A vampire bat's wingspan is typically 8 inches, though its body is about the size of an adult human thumb. It feeds on the blood of horses, pigs and even birds. The common vampire bat, Desmodus rotundus, prefers cows, however.
The vampire hunts at night, when other animals are sleeping. It doesn't suck blood. It uses heat sensors to find a victim's veins. Sharp teeth cut the animal, and the bat simply laps up what oozes out.
A chemical in the bat's saliva keep the blood from clotting, so it keeps flowing (a blood-thinning drug developed from vampire bat saliva helps prevent strokes and heart attacks). Another chemical numbs the victim's skin so it won't wake up.
"They sit there licking the wound for up to a half hour," says Daniel Riskin of Cornell University. A bat will drink about a tablespoon of blood in a sitting.
Vampires have attacked humans, but such reports are rare.