Infrared sensing in snakes

The ability to sense infrared thermal radiation evolved independently in several different families of snakes. Essentially, it allows these animals to “see” radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes.Kardong KV, Mackessy SP. 1991. The strike behavior of a congenitally blind rattlesnake. Journal of Herpetology 25: 208-211.] It was previously thought that the organs evolved primarily as prey detectors, but recent evidence suggests that it may be used in thermoregulation and predator detection, making it a more general-purpose sensory organ than was supposed.Krochmal AR, Bakken GS, LaDuc TJ. 1994. Heat in evolution’s kitchen: evolutionary perspectives on the function and origin of the facial pit of pitvipers (Viperidae:Crotalinae). The Journal of Experimental Biology 207: 4231-4238.] Greene HW. 1992. The ecological and behavioral context for pitviper evolution. "In" Campbell JA, Brodie ED Jr. 1992. Biology of the Pitvipers. Texas: Selva. 467 pp. 17 plates. ISBN 0-9630537-0-1.]

Phylogeny and evolution

The facial pit underwent parallel evolution in pitvipers and some boas and pythons. It evolved once in pitvipers and multiple times in boas and pythons.Pough "et al." 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Education, Inc., 2002.] The electrophysiology of the structure is similar between the two lineages, but they differ in gross structural anatomy. Most superficially, pitvipers possess one large pit organ on either side of the head, between the eye and the nostril (loreal pits), while boas and pythons have three or more comparatively smaller pits lining the upper and sometimes the lower lip, in or between the scales (labial pits). Those of the pitvipers are the more advanced, having a suspended sensory membrane as opposed to a simple pit structure. In vipers, the pit organ is seen only in the subfamily Crotalinae: the pitvipers. The organ is used extensively by them to detect and target warm-blooded prey such as rodents and birds, and it was previously assumed that the organ evolved specifically for that purpose. However, recent evidence shows that the pit organ may also be used for thermoregulation. In an experiment that tested snakes' abilities to locate a cool thermal refuge in an uncomfortably hot maze, all pitvipers were able to locate the refuge quickly and easily, while true vipers were unable to do so. This suggests that the pitvipers were using their pit organs to aid in thermoregulatory decisions. It is also possible that the organ may even have evolved as a defensive adaptation rather than a predatory one, or that multiple pressures may have potentially contributed to the organ's development. The use of the heat pit to direct thermoregulation in pythons and boas has not yet been determined. Viperine snakes (which lack pit organs) also use thermal cues to guide strike behavior, but not to guide thermoregulation.Breidenbach CV. 1990. Thermal cues influence strikes in pitless vipers. Journal of herpetology 24:448-450]

Anatomy

In pitvipers, the heat pit consists of a deep pocket in the rostrum with a membrane stretched across it. Behind the membrane, an air-filled chamber provides air contact on either side of the membrane. The pit membrane is highly vascular and heavily innervated with numerous heat-sensitive receptors formed from terminal masses of the trigeminal nerve (terminal nerve masses, or TNMs). The receptors are therefore not discrete cells, but a part of the trigeminal nerve itself. The labial pit found in boas and pythons lacks the suspended membrane and consists more simply of a pit lined with a membrane that is similarly innervated and vascular, though the morphology of the vasculature differs between these snakes and

=Neuroanatomy= In all cases, the facial pit is innervated by the trigeminal nerve. In crotalines, information from the pit organ is relayed to the nucleus reticularus caloris in the medulla via the lateral descending trigeminal tract. From there, it is relayed to the contralateral optic tectum. In boas and pythons, information from the labial pit is sent directly to the contralateral optic tectum via the lateral descending trigeminal tract, bypassing the nucleus reticularus caloris.Newman EA, Gruberd ER, Hartline PH. 1980. The infrared trigemino-tectal pathway in the rattlesnake and in the python. The Journal of Comparative Neurology 191: 465-477.]

It is the optic tectum of the brain which eventually processes these infrared cues. This portion of the brain receives other sensory information as well, most notably optic stimulation, but also motor, proprioceptive and auditory. Some neurons in the tectum respond to visual or infrared stimulation alone; others respond more strongly to combined visual and infrared stimulation, and still others respond only to a combination of visual and infrared. Some neurons appear to be tuned to detect movement in one direction. It has been found that the snake’s visual and infrared maps of the world are overlaid in the optic tectum. This combined information is relayed via the tectum to the forebrain.Hartline PH, Kass L, Loop MS. 1978. Merging of modalities in the optic tectum: infrared and visual integration in rattlesnakes. Science 199: 1225-1229.]

The nerve fibers in the pit organ are constantly firing at a very low rate. Objects that are within a neutral temperature range do not change the rate of firing; the neutral range is determined by the average thermal radiation of all objects in the receptive field of the organ. The thermal radiation from warm objects causes an increase in the temperature of the nerve fiber, resulting in stimulation of the nerve and subsequent increase in firing rate. Thermal radiation from colder objects cools the nerve, causing an inhibition and firing rate depression.Bullock TH, Cowles RB. 1952. Physiology of an infrared receptor: the facial pit of pit vipers. Science 115: 541-543.] The sensitivity of the nerve fibers is estimated to be >0.001 °C.Bakken GS, Krochmal AR. 2007. The imaging properties and sensitivity of the facial pit of pitvipers as determined by optical and heat-transfer analysis. The Journal of Experimental Biology 210: 2801-2810.]

The pit organ will adapt to a repeated stimulus; if an adapted stimulus is removed, there will be a fluctuation in the opposite direction. For example, if a warm object is placed in front of the snake, the organ will increase in firing rate at first, but after a while will adapt to the warm object and the firing rate of the nerves in the pit organ will return to normal. If that warm object is then removed, the pit organ will now register the space that it used to occupy as being colder, and as such the firing rate will be depressed until it adapts to the removal of the object. The latency period of adaptation is approximately 50-150 msec.

The facial pit actually visualizes thermal radiation using the same optical principals as a pinhole camera, wherein the location of a source of thermal radiation is determined by the location of the radiation on the membrane of the heat pit. However, studies that have visualized the thermal images seen by the facial pit using computer analysis have suggested that the resolution is actually extremely poor. The size of the opening of the pit results in poor resolution of small, warm objects, and coupled with the pit's small size and subsequent poor heat conduction, the image produced is of extremely low resolution and contrast. It is known that some focusing and sharpening of the image occurs in the lateral descending trigeminal tract, and it is possible that the visual and infrared integration that occurs in the tectum may also be used to help sharpen in the image. In addition, snakes may deliberately choose ambush sites with low thermal background radiation (colder areas) to maximize the contrast of their warm prey in order to achieve such a high degree of accuracy from their thermal “vision”.

ee also

*Crotalinae
*Neuroethology
*Thermoception

References