To understand this phenomenon you must first understand #bats are not birds or insects, they are mammals. Thus meaning, their bones are not hollow and are significantly heavier than other flying species. Their build does not support their lifestyle in the frame we see, but turn everything upside down and it is highly advantageous. This take off disadvantage is avoided by using gravity to assist in flight. By starting from an inverted position all the bat has to do is release its grip and open its wings. This gives them instant flight allowing them to get to top speeds quickly. In addition to flight, due to their mammalian make up, #bats do not have tendons that apply pressure in a downward force such as a bird perched on a limb. A bat will get its grip strength from tendons stretching and pulling their toes closed. It is a very similar movement to a bird perch, just backwards. Many wonder, how does a bat defecate or urinate upside down? Simple answer is bat fecal matter is ovular like a small grain of rice, this is why you typically see it fall into piles. Urination on the other hand is a little more tricky, they typically wait until they are in flight. These patterns are not ideal in a residential environment. While we love having bats in nature, your attic should not harbor them.