Blood: A Difficult Meal to Live on

Despite how they are portrayed in movies, vampire bats do not want to hurt you. But they do drink blood. In fact, it is their only food source. This unusual feeding strategy is called hematophagy. But rarely, if ever, do they feed on human blood.

Vampire bats, which are found in Latin America, typically feed at night by sneaking up on sleeping animals to drink a tiny amount of warm blood. Despite being expert fliers, when it is time to feed common vampire bats turn to a stealthier kind of movement: they crawl and even run on the ground with a fast-galloping gait to approach their unsuspecting victims [1]. Their bites are small and painless, so that the host does not wake up. Their saliva contains a natural blood thinner called an anticoagulant protein. Normally blood clots, or turns solid, to seal a wound, but the anticoagulant stops that from happening, so the blood keeps flowing as they drink [2].

This way of feeding is truly unique among mammals! Most animals, including humans, get their energy from sugars (carbohydrates) and fats. But while humans need some time to digest and process a meal before that energy becomes available, some animals can tap into fresh fuel almost instantly. Nectar-feeding bats and fruit bats, for example, drink sugary nectar and then burn the sugars almost immediately to power flight. Vampire bats appear to do something similar even though their diet is very different. Their bodies are specialized for consuming and digesting blood, not fruit or insects.

Blood, however, is mostly water and protein and contains very little sugar or fat. Imagine if you only had water and protein to eat, but you still needed to play sports. Your muscles would scream: “I need fast fuels like sugars and fats. What am I supposed to do with nothing but protein?!” Interestingly, the stomach and intestines of vampire bats can quickly process a blood meal thanks to specialized transport proteins and strong stomach acids, which rapidly break down blood proteins and absorb water [3, 4]. Unlike most other animals, vampire bats have muscles tailored to burn amino acids (the building blocks of proteins) rather than sugars. In other words, vampire bats can rapidly convert blood protein into usable energy. The key question is not whether blood gives vampire bats enough energy, but rather how quickly they can absorb and turn nutrients from fresh blood into power for intense activities like running and flying.

To answer this question, scientists developed a clever experiment involving tiny bat treadmills (Figure 1). It might sound like something out of a silly cartoon, but this is exactly what they did to crack the vampire bat fueling mystery.

The Big Energy Question

In the wild, vampire bats often run on the ground to chase after their prey. Knowing this, researchers designed a mini treadmill inside a chamber to measure vampire bat metabolism. The researchers captured 24 healthy vampire bats and fed them cow’s blood. In some tests, they added the amino acids glycine or leucine. Glycine is a non-essential amino acid, meaning the bat’s body can make it on its own, whereas leucine is an essential amino acid, meaning the bat must acquire it from food.

Both amino acids were labeled with a tiny traceable marker—kind of like giving a meal a harmless molecular GPS tag—that allowed the scientists to track whether the amino acids were being used for energy inside the bat’s body. Also, a few bats were given normal blood without any labels, so they could be compared to those that ate the “special” blood.

After the blood meal, each bat walked or ran on the tiny treadmill at three different speeds: slow, medium, and fast. While the bat moved, the machine measured how much oxygen (O₂) it was consuming and how much carbon dioxide (CO₂) it was breathing out (Figure 2). Scientists care about these gases because they can reveal what types of metabolism are occurring in the body.

When our cells burn fuel (sugar, fat, or protein) for energy, they use O₂ and produce CO₂. The ratio of CO₂ produced to O₂ used is called the respiratory exchange ratio (RER). In other words, the gases an animal breathes out can reveal what fuel it is burning inside. RER is like a fingerprint for scientists: about 7 CO₂ for every 10 O₂ (RER of approximately 0.7) means mostly fat is being burned; 10 CO₂ for 10 O₂ (RER of about 1.0) means mostly carbohydrates (sugars) are being burned, and a RER value between 0.8 and 0.9 is an indication that protein is being burned. The researchers also looked for the special label from glycine or leucine in the CO₂ that the bats breathed out. If those labels show up in the breath, it means the bat is oxidizing (“burning”) those amino acids for energy and turning them into CO₂.

Burning Protein: A New Fuel for Mammals?

What did the vampire bat workouts reveal? The bats were, indeed, using amino acids from blood as their primary fuel during exercise. Within minutes after the meal, the bats’ breath already contained a lot of CO₂ coming from the burning of those labeled amino acids. As the exercise continued, this dropped to 41% after 25 min, and then 28% after 40 min, as the meal was used up. This showed that bats were quickly burning the protein from their blood to power their workout.

Even more interesting, the bats did not care which amino acid they were burning. They used glycine and leucine at about the same rate. As the bats ran faster, they breathed harder—O₂ use went up and they produced more CO₂–but the RER stayed steady at around 0.8–0.9, no matter how fast they ran (Figure 3).

This means they kept using protein as their main fuel even as they needed more energy to run faster. Most other animals would switch fuels when running fast, usually burning more carbohydrates (sugar) at high intensity to get quicker access to energy. But not these bats—they stuck with burning protein from their blood meal for energy, which is highly unusual for a mammal.

Why is this Discovery Important?

These findings reveal unique adaptations in vampire bats. No other mammals have been shown to fuel intense exercise primarily with proteins from a recent meal. This is something only previously seen in blood-sucking insects, like the tsetse fly or certain mosquitoes, and makes vampire bats an extreme outlier in the mammalian world.

However, there are tradeoffs to this unusual lifestyle. Because vampire bats cannot store much sugar or fat, their bodies have very low energy reserves [6]. This means that if they miss more than a day or two of food, they quickly starve. In fact, studies show that a vampire bat can die within 48–72 h without blood. To survive this danger, vampire bats have evolved an amazing behavior: they share blood with friends and family [7]! After a successful hunt, a well-fed bat will sometimes spit blood into the mouth of a hungry roost-mate. This gives the famished friend a life-saving meal.

All these traits make vampire bats an evolutionary wonder. As the only known blood-feeding mammals, they show how a very unusual diet can reshape an animal’s body and behavior. Who knows what other secrets are waiting to be discovered? Maybe the next time you hear about a strange animal adaptation, you will remember this story of the vampire bat and think about what other surprises nature has in store!

Glossary

Hematophagy: ↑ A way some animals, like vampire bats, survive by feeding on the blood of other animals.

Anticoagulant: ↑ A protein that stops blood from clotting, helping it flow smoothly and stay liquid and easy to drink while bats feed.

Amino Acids: ↑ Tiny molecules that act as the building blocks of proteins. Animals use amino acids to build and repair their bodies, and sometimes to produce energy.

Metabolism: ↑ The process the body uses to turn food into energy to grow, move, and stay alive.

Respiratory Exchange Ratio: ↑ A number that shows what type of fuel an animal is burning for energy by comparing how much carbon dioxide (CO₂) it breathes out to how much oxygen (O₂) it uses.

Adaptation: ↑ A feature or behavior that helps an animal survive better in its environment.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

AI Tool Statement

The author(s) declared that Generative AI was not used in the creation of this manuscript.

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