Bats on the Run

On a warm summer night in Mexico, a herd of cattle slumbers in a field. With hardly a sound, a tiny, winged creature flutters down to the ground from a nearby tree. It’s a vampire bat! The small animal could fit in the palm of your hand, but it has a big appetite—for blood. The bat slowly creeps along the grass, toward an unsuspecting cow. Then the bat uses its needle-like fangs to pierce the cattle’s ankle and lap up the blood from the wound.

It's a warm summer night in Mexico. A herd of cattle sleeps in a field. A tiny winged creature flutters down to the ground from a nearby tree. It hardly makes a sound. The creature is a vampire bat! The small animal could fit in the palm of your hand. But it has a big appetite—for blood. It slowly creeps along the grass, toward a sleeping cow. The bat pierces the cattle’s ankle with its needle-like fangs. Then it laps up the blood from the wound.

Vampire bats are the only known mammals that feed solely on blood. This behavior is called hematophagy, explains Kenneth Welch. He’s a biologist at the University of Toronto Scarborough in Canada who studies animals with unusual diets. Most mammals get their energy from sugars and fats. But blood contains mainly protein—so how do vampire bats get energy?

Welch wondered if bats’ unique diet meant the animals metabolize nutrients, breaking them down to fuel their bodies’ many processes, differently than other mammals. To find out, he decided to study how vampire bats burn energy after a blood meal . . . by having them run on tiny treadmills!

Vampire bats feed only on blood. They’re the only mammals known to do this. This behavior is called hematophagy, explains Kenneth Welch. He’s a biologist at the University of Toronto Scarborough in Canada. Welch studies animals with unusual diets. Most mammals get their energy from sugars and fats. But blood contains mainly protein. So how do vampire bats get energy?

Animals break down nutrients to fuel their bodies’ many processes. Welch wondered if the bats metabolize nutrients differently from other mammals. That might explain their unique diet. To find out, he decided to study how vampire bats burn energy after a blood meal. He would have them run on tiny treadmills!

Vampire bats have cells in their noses that sense infrared energy. The bodies of the warm-blooded animals the bats feed on give off this energy as heat. These include pigs, cattle, birds (especially chickens), dogs, and occasionally even humans (see “What’s in Blood?”). Bats use their infrared-sensing ability to detect blood pulsing through the bodies of their victims. This sense is so sensitive that a vampire bat near an animal’s ankle can pinpoint blood pumping within a tiny vessel beneath the skin. Once the bat finds a source of blood, it makes an incision and drinks up. To learn more about how vampire bats get their energy, Welch would need to observe the animals right after they’d eaten.

Vampire bats have cells in their noses that sense infrared energy. The bats feed on warm-blooded animals. These animals’ bodies give off infrared energy as heat. They include pigs, cattle, birds (especially chickens), dogs, and sometimes humans (see “What’s in Blood?”). Bats detect infrared energy in the blood flowing through their victims’ bodies. This sense is very strong. If a vampire bat is near an animal’s ankle, it can detect blood in a tiny vessel under the skin. The bat finds a source of blood. Then it makes a cut and drinks up. Welch needed to observe the bats right after they ate. Then he might learn more about how they get their energy.

First Welch needed to collect vampire bats to study. In the spring of 2023, he traveled from Canada to the tropical jungles of Belize in Central America to collect a couple dozen of them from the wild. He and his collaborator Giulia Rossi hung nearly invisible net traps between trees and across hiking paths in hopes that bats would fly into them and get caught. They captured 24 adult common vampire bats (Desmodus rotundus)—one of the three vampire bat species known to practice hematophagy.

Other animals, including insects, worms, fish, and birds, also practice hematophagy. They gain energy by quickly breaking down amino acids—the building blocks of proteins—in the blood they consume. Welch suspected vampire bats might get their energy from blood in a similar way.

To test this hypothesis, or proposed answer to a research question, the scientists mixed a special concoction of amino acids into cows’ blood. Amino acids occur naturally in cows’ blood, but the ones Welch and his team used in the experiment contained carbon (C) isotopes, or variants of a chemical element. The researchers used syringes to hand-feed the blood mixture to the bats. As their bodies broke down the meal, the bats would exhale particles containing the isotope. If the researchers detected this isotope in the bats’ exhaled gases, they would know the animals were breaking down the amino acids to extract energy.

First Welch needed to collect vampire bats to study. In the spring of 2023, he traveled from Canada to the tropical jungles of Belize in Central America. He hoped to collect a couple dozen bats from the wild. He and fellow scientist Giulia Rossi hung nearly invisible net traps. They placed them between trees and across hiking paths. Bats flew into the nets and got caught. The scientists captured 24 adult common vampire bats (Desmodus rotundus). That’s one of three vampire bat species known to practice hematophagy.

Other animals also practice hematophagy. They include insects, worms, fish, and birds. To gain energy, they eat blood and quickly break down its amino acids. These are the building blocks of proteins. Welch thought vampire bats might get their energy from blood in this way too.

The scientists needed to test this hypothesis, or proposed answer to a research question. So they mixed certain amino acids into cows’ blood. Amino acids occur naturally in cows’ blood. But Welch and his team used ones that contained special carbon (C) isotopes, or different forms of a chemical element. The researchers hand-fed the blood mixture to the bats with syringes. The bats’ bodies would break down the meal. Then they would exhale particles that contained the isotope. The researchers would look for this isotope in the bats’ exhaled gases. If they detected it, the animals were breaking down the amino acids to gain energy.

After feeding the bats, the scientists immediately transferred the animals to bat-sized motorized treadmills. The treadmills were enclosed in airtight chambers. These chambers would allow Welch’s team to capture particles containing the carbon isotopes that the bats exhaled as they metabolized their food. Welch got the idea from studies testing how people metabolize food. In these experiments, participants run on treadmills or pedal stationary bicycles while wearing face masks to capture gases they exhale. But there aren’t any masks designed to capture gases from bats.

Right after the bats ate, the scientists moved them to bat-sized motorized treadmills. The treadmills were inside airtight chambers. As the bats metabolized their food, they exhaled particles that contained the carbon isotopes. The chambers allowed Welch’s team to capture these particles. Welch got the idea from studies on how people metabolize food. In these experiments, people run on treadmills or pedal stationary bicycles. They wear face masks to capture their exhaled gases. But no masks are designed to capture gases from bats.

The findings proved that, unlike any other mammal, vampire bats have evolved a unique and extremely quick way of digesting proteins to create energy they need to survive. But there are some major downsides to vampire bats’ speedy, protein-powered metabolism. They use up their energy so quickly that they don’t store fuel in their bodies. That puts vampire bats at risk of starvation if they go more than a few nights without feeding on a blood meal, explains Welch.

However, the animals have a solution: “Bats are very, very social animals,” says Welch. “To avoid starvation, they engage in a sort of energy sharing—one bat will barf up some blood into the mouth of another to give it a meal.” That’s true friendship!

The findings proved that vampire bats digest proteins to create energy they need to survive. And they do it in a unique and extremely quick way. This makes them unlike any other mammal. But their speedy, protein-powered metabolism has some major downsides. They use up their energy very quickly, so they don’t store fuel in their bodies. Vampire bats could starve if they go more than a few nights without a blood meal, explains Welch. But the animals have a solution. “Bats are very, very social animals,” says Welch. “To avoid starvation, they engage in a sort of energy sharing—one bat will barf up some blood into the mouth of another to give it a meal.” That’s true friendship!