Some snakes, including pit vipers and pythons, are known to hunt in the dark by sensing the heat their prey radiates. But how do snakes convert this warmth into the thermal images they “see”? A model proposed by University of Houston and Rutgers University researchers suggests a potential answer. Their paper, published in Matter, may also help in developing soft artificial materials that convert heat to electricity, useful for applications such as sensors and energy harvesting.
The snakes' pit organ—a vase-shaped indentation with a thin membrane stretched across it, positioned near each nostril—seems to act like a thermal “eye.” The organ is exquisitely sensitive and detects animals about 40 centimeters away within half a second in pitch darkness. Biologists had previously identified channels for conducting charged ions, activated by temperature changes, in the membrane's nerve fibers. Scientists knew this membrane heats up very rapidly, but it was still unclear how thermal variations in the pit organ became electric signals that travel to the brain.
“Pyroelectric materials, which convert heat to electricity, do exist in nature. But they're rare, and they're hard crystals; no such crystals have been found in snakes,” says Pradeep Sharma, a mechanical engineer at the University of Houston and co-author of the paper. “What we show is that soft materials like biological cells can also act as weak pyroelectrics under some special circumstances.”
Sharma and his team developed a mathematical model to show how static charges would move in a material that is deformable and responsive to heat. They modeled the pit membrane as a film that is made up of such a material and that thickens if heated. Most biological cells (including those that make up the real membrane) naturally generate a small electric voltage across their outer surface. The researchers found that when the membrane thickens, the charges on its cells should shift slightly, resulting in a voltage change that can be picked up by nerve cells.
They tested this theoretical model with real-world values, and found that it corresponded with how quickly real snakes can detect prey—as well as how close, and how much warmer than its environment, the prey animal must be.
Yale University neuroscientist Elena Gracheva's work had previously exposed the role ion channels play in snakes' heat-sensing abilities. Now, says Gracheva (who was not involved in the new study), this pioneering look at signal conversion “lays the foundation for future experimental work by biologists to verify the model.”
It could also lead to new technology, Sharma says: “We can use the same model to create artificial materials that have pyroelectric properties for exciting applications in materials science.”