Genes that make mustard hot and spicy on human tongues also let snakes "see" heat, explaining the remarkable ability of some species to strike prey in total darkness.
Until now, scientists knew that snakes' heat perception involved the pit organ -- a cavity found between the eyes and nostrils of boa constrictors, pythons and pit vipers. These species can hunt if blindfolded, but cover their pit organ and they can't. Beyond that, however, the snakes' sixth sense was a mystery.
"Although the role of the pit organ as an infrared sensor is well established, fundamental questions remain about its mechanism of stimulus detection," wrote University of California, San Francisco biologists Elena Gracheva, Nicolas Ingolia and David Julius in a study published 14 March in Nature.
Some scientists have proposed that infrared photons hit light-sensitive cells in the pit organ, making it work like a rudimentary eye. But others, including the UCSF team, suspected that pit organs detect heat directly.
When they measured gene activity in nerve cells that run from pit organs to snake brains, the researchers found that a gene called TRPA1 was about 400 times higher than in other snake tissues. In humans, TRPA1 produces proteins that let people detect chemical irritation and temperature difference, producing the distinctive sensation of mustard and peppers.
When the researchers heated pit viper nerve tissue samples in a dish to about 82 degrees -- close to the surface temperature of an unsuspecting mouse -- their TRPA1 activity shot up.
After the changes in ambient temperatures trigger TRPA1 activity in pit organ membranes, specialised brain structures process the signals and turn them into spatial images.
According to the researchers, the findings illustrate the ability of evolution to use common components for different, highly specialised functions. As for what a mouse slathered in mustard would taste like to a snake, they do not speculate.