Infrared sensing in snakes

Last updated
A python (top) and rattlesnake illustrating the positions of the pit organs. Arrows pointing to the pit organs are red; a black arrow points to the nostril. The Pit Organs of Two Different Snakes.jpg
A python (top) and rattlesnake illustrating the positions of the pit organs. Arrows pointing to the pit organs are red; a black arrow points to the nostril.

The ability to sense infrared thermal radiation evolved independently in three different groups of snakes, consisting of the families of Boidae (boas), Pythonidae (pythons), and the subfamily Crotalinae (pit vipers). What is commonly called a pit organ allows these animals to essentially "see" [1] radiant heat at wavelengths between 5 and 30  μm. The more advanced infrared sense of pit vipers allows these animals to strike prey accurately even in the absence of light, and detect warm objects from several meters away. [2] [3] It was previously thought that the organs evolved primarily as prey detectors, but recent evidence suggests that it may also be used in thermoregulation and predator detection, making it a more general-purpose sensory organ than was supposed. [4] [5]

Contents

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. [6] 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.

Anatomy

In pit vipers, 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 crotalines. 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. [7]

Diagram of the Crotaline pit organ. Diagram of the Crotaline Pit Organ.jpg
Diagram of the Crotaline pit organ.

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. [8]

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. [9]

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 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. [10] The sensitivity of the nerve fibers is estimated to be <0.001 °C. [11]

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 to 150 ms. [10]

The facial pit actually visualizes thermal radiation using the same optical principles 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 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 the image.

Molecular mechanism

In spite of its detection of infrared light, the infrared detection mechanism is not similar to photoreceptors - while photoreceptors detect light via photochemical reactions, the protein in the pits of snakes is a type of transient receptor potential channel, TRPA1 which is a temperature sensitive ion channel. It senses infrared signals through a mechanism involving warming of the pit organ, rather than chemical reaction to light. [12] In structure and function it resembles a biological version of warmth-sensing instrument called a bolometer. This is consistent with the very thin pit membrane, which would allow incoming infrared radiation to quickly and precisely warm a given ion channel and trigger a nerve impulse, as well as the vascularization of the pit membrane in order to rapidly cool the ion channel back to its original temperature state. While the molecular precursors of this mechanism are found in other snakes, the protein is both expressed to a much lower degree and is much less sensitive to heat. [12]

Behavioral and ecological implications

Infrared sensing snakes use pit organs extensively to detect and target warm-blooded prey such as rodents and birds. Blind or blindfolded rattlesnakes can strike prey accurately in the complete absence of visible light, [13] [14] though it does not appear that they assess prey animals based on their body temperature. [15] In addition, snakes may deliberately choose ambush sites that facilitate infrared detection of prey. [16] [17] It was previously assumed that the organ evolved specifically for prey capture. [11] 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 pit vipers 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. [4] 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 contributed to the organ's development. [5] The use of the heat pit to direct thermoregulation or other behaviors in pythons and boas has not yet been determined.

See also

Related Research Articles

<span class="mw-page-title-main">Infrared</span> Form of electromagnetic radiation

Infrared is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with waves that are just longer than those of red light, so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750 nm (400 THz) to 1 mm (300 GHz). IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the solar spectrum. Longer IR wavelengths (30–100 μm) are sometimes included as part of the terahertz radiation band. Almost all black-body radiation from objects near room temperature is in the IR band. As a form of electromagnetic radiation, IR carries energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon.

<span class="mw-page-title-main">Snake</span> Limbless, scaly, elongate reptile

Snakes are elongated, limbless reptiles of the suborder Serpentes. Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads. To accommodate their narrow bodies, snakes' paired organs appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca. Lizards have independently evolved elongate bodies without limbs or with greatly reduced limbs at least twenty-five times via convergent evolution, leading to many lineages of legless lizards. These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal.

<span class="mw-page-title-main">Cranial nerves</span> Nerves that emerge directly from the brain and the brainstem

Cranial nerves are the nerves that emerge directly from the brain, of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck, including the special senses of vision, taste, smell, and hearing.

<span class="mw-page-title-main">Visible spectrum</span> Portion of the electromagnetic spectrum that is visible to the human eye

The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light. The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well, known collectively as optical radiation.

<span class="mw-page-title-main">Viperinae</span> Subfamily of snakes

Viperinae, or viperines, are a subfamily of vipers endemic to Europe, Asia and Africa. They are distinguished by their lack of the heat-sensing pit organs that characterize their sister group, the subfamily Crotalinae. Currently, 13 genera are recognized. Most are tropical and subtropical, although one species, Vipera berus, even occurs within the Arctic Circle. Like all vipers, they are venomous.

In physiology, thermoception or thermoreception is the sensation and perception of temperature, or more accurately, temperature differences inferred from heat flux. It deals with a series of events and processes required for an organism to receive a temperature stimulus, convert it to a molecular signal, and recognize and characterize the signal in order to trigger an appropriate defense response.

<span class="mw-page-title-main">Rattlesnake</span> Group of venomous snakes of the genera Crotalus and Sistrurus

Rattlesnakes are venomous snakes that form the genera Crotalus and Sistrurus of the subfamily Crotalinae. All rattlesnakes are vipers. Rattlesnakes are predators that live in a wide array of habitats, hunting small animals such as birds and rodents.

<span class="mw-page-title-main">Animal communication</span> Transfer of information from animal to animal

Animal communication is the transfer of information from one or a group of animals to one or more other animals that affects the current or future behavior of the receivers. Information may be sent intentionally, as in a courtship display, or unintentionally, as in the transfer of scent from predator to prey with kairomones. Information may be transferred to an "audience" of several receivers. Animal communication is a rapidly growing area of study in disciplines including animal behavior, sociology, neurology and animal cognition. Many aspects of animal behavior, such as symbolic name use, emotional expression, learning and sexual behavior, are being understood in new ways.

<span class="mw-page-title-main">Thermoreceptor</span> Receptive portion of a sensory neuron

A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous system, warmth receptors are thought to be unmyelinated C-fibres, while those responding to cold have both C-fibers and thinly myelinated A delta fibers. The adequate stimulus for a warm receptor is warming, which results in an increase in their action potential discharge rate. Cooling results in a decrease in warm receptor discharge rate. For cold receptors their firing rate increases during cooling and decreases during warming. Some cold receptors also respond with a brief action potential discharge to high temperatures, i.e. typically above 45 °C, and this is known as a paradoxical response to heat. The mechanism responsible for this behavior has not been determined.

<span class="mw-page-title-main">Pit viper</span> Subfamily of snakes

The Crotalinae, commonly known as pit vipers, or pit adders, are a subfamily of vipers found in Asia and the Americas. Like all other vipers, they are venomous. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on both sides of the head. Currently, 23 genera and 155 species are recognized: These are also the only viperids found in the Americas. The groups of snakes represented here include rattlesnakes, lanceheads, and Asian pit vipers. The type genus for this subfamily is Crotalus, of which the type species is the timber rattlesnake, C. horridus.

<span class="mw-page-title-main">Viper</span> Family of snakes

The Viperidae (vipers) are a family of snakes found in most parts of the world, except for Antarctica, Australia, Hawaii, Madagascar, New Zealand, Ireland, and various other isolated islands. They are venomous and have long, hinged fangs that permit deep penetration and injection of their venom. Three subfamilies are currently recognized. They are also known as viperids. The name "viper" is derived from the Latin word vipera, -ae, also meaning viper, possibly from vivus ("living") and parere, referring to the trait viviparity common in vipers like most of the species of Boidae.

<span class="mw-page-title-main">Snake venom</span> Highly modified saliva containing zootoxins

Snake venom is a highly toxic saliva containing zootoxins that facilitates in the immobilization and digestion of prey. This also provides defense against threats. Snake venom is usually injected by unique fangs during a bite, though some species are also able to spit venom.

<span class="mw-page-title-main">Superior colliculus</span> Structure in the midbrain

In neuroanatomy, the superior colliculus is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum or optic lobe. The adjective form tectal is commonly used for both structures.

Neuralgia is pain in the distribution of a nerve or nerves, as in intercostal neuralgia, trigeminal neuralgia, and glossopharyngeal neuralgia.

<i>Bitis</i> Genus of snakes

Bitis is a genus of venomous vipers found in Africa and the southern Arabian Peninsula. It includes the largest and the smallest vipers in the world. Members are known for their characteristic threat displays that involve inflating and deflating their bodies while hissing and puffing loudly. The type species for this genus is B. arietans, which is also the most widely distributed viper in Africa. Currently, 18 species are recognized.

A sense is a biological system used by an organism for sensation, the process of gathering information about the surroundings through the detection of stimuli. Although, in some cultures, five human senses were traditionally identified as such, many more are now recognized. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.

Feature detection is a process by which the nervous system sorts or filters complex natural stimuli in order to extract behaviorally relevant cues that have a high probability of being associated with important objects or organisms in their environment, as opposed to irrelevant background or noise.

<span class="mw-page-title-main">Caudal luring</span> Form of aggressive mimicry where the predator attracts prey using its tail

Caudal luring is a form of aggressive mimicry characterized by the waving or wriggling of the predator's tail to attract prey. This movement attracts small animals who mistake the tail for a small worm or other small animal. When the animal approaches to prey on the worm-like tail, the predator will strike. This behavior has been recorded in snakes, sharks, and eels.

<span class="mw-page-title-main">Infrared sensing in vampire bats</span>

Vampire bats have developed a specialized system using infrared-sensitive receptors on their nose-leaf to prey on homeothermic (warm-blooded) vertebrates. Trigeminal nerve fibers that innervate these IR-sensitive receptors may be involved in detection of infrared thermal radiation emitted by their prey. This may aid bats in locating blood-rich areas on their prey. In addition, neuroanatomical and molecular research has suggested possible similarities of IR-sensing mechanisms between vampire bats and IR-sensitive snakes. Infrared sensing in vampire bats has not yet been hypothesized to be image forming, as it was for IR-sensitive snakes. While the literature on IR-sensing in vampire bats is thin, progress continues to be made in this field to identify how vampire bats can sense and use infrared thermal radiation.

References

  1. Newman, EA; Hartline, PH (1981). "Integration of visual and infrared information in bimodal neurons in the rattlesnake optic tectum". Science. 213 (4509): 789–91. Bibcode:1981Sci...213..789N. doi:10.1126/science.7256281. PMC   2693128 . PMID   7256281.
  2. Goris, RC; Terashima, S (1973). "Central response to infra-red stimulation of the pit receptors in a crotaline snake, Trimeresurus flavoviridis". Journal of Experimental Biology. 58 (1): 59–76. doi:10.1242/jeb.58.1.59. PMID   4350276.
  3. "Snake infrared detection unravelled". Archived from the original on 28 December 2016. Retrieved 20 January 2017.
  4. 1 2 Krochmal, Aaron R.; George S. Bakken; Travis J. LaDuc (15 November 2004). "Heat in evolution's kitchen: evolutionary perspectives on the functions and origin of the facial pit of pitvipers (Viperidae: Crotalinae)". Journal of Experimental Biology. 207 (Pt 24): 4231–4238. doi: 10.1242/jeb.01278 . PMID   15531644.
  5. 1 2 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.
  6. Pough et al. 1992. Herpetology: Third Edition. Pearson Prentice Hall: Pearson Education, Inc., 2002.
  7. Goris, CR; et al. (2003). "The microvasculature of python pit organs: morphology and blood flow kinetics". Microvascular Research. 65 (3): 179–185. doi:10.1016/s0026-2862(03)00003-7. PMID   12711259.
  8. 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 (3): 465–477. doi:10.1002/cne.901910309. PMID   7410602. S2CID   10279222.
  9. Hartline, PH; L Kass; MS Loop (1978-03-17). "Merging of modalities in the optic tectum: infrared and visual integration in rattlesnakes". Science. 199 (4334): 1225–1229. Bibcode:1978Sci...199.1225H. doi:10.1126/science.628839. PMID   628839.
  10. 1 2 Bullock, TH; Cowles, RB (1952). "Physiology of an infrared receptor: the facial pit of pit vipers". Science. 115 (2994): 541–543. Bibcode:1952Sci...115..541B. doi:10.1126/science.115.2994.541-a. PMID   17731960. S2CID   30122231.
  11. 1 2 Bakken, George S.; Krochmal, Aaron R. (2007), "The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis", Journal of Experimental Biology, 210 (16): 2801–2810, doi: 10.1242/jeb.006965 , PMID   17690227
  12. 1 2 Gracheva, Elena O.; Nicholas T. Ingolia; Yvonne M. Kelly; Julio F. Cordero-Morales; Gunther Hollopeter; Alexander T. Chesler; Elda E. Sánchez; John C. Perez; Jonathan S. Weissman; David Julius (15 April 2010). "Molecular basis of infrared detection by snakes". Nature. 464 (7291): 1006–1011. Bibcode:2010Natur.464.1006G. doi:10.1038/nature08943. PMC   2855400 . PMID   20228791.
  13. Chen, Q; Liu, Y; Brauth, SE; Fang, G; Tang, Y (2017). "The thermal background determines how the infrared and visual systems interact in pit vipers". Journal of Experimental Biology. 220 (Pt 17): 3103–3109. doi: 10.1242/jeb.155382 . PMID   28855322.
  14. Kardong, KV; Mackessy, SP (1991). "The strike behavior of a congenitally blind rattlesnake". Journal of Herpetology. 25 (2): 208–211. doi:10.2307/1564650. JSTOR   1564650.
  15. Schraft, HA; Goodman, C; Clark, RW (2017). "Do free-ranging rattlesnakes use thermal cues to evaluate prey?". Journal of Comparative Physiology A. 204 (3): 295–303. doi:10.1007/s00359-017-1239-8. PMID   29218413. S2CID   3370317.
  16. Schraft, HA; Bakken, GS; Clark, RW (2019). "Infrared-sensing snakes select ambush orientation based on thermal backgrounds". Scientific Reports. 9 (1): 3950. Bibcode:2019NatSR...9.3950S. doi:10.1038/s41598-019-40466-0. PMC   6408448 . PMID   30850649 via in review.
  17. Shine, R; Sun, L; Kearny, M; Fitzgerald, M (2002). "Thermal correlates of foraging-site selection by Chinese pit-vipers (Gloydius shedaoensis, Viperidae)". Journal of Thermal Biology. 27 (5): 405–412. doi:10.1016/S0306-4565(02)00009-8.