Mating takes place in fall and winter, but the male's sperm is stored and does not fertilize the egg until the following spring. The populations of maternity colonies range from 120 to 1,350 bats. Most young are born in June after a gestation period of 56-100 days. Each female has a single pup, which is capable of flying at three weeks old. By December the bats enter hibernation after having stored enough body fat to get through the winter. They hibernate in caves with cold, but above freezing, temperatures.