Vampire bats must consume 70% to 80% of their body weight in blood almost every night. To satisfy this never-ending thirst, they bite their prey—typically sleeping livestock, but also the occasional human toe poking out from under the covers—in areas where warm blood courses close to the surface. Now scientists have discovered a molecular heat sensor that helps the bats home in on their dinner.
Researchers led by neuroscientist David Julius of the University of California, San Francisco, searched for genes related to known molecular heat sensors in several bat species collected by colleagues in Venezuela. In the vampire bat Desmodus rotundus, the researchers found evidence of a change in how cells use the gene for an ion channel called TRPV1. This molecular pore resides on the surface of sensory neurons, and in other animals it stimulates the neurons in response to painful heat or capsaicin, the compound that gives chili peppers their sting.
In Desmodus, neurons in the nerve connected to the small heat-sensing pits near the bat's nose splice together different parts of the Trpv1 gene to produce a version of the ion channel that's shorter at one end than the version made by other animals, including bats that feed on fruit, nectar, or insects, the team reports in tomorrow's Nature.
To investigate the workings of the shorter TRPV1 channel, Julius and colleagues inserted the genetic instructions into frog egg cells, causing the cells to make the channels and stick them on their surface. Probing the egg cells with electrodes, the researchers discovered that the short version of TRPV1 opens at about 31˚C, which in a neuron would increase firing. That's well below the 40˚C or higher threshold for the long version of TRPV1. In effect, Julius says, vampire bats have converted the channel from a detector of things that are painfully hot to one that reacts to things at body temperature--like blood. Julius notes that his team found the short version of TRPV1 in only the vampire bat's facial nerves; nerves in other parts of the bat's body make the longer, pain-sensing version of TRPV1.
"It's really fascinating to know how this works," says Brock Fenton, a bat biologist at the University of Western Ontario in London, Canada. The findings are an important clue to how vampire bats' unusual lifestyle evolved, Fenton says.