The Pit Organs of Snakes

It is believed that the facial pits, or pit organs, located on the heads of some snakes function as specialized receptors for infrared (heat) radiation. When a rattlesnake strikes, it appears that the direction of the strike is determined by detecting infrared radiation emitted by its prey. Interestingly, a rattlesnake only targets warm-blooded prey, and if the prey is at room temperature, the snake will not strike. However, a blindfolded snake will accurately strike at a dead rat that is warmer than its surroundings when it is pulled across the cage. Since the snake cannot rely on vision due to the blindfold, and still strikes accurately at objects like a moving, cloth-wrapped electric bulb, it is evident that the pit organs are responsible for detecting the heat of warm objects. All snakes with pit organs prefer to feed on warm-blooded prey, further supporting the belief that these organs function as infrared detectors. In rattlesnakes, the pit organs are positioned between the nostrils and eyes, one on each side, and are connected to numerous nerves, suggesting a sensory role for these organs.

The sensitivity of the pit organ has been studied by measuring nerve activity from the organ. Stimuli such as sound, vibration, or moderate-intensity light (with the infrared part of the spectrum filtered out) show no detectable impact on the nerve activity. However, when objects with a different temperature from their surroundings are introduced into the receptive field around the snake’s head, there is a marked change in nerve activity, regardless of the air temperature between the object and the snake.

How is infrared radiation detected? The pit is covered by a thin transparent membrane, and one theory proposes that a temperature increase behind the membrane causes the gas to expand, leading to membrane deformation, which could be sensed by a receptor. However, this theory is unlikely since cutting the membrane and exposing the pit to outside air does not reduce its responsiveness. This result is inconsistent with the hypothesis that pressure changes are detected.

Two other possibilities remain: either the process is photochemical, meaning infrared radiation is absorbed by a specific compound similar to the light-sensitive pigments in the eye, or the pit organs detect a slight rise in temperature caused by the infrared radiation. The low quantum energy of infrared radiation emitted from a mammal’s body makes a photochemical effect on a pigment unlikely. Experiments with infrared radiation produced by lasers of known wavelengths strongly suggest that the pit organs respond to temperature changes rather than a photochemical process.

Can the pit organs be used to perceive the source of infrared radiation similarly to how our two eyes provide stereoscopic vision? This seems likely, based on the accuracy with which snakes can strike their prey and research on brain activity. When infrared radiation reaches the pit organs, electric activity is recorded in the optic tectum, the part of the brain associated with the optic nerve. This is significant, as although the pit organ nerves are separate from the optic nerves, the same brain region processes both visual and infrared information. The optic tectum is divided into left and right sections, each receiving input from both eyes through the optic chiasma, where the optic nerves cross. Many neurons in the optic tectum respond to stimulation of the pit organ on the opposite side of the head. This mirrors how visual information is processed, as the crossing of optic nerves is crucial for stereoscopic vision and depth perception. Information from the two pit organs is seemingly coordinated and interpreted similarly, as shown by neural activity changes in the optic tectum when infrared radiation affects both pits at the same time. Thus, it seems that the pit organs provide stereoscopic perception, substantially improving the snake’s ability to pinpoint the location of its prey.