Infrared vision in snakes
Often touted as the 6th sense of the snake, this trait is special to the boas, pythons and pit vipers (a mere 7.3% of all snakes). The trait is actively involved in hunting, and recent evidence suggests it may play a role in thermoregulation. Infra-red vision is an excellent example of parallel evolution, whereby the same trait evolves independently in two separate lineages - in this case it evolved once in the boas and pythons, and several times in the pit vipers. The trait is similar in the two lineages, and whilst there are some subtle nuances between the two groups, we will focus upon the mechanism in the boas and pythons which is more simplistic (infra red vision in pit vipers is more advanced than in boas and pythons).
Ball python showing distinct labial, or "heat seaking pits"
Clearly visible down each side of the above ball pythons head, just above the mouth is a set of four pits, so eight in total. These are the heat seeking pits, or labial pits to give them their proper name. Within the labial pits is a highly vasculated (well supplied with blood) membrane, upon which sits a high density of nerves which are highly sensitive to heat. The constant blood flow in the area has two benefits, it keeps the heat sensitive nerves bathed in oxygen, and it helps to cool them down again after they have been excited. This cell cooling is imperative to the heat sensitive mechanism because were the cells not cooled, they would continue to send the message of heat to the brain (think of it in a similar way to when you stare at a bulb and the image is 'burnt' in to your retina, were the heat sensitive cells not quickly cooled, the snake would have the images constantly 'burnt in').
The nerve fibres within the labial pits have a 'base rate' at which they fire, ie the cells are constantly sending impulses, but at a low rate. The snakes have a 'neutral temperature range' (determined by the average temperature of all local objects), when an object is within this neutral range it does not effect the rate of firing within the cell, ie no information is transmitted, the signal is neutral. Thermal radiation (heat) which is above this neutral temperature range causes the rate at which the cells send impulses to increase. This change in firing rate is interpreted by the brain as a temperature change and the snake is able to map the environment using this information, thus creating the infra red, or heat sensitive vision.
Of course it is impossible for us to say how the snake makes use of this information, it is unlikely the snakes witness the typical portrayal of infra red vision seen in a thermogram, with warm items displaying visually as red, and cool items as blue. Its difficult to say whether there is a visual element to it at all. Trying to define it is probably akin to defining sound to the deaf, or vision to the blind.
Typical thermogram of a mouse representing "heat seaking vision" - Is this realistic?
Image source www.visualphotos.com, photographer Ted Kinsman
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