Some snakes, like rattlesnakes, have a small hole between each eye and nose. This hole is called a pit organ. It can feel heat from warm animals.

So the snake can find a mouse even in the dark. Inside the pit, there is a very thin skin. When it gets a little warm, the nerves send a message. Scientists found the special protein that feels the heat in 2010.

Rattlesnakes and other pit vipers have a small hollow between each eye and nostril, and some pythons and boas have similar pits along their lips. These pit organs sense infrared radiation, the heat given off by warm bodies. Deep inside a viper's pit is a thin membrane full of nerves, which warms slightly when heat reaches it.

In 2010, Elena Gracheva and David Julius discovered that the key sensor is a protein called TRPA1, which acts like a door that opens when it is heated. In humans, the same type of protein helps us feel the sharp sting of wasabi. Julius later won a Nobel Prize for his research on proteins that sense heat and pain.

Between the eyes and nostrils of pit vipers such as rattlesnakes sits a pair of small pits. Some pythons and boas have rows of similar pits along their lips, and the two designs are thought to have evolved separately. Inside each viper pit, a thin membrane packed with nerves is stretched over an air chamber. When infrared radiation from a warm body hits it, the membrane warms slightly.

In 2010, Elena Gracheva, David Julius, and colleagues identified the molecular sensor as TRPA1, an ion channel that in these snakes is more sensitive to heat than any vertebrate channel known before. So snakes detect infrared by being heated, not through the photochemical reactions that eyes use. Julius later shared the 2021 Nobel Prize for work on such channels.

Vampire bats sense the body heat of their prey with a modified version of a different channel, TRPV1. How sharp is the image a snake gets? Calculations by George Bakken and Aaron Krochmal suggested that the thermal image on the membrane is blurry, so the nerves would have to respond to differences of 0.001 degrees Celsius or less. That number came from a model rather than from direct measurement, and the brain may sharpen the image.