All but two Montana species roost in caves or man-made structures. The hoary bat roosts in tree foliage, and the silver-haired bat roosts in bark crevices. These roost preferences may explain the fact that all but these three species give birth to more than one pup per year. Cave roosting species nearly always give birth to one offspring, putting their energy into a single, large pup which may have a high chance of survival due to the relative safety of the cave environment. Foliage and bark roosting bats often give birth to twins, and triplets or quadruplets may occasionally be seen.
Many bats use a reproductive strategy called delayed fertilization.
Mating occurs in the late fall, but the female's egg is not fertilized until early spring. Parturition takes place from early June to mid-July. The gestation period varies widely in response to environmental conditions. A cold, rainy spring may result in a later parturition date, while a warm, dry spring may result in an earlier one. Pups grow rapidly.