The female bat is possessed of a unique problem - that of flying while burdened by the additional weight of the developing foetus. Birds, by laying eggs, have to carry extra weight only briefly. But the bat carries it for the six weeks or so that elaps between the implantation of the egg in the uterus and the young bat's birth. Although, in temperate climates, bats mate before or during hibernation, the female stores the sperm and fertilisation may be delayed until spring. In one or two species the fertilised egg divides but does not implant until weeks or months later.
Female bats have a second problem. Since they roost hanging by their feet, they must give birth upside-down. A few species turn head uppermost to give birth, and hang by claws on the front of their wings, but most of them do it in the reversed position. The mother catches her newborn offsping, naked and blind, in the folds of her wings. It clings to her fur with its strong claws, and then crawls down to take its first draught of milk from a teat in one of her armpits. Only occasionally does the mother leave the young, and that is to feed herself.