Bats. They see the world upside down. Literally. But why do bats hang upside down? Is it their way to seek protection? Is it their way to hibernate? Is hanging upside down just instinctive? Or is it an adaptation? The answer to all these is a 'well, yes'.
Hanging upside down is not the only different-from-normal behavior of bats. Something equally strange yet interesting is that they are the only flying mammals. Plus, they have very good echolocation skills. This means, they emit ultrasonic signals, which on echoing back, help them gauge distances. This skill helps the bats in locating their prey and also in protecting themselves from their predators. Bats' wings and legs have a unique structure. They can lock their feet to a support and suspend their bodies down, without the fear of falling. In this mechanism, the bats open their claws (like you would hold your fist open), then find a support to grip to (like you would hold an object in your clenched fist). From the support, when a bat suspends itself down, its weight causes the tendons to clench. The talons connected to the tendons lock themselves to the support and this is how bats can remain in the hanging position. The wings are like a webbing that extends from their legs and up, which can be wrapped around their bodies when roosting. This structure of their wings helps them in quick movement.
There's something else that helps bats take quick flight. And it is, yes, their hanging upside down! How does it help? In the upside down posture, it's easier for the bats to take flight by just dropping themselves down and gliding. They cleverly use the gravitational pull to their advantage by dropping from a height to take flight. They cannot launch into flight from the ground. Their body mass does not allow them to. Plus, neither their wings are that developed nor their legs, meaning they can neither run to take the lift, not their wings are capable of giving them flight.
The second reason why bats hang is that they cannot stand! It's strange but true. Bats are flying mammals. Unlike other birds, bats cannot stand firmly or walk. So their resting position is hanging, that too, upside down! This posture helps their muscles relax. Imagine a tired you hanging down from a support. No, that wasn't a joke. It was meant to help you realize how you would feel then. Won't you feel a relaxing stretch in your body? That's how bats feel!
Bats hang upside down during torpor. Torpor is temporary hibernation. During this period, bats conserve energy. Their breathing and heart rates are lowered, which helps the bats further save their energy. Now hanging upside down would mean that the body is suspended in the direction of gravity, which means less or no consumption of bodily energy. Locking their feet to a support, bats hang down and enter torpor.
This was precisely why bats hang in the inverted position. The most prominent reason why bats hang that way seems to be, to take a quick flight. They have to 'fall down' to 'fly up'. Actually, in the bottom up position that they hang in, our 'down' is their 'up'! Interesting.