Now bats have a distinctive system of tendons in their legs and when they hang upside down, the tendons pulls the claws together, which gives the bat grip. The heavier the bat is, the harder the tendons will pull on the claws, and the better the grip will be. So by hanging upside down, a bat can hold onto a branch, the side of a wall, or the ceiling of a cave without having to remember to grip. Therefore it can safely go to sleep like this (just like an umbrella hanging from a peg).

This is just one of many fascinating things to learn about bats.