Bats usually mate in the autumn, in some species they fly about cave or building entrances to meet each other, in others the males call to females from a perch. Occasionally mating also occurs in the winter. The females store the sperm throughout the entire winter, and wait until the spring to ovulate and fertilize the egg. This enables the females to wait until they perceive the conditions to be right to become pregnant. They can also slow pregnancy if conditions get worse and food is scarce or the weather very cold. This reproductive strategy is unique to bats.In winter, when insects are scarce, bats hibernate. They choose places that heat and cool slowly, usually in caves, cold parts of buildings or hollow trees. Warm weather or disturbance can cause them to wake, using precious reserves of stored energy, which may cause them to run out before spring and die. Studies indicate nearly half of young bats do not survive their first hibernation. We still only know where a very small proportion of the bat population in Britain go to roost and hibernate, unfortunately some of the ones we know about are because people have accidentally discovered the bats during hibernation and the roost has run into problems.
