The males and females of some species even live in different regions of a continent except during the mating season. The mating season, which lasts for only a few weeks, may occur during the spring, fall, or winter, depending on the species. The female may carry the male's sperm within her body for several months before she becomes pregnant. In the spring, pregnant bats may leave their usual colony and join together in a nursery colony, where they give birth and rear their young. The females of most species of bats have one baby a year.
Some bats have a baby twice a year.
A few species have up to four young at a time. A newborn bat weighs about a fifth as much as an adult bat. Bats do not build nests, and so the infant must hold on to its mother or its home roost. Among some species, the infant clings to the mother for several weeks. The infants of other species stay in their roost, where they hang from a rock alone or with a group.