Most bats have only one baby a year.
Many types of female bats group together to make a nursery before they have their babies. They usually make their nurseries in a cave. Many babies and their mothers crowd together in the nursery. With all of the bats together the temperature is raised to over 100 F. That is good for the babies, they need a warm and humid place to keep them safe. They have no fur to protect them and keep them warm.