After arriving at winter caves, copulation occurs. Females immediately enter hibernation, while males may remain active for a few more weeks. Males use this extra time before entering hibernation to replenish fat reserves used during breeding. Males begin hibernation by early November. During hibernation, the body temperature of gray bats drops close to the ambient temperature, allowing the body to conserve fat. These fat reserves must last the approximately six months of hibernation and spring migration. Adult mortality is especially high during spring migration, as bats that do not have sufficient fat reserves have difficulties surviving the stress and energy-intensive migration period. After copulation, females store sperm in their uteri, ovulating only after they have emerged from hibernation. Gestation in gray bats lasts 60 to 70 days, with birth occurring in late May and early June. Gray bat females give birth to one offspring per clutch (bout of reproduction), thus giving birth to one offspring per year. Therefore, gray bats demonstrate an iteroparous life-history strategy. The young clings to the mother for about a week, after which they remain in the maternity colony until they are able to fly. Most young take flight by four weeks of age (late June to mid-July).