Bats have several reasons why they hang upside-down. The first one is to launch themselves into flight quickly if they need to. The structure of the bats wing does not provide enough lift to take off from the ground, and their legs are not developed enough to thrust themselves into the air. So they roost in high places where they can take advantage of the force of gravity to launch them into flight. Another reason is the way the bats talon, or foot, works. The bat has to exert energy to get the talon to open, so no energy is exerted to get the talon to close. All the bat has to do is get the talon to open, then relax for it to close and grasp something. This is how bats are able to sleep hanging from their roost, because the natural relaxed position for the talon is to be closed. Another reason bats hang upside down is to avoid predators who would not think to look up, and even if they did, probably could not get to them.