At night, bats fly through the air, catching hundreds of insects and other small animals. But during the day, they hardly move at all. Instead, bats pass the time hanging upside down from a secret spot. There are a couple of reasons why bats rest this way. First of all, it puts them in a position for takeoff. Unlike birds, bats can't fly into the air from the ground. Their wings don't produce enough lift to take off from a dead stop, and their hind legs are so small and underdeveloped that they can't run to build up the necessary takeoff speed. Instead, they use their front claws to climb to a high spot, and then fall into flight. During the hours when most enemies are active, bats gather where few animals would think to look and most can't reach. This allows them to disappear from the world until night comes again. There's also little competition for these resting spots, as other flying animals don't have the ability to hang upside down. Bats have a unique physiological adaptation that lets them hang around this way without using any energy. For you to hold your fist around an object tight, you contract  several muscles in your arm, which are connected to your fingers by tendons ;as one muscle contracts, it pulls a tendon, which pulls one of your fingers closed. A bat's talons  close in the same way, except that their tendons are connected only to the upper body, not to a muscle. To hang upside down, a bat pulls its claws open with other muscles. To get the talons to take hold of the surface, the bat simply lets its body relax. The weight of the upper body pulls down on the tendons connected to the talons, causing them to hold tight. Therefore, the bat doesn't have to do anything to hang upside down. Why can bats hang upside down easily?
Because their upper body is light. Because they have strong muscles. Because their talons are linked to muscles tightly. Because their tendons are linked to their upper body.