Bats hang upside down because it is a defense mechanism. Also, because their wings and hind legs are too weak to help them take flight from stop; hence by hanging upside down the bats can just fall into flight.
When we try to imagine bats, two images pop into our minds. One is of a bat soaring through the air of a dark night, and the second is of them hanging upside down in a cave. However, there is a method to their madness, and even though hanging upside down seems like a very uncomfortable position to sleep in, it actually helps them survive.
Bats are fantastic creatures; they are the only mammals that are capable of true and sustained flight. Other mammals don’t fly, they glide. However, bats truly fly similar to birds. Just like their flight, their ability to hang upside down is an evolutionary trait that has allowed them to survive and thrive.
Unlike birds, the bats cannot launch themselves from the ground, or from a complete stop. This is mainly because their wings are more delicate, as they are just skin stretched between elongated fingers, and cannot generate enough lift to take off from a complete stop. Additionally, bats also have very weak hind legs, which means that they cannot run to generate enough speed to take off. This is were the hanging helps the bats. The bats can just let go from their posts and gravity will help them gain enough speed and momentum to generate flight.
This is also helps them survive, as it allows them to avoid danger. Due to their delicate wings and weak hind legs, the bats are more vulnerable on the ground. Hence, by hanging upside down, the bats are ready to take off at a moment’s notice incase of danger, such as a predator’s attack.
Additionally, it is the best and easiest manner of survival for bats, as it takes them practically no energy to hang upside down. Unlike humans, bats do not have to flex various tendons and muscles to clench something, instead all the bats have to do is to let their body weight close their claws and grip something. Then it takes no effort to hang on, only some effort to open their claws, let go and fall into flight. Also, because their blood circulatory systems differ, they do not get the problem of lightheadedness caused by the blood rushing to the head. Bat have various small arteries and veins in their brains that allows the blood to flow away from the brain faster, hence avoiding the problem.
This behavior of hanging down is called roosting. While bats do dominantly roost in caves, the truth is that they can roost in any damp dark place. This includes the inside of wells, attics, any hollows, crevices, foliage, tree branches, as well as tents that the bats make by biting leaves. There are also many man-made roosts for bat, built mainly by bat-enthusiasts and conversationalists.