I had earlier displayed these following on the Poll of this blog: Why do bats hang upside down in the ceiling?
For hibernating easily
For easy take off to flight
Predators cannot access them from below
Bones are weak for upright posture
For all the above reasons
Easier to locate the predator ultrasonically
Reason not yet found or proved
What could be your choice from these? You can click on comments placed below this post and check the answer. I have revealed it there!