Reproduction – Bats are very unusual for mammals their size…they have small numbers of young. Many bats have only one baby per year, others one to two, and just a few have as many as four.
Bats mate in the fall but give birth in the spring. Many females store the male’s sperm until spring, others keep the fertilized egg from growing until then. The advantage to fall mating has to do with the small size of bats. In the spring, they are coming out of winter at the end of hibernation, or at least, feeding less from at least some days of torpor. That means males are not as strong or healthy which would make mating behaviors more difficult. They avoid that problem by mating in the fall.
Then the females give birth in the spring when their food comes alive again. They need a lot of food to feed both themselves and their babies.
In many bat species, females come together to the same roost to stay as a group when giving birth and raising their young. This gives some added warmth for the group and young bats need more warmth to develop properly. In addition, it gives more protection from predators because the number of individuals can be distracting to many predators.
Babies grow to flying size within a couple of months, so by mid-summer or so, they start leaving the roost. At this time, the young are 95% the size of adults, so size cannot help you tell them apart.
Longevity – Also unusual for mammals their size, bats have relatively long lives. Most bats can live at least 10–14 years, and some have been recorded as living 30–40 years. No other mammal of their size lives that long.