The snake pit receptor is an imaging sensory organ used to target an infrared-emitting prey moving in three-dimensional space. This system is the most sensitive infrared detector currently known. Crotaline snakes and certain genera of boids each possess a type of facial organ, believed to have evolved separately, which are used to image infrared radiation. Information obtained from the use of these organs can be overlayed with images of radiant electromagnetic energy from the lateral eye, which responds to visible light
Crotalidae is a subfamily of the family viperidae. These snakes are characterised by infrared sensing pit organs situated within deep invaginations, one on either side of the head, between the eye and nasal passage (Fig. 1).
Some genera of Boidae also possess a series of thremosensitive pits on the scales of the maxilla and mandible (Fig. 1). Within boids, there is great interspecies differentiation in the organisation pit organs (Safer, & Grace, 2004). Species belonging to the genus Aspedites are the only boids known to be lacking heat sensing pits. This is one of the primary characteristic that has lead taxonomists to believe these snakes are primitive forms
Top: head of a jungle carpet python (Morelia spilota cheynei) showing three pit organs on maxilla (arrowheads) and a series of labial pits on the mandible (arrows). Bottom: Western diamondback rattlesnake (Crotalus atrox) showing the location of a loreal pit organ (arrow).
The labial pits of boids differ from the facial pits of crotalids in that they lack an inner chamber and thin sensory membrane stretched across their bottom. These anatomical differences have resulted in hypothesis of convergent evolution, suggesting that the pit organ systems of crotalids and boids evolved independently of one another. These differences, however, do not affect the ability of snakes possessing these labial pits to strike at warm objects in the absence of visual, olfactory, and tactile cues. In the absence of vision, thermal receptors are the most important sensory systems for directing a strike towards endothermic prey.
Microstructure
The different pit organs of snakes are all innervated by the fifth cranial nerve, which forms a cluster of infrared-sensitive neuronal terminals in close proximity to the epidermal membrane of the organ. The dense arrays of neuronal terminals within pit organs originate from soma situated in the trigeminal ganglia. Via the trigeminal, infrared information is conveyed to the nucleus of the lateral descending trigeminal tract, a brainstem nucleus exclusively found in infrared imaging snakes.
Snake thermal receptors have been found to show highly developed arrangements of surface nanopits tens of nanometres in diameter. The average spacing of the nanopit arrays is approximately 500nm.
Infrared information is believed to be mapped in the optic tectum, coinciding with spatial maps of visual space. Ultimately, crotaline snakes and infrared imaging boids simultaneously image two distinct ranges of the electromagnetic spectrum, visible light and infrared radiation.
Thermal receptor function and properties
Being a natural infrared imager, the thermal sensitive pit organs of snakes combine high sensitivity, ambient temperature function, microscopic dimensions, and self-repair. Snake pit organ absorbs infrared radiation in both the 3-5 μm and 8-12 μm atmospheric transmission windows. Absorption, however, has been shown to be greater in the 8-12 μm range of the atmospheric transmission window. This correlates with the peak infrared emission (10 μm) of target endothermic prey, such as mammals and birds.
It is important to note that pit organ thermal receptors do not operate on a photic system. This was concluded by after two types of opsin antisera, capable of recognising epitopes of opsin proteins found in retinal photoreceptors, failed to mark the infrared terminals of pit organ. These results were somewhat expected, however, as infrared radiation is generally of insufficient energy to photoisomerize the 11-cis retinaldehyde within opsin proteins, activating the retinal photoreceptors. It is likely that pit organ terminals do not utilize a protein closely related to retinal opsins.
speculated that the high sensitivity of thermal receptors owes strongly to a combination of two morphological features of the pit organ, the proximity of nerve terminals to the surface membrane, and a highly reflective nanopit array, which serves as a selective infrared filter. Nanopit arrays are capable of acting as selective filters due to the small spacing, approximately 500 nm, between pits. 500nm is close to the grating space required to effectively reflect the electromagnetic radiation of natural sunlight without affecting infrared radiation.
As the thermal conductivity of air is lower than any organic material, the air filled nanopits in close proximity to the surface of infrared receptors significantly reduce the overall thermal conductivity of the organ. Low thermal conductivity of infrared receptor surfaces significantly reduces heat dissipation from the sites. This allows for a rapid localized increase in the temperature of pit organ surface membranes. Skin surrounding the thermal receptors is composed of non-specific cells which have a higher thermal conductivity than the receptor area, this results in an increased rate of heat dissipation. Consequently, a thermal gradient is formed between the area of infrared reception and neighbouring non-specific skin. This thermal gradient combined with the reduced proximity of nerve terminals to the pit surface is conducive for high-sensitivity photothermal detection.
The special resolution of snake infrared imaging systems is enhanced by low lateral transfer rate of thermal energy across the surface of the receptor. This characteristic impedes the rate of conductivity between adjacent receptor terminal units, reducing the likelihood of overheating and allowing the receptors to function in the presence of a high thermal background.
Environmental Application
Crotalids and boids are specialist ambush predators that show a preference for targeting homeothermic prey. This behaviour is facilitated by the presence thermal imaging facial pit organs. Pit organs significantly enhance a snake's ability to detect and localize prey.
Facial pit organs are highly sensitive infrared receptors that form part of an elaborate spatial imaging system capable of mapping the thermal environment and provide an additional imaging channel independent of ocular vision. Pit receptors have been found to efficiently recognise infrared photons emitted at 10 μm. This matches the maximal infrared radiation emitted by homeothermic prey.
As a result of photophysical studies, it has been found that snake infrared imaging systems can discern changes in environmental temperature as low as 0.003°C. The ability to detect such minute changes in temperature permits snakes that poses thermal receptors to target prey, locate suitable retreats and basking spots, and identify predators.
A prey item can elicit a response in a snake's thermal receptors if the infrared radiation emitted by the prey contrasts with the background radiation. have shown that a snake that has been deprived of all sensory systems, except its pit organs, maintains the ability to strike correctly at warm moving objects. This capacity for directional analysis is an extremely useful tool for both locating prey and defence against predators.
Case studies conducted on tiger snakes, Notechis scutatus, living on Carnac Island, Western Australia, have found that many adult specimens have apparently been blinded by silver gulls, Larus novaehollandiae, while attempting to feed on nesting chicks. These studies concluded that there was no difference in body condition (mass relative to length) between snakes with functioning eyes, and those without. The ability of these snakes to survive without a visual system is a result of possessing a combination of highly sensitive infrared receptors, an acute olfactory system, and mechanoreceptors lining the bottom of the mandible, which allow snakes to sense vibrations in the ground.
note that non-crotaline vipers, such as members of the viperine sub-family Causinae and genus Bitis, possess supranasal sacs beneath the supranasal scales (Fig. 2). It is hypothesised that these sacs may have infrared imaging capabilities. Safer, and Grace concluded that snakes possessing supranasal sacs did not demonstrate any obvious predatory or defensive response when presented with thermal cues. Due to a lack of research into the function of supranasal sacs, however, there is still the possibility that they may act as a sensory system analogous to thermal receptive pit organs.
Gaboon viper (Bitis gabonica) possessing a supranasal sac (arrow).
As the pit organs of snakes are the most sensitive natural infrared receptors currently known. The study of these organs may provide insight into developing more accurate artificial thermal detectors. These detectors would serve to replace current, less efficient thermal sensors which are used in situations such as home security, military operations, and environmental research. Further research into the physiology of pit organs may also secondarily provide a method of deterring snakes from entering areas populated by humans.
