Hunting in the Dark: How Pit Vipers See Heat

In the middle of a moonless night, a Western diamondback rattlesnake lies motionless on a desert rock. A mouse scurries past in total darkness — invisible to human eyes. The snake strikes with pinpoint accuracy within 0.5 seconds. Its secret weapon? A pair of tiny pit organs between its nostril and eye that detect the warmth of the mouse's body, creating a heat image even when there is zero light. Heat pit organs are one of the most remarkable sensory systems in the animal kingdom. Snakes in the family Viperidae — pit vipers — have perfected them over millions of years of evolution.

By the end of this lesson, you will be able to: • Describe the structure of a snake's heat pit organ and where it sits on the snake's head. • Explain how heat pit organs detect infrared radiation from warm prey. • Name at least three species of snakes that have heat pit organs. • Compare pit vipers with pythons and boas, which also have heat-sensing organs but in a different location.

A heat pit organ is a small hollow pocket on a snake's face that acts like a miniature infrared camera. In pit vipers — rattlesnakes, copperheads, cottonmouths, and many tropical vipers — these pits sit in a depression called the loreal pit, located between the nostril and the eye on each side of the head. Look at a rattlesnake's face and you will see two nostrils near the snout, then two small elliptical holes a little further back — those are the heat pits. Each pit is divided into an outer chamber and an inner chamber by a thin membrane just 0.002 mm thick. This membrane is packed with temperature-sensitive receptors that can detect temperature differences of as little as 0.003 degrees Celsius.

All warm objects emit infrared (IR) radiation — a type of electromagnetic wave we feel as heat but cannot see with our eyes. A mouse's body, at about 37 degrees Celsius, radiates IR energy continuously. When a mouse walks past, IR radiation enters the outer chamber of the snake's loreal pit and strikes the thin membrane inside. The membrane heats up slightly, and heat-sensitive receptors send electrical signals through the trigeminal nerve straight to the snake's brain. Because the snake has one pit on each side of its head, the brain can compare signals from both sides and judge the direction and distance of the heat source. Scientists estimate pit vipers can accurately strike targets up to 30 cm away using heat sensing alone in absolute darkness. The brain processes heat-pit signals in the optic tectum — the same region that handles vision. In a very real sense, snakes see heat.

About 200 species of pit vipers live across Asia and the Americas. Well-known examples include: • Western Diamondback Rattlesnake (Crotalus atrox) — desert scrub of the American Southwest and Mexico. • Copperhead (Agkistrodon contortrix) — wooded hillsides of eastern North America. • Cottonmouth / Water Moccasin (Agkistrodon piscivorus) — swamps and rivers of the southeastern US. • Green Pit Viper (Trimeresurus spp.) — a diverse genus of 30+ tree-dwelling species across Asia. • Bushmaster (Lachesis muta) — the longest venomous snake in the Americas, up to 3 meters, in Central and South American rain forests. All true pit vipers are venomous and bear live young. The heat pits are the defining feature of the subfamily Crotalinae.

Most pit vipers are nocturnal hunters — they come out at night when warm-blooded prey like mice and rats are active. Total darkness does not slow a pit viper down at all. Heat sensing also helps after a venomous strike: prey animals sometimes run before the venom takes effect. The snake follows the heat trail left by the fleeing animal — a trail that lingers for several minutes — to locate and swallow its meal. Researchers at UC Davis found that rattlesnakes following heat trails located prey about 80 percent of the time in complete darkness, compared to only about 30 percent using tongue-flicking chemical cues alone.