A bat's life cycle begins when the mother gives birth to a single bat, which then feeds on the mother's milk for six weeks. Within two months, bats fly and forage for food, reaching maturity after one or two years. Bats in moderate climates can live more than 10 years.
Big brown bats can give birth to twins, and the Eastern red bat bears litters. Infant mortality rates are high among young bats due to genetic defects, accidents and disease, and many young bats fail to become independent in the wild. However, adult bats have low mortality rates because they are safeguarded from harsh weather and predators during the day. Bats also develop immunity to diseases when living in colonies. Records indicate that some big brown, little brown and greater horseshoe bats live for over 20 years. Other bats have lived for over 30 years.