greater horseshoe bats, for instance, have swiftly accumulated a mass of body fat, almost doubling their weight, from about 17 to 32 grams.