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Vampires’ gift of ‘blood honey’

A Maryland biologist probes the unusual dining behaviors of a blood-thirsty bat.

Carter’s group at Maryland has been studying meal sharing among common vampire bats, the species pictured here.

belizar/Shutterstock

Going as a vampire for Halloween? To be truly “authentic,” you might want to pee a lot. And often. That’s what vampire bats do, explains Gerald Carter. A bat ecologist, he works at the University of Maryland, in College Park. There he manages a small colony of the blood diners, studying their feeding behaviors.

To get a decent meal, bats must drink a lot. But blood is mostly water. So to extract nutrients from their soupy meal, these animals need to eliminate a lot of liquid waste. Pee. In fact, Carter observes: “Often they’ll be peeing while they’re feeding.”

Going even a day without food is a big deal for these animals. Missing just three meals in a row kills them. Fortunately, vampire bats lead complex social lives. And the bonds they form with others can provide a safety net against death by starvation. Carter is studying this — how vampire bats deliberately throw up a little of their last meal to share with another who is starving. A donor doesn’t really hurl. Its regurgitated blood stays right at its mouth where a hungry vampire can spend a few minutes lapping it up.

A vampire bat’s willingness to share blood has been recognized for decades. What’s remained a mystery is why. If a bat has to drink so much blood to derive enough energy to survive, why would it give away some of that fuel?

Perhaps it’s not surprising that a bat might cough up some food to help a starving child or other family member. But years ago Maryland scientist Gerald Wilkinson observed vampires also giving blood to individuals outside their families. He suggested vampires might be working from the idea that: If I help you, then maybe one day you’ll return the favor. People certainly do this. It’s actually called trading favors. But there were few data to show that animals, such as bats, also trade favors. So, several years ago, Carter and Wilkinson began tests to see if their vampires might.

Periodically the biologists now make a healthy bat miss a meal. The next day they release it into a cage of roost mates, all of whom had finished eating a couple of hours earlier.  Then the scientists watch to see whether the hungry bat gets a meal donation — and if so, from whom.

More often than not, a well-fed bat volunteered some of its last meal to a hungry bat. Who got the gift could be a donor’s child — or just a friend, the scientists found. In fact, a starving vampire would typically receive donations from about three or four bats. That meant that no one animal paid too high a price for being generous.

Getting back to the issue of water: Once bats have filtered the excess from blood, they’re left with a thick and rich liquid.  It isn’t much different from what honeybees do with the thin nectar they collect from flowers. Over time, they remove its extra water. The final product is honey. So, Carter observes,  what the bat throws up as a gift might be considered “a kind of blood honey.”

If the bat getting this gift wasn’t a family member, it often was a bat that tended to groom the donor. Since friends groom friends, this suggests that vampire bats tend to reserve gifts of blood for their pals. But even a starving bat can be picky, Carter and Wilkinson found.  Hungry bats sometimes rejected an offered meal when it came from certain individuals (perhaps not a pal). Carter and Wilkinson reported their findings earlier this year in a British journal, the Proceedings of the Royal Society B.

The ‘super’ bats

Speaking of dining habits, even in the wild “Vampire bats don’t suck,” Carter observes. They carefully lick blood from a host’s skin. To get at it, the bat nicks its grocer with razor sharp teeth. Most bitten animals never know their blood supply is being tapped as the super-stealthy bat daintily licks up its salty red soup.

Surviving on nothing but somebody else’s blood can be challenging for a flying animal. A decent meal of the watery food leaves bats swollen and heavy.  After each dinner “it’s like they’re all pregnant,” Carter says. Not surprisingly, that makes it hard for them to fly. So to get into the air, “they do like a flying push-up,” he says. They have huge thumbs, two times the length of other bats’ thumbs. “They smack thumbs against the ground,” and their bloated bodies lurch upward. Once they get this head start into the air, they flap away.

Even with these tricks, they don’t fly away from a meal with much extra food. They need to feed every night. Many animals (such as mosquitoes) also drink blood. But this food remains just one part of a bigger menu.  Indeed, very few animals have evolved what it takes to live on nothing but blood. Among mammals, from mice and cats to bears and people, only three species manage to do this. (To find out what the other two are, read on.)

So where do Carter’s bats get their meals? Maryland is hundreds of kilometers too far north for any of these animals to survive outdoors. So Carter maintains an indoor colony. Once a month he goes to a slaughterhouse and fills up gallon jugs with blood. He has to stir it around and scoop out clots so it will make smooth drinking. Then he freezes the supply. “It’s pretty gross,” he admits. But that’s a small price, he adds, for getting a chance to work with such remarkable and alert social animals.

“They really watch you,” he notes. “If you’re walking around the room, the vampire bat will track you. It will turn its head and watch what you’re doing.”

“I think of vampire bats as super bats,” Carter says. “They’re super strong, super fast and super intelligent.”

Dodge and sprint artists

In fights, bats have great moves. That’s important for an animal that needs to bite strong, dangerous animals many times its size. Carter found an old research report that described a creepy experiment: Someone trapped a vampire bat in a cage with a rat snake. Again and again the snake struck at the mammal. But the bat dodged every strike — and then ended up feeding on the snake’s face.

Also rare — at least among bats — the vampires are nimble on their feet. In fact, they can manage a quick, if strange-looking sprint. “It’s this completely novel form of running,” Carter says.  Vampire bats swing their folded wings and front limbs forward, a little like somebody swinging a pair of crutches out in front of them. Then the bat hauls it hind legs forward to catch up.  

As far as most running animals go, this is “sort of backwards,” Carter explains. Usually rear legs power the run.  Bat ancestors, however, developed the front-limb bones and upper body muscle bulk to drive their flight. When some of these long-ago fliers evolved into modern vampire bats, the animals came up with a new way to run — using front-leg drive

Oh, and those other exclusively blood feeding mammals: They’re two other types of bat, both of which prey on birds. Says Carter: “They’re actually much cuter” than the common vampire bat (Desmodus rotundus) that he studies. One of those other vampire bats climbs trees, dangles under branches and waits for a chance to bite bird toes. The other keeps climbing up a bird’s body, eventually wriggling down amongst its feathers to feed. Yum!

Power Words

clot  A thickened clump of material within a solution, created in part from the solution.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

evolve  To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs).

groom  (in zoology) The practice of some animals to clean another, usually in places the groomed animal can’t see or reach, such as the back, head or face. Sometimes a groomer will remove ticks or other parasites. Other times it might remove tangles in fur or debris such as leaves. The attention the groomed animal receives can be calming and is usually accepted only from a family member or close member of its social group.

mammal  A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

nectar  A sugary fluid secreted by plants, especially within flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.

regurgitate  To bring up something from the stomach that had been swallowed but not fully digested.

roost  To rest on a branch or above-ground perch. A roost is also the name for the place where a winged animal perches.

species  A group of similar organisms capable of producing offspring that can survive and reproduce.

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