How Does a Snake Strike in Total Darkness? Seeing Heat
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Pits on the face of certain snakes detect infrared radiation precisely enough to aim a strike at a warm animal in complete darkness. The detection is genuinely a form of vision rather than a vague sense of warmth.
What the structure is
The organ is a shallow cavity on each side of the face, spanned partway down by an extremely thin membrane suspended in the opening with air on both sides. That membrane carries an enormous density of nerve endings, and the suspension means it has almost no thermal mass, so radiation falling on it warms it rapidly and it cools again just as fast. The nerve endings detect that temperature change directly, using a channel protein that responds to heat, and the response is fast enough to track a moving source. Temperature differences of a small fraction of a degree are detectable, which corresponds to a warm-blooded animal at a distance of roughly a metre.
Why it counts as an image
The arrangement produces spatial information rather than a single reading:
- •The opening acts as a pinhole, projecting a crude image onto the membrane
- •Different parts of the membrane therefore receive radiation from different directions
- •Two pits give overlapping fields and therefore a sense of distance
- •Signals travel to a brain region that combines them with visual input
- •Recordings show cells responding to the position of a heat source
- •Snakes with their eyes covered still strike accurately at warm targets
Which snakes have them
The sense is found in two groups that are not closely related, which makes it an independent invention twice over. Pit vipers, including rattlesnakes, copperheads and the lanceheads of the Americas, have a single large pit on each side between the eye and the nostril, and theirs is the more sensitive arrangement. Pythons and some boas have rows of smaller pits along the lips, which are less sensitive individually and cover a wider field. The underlying molecular machinery differs between the groups, using different proteins recruited for the same purpose, which is a clear case of the same problem being solved twice from different starting materials.
The other animals that sense heat
Infrared detection has appeared several times in unrelated animals, and the variety of mechanisms is striking. Vampire bats have heat-sensitive pits around the nose that locate blood vessels close to the skin of a sleeping animal, using a modified version of the same protein family the snakes use. Certain beetles detect forest fires from many kilometres away using pit organs on the thorax, and fly towards them to lay eggs in freshly burnt wood, which makes them a nuisance at industrial sites that produce heat. Some bloodsucking bugs locate hosts by warmth. Several fish detect small temperature differences in water. None of these produces an image the way the snake arrangement does, since only that one has a pinhole and a suspended membrane.
What it is actually for
The obvious assumption is hunting, and the evidence supports a broader set of uses. Detecting and striking warm prey in darkness is well demonstrated, with blindfolded snakes striking accurately and with strike accuracy falling when the pits are blocked. Choosing an ambush site appears to be at least as important, since a snake can assess whether a location shows signs of warm animals passing. Detecting predators works the same way in reverse. Thermoregulation is plausible, since an animal that must manage its own temperature benefits from sensing warm and cool surfaces at a distance, and rattlesnakes have been shown to use the sense in selecting where to rest.
The takeaway
An extremely thin suspended membrane with air on both sides warms and cools fast enough to track a moving heat source, and dense nerve endings detect fractions of a degree. The pit opening projects a crude image, so the information is spatial rather than a single reading, and two pits give distance. Pit vipers and pythons evolved it independently using different proteins, and it serves ambush selection as much as striking.