How vampire bats tuned their thermometers to evolve a heat-seeking face

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Mythology imbues the vampire bat with supernatural powers, but its real abilities are no less extraordinary. Aside from its surprising gallop and its anti-clotting saliva, the bat also has a heat-seeking face. From 20 centimetres away, it can sense the infrared radiation given off by its warm-blooded prey. It uses this ability to find hotspots where blood flows closest to the skin, and can be easily liberated by a bite. Now, Elena Gracheva and Julio Cordero-Morales from the University of California, San Francisco have discovered the gene behind this ability.

Among the back-boned vertebrates, there are only four groups that can sense infrared radiation. Vampire bats are one, and the other three are all snakes – boas, pythons, and pit vipers like rattlesnakes. Last year, Gracheva and Cordero-Morales showed that the serpents’ sixth-sense depends on a gene called TRPA1, the same one that tells us about the pungent smells of mustard or wasabi. Boas, pythons and vipers have independently repurposed this irritant detector into a thermometer.

Vampire bats evolved their ability in a similar way, but they have tweaked a different protein called TRPV1 that was already sensitive to heat. Like TRPA1, TRPV1 also alerts animals to harmful substances. It reacts to capsaicin, the chemical that makes chillies hot and allyl isothiocyanate, the pungent compound that gives mustard and wasabi their kick. In humans, it also responds to any temperature over 43 degrees Celsius. The vampire has simply tuned it to respond to lower temperatures, such as those of mammal blood.

The bats have three leaf-shaped pits around their nose that are riddled with unusually large nerves, like those found in the pits of heat-seeking snakes. If they detect anything over 29 C, they fire. The pits and the large nerves are a vampire speciality – the silky short-tailed bat, a closely related fruit-eating species, has neither.

Gracheva and Cordero-Morales found that these large nerves contain a shortened form of TRPV1 that is very rarely found in the normal-sized nerves elsewhere in the vampire’s face, or in the nerves of the silky bat. The protein normally comes in at 848 amino acids. But the bat’s cells can process the instructions for making TRPV1 in a different way, which lops off the final 62 amino acids.

This simple tweak tunes the bat’s thermometer to lower temperatures. The long version of TRPV1 responds to anything over 40 C; the body heat of a warm-blooded mammal would be invisible to it. But the abridged form responds to anything over 31 C. And since the bat’s leaf-pit nerves contain both versions in equal proportions, they are activated by temperatures of 33 C or more. The warmth of a mammal’s blood is enough to set them off.

Other mammals including fruit bats and moles have the ability to produce the shorter version of TRPV1, complete with its lower temperature sensitivity. But none of them actually do so to any great extent. It’s only in the vampire’s face, where the ability to detect lower temperatures is useful, that the abridged TRPV1 has been put to use.

Reference: Gracheva, Cordero-Morales, Gonzalez-Carcacia, Ingolia, Manno, Aranguren, Weissman & Julius. 2011. Ganglion-specific splicing of TRPV1 underlies infrared sensation in vampire bats. Nature

Image from Wikipedia

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