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Has anyone seen a Bat sleeping upright? despite the fact that they have legs like normal birds do?
From what i think, it's because their feet is too fragile and tiny to support their weight if they stood on them. They can't actually grip with their hands, so use feet instead. And from studies about Bats, they can't launch themselves from the ground to take off like birds do, so from a hanging position they can use gravity to their advantage to get airborne and fly off.
Any other ideas is welcomed. Thank you