Dracula’s Children

Millions of years ago, some bats gave up their old habits of hunting for insects and tried something new: drinking blood. These creatures evolved into today’s vampire bats, and it’s mind-boggling to explore all the ways that they evolved to make the most of their sanguine meal.

A lot of the adaptations are easy enough to see with the naked eye. Vampire bats have Dracula-style teeth, for example, which they use to puncture the tough hide of cows. When they open up a crater-shaped wound, they dip in their long tongue, which contains two straw-shaped ducts that take up the blood.

Finding these prey has led to another remarkable adaptation that you can see–at least if you’re a scientist who studies how vampire bats move. Like other bats, they can fly, but on top of that, they can also walk and, yes, even gallop. Here is a video of a running vampire bat made by Dan Riskin (see this Loom post for details). Of the 1200 or so species of bats, vampire bats are among the very few that can move quickly on the ground.

Vampire running! from Carl Zimmer on Vimeo.

But vampire bats have many other adaptations for drinking blood that are invisible. They use their combined senses–long-range vision, a sharp sense of smell, acute hearing, and echolocation–to find their victims. In their noses, they even have heat-sensitive pits that detect the heat of warm-blooded animals. Once they land on an animal, they apply those pits to the skin to locate capillaries full of hot blood close to the surface.

Photo by Bruce Dale/National Geographic
Photo by Bruce Dale/National Geographic

When vampire bats dip their tongue into a wound, they don’t just draw out blood. They also put their saliva into their victim. And in this liquid are still more invisible adaptations for a blood-feeding life. Vampire bats, you see, are venomous.

This may sound odd. That’s because we usually think of venom as a chemical an animal sticks in your body to cause you pain or death. But biologists define venom more broadly than that: it’s a secretion produced in a specialized gland in an animal, which is delivered to another animal by inflicting a wound, where it can disrupt its victim’s physiology.

Snake venom, the sort we’re all most familiar with, can disrupt physiology to the point of death. And it does so in several ways–jamming neurons, for example, or causing tissue to rot. But other animals that don’t set out to kill their victims also produce venom. Vampire bats, for example, don’t want eat a whole cow. They just want to take a sip.

Unfortunately, drinking blood has some drawbacks. Vertebrates come equipped with lots of molecules and cells that plug up wounds. As soon as they sense even a tiny tear in a blood vessel, they start making clots to staunch the flow.

Vampire bats use venom to keep the blood flowing. In a new paper with a title worth quoting in full–“Dracula’s Children: Molecular Evolution of Vampire Bat Venom”–an international team of scientists explore the molecules that vampire bats use to subvert blood’s defenses.

What’s most striking about vampire bat venom is how it goes after its victim from so many directions. Blood clots develop through a series of reactions that involve a chain of enzymes. Vampire bats produce different proteins to go after different enzymes in that chain. Platelets, which are cell fragments, also clump around wounds to help heal wounds. Vampire bats make separate compounds that attacks platelets.

To make their venom cocktail, vampire bats have repurposed old molecules for new jobs. When any vertebrate formed a blood clot to stop a wound, it needs to break that clot down once the wound is healed. An enzyme called plasminogen activator creates a supply of molecules called plasminogen, which chops up the clots. Vampire bats produce plasminogen activators in their blood for this job. But they also produce an extra supply in their mouth glands. When the plasminogen activators get into a wound, they use the victim’s own plasminogen to keep the blood flowing.

Once bats borrowed plasminogen activators to use in their venom, the molecules became better adapted to that new job. Normal plasminogen activators get cleared from the blood stream by other enzymes. That’s important for our survival, because otherwise they would hang around and make it hard to form new clots. Vampire bat plasminogen activators have a slightly different shape that shields them from their victim’s enzymes.

Together, these molecules are so effective that a cow will keep bleeding long after a vampire has flown away. While scientists have been studying vampire bat venom for decades, they’re still finding new molecules in the cocktail. The authors of “Dracula’s Children” applied a new method to the search. They caught two vampire bats and cataloged all the genes that were highly active in their mouth glands. The scientists then identified the genes and studied the properties of the proteins they encoded. They discovered dozens of new proteins. Some of them kill microbes, keeping the bat’s food supply clean. Some expand blood vessels, increasing the flow into the wound.

When a cow gets attacked by a vampire bat, it’s not entirely helpless. Ranchers have noticed that when bats feed over and over again on their herds, the cows bleed for a shorter period of time. Scientists have found that this happens because the immune systems of the animals learn to recognize some of the venom molecules and attack them. In the new study, the researchers found venom molecules that can ward off the immune system. But the venom itself is evolving to escape the immune system’s recognition, taking on new shapes that may allow them to go unnoticed.

Reading “Dracula’s Children” gives me a potent sense of deja vu. I recently wrote a feature about ticks for Outside, and in the research for the piece I learned all about how ticks produce saliva loaded with proteins that, among other things, open blood vessels, use our own molecules to break up clots, and do many of the things that vampire bat venom does.

Vampire bats are what you get if you turn a mammal into a tick. And I mean that as the highest compliment.

(For more on the convergences of parasites, see my book, Parasite Rex.)

22 thoughts on “Dracula’s Children

  1. I love little vampire bats – they practice reciprocal altruism and have strong bonds with others in their group. Of the 3 species, only one is interested in mammal blood – and they can dine without killing. Bat Conservation International has more information, if you’re interested.

  2. Another fun effect of the efficient terrestrial locomotion and galloping: vampire bats can launch with quite a bit of extra weight on board from a blood meal.

  3. “Vampire running! from Carl Zimmer on Vimeo.”
    For a split second there I thought you were chasing the poor thing! 🙂
    Watching its motion somehow reminded me of swimming!?

    Thanks for another excellent article.

  4. It’s really cool how the vampire bats are like leeches, using their saliva to keep the blood flowing. That was a funny video of the vampire bat galloping, but you could also see that the bat was changing the pattern in which it was running (the timing of foot placement).
    -great article

  5. Vampire bats do not land on their prey – they land nearby and walk. Hence the adaptions for walking and jumping. Landing directly on their prey would alert the animal.

  6. Interesting. So technically, anticoagulants and antibacterials are “poisons” in the receiving or prey animals.

    Accepting that, animals with similar anticoagulant properties are: ticks (also protein dissolving chemicals in their saliva); colubrid snakes — black snakes and their relatives; leeches, and vampire bats.

    Back when i was 10 -14 years old, i caught a black racer and it struck and bit me. The catching wasn’t an issue — i caught many snakes (Very Carefully with the poisonous ones) and this was the first time i had been bitten. The snake had two ridges of serrated bone, and the bite mark was a crescent of little dots, some connected, some not. And that little crescent started dripping blood

    Normally, when i get a small cut which goes drip, drip, drip — it coagulates within 3 to 5 minutes. This one dripped for more than half an hour, probably close to an hour of a drip every two or three seconds. If it bit a mouse like that, the mouse would leave a trail of blood and bleed to death in just a few minutes. Not many predators can do something that effective.

    The important part of all this is: gee now i can brag i’ve been bitten by a poisonous snake, and it was no big deal! (grin)

  7. Vampire bats will actually share their blood meals with non-relatives that were not successful in obtaining food. This makes them one of the few species that exhibit seemingly altruistic behavior, since sharing does not increase their own fitness,nor the fitness of kin.

  8. Very Informative, for past couple of days I am reading many post which changed my idea about VENOM. How different animals use it and how it works, Simple and yet very effective. Evolution’s remarkable way to adapt the ever changing environment.

  9. @Donald Young: So close! I nearly got the whole way through the comments without someone trying to bring god into it.

  10. Nice article. I just wish it wasn’t written in such a way that hints that animals somehow choose to evolve these various adaptations.

  11. While Reading this Particular article I had a few questions come to mind and one that kind of stirred me the most was a intriguing one indeed. The article spoke of the Venom Adapting to basically keep ahead of the immune system of the victims. How is the Bats body knowing to make these changes to the Venom, Does it analyze the blood that it drinks threw different sensors as it drinks it?

  12. It is biochemistry. The bat venom reacts to the increased clotting and modifies to increase blood flow. A very efficient mammal parasite.

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