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Life Cycle of the Little Brown Bat

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Little brown bats are aptly named for their tiny size. The bats can weigh as much as a half-ounce and average almost 3 1/2 inches long. Little brown bats live in most parts of North America, making their homes in caves, trees, wood piles or under rocks. Little brown bats have an uncommon life cycle because they can mate even while hibernating and delay pregnancy.


Little brown bats generally mate before winter in the same site where they will soon hibernate. While male and female bats usually are awake during mating, male bats can also mate with females who already started hibernating. Mating by little brown bats is random, and both sexes can copulate with several bats during mating season. Although the animals mate in late fall, female bats delay ovulation and keep sperm for nearly seven months before becoming pregnant in the spring. Females are pregnant for between 50 and 60 days.

Birth and Early Life

Little brown bats are born in the spring in nursery colonies that can house thousands of mother and baby bats. Females give birth to only one bat during each reproduction cycle. During birth, the mother bat hangs right-side up. After being born, the infant bat attaches himself to his mother's teat to nurse. Little brown baby bats nurse for about two weeks. The baby bats can hear as well as their adult counterparts by the time they reach 13 days old. At 3 weeks old, the baby has teeth and is able to fly away and find his own food.

Adult Life

Adult little brown bats live in day roosts during the light hours and night roosts after dark. For a roost to be adequate, the air temperature must remain nearly the same at all times. Day roosts usually have little to no light and provide a protected shelter, while night roosts are usually closed-in areas where many bats cluster. The animals hunt during the night, primarily preying on small insects such as stone flies and mayflies. They capture their food with their teeth or by using their tail or wings to catch insects.


By late fall, little brown bats prepare for hibernation by finding a suitable roost such as a cave or mine. The first animals to arrive begin swarming around the roost to help other bats find it. Adult bats usually are the first to arrive, followed by females. Little brown bats hibernate between September and October, emerging around April. The animals sometimes wake up every few weeks during hibernation and will fly around if the night is warm, but they won't feed. Fat reserves provide the animals with energy during hibernation.


Little brown bats live an average of six to seven years, although some have been known to survive past 10 years. The bats' ability to find food in many areas and use various places as roosts allows them to survive changing conditions and climates. Males generally live longer than female bats. A little brown bat is most likely to perish during its first winter since younger bats weigh less than adults and have less fat stored. In addition, disturbances during hibernation such intrusions by humans can sometimes cause mortality.