Infrared sensing in snakes
Encyclopedia
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
evolved independently in several different families of snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s. Essentially, it allows these animals to “see” radiant heat at wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s between 5 and 30 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
to a degree of accuracy such that a blind rattlesnake
Rattlesnake
Rattlesnakes are a group of venomous snakes of the genera Crotalus and Sistrurus of the subfamily Crotalinae . There are 32 known species of rattlesnake, with between 65-70 subspecies, all native to the Americas, ranging from southern Alberta and southern British Columbia in Canada to Central...
can target vulnerable body parts of the prey at which it strikes. It was previously thought that the organs evolved primarily as prey detectors, but recent evidence suggests that it may also be used in thermoregulation
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
and predator detection, making it a more general-purpose sensory organ than was supposed.
Phylogeny and evolution
The facial pit underwent parallel evolutionParallel evolution
Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...
in pitviper
Crotalinae
The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...
s and some boas and pythons. It evolved once in pitvipers and multiple times in boas and pythons. The electrophysiology
Electrophysiology
Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
of the structure is similar between the two lineages, but they differ in gross structural anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
. Most superficially, pitvipers possess one large pit organ on either side of the head, between the eye and the nostril (loreal pit
Loreal pit
The Loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes . It is located behind the nostril and in front of the eye, but below the line that runs between the centers of each. It is the external opening to an extremely sensitive infrared...
s), 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
Viperidae
The Viperidae are a family of venomous snakes found all over the world, except in Antarctica, Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four...
, the pit organ is seen only in the subfamily Crotalinae
Crotalinae
The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...
: the pitvipers. The organ is used extensively by them to detect and target warm-blooded
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
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
Viperinae
The Viperinae, or viperines, are a subfamily of venomous vipers found in Europe, Asia and Africa. They are distinguished by their lack of the heat-sensing pit organs that characterize their sister group, the Crotalinae. Currently, 12 genera and 66 species are recognized...
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
Viperinae
The Viperinae, or viperines, are a subfamily of venomous vipers found in Europe, Asia and Africa. They are distinguished by their lack of the heat-sensing pit organs that characterize their sister group, the Crotalinae. Currently, 12 genera and 66 species are recognized...
snakes (which lack pit organs) also use thermal cues to guide strike behavior, but not to guide thermoregulation.
Anatomy
In pitvipersCrotalinae
The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...
, the heat pit consists of a deep pocket in the rostrum with a membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...
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
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...
(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 crotalines
Crotalinae
The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...
. The purpose of the vasculature, in addition to providing oxygen to the receptor terminals, is to rapidly cool the receptors to their thermo-neutral state after being heated by thermal radiation from a stimulus. Were it not for this vasculature, the receptor would remain in a warm state after being exposed to a warm stimulus, and would present the animal with afterimages even after the stimulus was removed.
Neuroanatomy
In all cases, the facial pit is innervated by the trigeminal nerve. In crotalines
Crotalinae
The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...
, information from the pit organ is relayed to the nucleus reticularus caloris in the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply 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.
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
Optic
Optic may mean:* optic, a British term for a device for dispensing fixed amounts of alcoholic spirits* Optics, the study of the behavior and properties of light* An optical element or component, such as a lens, prism, or mirror...
stimulation, but also motor, proprioceptive and auditory
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
. 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.
The nerve fibers in the pit organ are constantly firing at a very low rate. Objects that are within a neutral temperature range
Temperature range
Atmospheric temperature range is the numerical difference between the minimum and maximum values of temperature observed in a given location....
do not change the rate of firing; the neutral range is determined by the average thermal radiation of all objects in the receptive field
Receptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
of the organ. The thermal radiation above a given threshold causes an increase in the temperature of the nerve fiber, resulting in stimulation of the nerve and subsequent firing, with increased temperature resulting in increased firing rate. The sensitivity of the nerve fibers is estimated to be >0.001 °C.
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
Incubation period
Incubation period is the time elapsed between exposure to a pathogenic organism, a chemical or radiation, and when symptoms and signs are first apparent...
of adaptation is approximately 50-150 msec.
The facial pit actually visualizes thermal radiation using the same optical principles as a pinhole camera
Pinhole camera
A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...
, 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
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
, 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
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
(colder areas) to maximize the contrast of their warm prey in order to achieve such a high degree of accuracy from their thermal “vision”.