making them too heavy to fly. So when feeding on liquid rich blood they begin urinating almost immediately.
https://en.wikipedia.org/wiki/Vampire_bat#Feeding
ref - https://www.reddit.com/r/todayilearned/comments/f7j5jj/til_vampire_bats_can_consume_50_of_their_weight/
origin - https://www.pipiscrew.com/?p=17139 vampire-bats-can-consume-50-of-their-weight-in-blood