A Sixth Sense for Heat
Pit vipers, a group of snakes in the subfamily Crotalinae, have a sensory system that allows them to perceive the world in a way humans cannot. Flanking the snout, between the eye and the nostril, are two openings known as loreal pits. These are not for smelling, but are sophisticated infrared detectors. Each pit contains a thin, suspended membrane, roughly 15 micrometers thick, that is densely packed with nerve endings from the trigeminal nerve. This membrane is so sensitive it can detect temperature fluctuations as small as 0.001 to 0.003°C. This allows the snake to register the faint body heat of a small mammal against a cooler background, creating a thermal image of its surroundings.
The physical structure of the pit organ functions like a pinhole camera, but for heat instead of light. Infrared radiation, with wavelengths between 5 and 30 micrometers, enters the opening and warms the sensory membrane, creating a low-resolution thermal pattern. This system allows to allow for accurate strikes on warm-blooded prey in complete darkness from several meters away. In Connecticut, this sensory advantage is employed by native pit vipers like the Northern Copperhead (Agkistrodon contortrix). These snakes use their thermal sense for hunting nocturnal rodents and for thermoregulation and detecting potential predators.
Fusing Two Realities in the Brain
The data from these heat-sensing pits does not remain a separate stream of information. Instead, it merges with the snake's sense of sight in a brain region called the optic tectum. Information from the eyes and the pit organs projects to this midbrain structure, where visual and thermal "maps" of the world are overlaid. Within the optic tectum are specialized "bimodal neurons" that respond to both light and thermal stimuli.
Neurophysiological studies show that these bimodal neurons process the inputs in complex ways. Some neurons, called 'OR' units, will fire if they receive a signal from either the eye or the pit organ. Other neurons, dubbed 'AND' units, fire only when they receive simultaneous visual and infrared stimulation. Still others show enhanced or depressed responses when one sense is stimulated in the presence of the other. This integration creates a single, unified perception of the environment. A mouse in the dark is a scent or a sound; it is a fused visual and thermal image, a warm shape moving through the snake's perception of the world. While the thermal image from the pits alone is low-resolution, integrating it with visual data may help sharpen the snake's overall perception. This combined sense provides a significant advantage for an ambush predator.