Vipers aresnakes in the familyViperidae, found in most parts of the world, except forAntarctica,Australia,[2]Hawaii,Madagascar,New Zealand,Ireland, and various other isolated islands. They arevenomous and have long (relative to non-vipers), hingedfangs that permit deep penetration andinjection of their venom.[3] Three subfamilies are currently recognized.[4] They are also known asviperids. The name "viper" is derived from the Latin wordvipera, -ae, also meaning viper, possibly fromvivus ("living") andparere ("to beget"), referring to the traitviviparity (giving live birth) common in vipers like most of the species ofBoidae.[5]
Arattlesnake skull, showing the long fangs used to inject venom
All viperids have a pair of relatively longsolenoglyphous (hollow) fangs that are used to injectvenom from glands located towards the rear of the upper jaws, just behind the eyes. Each of the two fangs is at the front of the mouth on a shortmaxillary bone that can rotate back and forth. When not in use, the fangs fold back against the roof of the mouth and are enclosed in a membranous sheath. This rotating mechanism allows for very long fangs to be contained in a relatively small mouth. The left and right fangs can be rotated together or independently. During a strike, the mouth can open nearly 180° and the maxilla rotates forward, erecting the fangs as late as possible so that the fangs do not become damaged, as they are brittle. The jaws close upon impact and the muscular sheaths encapsulating the venom glands contract, injecting the venom as the fangs penetrate the target. This action is very fast; in defensive strikes, it will be more a stab than a bite. Viperids use this mechanism primarily for immobilization and digestion of prey. Pre-digestion occurs as the venom containsproteases, which degrade tissues. Secondarily, it is used for self defense, though in cases with nonprey, such as humans, they may give adry bite (not inject any venom). A dry bite allows the snake to conserve its precious reserve of venom, because once it has been depleted, time is needed to replenish it, leaving the snake vulnerable. In addition to being able to deliver dry bites, vipers can inject larger quantities of venom into larger prey targets, and smaller amounts into small prey. This causes the ideal amount of predigestion for the lowest amount of venom.
Almost all vipers havekeeled scales, a stocky build with a short tail, and a triangle-shaped head distinct from the neck, owing to the location of the venom glands. The great majority have vertically elliptical, or slit-shaped,pupils that can open wide to cover most of the eye or close almost completely, which helps them to see in a wide range of light levels. Typically, vipers are nocturnal andambush their prey.
Arabian Horned Viper from Arabian Peninsula
Compared to many other snakes, vipers often appear rather sluggish. Most areovoviviparous: the eggs are retained inside the mother's body, and the young emerge living. However, a few lay eggs in nests. Typically, the number of young in a clutch remains constant, but as the weight of the mother increases, larger eggs are produced, yielding larger young.
Viperid snakes are found in the Americas, Africa, Eurasia, and South Asia. In the Americas, they are native from south of48°N. In theOld World, viperids are located everywhere exceptSiberia, Ireland, and north of the Arctic Circle in Norway and Sweden.[1] Wild viperids are not found inAustralia. Thecommon adder, a viperid, is the only venomous snake found inGreat Britain.
Viperid venoms typically contain an abundance ofprotein-degrading enzymes, calledproteases, that produce symptoms such as pain, strong local swelling andnecrosis, blood loss from cardiovascular damage complicated bycoagulopathy, and disruption of the blood-clotting system. Also being vasculotoxic in nature, viperine venom causes vascularendothelial damage andhemolysis. Death is usually caused by collapse in blood pressure. This is in contrast toelapid venoms, which generally containneurotoxins that disable muscle contraction and cause paralysis. Death from elapid bites usually results fromasphyxiation because the diaphragm can no longer contract, but this rule does not always apply; some elapid bites includeproteolytic symptoms typical of viperid bites, while some viperid bites produceneurotoxic symptoms.[3]
Proteolytic venom is also dual-purpose: first, it is used for defense and to immobilize prey, as with neurotoxic venoms; second, many of the venom's enzymes have a digestive function, breaking down molecules such aslipids,nucleic acids, and proteins.[6] This is an important adaptation, as many vipers have inefficient digestive systems.[7]
Due to the nature of proteolytic venom, a viperid bite is often a very painful experience and should always be taken seriously, though it may not necessarily prove fatal. Even with prompt and proper treatment, a bite can still result in a permanentscar, and in the worst cases, the affected limb may even have to beamputated. A victim's fate is impossible to predict, as this depends on many factors, including the species and size of the snake involved, how much venom was injected (if any), and the size and condition of the patient before being bitten. Viper bite victims may also beallergic to the venom or theantivenom.
These snakes can decide how much venom to inject depending on the circumstances. The most important determinant of venom expenditure is generally the size of the snake; larger specimens can deliver much more venom. The species is also important, since some are likely to inject more venom than others, may have more venom available, strike more accurately, or deliver a number of bites in a short time. In predatory bites, factors that influence the amount of venom injected include the size of the prey, the species of prey, and whether the prey item is held or released. The need to label prey for chemosensory relocation after a bite and release may also play a role. In defensive bites, the amount of venom injected may be determined by the size or species of the predator (or antagonist), as well as the assessed level of threat, although larger assailants and higher threat levels may not necessarily lead to larger amounts of venom being injected.[8]
The western diamondback rattlesnakeCrotalus atrox, the venom of which contains proteins allowing the snake to track down bitten prey
Hemotoxic venom takes more time than neurotoxic venom to immobilize prey, so viperid snakes need to track down prey animals after they have been bitten,[8] in a process known as "prey relocalization". Vipers are able to do this via certain proteins contained in their venom. This important adaptation allowedrattlesnakes to evolve the strike-and-release bite mechanism, which provided a huge benefit to snakes by minimizing contact with potentially dangerous prey animals.[9] This adaptation, then, requires the snake to track down the bitten animal to eat it, in an environment full of other animals of the same species.Western diamondback rattlesnakes respond more actively to mouse carcasses that have been injected with crude rattlesnake venom. When the various components of the venom were separated out, the snakes responded to mice injected with two kinds ofdisintegrins, which are responsible for allowing the snakes to track down their prey.[9]
Pit vipers have specialized sensory organs near the nostrils called heat-sensing pits.[10] The location of this organ is unique to pit vipers. These pits have the ability to detectthermal radiation emitted bywarm-blooded animals, helping them better understand their environment.[11] Internallythe organ forms a small pit lined with membranes, external and internal, attached to thetrigeminal nerve.[10][12]Infrared light signals the internal membranes, which in turn signal the trigeminal nerve and send the infrared signals to the brain, where they are overlaid onto the visual image created by the eyes.[13]
Whether family Viperidae is attributed to Oppel (1811), as opposed to Laurenti (1768) or Gray (1825), is subject to some interpretation. The consensus among leading experts, though, is that Laurenti usedviperae as theplural ofvipera (Latin for "viper", "adder", or "snake") and did not intend for it to indicate a family group taxon. Rather, it is attributed to Oppel, based on his Viperini as a distinct family group name, despite the fact that Gray was the first to use the form Viperinae.[1]
^abcdeMcDiarmid RW, Campbell JA, Touré T. 1999. Snake Species of the World: A Taxonomic and Geographic Reference, vol. 1. Herpetologists' League.ISBN1-893777-00-6 (series).ISBN1-893777-01-4 (volume).
^Fender-Barnett, Arli (27 May 2019)."Are Australian snakes really the most dangerous in the world?".CSIRO. Archived fromthe original on 9 December 2023.Yes, we have some scary snakes in Australia but none are close to what's happening in tropical parts of Asia, Africa and South America – they have a group of snakes called Vipers (which we don't have, phew!).
^ab"Viperids".Snakesuntamed.webr.ly. Archived fromthe original on 8 January 2015. Retrieved8 January 2015.
^abHayes WK, Herbert SS, Rehling GC, Gennaro JF. 2002. Factors that influence venom expenditure in viperids and other snake species during predatory and defensive contexts.In Schuett GW, Höggren M, Douglas ME, Greene HW. 2002. Biology of the Vipers. Eagle Mountain Publishing, LC. 580 pp. 16 plates.ISBN0-9720154-0-X.
^Lynn, W. Gardner (1931). "The Structure and Function of the Facial Pits of Pit Vipers".American Journal of Anatomy.49:97–139.doi:10.1002/aja.1000490105.
Gray JE. 1825. A synopsis of the genera of reptiles and Amphibia, with a description of some new species.Annals of Philosophy, new ser., 10: 193–217.
Laurenti JN. 1768. Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena et antidota reptilium Austriacorum. J.T. de Trattnern, Wien.
Oppel M. 1811. Mémoire sur la classification des reptiles. Ordre II. Reptiles à écailles. Section II. Ophidiens. Annales du Musée National d'Histoire Naturelle, Paris 16: 254–295, 376–393.
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