Vampire bats use infrared sensors on their lips to locate blood vessels in their prey, scientists have discovered. The specialised nerve cells are similar to the pain-sensing cells in the human tongue, skin and eyes, which allow us to sense the sting of chilli peppers and high temperatures.

Vampire bats are the only mammals known to feed entirely on blood, and they need a daily supply to survive. They eat at night, creeping up on sleeping cows, goats or birds before sinking their razor-sharp teeth into veins and sucking up several tablespoons of blood. The animals have excellent hearing and eyesight and use high-pitched calls to help them navigate.

Biologists have known for some time that the bats can detect blood vessels from up to 20cm away, but it was not clear how they did it quickly enough to strike and retreat without alerting their prey.

David Julius, a physiologist at the University of California, San Francisco, led a team of scientists examining tissue from wild vampire bat populations in South America. They found that the upper lips of the animals were covered with a specific form of the heat-detecting molecule TRPV1. Whereas this molecule normally helps animals detect temperatures above 43C, in vampire bats the threshold was reduced to around 30C, allowing them to pinpoint the heat radiating from warm veins near the surface of their prey.

"The vampire bat makes these two different forms of the molecule, one of which is specifically involved in detection of infrared radiation," said Julius. His results were published in Nature on Thursday.

Humans have a longer version of the gene that makes TRPV1, which contributes to our ability to detect levels of heat that could result in pain.

The molecule also makes damaged tissue hypersensitive to temperature and pressure. "When you have a tissue injury such as sunburn, there are all these inflammatory mediators that get produced and then they act on different molecules on the sensory nerve ending to increase our sensitivity to pain," said Julius. "The reason you want to do that is because, when you have an injury, you want to make sure you really protect that area and guard it."

Julius added that the discovery of TRPV1 in vampire bats had helped biologists place the animal more accurately on the evolutionary tree, by looking for other animals that have a similar version of the gene that encodes it.

"For some time, people thought that bats were more closely related to rodents because of their anatomical features," he said. "In more recent years, with the advent of genomic methods, there have been a number of groups that postulated the fact that bats are more closely related to this other superorder. Bats are not as closely related to rodents and humans as they are to dogs, cows and whales."