Now researchers have figured out how they do it: The bats can hear you breathing.
Bats are known to have a well-developed sense of smell, as well as organs that can detect the infrared radiation emitted by warm-blooded animals.
But Lutz Wiegrebe, a neurobiologist at Ludwig-Maximilians University in Munich, Germany, and a colleague have found that bats are also good listeners.
Vampire bats have brain cells that are stimulated exclusively by breathing sounds, the researchers found.
Wiegrebe and study co-author Udo Groger suggest that listening is an important element in bats' prey selection. Their report appears in the June 22 issue of the journal BMC Biology.
The researchers trained vampire bats to discriminate between three sequences of breathing sounds recorded from three different human subjects.
The scientists then randomly interspersed additional breathing sounds recorded from the same three individuals and let the bats listen.
The bats were able to figure out which individuals produced the new sounds.
Human subjects were unable to make the same distinctions among the recordings.
This led the researchers to believe that the bats are able to hear and remember the unique ultrasonic components of an individual's breathing sounds.
This aptitude, the scientists say, could provide a valuable clue in determining how bats are able to return to a good source of blood.
"We think that [vampire bats] memorize the breathing sounds associated with a successful feed on a prey individual," Wiegrebe said, "although we cannot rule out the possibility that smell can be used to recognize individuals."
Vampire Bats and Humans
Humans are not immune from the bats' repeat attacks, according to Wiegrebe.
"There are reports that the same humans have been bitten on subsequent nights by the same animals," he said.
The scientist says he doubts that there's any way for a person to alter his or her breathing in order to prevent return bat attacks.
"I don't think that humans can voluntarily change their breathing pattern during sleep," Wiebrege said. Vampire bats generally feed at night, he explained.
"Moreover, the individual vocal tract imposes its signature on the breathing sounds, and this signature cannot be manipulated, because it is anatomically defined."