A real-life vampire probably couldn’t survive on blood alone

Vampires have been stalking the human imagination for centuries. These fictional creatures take many forms, including the famous Count Dracula and Morbius of the Marvel Universe. But most share one feature: long, razor-sharp teeth. These fangs help vampires bite the necks of victims to feast on their blood.

Many real bloodthirsty creatures exist in nature. Vampire bats are perhaps the most famous. Mosquitoes and ticks also make the list, as do some spiders and moths. But it would be hard to turn a human body — even an undead one — into a blood-drinking monster. 

Let’s learn about vampire bats

“We can’t survive the way vampires do,” says David Begun. This paleoanthropologist studies how apes, including humans, evolved. He works at the University of Toronto in Canada. In real life, no primates survive on blood, Begun notes. To do so, vampires would need to rework both the mouths and guts that humans inherited from our plant-eating ancestors.

My, what big teeth you have

In popular media, vampires puncture skin using knifelike upper canine teeth. Humans occasionally use their canines to tear food, too. But any vampire would probably struggle to reliably break skin. “Our canines are completely unsuited to piercing anything,” says Begun. “They look more like incisors, our front teeth.”

Human canines have slowly shrunk over time. Like many primates today — including our close cousins, chimpanzees and gorillas — our ancestors once sported large, pointed canines. These impressive teeth would have even sharpened themselves by rubbing against neighboring molars.

But anthropologists suspect that these flashy teeth weren’t primarily for eating. Instead, ancient apes probably used them to intimidate rivals and defend themselves. Over time, these teeth grew smaller as our ancestors evolved to live in cooperative groups. Less fighting between males left little need for such large canines.

Vampires would have to revert to big canines, says Begun. Those teeth would also need to be much sharper, more like the teeth of the only real blood-drinking mammals: vampire bats. 

These blood sippers use their two front incisors to create tiny incisions in their prey. Unlike human teeth, these incisors don’t have enamel. This allows them to remain permanently sharp and to not wear down over time. “No primate has such sharp canines that would allow that kind of thing to happen,” says Begun. 

Unlike their fictional counterparts, vampire bats (such as the one seen here) don’t suck blood. Instead, these nocturnal predators use their upper incisors to pierce skin. Saliva containing anticoagulants then enters the wound, which prevents blood from clotting. The small bats then use their tongues to lap up the oozing blood. Gabriel Mendes/iStock/Getty Images Plus

Even movies that show vampires growing sharp canines at will would be unrealistic. If taken care of, our permanent teeth don’t change much over the course of our lives. Instead, the size, number and shape of our teeth are all preset by our genes.  

“There are rodents that have continuously growing incisors,” says Begun. Beavers, squirrels and rats are a few examples. “But they can’t do it at will.” In fact, these rodents must grind their teeth or gnaw on hard items such as wood and bone to keep them short. If left unchecked, overgrown incisors can interfere with eating, injure their mouths and even grow through their skulls.

A stomach for blood

Finding ways to draw blood from victims isn’t the only hurdle real-life vampires would face. They’d also need to be able to digest it — and use that food as fuel for their undead activities.  

Blood isn’t particularly nutritious. Low in calories, it’s mostly made up of water and protein. Because of their humanlike guts, any vampire would probably suffer from malnutrition or even poisoning. 

“Humans can’t survive on blood alone,” says Begun. “We need a lot more diversity of nutrients, of vitamins and minerals, that aren’t present in blood.” 

That’s because we inherited our digestive systems from a long line of omnivores, or animals that eat both plants and other animals. Like today’s chimpanzees, our ape ancestors would’ve likely eaten mostly plants with an occasional helping of meat, says Begun. Long, coiled intestines probably helped them break down tough plants. And bacteria in their guts likely helped further digest this food and absorb nutrients.  

By about 2.5 million years ago, our ancestors started eating more meat. Later, humans started cooking with fire. That made digesting foods such as meat and tough plants much easier. Even more recently, people began growing starches such as grains to eat. 

This led to tweaks in our digestive tracts. We started producing extra enzymes that could break down starches. Meat and other easy-to-digest foods could be broken down in shorter intestines. This resulted in bodies best suited to eating many different foods, says Begun. 

Even vegans who don’t eat any animal products must have a varied diet. “They can’t just eat lettuce all day long,” Begun says. “They have to have beans. They have to have rice. They have to have complete proteins.”

While low in most nutrients, blood does contain lots of iron. Red blood cells hold about 70 percent of the iron in our bodies, where it helps carry oxygen to organs. But consuming too much iron can be toxic to humans, who have no way to digest or excrete large amounts of it. This iron overload can damage body organs, the nervous system and even cause death.

Unlike humans, vampire bats have evolved ways to digest blood. Research suggests that vampire bats lost a gene that blocks intestinal cells from absorbing too much iron. Those gut cells are also short-lived and regularly shed. This lets the bats quickly absorb lots of iron and discard it through their waste. Meanwhile, their gut bacteria help break down proteins. These microbes may also produce extra vitamins not found in blood. 

Just not in our blood 

Scientists are still trying to understand why vampire bats started feeding on blood. Some suspect the habit arose from bats that fed on parasites such as ticks. Perhaps the bats eventually started feeding on the blood oozing from wounds left behind when they scarfed down these pests.  

Whatever led bats to their taste for blood, it’s clear humans weren’t shaped by those same evolutionary pressures. And changing how a real species eats usually requires tens of thousands, if not millions, of years of evolution. That’s a lot more involved than the single bite needed to make a fictional vampire.  

Still, this reality shouldn’t stop movie-goers from enjoying vampires, Begun says. “It’s still OK to enjoy a vampire movie or a vampire TV show, even if it’s not actually feasible.”