Do Vampire Bats Really Drink Blood? How They Feed

Which bats drink blood?

Only three of the world’s more than 1,400 bat species feed on blood, and all live in Mexico, Central America and South America. They are the only mammals with a diet made up entirely of blood.

The common vampire bat is by far the most numerous and the best studied, so most of what follows describes it. Bats in Europe, the US and Canada eat insects, and fruit bats elsewhere eat fruit and nectar; none of them drink blood.

How do vampire bats find a meal?

Common vampire bats leave their roost only after it is completely dark, usually flying no more than about 5–8 km (3–5 mi) to feed. They locate sleeping animals using smell, hearing, eyesight and quiet echolocation calls. Laboratory experiments suggest they can even recognize individual animals by the sound of their breathing, which may help them return to the same host night after night.

Unlike most bats, the common vampire bat is agile on the ground. It often lands near its target and walks or hops up to it on all fours, using its folded wings as front legs, then climbs onto a leg, shoulder or flank. If the animal moves, the bat can jump sideways or backward to safety.

Heat sensors in the nose

Before it bites, the bat searches for a spot where blood runs close to the skin. It does this with heat sensors on its nose leaf and upper lip, which detect infrared radiation from warm skin at short range. In 2011 researchers reported in the journal Nature that vampire bats tune a heat-sensitive channel called TRPV1 so that it responds at about 30 °C (86 °F), much lower than the threshold that signals painful heat in other mammals.

This makes the vampire bat one of the very few vertebrates known to sense infrared heat, alongside pit vipers, pythons and boas. Favorite bite sites on cattle and horses include the neck, flanks, rump, ears and around the hooves.

The bite: razor teeth and a grooved tongue

If the skin is hairy, the bat first trims the hair away with its teeth. Then its upper incisors, which are large, blade-like and lack enamel so that they stay razor-sharp, remove a tiny flap of skin. The wound is only a few millimeters across and deep.

The bat then laps the blood that wells up. Its tongue has grooves along the sides that open and close as the tongue moves in and out, drawing blood into the mouth. The cut is so neat that hosts often keep sleeping, and only one bat usually feeds at a wound at a time, though a mother may share a wound with her pup.

Draculin: saliva that keeps blood flowing

Blood normally starts to clot within minutes. Vampire bat saliva contains several substances that prevent this, including an anticoagulant protein named draculin, after Count Dracula, and compounds that keep small blood vessels from narrowing. Other saliva proteins dissolve clots that have already begun to form.

These substances are so effective that a vampire bat bite may keep oozing blood after the bat has gone. A clot-dissolving protein from the saliva was later engineered into an experimental stroke drug called desmoteplase, although it was not approved for use.

How much blood does a vampire bat drink?

A common vampire bat typically drinks about 20 ml (4 teaspoons) of blood in one meal. That sounds small, but for an animal weighing around 25–40 g (0.9–1.4 oz) it is roughly half its body weight, enough to make flight difficult.

The bat solves this problem quickly. Its stomach and gut absorb water from the blood almost immediately, and the bat starts to pass dilute urine within about two minutes of starting to feed. Even so, a well-fed bat is so heavy that it uses its powerful wings to spring almost straight up into the air to take off.

Blood is a strange food: it is mostly water and protein, with very little fat or sugar and a lot of iron. Vampire bats have special kidneys and genes that help them cope with the iron and protein load, and they burn amino acids for energy. Because they store little fat, they can starve after about two to three nights without a meal, which is why they share food with roost-mates.

Do vampire bats prefer livestock?

In the wild, common vampire bats once fed mainly on tapirs, deer, peccaries and other large mammals, and on parts of the Peruvian coast they still bite sea lions and even penguins. Today most of their meals come from cattle, horses, pigs, goats and donkeys, which are numerous, large and often sleep in the open. Given a choice, they seem to prefer horses over cattle.

This switch to livestock has helped vampire bats become very common in ranching regions, from Mexico to Argentina. It also makes them a serious problem for farmers, because the bats spread rabies to cattle. You can read about where the three species live in Where do vampire bats live?