Scientists have found the genetic pathway that enables vampire bats to sink their razor-edged teeth into \"hot spots\" likely to yield a quick and copious liquid meal, reports a study released Wednesday.The protein encoded by the gene, known as TRVP1, allows the blood-thirsty beasts to detect tiny pulses of warmth emitted by a prospective victim\'s exposed veins, the researchers said. In most mammals, the equivalent gene -- which regulates sensitivity to temperature -- acts like an alarm by triggering pain, prompting an animal to recoil from heat above a certain threshold, often 43 degrees Celsius (109 degrees Fahrenheit).But for vampire bats it works as a homing device, turning the night-flying mammal into a heat-seeking missile with fangs.Within a range of about 20 centimetres (eight inches), anything above 30 C (86 F) shows up on its infrared radar as a potential meal.\"The bat can distinguish areas of skin where a vein is closest to the surface,\" explained David Julius, a scientist at the University of California at San Francisco and lead researcher for the study, published in Nature.Desmodus rotundus -- the only bat to feast on fresh blood -- needs a regular supply but is not greedy, never taking more than about two tablespoons per serving.A chemical in the bat\'s saliva numbs the victim\'s skin, and another prevents blood from clotting.All bats use an impressive array of tools to navigate and find food, including a highly developed sense of smell, sharp hearing and -- despite a reputation for blindness -- keen vision.Bats are also masters of echolocation, the ability to interpret the return echoes of sounds they emit to detect, identify and capture living prey that is, more often than not, also in flight.But only vampire bats, among the order\'s 1,200 known species, are able to \"feel\" fine gradients in infrared radiation, more commonly known as heat.Julius set out to discover how they do it.Having explored a similar capacity in rattlesnakes that can detect body heat exuded by a mouse a metre (three feet) away, he knew where to look.The diamondback rattler has hollow, bony cavities above the nose called pit organs containing ultra-thin, heat-sensing membranes that activate receptors connected to the brain.In vampire bats, these receptors are distributed around the animal\'s upper lip and upturned snout.They sense infrared waves -- just beyond visible light on the scale of electromagnetic radiation -- through the TRPV1 channel.But only a truncated version of the TRPV1 protein, Julius found, gives vampire bats their signature talent for tapping into the blood veins of snoozing mammals.\"Other bat species have a gene structure that makes it possible to produce the short variant, but they apparently don\'t exploit this possibility to the extent that the vampire bat does,\" Julius said in a email exchange.The study also bolsters recent research suggesting that bats are more closely related on the tree of evolution to horses, dogs, cows, moles and dolphins -- all members of the superorder Laurasiatheria -- than humans, monkeys, flying lemurs and mice, which belong to the Euarchontoglires superorder.Only animals in the first group have the genetic potential to produce the \"short\" variant of TRPV1.Still unknown is when the vampire bat developed its remarkable heat-seeking skills. \"We assume that the ability ... expanded over the course of about 25 million years, when vampire bats diverged from their insect-eating cousins,\" said co-author Nicholas Ingolia, a researcher at the Carnegie Institution in Baltimore, Maryland.