Is thermal radiation a visual color for pitvipers? A review of crotalid ecology, neurophysiology, behavior and image processing

Pitvipers have four 'eyes': two visual eyes and two flask-shaped facial pits resembling a pinhole camera that images environmental surface temperatures using thermal radiation. Pit apertures define the sharpness and brightness of the thermal image, and vary substantially among species, suggesting adaptation to specific habitats. Thermal radiation enters the facial pit through its external aperture and heats a suspended membrane covered with thousands of temperature sensors that are sensitive to minute changes. Sensor responses are preprocessed and mapped onto the tectum, where vision, sound and touch are also represented and combined, providing unified spatial awareness. Neurological evidence suggests that visual and thermal images might be merged, with surface temperatures as an additional 'color'. However, the eye lens focuses a sharp visual image, while the lens-less facial pit forms an indistinct blurry image. Mathematically, their greatly differing information content apparently prevents formation of a meaningful combined image. More likely, thermal radiation information is presented spatially but non-visually, analogous to touch and sound. Regardless, the spatial detail of the thermal 'image' determines how it may inform behavior, and must be examined in the snake's ecological context. Pits do more than direct strikes toward warm-blooded prey. Snakes are cold blooded and use pits to locate cool refuges to escape hot conditions. However, pit performance declines as body temperature increases. As refuges become less available and more important as temperatures increase, pitvipers will be doubly impacted by global warming. This Review will summarize what is known about their thermal vision and identify future directions for research.