How a western diamondback rattlesnake senses heat

Twelve wild-caught western diamondback rattlesnakes were set at the stem of a Y-shaped tube. One arm was 30°C (86°F). The other was 40°C (104°F), the same as the stem. According to Science News, which covered the 2003 study, the snakes slipped into the cool arm about 75% of the time. Then the researchers covered each snake’s facial pits with tiny plastic balls and aluminum foil, and the snakes picked the cool arm only about half the time, which is roughly what a coin flip gives. With the covers off, they got good at it again.

That experiment is a neat doorway into a sense we do not have. The western diamondback (Crotalus atrox), a big rattlesnake of the American Southwest, carries two small pits on its face that read heat. This article follows the signal from the pit to the brain: what the pit is made of, how faint warmth becomes a nerve message, and how that message turns into something like a picture. The picture, it turns out, is a blurry one. We also look at what the pits are for, because finding dinner is only part of the story.

Meet the western diamondback

The Animal Diversity Web (ADW), a University of Michigan database, describes dry, rocky, shrubby country, where the snake hides in rock crevices or mouse holes. It places the species from central and western Texas through southern New Mexico and Arizona into southern California, and well into central Mexico. The Reptile Database adds records from southern Nevada, Oklahoma and Arkansas. ADW calls the body plump, with a short tail and a broad, triangular head, and gives a body length of about 1.5 meters (5 feet). It eats mainly small mammals and birds. According to ADW, a wild adult eats about every two to three weeks, which is a lot of patience for a snake that carries its own thermometer.

This article is not a guide to handling rattlesnakes. They are venomous, and the right way to meet one is from a distance.

A pit, a membrane and a heat switch

Each pit is a small hollow in the face. Bakken and colleagues, writing in 2012, describe a pitviper’s pit as a flask-shaped cavity split into two chambers by a suspended membrane, a bit like a pinhole camera with a thin sheet stretched across its back wall. Their paper modeled four pitviper species from X-ray scans, so the layout is a family trait, not a diamondback exclusive.

The membrane does not catch light. A 2010 Nature study led by Elena Gracheva concluded that it works as a passive antenna. Warm objects send infrared radiation, which is simply heat, into the pit. The radiation warms the membrane, and the warmth heats the nerve endings packed into it. The paper describes that membrane as full of mitochondria (a cell’s power plants), rich in blood vessels and densely wired with nerve fibers. The fibers belong to the trigeminal nerve, which serves the face, not to the optic nerve that serves the eyes.

The heat sensor itself is a protein called TRPA1, an ion channel, which is a tiny gate in a cell’s surface that opens in response to a trigger. In rattlesnakes the study found it about 400 times more abundant in the trigeminal nerve cluster than in a nerve cluster serving the body. In lab cells, the rattlesnake version opened at about 28°C. A rat snake, which has no pits, needed about 37°C. The authors call pit viper TRPA1 the most heat-sensitive vertebrate ion channel found so far.

Two limits are worth knowing. Most of the channel tests were done in lab cells, not in living pit nerves. And the antenna idea is inferred from how the channel behaves and where it sits, not measured on the membrane itself. Think of it as a very well-supported sketch.

From heat to a picture, but a blurry one

A pinhole camera makes a sharp image only when the hole is much smaller than the camera is deep. In the snake pit it is not. Physics Today, reporting on a 2006 model by Sichert and colleagues, notes that in a temple viper the opening and the depth are both about 2 millimeters, so even a single hot point spreads into “a big blur” across the membrane. The membrane holds a few thousand sensor cells and sees a field of about 100 degrees. The same report says snakes find prey with about 5 degrees of precision, and the model proposes that the brain applies something like a virtual lens that sharpens the blur. That was a computer model of another species, so read it as a plausible explanation, not a measurement of the diamondback.

Bakken’s team added a second check. They built 3D models of four pitviper pits and worked out how heat from a point source spreads over each membrane. The computed images match what snakes do only if the membrane’s receptors are as sensitive as the best laboratory estimates, about 0.001°C.

Where does the signal go? In 1981, Eric Newman and Peter Hartline reported neurons in the rattlesnake optic tectum, a brain region that handles vision, that receive input from both the eye and the pit. Some fire only when both senses are stimulated together. Others answer to one sense but get stronger or weaker when the second is added. The authors suggested this may help the snake recognize and orient toward important objects. So the diamondback does not seem to keep a visual picture and a heat picture in separate drawers. It blends them.

More than a hunting tool

The popular story says pits exist to find warm prey. ADW does say the pits sense slight temperature differences. The 2003 study by Alan Krochmal and George Bakken asked a different question: can a snake use its pits to choose a good place to hide? They tested western diamondbacks in arenas of rising complexity, with pits working or blocked, and the snakes made successful thermoregulatory moves (moves that manage body temperature) only when the pits worked.

Here is what the simplest version of that test looked like, as Science News described it.

The researchers repeated the pattern with a four-box choice and with four natural-looking burrows, and performance dropped with blocked pits both times. The authors conclude that thermoregulation is a plausible alternative to prey capture as the reason pits first evolved. They do not claim to have settled it. Harry Greene of Cornell University, quoted in Science News, added another idea: pits may help a snake judge predator danger. Both are open questions.

Common mistakes and myths about snake heat vision

Myth: pits are tiny night-vision goggles. The pit’s image is a coarse one, according to the modeling work above, and its signal travels on facial nerves rather than the optic nerve. The brain then mixes it with vision. A goggle image is sharper than what the modeling suggests the pit delivers.

Mix-up: the pit is a nostril. ADW places the pit between the nostril and the eye. It is its own organ.

Mix-up: every snake has heat pits. ADW says the Boa constrictor has no heat-sensing pits, and the Reptile Database says its diagnosis includes no labial pits. Some other boas and pythons do have them. The 2010 paper tested a python and a different boa species, and it links the similar channel in those groups to convergent evolution, a tool that arose separately.

Mix-up: heat is the only sense a rattlesnake uses. ADW’s account of the Sidewinder, a smaller desert rattlesnake, lists heat-sensing pits together with chemical detection and touch, with sight as a lesser sense. We did not find an equivalent list for the diamondback in our sources, but the lesson carries: pits work as part of a team.

Myth: pits are only for hunting. The Y-tube results above point to a second job, picking cooler retreats.

Conservation status

The western diamondback rattlesnake is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

The Animal Diversity Web describes a hardy species with many young, and says it is not currently threatened by mass destruction. It also lists real pressures: control efforts such as bounties, poisons and traps, plus destruction of habitat and food. We have no confirmed population trend, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.