MELISSA BLOCK, host: A vampire bat may sound like something only a scientist can love. We're going to hear now from some scientists who study them. Vampire bats live on blood. They tend to get that blood from unsuspecting donors - cows or pigs or even humans. Finding a donor isn't all that hard; finding out where to bite to get the best meal is. As NPR's Joe Palca explains, scientists think they know how bats do it.

JOE PALCA: Don't worry about bumping into a vampire bat if you're coming home late from a party, unless you happen to live in the tropics, and even then they're pretty rare. But some people do find them lovable.

Dr. BROCK FENTON: Vampire bats are totally fascinating, and very easy to love.

PALCA: Brock Fenton is a bat biologist at the University of Western Ontario. He says vampire bats are charming creatures. They're itty bitty things - only weigh about an ounce - and they have simple needs.

FENTON: Vampire bats need to get about two tablespoons full of blood a day.

PALCA: They can only go about two days without a blood meal.

FENTON: So, the vampire bat that came home last night without having fed, if he doesn't get something to eat tonight, he's going to be dead. So, this puts a huge pressure on these animals to be able to find blood donors.

Dr. DAVID JULIUS: They really have evolved some amazing features that fit in with this bloodthirsty lifestyle.

PALCA: That's David Julius. He's a molecular biologist at the University of California, San Francisco. Vampire bats are not his first love. He's interested in the biological basis of pain. OK, bear with me, we'll get back to bats in a moment. One kind of pain we've all experienced is burning pain. There is a particular kind of molecule that's essential for our heat sensation called a TRP receptor. These TRP receptors don't just respond to hot heat, they also respond to the heat we get from biting into a red hot chili pepper.

JULIUS: From a nerve-endings standpoint and a molecular standpoint, they're quite similar. I think we perceive them as being different because, you know, we recognize the difference between touching something hot and eating a hot chili pepper or getting the oil from chili pepper in our eyes or in our fingers.

PALCA: So, what's this got to do with vampire bats? Well, vampire bats have these same heat sensing TRP receptors but they use them differently. As Julius reports in the journal Nature, bats use TRP receptors to detect heat from a distance. They have nerve cells on their faces that have a particular form of these TRP receptors. This means they can sense a warmer patch of skin on a nearby animal. And warmer skin means there's lots of blood vessels near the skin surface. Now you see the connection? TRP receptors sense the warmth of blood and tell the bat where to bite. It's as if they're seeing the world in hot and cold. And it's not just bats that can do this trick. Pit vipers can do it too. Michael Grace is a snake guy at the Florida Institute of Technology.

MICHAEL GRACE: You can cover the eyes of the snake and the snake looks out on the world and literally sees the world in heat. It forms an image of the thermal environment in the brain.

PALCA: So, if you come across a rattlesnake wearing a blindfold, don't count on being safe. There's one other thing David Julius has learned about vampire bats.

JULIUS: Most people look at a bat and they presume that it's closely related to rodents.

PALCA: But recently, there've been hints they are more closely related to a grouping of animals that includes moles, cows and horses. Julius has looked at the DNA sequence of the TRP receptors in all these animals.

JULIUS: And our data suggest that bats are more closely related to those animals.

PALCA: So, don't think of bats as flying rodents, think of them as itty bitty flying horses. Maybe that will make them seem more lovable. Joe Palca, NPR News, Washington.

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