Bats copulate indiscriminately just prior to fall hibernation, but a female bat delays her ovulation and stores the sperm for approximately seven months before allowing fertilization. In relation to their size, bats have the slowest gestation period of any mammal, from 40 days to six months.
During birth, females reverse the normal "head down" position and catch the pup in a flexible sack between their legs. A newborn hangs onto a nipple using its teeth, thumbs and hind feet. Pups develop quickly with eyes and ears that open shortly after birth. Within 10 days, they control their own body temperature, and by 13 days, their hearing is fully developed. Pups achieve adult size, are independent and fend for themselves after approximately a month.