Vampire movies skip the indignities of the all-blood diet. (The endless peeing, the bloating…) Only three mammal species, all bats, have triumphed at vampire living.
Evolving as blood feeders “has changed everything about them: how they move, how they think, what their social life is like,” says Gerald Carter of the University of Maryland in College Park. They’ve even developed a system for sharing blood meals.
One major downside of blood is its high water content. To get sustenance, vampire bats have to swallow a lot of blood and filter out the excess liquid. “Often they’ll be peeing while they’re feeding,” Carter says.
A big meal leaves a bat quite plump. “They’re like little water balloons of blood,” Carter says. Roly-poly as they get, they don’t fly away with much extra nutrition. A vampire bat drinks one meal a night, and missing just three nights in a row would probably kill the animal, Carter says.