Evening bats are promiscuous animals, as a male evening bat often mates with up to 20 different female bats. Females will eventually form colonies with other females and rear their young in these colonies. Mating usually occurs in the early fall, and then in the spring, the female evening bats usually give birth to two pups. These little pups are entirely helpless until they are about three weeks old. The male pups leave at around six weeks old and the female pups stay in the colony.