Vampire bats locate their blood meals with the help of three heat-sensing organs in their faces, so-called "leaf pits" surrounding their noses.
Now, new research shows the bats evolved these heat detectors by co-opting the same sensors all mammals use when something is too hot -- like a fire or boiling water -- and re-tuning them to detect body heat.
Only three other vertebrates, all snakes, are able to detect infrared radiation to seek their prey. David Julius of the University of California, San Francisco and colleagues uncovered how the snake systems work in a paper published last year.
"We wanted to ask how this same process might occur in a warm-blooded mammal," Julius told Discovery News. "There's only one that's known to do this, and that's vampire bats."
With the new study, published today in Nature,the team now knows that both the snakes (pit vipers, boas and pythons) and vampire bats have achieved this feat by modifying ion channels that open and send a nerve signal when triggered by a particular environmental stimulus.
In the case of vampire bats, they modified ion channels that typically respond to painful heat -- and which also respond to molecules in hot chili peppers and menthol in mint leaves--so that their leaf pits can detect lower temperatures, closer to body temperature, rather than the higher temperatures that cause pain.
"If you take an ion channel that's involved in sensing, say, (painful) heat and you use it to sense infrared radiation, you probably have to change the threshold quite a bit--in other words somehow change the structure of the channel so that it can sense lower temperatures and have the sensitivity to detect body heat," Julius said. "So, how does the mammal do that and maintain its ability to detect painful heat at the same time?"
The answer, it turns out, hinges on the fact that vertebrates have one set of sensory neurons for the neck and up, and another for the neck down. Vampire bats configure the neck-up receptors in the leaf pits to have a lower heat-sensing threshold, while the neck-down receptors continue business as usual, protecting the bats from dangerously hot temperatures.
The snakes' systems work similarly, Julius said, but the pit vipers (like rattlesnakes), boas and pythons derived their systems from a receptor that responds not to chili peppers and heat, but rather from a receptor that in humans responds to wasabi, which researchers believe is primarily a receptor to chemical irritants.
"If you think about it, if you're warm blooded, feeding on blood is a bit of a challenge," said Brock Fenton of the University of Western Ontario, in London, Ontario. "It can be hard to get because most animals protect their blood rather tenaciously."
But vampire bats need two tablespoons a day to survive, and two nights without feeding is fatal to the animals. "There's a lot of pressure on them to find the right prey."
Vampire bats have several tricks to help them succeed. For one, Fenton said, they have neurons that are attuned to the deep breathing of sleeping mammals -- much easier to tap for a meal than those that are awake.
"The other thing is they have to find a place on an animal where it's easy to get at the blood," he said. "They're looking for a place where the blood is close to the skin and that’s where the heat detection comes in."