High-speed video shows that bats achieve some of their efficiency in flight by pulling their wings inward on the upstroke, as seen above. While this does affect drag forces on the wing slightly, the primary energy savings comes from the inertial ease of lifting the folded wing. Much the way it is easier to lift your arm when it is folded than when you stretch it outright, it takes less energy for the bat to lift a folded wing than one that is fully extended. (via Wired Science)
Flapping to Fly Efficiently
