With gargoyle-like faces, razor-sharp fangs and an insatiable thirst for blood, vampire bats are nightmare fuel. And that's before they start running.
Unlike most bats, which largely avoid the ground, vampire bats are capable runners, using their folded wings to propel them forward. This helps them stealthily stalk livestock -- and occasionally unsuspecting humans.
"They don't want to flutter down and drop right on the back of a cow," said Kenneth Welch, a biologist at the University of Toronto Scarborough, who studies vampire bats and other animals with specialized diets. "Instead, they land a few feet away, silently approach the cow's leg and make a tiny, painless incision with the cow none the wiser."
These pursuits of prey can be energy draining. And the palm-size bats' blood-based diet is lean on carbohydrates and fats, which most mammals rely on to generate energy.
Because vampire bats can't carbo-load, they seem to rely on the proteins in the blood they slurp up. In a paper published Wednesday in the journal Biology Letters, Dr. Welch and his colleague discovered that vampire bats rapidly generate energy by burning protein-building amino acids.
Vampire bats are the only mammals that engage in hematophagy, or feeding exclusively on blood. But this behavior is practiced by other animals, including tsetse flies, which are known on the African continent for spreading sleeping sickness. To fuel their flight between meals, these flies oxidize an amino acid, proline, from their blood meals.
Dr. Welch suspected that vampire bats could also break down amino acids for energy. To test his hypothesis, his colleague Giulia Rossi, currently a postdoctoral researcher at McMaster University in Ontario, helped collect two dozen vampire bats in Belize.
The team then fed them cow blood from a local slaughterhouse. Before the bats lapped up the blood, the scientists enriched it with high concentrations of two amino acids, glycine and leucine. The heightened levels of amino acids, along with other chemical signatures like isotopes in the blood, would help the scientists track how the bats processed their meals.
After the vampire bats ate, they were each placed on a miniature treadmill. According to Dr. Welch, the dexterous bats initially used their thumbs to hook into crevices to avoid the moving belt. "They're like you or me," he said, "if they don't want to run on the treadmill, they'll cheat and stand on the side."
But once the vampire bats took to the treadmill, they flexed their fitness. As the team steadily increased the speed of the treadmill, the bats first walked, then trotted and eventually bounded along as the belt moved at nearly 100 feet per minute. Most of the bats kept moving for the entire 90-minute test period.
As the bats sprinted, the team collected samples of their breath to measure the intake of oxygen and the expulsion of carbon dioxide. By analyzing the exhaled CO2, the scientists could pinpoint traces of the amino acids from the blood the bats had ingested before running.
The researchers discovered that the breakdown of both glycine and leucine from the treated blood samples were responsible for as much as 60 percent of the bats' total energy production during their runs. This illustrated that vampire bats can turn amino acids into usable energy almost instantly.
According to Michael Hiller, a researcher at the LOEWE Center for Translational Biodiversity Genomics in Frankfurt who was not involved with the new paper, the researchers' observation of vampire bats' ability to metabolize amino acids in less than 10 minutes is a finding that is "unparalleled in mammals." It represents an intriguing case of convergent evolution where vampire bats and blood-feeding insects developed a similar mechanism to survive their extreme diet, Dr. Hiller said.
However, the ability for vampire bats to quickly break down amino acids has a downside. Because their bodies are specialized for quickly turning proteins into energy, they have largely lost the ability to process and store other fuel sources.
This makes vampire bats susceptible to starvation. They cannot store energy long term, which means going multiple nights without eating can be fatal.
But it turns out that vampire bats are willing to give blood. When one of these gregarious bats has a bellyful of blood, it will often regurgitate its meal to help a hungry roost mate, which often returns the favor in the future. This ensures vampire bats stay fueled up to run down their next feast.