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Michael: Vampires. They’re in all kinds of spooky stories — and certain Young Adult fantasy romance novels. There may not be undead, blood-sucking humans walking around. But blood-drinking mammals do exist, in the form of vampire bats.

Surviving on a diet of only blood, also known as hematophagy, isn’t easy, and vampire bats are one of the few mammals that have evolved a thirst for blood. Along the way, they also developed some useful techniques to help them survive. To be clear, vampire bats don’t exactly suck blood.

Instead, they drink blood using the physics of capillary action, where a liquid can move through a narrow tube or cylindrical space — often against the downward force of gravity. Vampire bats have special grooves on their tongues — which, along with their lower lip and incisors, can create a tube-like shape that easily wicks up the liquid. They do make a lapping motion with their tongue, which helps speed up the process, but it isn’t necessary for capillary action to work.

Another thing that helps keep the blood flowing is the bats’ saliva. Vampire bat saliva contains a protein called desmoteplase, which researchers nicknamed Draculin. It acts as an anticoagulant to stop blood from clotting. Researchers have actually isolated this protein from vampire bat saliva in the hopes of turning it into an anti-stroke medication — since it helps stop blood from clotting, it might also be good at preventing blood clots in human brains.

So when a vampire bat bites into a juicy blood vessel, the proteins in the saliva keep the animal’s blood free-flowing for a big meal. And vampire bats need a big meal every time they feed. Blood isn’t a very rich food source. It’s mostly made up of water, with small amounts of protein, sugar, and trace minerals.

It’s completely lacking in fat, and animals tend to need fat, which is a major type of energy storage in a balanced diet. So vampire bats need to drink about half their weight in blood every day to get enough nutrients to keep their metabolism running smoothly. Since that means they’re taking in a lot of water, their digestive and excretory systems quickly filter out water and leave the proteins, sugars, and minerals to be digested. In fact, vampire bats will often start urinating even before they’ve finished feeding, just to keep things flowing.

Finding food takes some special skills, though. Like other bats, vampires use echolocation to fly around and look for prey. They also have a keen sense of smell and decent eyesight. But the wrinkly faces of vampire bats have specialized nerve fibers loaded with proteins that allow them to detect the infrared radiation given off by warm-blooded animals. Once they find something to eat, the three known species of vampire bats all have unique feeding behaviors.

Common vampire bats feed only on blood from mammals, and they prefer livestock. So these bats evolved ways to quickly maneuver on the ground and reach their prey more easily. They have strong hind leg muscles and elongated thumbs at the tips of their wings, which they use to jump around. They’re agile enough to escape from ground predators.

Hairy-legged vampire bats, on the other hand, feed exclusively on bird blood and hunt from the tree tops. They jump onto unsuspecting birds from above, and usually try to bite the bird in the cloaca -- the multipurpose exit point for a bird’s digestive and urinary tracts, and also the place where it keeps its reproductive organs. For some reason, the birds tend not to like this very much. So, to stop the bird from fighting back or knocking it off, the hairy-legged vampire bat uses the big bony spurs on its ankles to help it hang upside-down and latch onto its prey.

White-winged vampire bats feed on both mammals and birds, but they evolved a few tricks to help them feed on chickens. Sometimes, the bat lands on the chicken’s back, so the chicken thinks the bat is really a mounting rooster. So the chicken just crouches down, giving the bat access to its neck for feeding. Other times, the white-winged vampire sneaks up from underneath the chicken and nuzzles against the brood patch -- a featherless spot of skin normally used to keep chicks warm. The chicken thinks it’s being nuzzled by a baby chick, and lets the vampire latch on and start drinking blood.

With all these complicated feeding techniques, you’d think vampire bats would have no trouble finding a meal. But sometimes they don’t get enough. Luckily for the bats that don’t find enough food, vampire bats live in large colonies that can contain thousands of bats, and they take care of each other using a behavior called reciprocal altruism. Often, one bat will sacrifice their own needs for another, with the assumption that they’ll be helped in return in the future. The main way they do this is very appetizing: they regurgitate blood to share with bats that weren’t able to get a meal. Bats who’ve shared blood meals in the past get larger donated meals in return, compared to more selfish bats.

So yes: like Dracula, vampire bats are skilled, blood-drinking hunters. But they also have to be extra nice to each other to help their colony survive.

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