Why does a bat need a toilet finger

The flying membranes, stretched in bats between the limbs and the tail, are fixed on a frame, which is formed by highly elongated fingers of the forelimbs, metacarpals and forearm, while the thumb, equipped with a claw, is free and helps the animals to climb. This bat thumb is sometimes called the toilet thumb for the following reason. Resting on the "roost" (for example, on a horizontal branch), the bat always hangs upside down, holding on to the "roost" with the fingers of its hind limbs, and when it needs to get rid of body secretions, it grabs the "roost" with its thumbs and turns over so as not to get dirty.