Vampire reality check

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.

In the face of such evolutionary pressures, vampire bats have become “superstrong, superfast and superintelligent,” Carter says. Unlike most bats, the common vampire, Desmodus rotundus, can run on the ground if it needs to dart among cattle and the other sizable mammals that it feeds on in Central and South America. And it’s agile. An old experiment caged a vampire bat with a rat snake, Carter says. The trapped bat dodged the snake’s strikes and ended up feeding on its reptilian face.

These bats, which can live several decades, switch among roosts and form complex social networks, a bit like dolphins’. Vampire society creates a safety net, as bats will regurgitate concentrated blood for a starving roostmate. “It looks like they’re French kissing,” Carter says.

Carter’s adviser, Gerald Wilkinson, stirred excitement and controversy in 1984 by reporting that bats sometimes regurgitate for nonrelatives. An animal example of altruism without family ties intrigued theorists. But critics suggested other
explanations, such as hungry bats harassing neighbors until they regurgitated.

So far, harassment doesn’t look like the explanation, Carter and Wilkinson reported in the Proceedings of the Royal Society B in January. Potential donors in the study colony were more likely to make the first approach to a pal in need of regurgitated favors. 

A montage of video shows vampire bats regurgitating blood for hungry pals. The bats can survive only a few days without blood.  Credit: G. Carter