Some snakes can see in the dark.
All mammals sense warmth. A few, such as vampire bats, can see heat images. As well as pit-bearing snakes, some insects possess thermavision.
Unlike visible light, the infrared range of the electromagnetic spectrum is richer in thermal radiation than in photonic waves. Thus, it is much easier to see in the dark thermally than photonically. Whence thermavision.
The ability to see via infrared heat radiation independently evolved in 2 groups of pit-bearing snakes: the boas and pit vipers. Thermal imaging arose once in pit vipers. Among boas and pythons, thermavision evolved multiple times.
The electrophysiology of thermavision is selfsame among snakes, but the anatomy differs. Pit vipers possess a single pit organ on either side of their heads, between the eye and nostril. In boas and pythons, there 3 or more smaller pits near the lips.
The pit organ is a hollow chamber enclosed by a thin membrane. The design is such that the pit organ rapidly warms and loses heat, thus affording optimal sensitivity.
The pit organ is part of the same somatosensory system used for touch. The eyes have their own physiological system. The mind merges light and thermal inputs to yield a coherent image from all available data.
Nerve cells in the pit organ produce an ion channel that acts as an infrared heat receptor. Proteins in nerve cell membranes collectively act as a pyroelectrical material: able to translate heat into a correspondent electrical signal. The greatest sensitivity is achieved at an infrared threshold that perfectly corresponds to the thermal signature potential prey animals provide: ~28 °C.
References:
Faezeh Darbaniyan et al, “Soft matter mechanics and the mechanisms underpinning the infrared vision of snakes,” Matter (21 October 2020).
“How do snakes ‘see’ in the dark? Researchers have an answer,” ScienceDaily (21 October 2020).
Jane Fang, “Snake infrared detection unravelled,” Nature (14 March 2010).
Shozo Yokoyama et al, “Molecular convergence of infrared vision in snakes,” Molecular Biology and Evolution (11 October 2010).
Angela L.Campbell et al, “Biological infrared imaging and sensing,” Micron (2002).