How Do Pit Vipers Sense Heat? Rattlesnake Pit Organs Explained

What is a pit organ?

The pit organ, also called the loreal pit, is the feature that gives pit vipers their name. It looks like a small second nostril just below and in front of each eye. On a large snake such as the western diamondback rattlesnake, it is a few millimeters across and easy to see in close-up photos.

The pit is a cavity divided into two chambers by a very thin membrane, only about 15 micrometers thick, much thinner than a sheet of paper. The outer chamber opens to the air at the front, and the inner chamber is connected to the outside by a tiny pore. Because the membrane has air on both sides, it heats up quickly when infrared radiation strikes it and cools quickly afterward.

The membrane contains a dense network of heat-sensitive nerve endings from the trigeminal nerve, the same nerve that carries touch sensations from the face. It is also full of tiny blood vessels, which carry heat away and reset the membrane so that the snake does not see lingering “afterimages.”

How the pit detects heat

Every warm object gives off infrared radiation, which is invisible to the human eye. Pit organs respond to infrared wavelengths of roughly 5 to 30 micrometers, the range given off by animals at body temperature. When a mouse or a bird passes in front of a pit viper, its radiation warms a small area of the pit membrane by a tiny amount.

The nerve endings are extraordinarily sensitive. They are estimated to respond to temperature changes of less than 0.001 °C. This lets a snake notice even a small mammal that is only slightly warmer than the ground around it, at close range, and larger warm objects from farther away.

The pit works like a pinhole camera. Because the opening is narrow and the membrane sits at the back, heat from the left side of the snake falls on one part of the membrane and heat from the right falls on another. This gives the snake a rough image of where the warm object is, not just a sense that something warm is nearby. The image is blurry, but it is good enough to aim a strike.

The molecule behind the heat sense: TRPA1

For a long time, scientists did not know which molecule in the pit actually detects the heat. In 2010, a research team led by scientists at the University of California, San Francisco reported in the journal Nature that the sensor is a protein called TRPA1, working in the pit nerves of the western diamondback and other heat-sensing snakes.

TRPA1 is an ion channel, a tiny gate in the nerve cell membrane. In people, a version of TRPA1 responds to irritating chemicals such as the compound that makes wasabi and mustard pungent. In pit vipers, the same family of protein has become extremely sensitive to warmth. When infrared radiation warms the membrane, the channels open, the nerve fires and the signal travels to the brain.

This makes the pit organ work less like an eye and more like a bolometer, an instrument that measures radiation by detecting how much it heats a surface. Eyes use light-sensitive chemicals to detect light; pits detect heat directly.

Combining heat and vision in the brain

Signals from the pits travel along the trigeminal nerve to the brain, where they reach the optic tectum, the same area that processes signals from the eyes. There, the infrared map from the pits is laid over the visual map from the eyes.

Some brain cells in this area respond only to heat, some only to light, and some respond most strongly when both arrive together. The result is a combined picture of the world in which a warm mouse stands out against cooler rocks. Experiments have shown that rattlesnakes with their eyes covered can still strike warm targets accurately, while snakes with their pits blocked rely on vision and smell. Smell matters too, and snakes also track prey with their tongues; see why snakes flick their tongues.

Which snakes have heat pits?

The ability to sense infrared radiation evolved separately in different snake groups:

Pit vipers live mainly in the Americas and Asia. In North America, the group includes all rattlesnakes plus the copperhead and cottonmouth. The true vipers of Europe and Africa, such as the adder, do not have heat pits, and neither do most other snakes, including cobras and garter snakes.

What pits are used for

Heat pits are best known as a hunting tool. Many pit vipers hunt by ambush at night, when warm-blooded rodents are active, and the pits help them detect, track and strike prey in the dark. Some snakes even seem to choose ambush sites where warm prey will stand out clearly against a cool background.

Pits have other uses too. Experiments have shown that pit vipers can use them to find cooler or warmer spots for regulating their own body temperature. They may also help detect predators. There is even evidence of a counter-move: California ground squirrels confronting rattlesnakes have been found to heat up their tails, producing an infrared signal aimed at the snake’s pits, but they do not do this when facing snakes that lack pits.

Myths and facts about heat pits

• Myth: the pits are ears or nostrils. Fact: snakes have no external ears, and the nostrils are separate openings; the pits are dedicated heat sensors.
• Myth: pit vipers see in full thermal color, like a thermal camera. Fact: the heat image is blurry and low in detail; it works together with vision and smell.
• Myth: all venomous snakes have pits. Fact: only pit vipers do among venomous snakes; cobras, mambas, coral snakes and true vipers do not.

Whatever they sense with, rattlesnakes and other pit vipers try to avoid people. Keep your distance from any venomous snake, and if someone is bitten, call 911 (US) or 112 (EU) and seek emergency medical help immediately.