With gargoyle-like faces7xm, 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.

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