New research shows that vampire bats, who solely feast on the blood of mammals, have evolved heat sensors in their face in order to detect blood with precision, a mystery left undiscovered until now.

These heat detecting organs called "leaf pits" have infrared radiation sensors, similar to the way humans feel heat. Located around their noses and lips, vampire bats have evolved the leaf pits to locate blood vessels in order to suck blood more precisely.

Vampire bats (Desmodus rotundus) are one of very few vertebrates with infrared radiation sensors. The other three with this capability are snakes, specifically pit vipers, boas and pythons.

The new study, published in Nature, ends a mystery as to how vampire bats precisely locate veins to extract blood and feed on.

It was known that snakes have modified ion channels that open and send nerve signals when stimulated by the environment. Similarly, vampire bats have adapted their ion channels that would normally respond to painful heat, giving the infrared sensors the ability to detect lower temperatures, like body heat, instead of high 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," said David Julius of the University of California, San Francisco and author of the study.

However, vampire bats do still feel painful heat, according to Julius, because they have evolved upper and lower sets of sensory neurons. The upper set which includes the leaf pits around the nose and lips, has adapted to sense low temperature heat while the sensory neurons elsewhere feel painful heat and protect against hot temperatures.

A group of researchers led by Julius used gene expression to compare the fibers of the nerves in both the upper and lower regions. They found two different variations of a protein are produced by the ion channels.

The protein, TRPV1 present in all vertebrates to sense heat, is used regularly throughout the body to detect harmful heat. But a shorter variation is produced in the nerve fibers of the leaf pit organs which can detect heat at cooler temperatures.

"The channel is like a little thermostat," Julius said.

According to the study, by excluding parts of the gene, vampire bats have evolved by splitting their functions to feel pain while detecting body heat in order to find prey to feed on. This function has helped them pinpoint exact spots to suck blood, as vampire bats only feed on blood, needing at least two tablespoons per day to survive.

The study also points out that the new findings regarding vampire bats' evolution to locate and suck blood with precision can give more information about the sensors in humans that act the same way in order to design and improve prescription drugs.