Why can't I recognise a face upside down?
Why can't I recognise a face that I've known for many years upside down?
I'm afraid I don't have a clue! Although what I can say is that although we can't do it and other animals like sheep and cows can't recognise other sheep and cows when they're upside down, monkeys can actually do it. So it would seem to imply that if you habitually hang upside down then it's something you can do. They've not looked at bat face recognition as far as I know!