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Vampires are real.
Of the more than 1,200 species of bats, four are vampires, meaning they subsist wholly on the blood of other animals.
The common vampire bat (Desmodus rotundus), like all vampire bats, lives in Central and South America. It has evolved a number of specializations to facilitate its blood-sucking lifestyle. Vampire bats use their razor-sharp upper incisors to make a small incision in the skin of a sleeping animal (often chicken, cattle or other livestock, but sometimes humans). They lap the blood from the wound, aided by saliva that contains anticoagulants to inhibit blood clotting. A vampire bat will typically consume half its body weight in blood in a single feeding. In order to be light enough to fly back to its roost, its specially adapted kidneys work overtime and it expels urine as it feeds.
Like many bats, vampires navigate by echolocation. But once they get close to a prey animal on the ground, they use a more unusual sense: thermoreception. From 20 centimeters away, a vampire bat can sense the infrared radiation given off by its warm-blooded prey. It uses thermoreceptors on its nose to locate the best spots to feed, areas where the blood flows close to the skin.
The only other vertebrates that can sense infrared radiation are certain snakes (boas, pythons, and pit vipers). Elena Gracheva and Julio Cordero-Morales from the University of California, San Francisco have identified the genes behind thermoreception in both snakes and bats. Snakes use a gene called TRPA1. Humans and other animals also have this gene—it reacts to the chemicals in mustard and wasabi, resulting in a burning sensation. But snakes have repurposed this molecular irritant alarm and use it instead to sense the body heat of their prey.
Common vampire bats evolved thermoreception in a similar way, but using a different gene. The protein TRPV1 is found in all mammals and senses heat on the skin and also the painful heat caused by exposure to capsaicin, the chemical that makes chillies hot. In humans, TRPV1 responds to any temperature over 43 degrees Celsius (100 degrees Farenheit). The vampire bat's TRPV1 has been tuned to be able to detect much lower levels of heat, like those of mammal blood (30 degrees Celsius, 86 degrees Farenheit).
If you look at a common vampire bat's face up close, you'll see three leaf-shaped pits around its nose, each one about 1 mm in diameter. A mass of nerve cells send messages to the leaf pits. These nerves are unusually large in vampire bats, but not in closely-related fruit bat species.
Gracheva and Cordero-Morales found that a simple tweak changed the structure of TRPV1 in those large facial nerves. A shorter version of the protein is expressed in these nerves, while the regular, longer version is made in the rest of the vampire's body. The shorter form, found only near the leaf pits, makes the vampire's face more sensitive to heat. Basically, it adjusts the bat's thermometer to lower temperatures.
This slight temperature adjustment has turned the common vampire bat's face into a body heat-sensing organ. A cool superpower, and one that comes in handy when trying to sneak up on sleeping mammals and birds.
