Females can mate during their first fall, but some do not breed until their second year. Males become reproductively active during their second year. Breeding occurs in and around hibernacula in fall. During the breeding season, Indiana bats undergo a phenomenon known as swarming. During this activity, large numbers of bats fly in and out of caves from sunset to sunrise. Swarming mainly occurs during August to September and is thought to be an integral part of mating. Bats have been observed copulating in caves until early October. During the swarming/breeding period, very few bats are found roosting within the hibernacula during the day. Limited mating may also occur at the end of hibernation.