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Snakes are elongated, , s of the Serpentes . Like all other , snakes are , s covered in overlapping . Many species of snakes have s with several more joints than their ancestors, enabling them to swallow prey much larger than their heads with their . To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and most have only one functional . Some species retain a with a pair of claws on either side of the . Lizards have evolved elongate bodies without limbs or with greatly reduced limbs about twenty-five times independently via , leading to many lineages of s. These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal (see , , and ). Living snakes are found on every continent except Antarctica, and on most smaller land masses; exceptions include some large islands, such as Ireland, Iceland, Greenland, the , and the islands of New Zealand, as well as many small islands of the Atlantic and central Pacific oceans. Additionally, are widespread throughout the Indian and Pacific oceans. More than twenty are currently recognized, comprising about 520 and about 3,900 . They range in size from the tiny, to the of in length. The fossil species ' was long. Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during the period, with the earliest known fossils dating to between 143 and 167  ago. The diversity of modern snakes appeared during the epoch ( 66 to 56 Ma ago, after the ). The oldest preserved descriptions of snakes can be found in the . Most species of snake are nonvenomous and use it primarily to kill and subdue prey rather than for self-defense. Some possess venom that is potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by .


Etymology

The English word ''snake'' comes from , itself from ( 'ring snake', Swedish 'grass snake'), from root 'to crawl to creep', which also gave ''sneak'' as well as 'snake'. The word ousted ', as ''adder'' went on to narrow in meaning, though in Old English was the general word for snake. The other term, ''serpent'', is from French, ultimately from Indo-European 'to creep', which also gave () 'I crawl'.


Evolution

The fossil record of snakes is relatively poor because snake s are typically small and fragile making ization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the period. The earliest known true snake fossils (members of the crown group Serpentes) come from the marine s, the oldest of which is the ( age) ', dated to between 112 and 94 million years old.Vidal, N., Rage, J.-C., Couloux, A. and Hedges, S.B. (2009). "Snakes (Serpentes)". Pp. 390–397 in Hedges, S. B. and Kumar, S. (eds.), ''The Timetree of Life''. Oxford University Press. Based on , there is consensus that snakes descended from s.Mehrtens JM. 1987. ''Living Snakes of the World in Color''. New York: Sterling Publishers. 480 pp. . s and s—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as s, which are used to grasp during mating. The families and also possess remnants of the pelvic girdle, appearing as horny projections when visible. Front limbs are nonexistent in all known snakes. This is caused by the evolution of their s, controlling limb . The axial skeleton of the snakes’ common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant. As a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the , , and 1–3 neck vertebrae). In other words, most of a snake's skeleton is an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very reduced in number (only 2–10 lumbar and pelvic vertebrae are present), while only a short tail remains of the caudal vertebrae. However, the tail is still long enough to be of important use in many species, and is modified in some aquatic and tree-dwelling species. Many modern snake groups originated during the , alongside the of mammals following the extinction of (non-avian) s. The expansion of grasslands in North America also led to an explosive radiation among snakes. Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of s and s in North America and the significant diversification of (including the origin of many modern genera such as , , , and ).


Origins

There is fossil evidence to suggest that snakes may have evolved from burrowing lizards, during the . An early fossil snake relative, ', was a two-legged burrowing animal with a , and was fully . One analog of these putative ancestors is the earless monitor ' of (though it also is ). species evolved bodies streamlined for burrowing, and eventually lost their limbs. According to this hypothesis, features such as the , fused eyelids () and loss of external ears evolved to cope with difficulties, such as scratched s and dirt in the ears. Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection to the vertebrae. These include fossil species like ', ' and ', which are slightly older than '. This hypothesis was strengthened in 2015 by the discovery of a 113-million-year-old fossil of a four-legged snake in Brazil that has been named '. It has many snake-like features, is adapted for burrowing and its stomach indicates that it was preying on other animals. It is currently uncertain if ''Tetrapodophis'' is a snake or another species, in the order, as a snake-like body has independently evolved at least 26 times. ''Tetrapodophis'' does not have distinctive snake features in its spine and skull. A study in 2021 places the animal in a group of extinct marine lizards from the Cretaceous period known as s and not directly related to snakes. An alternative hypothesis, based on , suggests the ancestors of snakes were related to s—extinct reptiles from the —forming the clade . According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment. This ultimately led to an animal similar to today's s. In the Late , snakes recolonized land, and continued to diversify into today's snakes. Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial ''Najash rionegrina''. Similar skull structure, reduced or absent limbs, and other anatomical features found in both mosasaurs and snakes lead to a positive correlation, although some of these features are shared with varanids. File:Tetrapodophis amplectus 3483.jpg, ' File:Pachyrhachis problematicus 45.JPG, ' File:Eupodophis descouensi Holotype.jpg, ' File:Eupodophis descouensi Holotype hind leg.jpg, '' descouensi'' hind leg Genetic studies in recent years have indicated snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented remains found from the and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis. In 2016, two studies reported that limb loss in snakes is associated with DNA mutations in the Zone of Polarizing Activity Regulatory Sequence (ZRS), a regulatory region of the gene which is critically required for limb development. More advanced snakes have no remnants of limbs, but basal snakes such as pythons and boas do have traces of highly reduced, vestigial hind limbs. Python embryos even have fully developed hind limb buds, but their later development is stopped by the DNA mutations in the ZRS.


Distribution

There are about 3,900 species of snakes, ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia. Snakes can be found on every continent except Antarctica, as well as in the sea, and as high as in the of Asia. There are numerous islands from which snakes are absent, such as , , and (although New Zealand's waters are infrequently visited by the and the ).


Taxonomy

All modern snakes are grouped within the Serpentes in , part of the , though their precise placement within squamates remains controversial. The two s of Serpentes are: and . This separation is based on characteristics and sequence similarity. Alethinophidia is sometimes split into and , with the latter consisting of "colubroid" snakes (, , , , and ) and acrochordids, while the other alethinophidian families comprise Henophidia. While not extant today, the , a family of giant, primitive, python-like snakes, was around until 50,000 years ago in Australia, represented by genera such as '. There are numerous debates in the systematics within the group. For instance, many sources classify and as one family, while some keep the and (sea snakes) separate for practical reasons despite their extremely close relation. Recent molecular studies support the of the of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (''Cylindrophis'', ''Anomochilus'', uropeltines), macrostomatans, booids, boids, pythonids and caenophidians.


Families



Legless lizards

While snakes are limbless reptiles, evolved from (and grouped with) lizards, there are many other species of lizards that have lost their limbs independently but which superficially look similar to snakes. These include the and . Other that are unrelated to snakes include (amphibians), (near-lizard squamates), and the extinct (amphibians).


Biology


Size

The now ' was in length. By comparison, the largest snakes are the , measuring about long, and the , which measures about long and is considered the heaviest snake on Earth at . At the other end of the scale, the smallest extant snake is ', with a length of about . Most snakes are fairly small animals, approximately in length.


Perception

Pit vipers, pythons, and some boas have in deep grooves on the snout, allowing them to "see" the radiated heat of warm-blooded prey. In pit vipers, the grooves are located between the nostril and the eye in a large "pit" on each side of the head. Other infrared-sensitive snakes have multiple, smaller labial pits lining the upper lip, just below the nostrils. A snake tracks its prey using smell, collecting airborne particles with its , then passing them to the or ''Jacobson's organ'' in the mouth for examination.Cogger (1991), p. 180. The fork in the tongue provides a sort of directional sense of smell and taste simultaneously. The snake's tongue is constantly in motion, sampling particles from the air, ground, and water, analyzing the chemicals found, and determining the presence of prey or predators in the local environment. In water-dwelling snakes, such as the , the tongue functions efficiently underwater. The underside of a snake is very sensitive to vibration, allowing the snake to detect approaching animals by sensing faint vibrations in the ground. Snake vision varies greatly between species. Some have keen eyesight and others are only able to distinguish light from dark, but the important trend is that a snake's visual perception is adequate enough to track movements. Generally, vision is best in tree-dwelling snakes and weakest in burrowing snakes. Some have , where both eyes are capable of focusing on the same point, an example of this being the . Most snakes focus by moving the back and forth in relation to the . Diurnal snakes have round pupils and many nocturnal snakes have slit pupils. Most species possess three visual pigments and are probably able to see two primary colors in daylight. It has been concluded that the last common ancestors of all snakes had -sensitive vision, but most snakes that depend on their eyesight to hunt in daylight have evolved lenses that act like sunglasses for filtering out the UV-light, which probably also sharpens their vision by improving the .


Skin

The skin of a snake is covered in . Contrary to the popular notion of snakes being slimy (because of possible confusion of snakes with s), has a smooth, dry texture. Most snakes use specialized belly scales to travel, allowing them to grip surfaces. The body scales may be smooth, , or granular. The eyelids of a snake are transparent "spectacle" scales, also known as , which remain permanently closed. The shedding of scales is called ' (or in normal usage, ''molting'' or ''sloughing''). Snakes shed the complete outer layer of skin in one piece.Smith, Malcolm A. '. Vol I, Loricata and Testudines. p. 30. Snake scales are not discrete, but extensions of the —hence they are not shed separately but as a complete outer layer during each molt, akin to a sock being turned inside out. Snakes have a wide diversity of skin coloration patterns which are often related to behavior, such as the tendency to have to flee from predators. Snakes that are at a high risk of predation tend to be plain, or have longitudinal stripes, providing few reference points to predators, thus allowing the snake to escape without being noticed. Plain snakes usually adopt active hunting strategies, as their pattern allows them to send little information to prey about motion. Blotched snakes usually use ambush-based strategies, likely because it helps them blend into an environment with irregularly shaped objects, like sticks or rocks. Spotted patterning can similarly help snakes to blend into their environment. The shape and number of scales on the head, back, and belly are often characteristic and used for taxonomic purposes. Scales are named mainly according to their positions on the body. In "advanced" (n) snakes, the broad belly scales and rows of s correspond to the e, allowing these to be counted without the need for .


Molting

(or "ecdysis") serves a number of purposes. Firstly, the old and worn skin is replaced, and secondly, it helps get rid of parasites such as s and s. Renewal of the skin by molting supposedly allows growth in some animals such as insects, but this has been disputed in the case of snakes. Molting occurs periodically throughout the life of a snake. Before each molt, the snake stops eating and often hides or moves to a safe place. Just before shedding, the skin becomes dull and dry looking and the snake's eyes turn cloudy or blue-colored. The inner surface of the old skin liquefies, causing it to separate from the new skin beneath it. After a few days, the eyes become clear and the snake "crawls" out of its old skin, which splits close to the snake's mouth. The snake rubs its body against rough surfaces to aid in the shedding of its old skin. In many cases, the cast skin peels backward over the body from head to tail in one piece, like pulling a sock off inside-out, revealing a new, larger, brighter layer of skin which has formed underneath. A young snake that is still growing may shed its skin up to four times a year, but an older snake may shed only once or twice a year. The discarded skin carries a perfect imprint of the scale pattern, so it is usually possible to identify the snake from the cast skin if it is reasonably intact. This periodic renewal has led to the snake being a symbol of and , as pictured in the . Scale counts can sometimes be used to identify the sex of a snake when the species is not distinctly . A probe is fully inserted into the , marked at the point where it stops, then removed and measured against the .Rosenfeld (1989), p. 11. The scalation count determines whether the snake is a male or female, as the of a male will probe to a different depth (usually longer) than the cloaca of a female.


Skeleton

The of most snakes consists solely of the skull, hyoid, vertebral column, and ribs, though snakes retain vestiges of the pelvis and rear limbs. The skull consists of a solid and complete , to which many of the other bones are only loosely attached, particularly the highly mobile jaw bones, which facilitate manipulation and ingestion of large prey items. The left and right sides of the lower jaw are joined together only by a flexible ligament at the anterior tips, allowing them to separate widely, and the posterior end of the lower jaw bones articulate with a , allowing further mobility. The and quadrate bones can pick up ground-borne vibrations; because the sides of the lower jaw can move independently of one another, a snake resting its jaw on a surface has sensitive stereo , used for detecting the position of prey. The jaw–quadrate– pathway is capable of detecting vibrations on the scale, despite the absence of an outer ear and the lack of an mechanism—provided by the in other vertebrates—for receiving vibrations from the air. The hyoid is a small bone located posterior and ventral to the skull, in the 'neck' region, which serves as an attachment for the muscles of the snake's tongue, as it does in all other s. The vertebral column consists of between 200 and 400 vertebrae, or sometimes more. The body vertebrae each have two ribs articulating with them. The tail vertebrae are comparatively few in number (often less than 20% of the total) and lack ribs. The vertebrae have projections that allow for strong muscle attachment, enabling locomotion without limbs. Caudal (self-amputation of the tail), a feature found in some lizards, is absent in most snakes. In the rare cases where it does exist in snakes, caudal autotomy is intervertebral (meaning the separation of adjacent vertebrae), unlike that in lizards, which is intravertebral, i.e. the break happens along a predefined fracture plane present on a vertebra. In some snakes, most notably boas and pythons, there are vestiges of the hindlimbs in the form of a pair of s. These small, claw-like protrusions on each side of the cloaca are the external portion of the vestigial hindlimb skeleton, which includes the remains of an ilium and femur. Snakes are s with teeth that are continuously replaced.


Internal organs

Snakes and other reptiles have a three-chambered heart that controls the via the left and right atrium, and one ventricle. Internally, the ventricle is divided into three interconnected cavities: the cavum arteriosum, the cavum pulmonale, and the cavum venosum. The cavum venosum receives deoxygenated from the right atrium and the cavum arteriosum receives oxygenated blood from the left atrium. Located beneath the cavum venosum is the cavum pulmonale, which pumps blood to the pulmonary trunk. The snake's heart is encased in a sac, called the ', located at the of the . The heart is able to move around, owing to the lack of a diaphragm; this adjustment protects the heart from potential damage when large ingested prey is passed through the . The is attached to the and and filters the blood. The , located in fatty tissue above the heart, is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is unique for the presence of a renal portal system in which the blood from the snake's tail passes through the kidneys before returning to the heart. The left is often small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. In the majority of species, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion that does not function in gas exchange. This 'saccular lung' is used for purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. Many organs that are paired, such as or , are staggered within the body, one located ahead of the other. Snakes have no s.


Venom

Cobras, vipers, and closely related species use to immobilize, injure, or kill their prey. The venom is modified , delivered through . The fangs of 'advanced' venomous snakes like viperids and elapids are hollow, allowing venom to be injected more effectively, and the fangs of snakes such as the boomslang simply have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey-specific, and their role in self-defense is secondary. Venom, like all salivary secretions, is a predigestant that initiates the breakdown of food into soluble compounds, facilitating proper digestion. Even nonvenomous snakebites (like any animal bite) cause tissue damage. Certain birds, mammals, and other snakes (such as s) that prey on venomous snakes have developed resistance and even immunity to certain venoms. Venomous snakes include three of snakes, and do not constitute a formal group. The term "poisonous snake" is generally an incorrect label for snakes. A poison is inhaled or ingested, whereas venom produced by snakes is injected into its victim via fangs. There are, however, two exceptions: ' sequesters toxins from the toads it eats, then secretes them from nuchal glands to ward off predators; and a small unusual population of s in the US state of retains enough toxins in their livers from ingested s to be effectively poisonous to small local predators (such as s and es).Freiberg (1984), p. 123. Snake venoms are complex mixtures of s, and are stored in venom glands at the back of the head. In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw.Freiberg (1984), p. 125. The proteins can potentially be a mix of s (which attack the nervous system), s (which attack the circulatory system), s, s, and many other toxins that affect the body in different ways. Almost all snake venom contains ', an enzyme that ensures rapid diffusion of the venom. Venomous snakes that use hemotoxins usually have fangs in the front of their mouths, making it easier for them to inject the venom into their victims. Some snakes that use neurotoxins (such as the ) have fangs in the back of their mouths, with the fangs curled backwards.Freiberg (1984), p. 126. This makes it difficult both for the snake to use its venom and for scientists to milk them. Elapids, however, such as cobras and kraits are '—they possess hollow fangs that cannot be erected toward the front of their mouths, and cannot "stab" like a viper. They must actually bite the victim. It has been suggested that all snakes may be venomous to a certain degree, with harmless snakes having weak venom and no fangs. According to this theory, most snakes that are labelled "nonvenomous" would be considered harmless because they either lack a venom delivery method or are incapable of delivering enough to endanger a human. The theory postulates that snakes may have evolved from a common lizard ancestor that was venomous, and also that venomous lizards like the , , s, and the now-extinct , may have derived from this same common ancestor. They share this "" with various other n species. Venomous snakes are classified in two taxonomic families: * s – s including s, , s, , s, and s. * – vipers, s, /, and . There is a third family containing the ''opistoglyphous'' (rear-fanged) snakes (as well as the majority of other snake species): * s – s, s, , , although not all colubrids are venomous.


Reproduction

Although a wide range of reproductive modes are used by snakes, all employ . This is accomplished by means of paired, forked , which are stored, inverted, in the male's tail.Capula (1989), p. 117. The hemipenes are often grooved, hooked, or spined—designed to grip the walls of the female's . Most species of snakes lay which they abandon shortly after laying. However, a few species (such as the king cobra) construct nests and stay in the vicinity of the hatchlings after incubation. Most pythons coil around their egg-clutches and remain with them until they hatch.Cogger (1991), p. 186. A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even "shiver" to generate heat to incubate the eggs. Some species of snake are and retain the eggs within their bodies until they are almost ready to hatch.Capula (1989), p. 118.Cogger (1991), p. 182. Several species of snake, such as the and green anaconda, are fully , nourishing their young through a as well as a ; this is highly unusual among reptiles, and normally found in or . Retention of eggs and live birth are most often associated with colder environments. in snakes is demonstrated by the 3,000 species that each use different tactics in acquiring mates. Ritual combat between males for the females they want to with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and force it downward. It is common for neck-biting to occur while the snakes are entwined.


Facultative parthenogenesis

is a natural form of reproduction in which growth and development of embryos occur without fertilization. ''Agkistrodon contortrix'' (copperhead) and ''Agkistrodon piscivorus'' (cottonmouth) can reproduce by , meaning that they are capable of switching from a mode of reproduction to an mode. The most likely type of parthenogenesis to occur is with terminal fusion, a process in which two terminal products from the same fuse to form a diploid . This process leads to genome-wide , expression of deleterious recessive s, and often to developmental abnormalities. Both captive-born and wild-born copperheads and cottonmouths appear to be capable of this form of parthenogenesis. Reproduction in reptiles is almost exclusively sexual. Males ordinarily have a ZZ pair of sex-determining chromosomes, and females a ZW pair. However, the Colombian Rainbow boa (') can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny. The WW females are likely produced by terminal automixis.


Behavior


Winter dormancy

In regions where winters are too cold for snakes to tolerate while remaining active, local species will enter a period of . Unlike , in which the dormant mammals are actually asleep, brumating reptiles are awake but inactive. Individual snakes may brumate in burrows, under rock piles, or inside fallen trees, or large numbers of snakes may clump together in .


Feeding and diet

All snakes are , on small animals including lizards, frogs, other snakes, small mammals, birds, eggs, fish, snails, worms, and insects.Behler (1979) p. 581 Snakes cannot bite or tear their food to pieces so must swallow their prey whole. The eating habits of a snake are largely influenced by body size; smaller snakes eat smaller prey. Juvenile pythons might start out feeding on lizards or mice and graduate to small deer or antelope as an adult, for example. The snake's is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, they have an extremely flexible , the two halves of which are not rigidly attached, and numerous other joints in the skull, which allow the snake to open its mouth wide enough to swallow prey whole, even if it is larger in diameter than the snake itself. For example, the has flexible jaws adapted for eating eggs much larger than the diameter of its head. This snake has no teeth, but does have bony protrusions on the inside edge of its , which it uses to break the shell when eating eggs. The majority of snakes eat a variety of prey animals, but there is some specialization in certain species. s and the Australian consume other snakes. Species of the family have more teeth on the right side of their mouths than on the left, as they mostly prey on snails and the shells usually spiral clockwise. Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by , while some swallow their prey when it is still alive. After eating, snakes become dormant to allow the process of to take place; this is an intense activity, especially after consumption of large prey. In species that feed only sporadically, the entire enters a reduced state between meals to conserve energy. The digestive system is then 'up-regulated' to full capacity within 48 hours of prey consumption. Being ("cold-blooded"), the surrounding temperature plays an important role in the digestion process. The ideal temperature for snakes to digest food is . There is a huge amount of energy involved in a snake's digestion, for example the surface body temperature of the South American rattlesnake (') increases by as much as during the digestive process. If a snake is disturbed after having eaten recently, it will often its prey to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient; the snake's digestive dissolve and absorb everything but the prey's hair (or feathers) and claws, which are excreted along with .


Hooding and spitting

Hooding (expansion of the neck area) is a visual deterrent, mostly seen in cobras (elapids), and is primarily controlled by rib muscles. Hooding can be accompanied by spitting venom towards the threatening object, and producing a specialized sound; hissing. Studies on captive cobras showed that 13 to 22% of the body length is raised during hooding.


Locomotion

The lack of limbs does not impede the movement of snakes. They have developed several different modes of locomotion to deal with particular environments. Unlike the gaits of limbed animals, which form a continuum, each mode of snake locomotion is discrete and distinct from the others; transitions between modes are abrupt.Cogger(1991), p. 175.


Lateral undulation

Lateral undulation is the sole mode of aquatic locomotion, and the most common mode of terrestrial locomotion. In this mode, the body of the snake alternately flexes to the left and right, resulting in a series of rearward-moving "waves". While this movement appears rapid, snakes have rarely been documented moving faster than two body-lengths per second, often much less. This mode of movement has the same net cost of transport (calories burned per meter moved) as running in lizards of the same mass. Terrestrial lateral undulation is the most common mode of terrestrial locomotion for most snake species. In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, irregularities in the soil, etc. Each of these environmental objects, in turn, generates a reaction force directed forward and towards the midline of the snake, resulting in forward thrust while the lateral components cancel out. The speed of this movement depends upon the density of push-points in the environment, with a medium density of about 8 along the snake's length being ideal. The wave speed is precisely the same as the snake speed, and as a result, every point on the snake's body follows the path of the point ahead of it, allowing snakes to move through very dense vegetation and small openings. When swimming, the waves become larger as they move down the snake's body, and the wave travels backwards faster than the snake moves forwards. Thrust is generated by pushing their body against the water, resulting in the observed slip. In spite of overall similarities, studies show that the pattern of muscle activation is different in aquatic versus terrestrial lateral undulation, which justifies calling them separate modes. All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the motion (moving backwards with forward-moving waves).


Sidewinding

Most often employed by colubroid snakes (, , and ) when the snake must move in an environment that lacks irregularities to push against (rendering lateral undulation impossible), such as a slick mud flat, or a sand dune, sidewinding is a modified form of lateral undulation in which all of the body segments oriented in one direction remain in contact with the ground, while the other segments are lifted up, resulting in a peculiar "rolling" motion.Cogger(1991), p. 177. This mode of locomotion overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping. The static nature of the contact points can be shown from the tracks of a sidewinding snake, which show each belly scale imprint, without any smearing. This mode of locomotion has very low caloric cost, less than ⅓ of the cost for a lizard to move the same distance. Contrary to popular belief, there is no evidence that sidewinding is associated with the sand being hot.


Concertina

When push-points are absent, but there is not enough space to use sidewinding because of lateral constraints, such as in tunnels, snakes rely on concertina locomotion. In this mode, the snake braces the posterior portion of its body against the tunnel wall while the front of the snake extends and straightens. The front portion then flexes and forms an anchor point, and the posterior is straightened and pulled forwards. This mode of locomotion is slow and very demanding, up to seven times the cost of laterally undulating over the same distance. This high cost is due to the repeated stops and starts of portions of the body as well as the necessity of using active muscular effort to brace against the tunnel walls.


Arboreal

The movement of snakes in arboreal habitats has only recently been studied. While on tree branches, snakes use several modes of locomotion depending on species and bark texture. In general, snakes will use a modified form of concertina locomotion on smooth branches, but will laterally undulate if contact points are available. Snakes move faster on small branches and when contact points are present, in contrast to limbed animals, which do better on large branches with little 'clutter'. Gliding snakes (') of Southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees.Freiberg (1984), p. 135. These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair.


Rectilinear

The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning.Cogger (1991), p. 176. In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them. Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin. The ribs of the snake do not move in this mode of locomotion and this method is most often used by large s, s, and s when stalking prey across open ground as the snake's movements are subtle and harder to detect by their prey in this manner.


Interactions with humans


Bite

Snakes do not ordinarily prey on humans. Unless startled or injured, most snakes prefer to avoid contact and will not attack humans. With the exception of large constrictors, nonvenomous snakes are not a threat to humans. The bite of a nonvenomous snake is usually harmless; their teeth are not adapted for tearing or inflicting a deep puncture wound, but rather grabbing and holding. Although the possibility of infection and tissue damage is present in the bite of a nonvenomous snake, venomous snakes present far greater hazard to humans. The (WHO) lists under the "other neglected conditions" category. Documented deaths resulting from snake bites are uncommon. Nonfatal bites from venomous snakes may result in the need for amputation of a limb or part thereof. Of the roughly 725 species of venomous snakes worldwide, only 250 are able to kill a human with one bite. Australia averages only one fatal snake bite per year. In , 250,000 snakebites are recorded in a single year, with as many as 50,000 recorded initial deaths. The WHO estimates that on the order of 100 000 people die each year as a result of snake bites, and around three times as many amputations and other permanent disabilities are caused by snakebites annually. The treatment for a snakebite is as variable as the bite itself. The most common and effective method is through (or antivenin), a serum made from the venom of the snake. Some antivenom is species-specific (monovalent) while some is made for use with multiple species in mind (polyvalent). In the United States for example, all species of venomous snakes are s, with the exception of the . To produce antivenom, a mixture of the venoms of the different species of s, copperheads, and cottonmouths is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted from the immunized horse. The serum is separated and further purified and freeze-dried. It is reconstituted with sterile water and becomes antivenom. For this reason, people who are allergic to horses are more likely to suffer an allergic reaction to antivenom. Antivenom for the more dangerous species (such as s, s, and s) is made in a similar manner in India, South Africa, and Australia, although these antivenoms are species-specific.


Snake charmers

In some parts of the world, especially in India, snake charming is a roadside show performed by a charmer. In such a show, the snake charmer carries a basket containing a snake that he seemingly charms by playing tunes with his flutelike musical instrument, to which the snake responds. The snake is in fact responding to the movement of the flute, not the sound it makes, as snakes lack external ears (though they do have internal ears). The in India technically prohibits snake charming on the grounds of reducing animal cruelty. Other types of snake charmers use a snake and show, where the two animals have a mock fight; however, this is not very common, as the animals may be seriously injured or killed. Snake charming as a profession is dying out in India because of competition from modern forms of entertainment and environment laws proscribing the practice. Many Indians have never seen snake charming and it is becoming a folktale of the past.


Trapping

The ''Irulas'' tribe of and in India have been hunter-gatherers in the hot, dry plains forests, and have practiced the art of snake catching for generations. They have a vast knowledge of snakes in the field. They generally catch the snakes with the help of a simple stick. Earlier, the ''Irulas'' caught thousands of snakes for the snake-skin industry. After the complete ban of the snake-skin industry in India and protection of all snakes under the , they formed the Irula Snake Catcher's Cooperative and switched to catching snakes for removal of venom, releasing them in the wild after four extractions. The venom so collected is used for producing life-saving antivenom, biomedical research and for other medicinal products.Whitaker, Romulus & Captain, Ashok. ''Snakes of India: The Field Guide''. (2004) pp 11 to 13. The ''Irulas'' are also known to eat some of the snakes they catch and are very useful in rat extermination in the villages. Despite the existence of snake charmers, there have also been professional snake catchers or . Modern-day snake trapping involves a using a long stick with a V- shaped end. Some television show hosts, like , , , and , prefer to catch them using bare hands.


Consumption

Although snakes are not commonly thought of as food, their consumption is acceptable in some cultures and may even be considered a delicacy. is popular in , consumed by locals in the autumn to warm their bodies. Western cultures document the consumption of snakes only under extreme circumstances of hunger, with the exception of cooked meat, which is commonly consumed in and parts of the . In n countries such as China, Taiwan, Thailand, Indonesia, Vietnam, and Cambodia, drinking the blood of a snake—particularly the cobra—is believed to increase sexual virility. When possible, the blood is drained while the cobra is still alive, and it is usually mixed with some form of to improve the taste. The use of snakes in alcohol is accepted in some Asian countries. In such cases, one or more snakes are left to steep in a jar or container of liquor, as this is claimed to make the liquor stronger (as well as more expensive). One example of this is the snake, which is sometimes placed in the n liqueur ' (ブ酒), also known as "Habu Sake". (蛇酒) is an alcoholic beverage produced by infusing whole snakes in or . First recorded as being consumed in China during the , this drink is considered an important curative and is believed to reinvigorate a person according to .


Pets

In the , some snakes are kept as pets, especially docile species such as the and . To meet the demand, a industry has developed. Snakes bred in captivity are considered preferable to specimens caught in the wild and tend to make better pets. Compared with more traditional types of companion animal, snakes can be very low-maintenance pets; they require minimal space, as most common species do not exceed in length, and can be fed relatively infrequently—usually once every five to 14 days. Certain snakes have a lifespan of more than 40 years if given proper care.


Symbolism

In , , the messenger god of , was represented as a serpent on ''s'', or . Representations of two intertwined serpents are common in and Neo-Sumerian artwork and still appear sporadically on s and amulets until as late as the thirteenth century BC. The horned viper (') appears in and kudurrus and is invoked in n texts as a magical protective entity. A dragon-like creature with horns, the body and neck of a snake, the forelegs of a lion, and the hind-legs of a bird appears in Mesopotamian art from the Akkadian Period until the (323 BC–31 BC). This creature, known in as the ', meaning "furious serpent", was used as a symbol for particular deities and also as a general protective emblem. It seems to have originally been the attendant of the Underworld god , but later became the attendant to the storm-god , as well as, later, Ninazu's son , the Babylonian , the scribal god , and the Assyrian national god Ashur. In , the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (). The was a well-known ian symbol of a serpent swallowing its own tail. The precursor to the ouroboros was the "Many-Faced", a serpent with five heads, who, according to the , the oldest surviving , was said to coil around the corpse of the sun god Ra protectively. The earliest surviving depiction of a "true" ouroboros comes from the gilded shrines in of . In the early centuries AD, the ouroboros was adopted as a symbol by Christians and chapter 136 of the ', an early Gnostic text, describes "a great dragon whose tail is in its mouth". In medieval alchemy, the ouroboros became a typical western dragon with wings, legs, and a tail. In the , King , whose name means "Snake", is depicted very negatively, as a particularly cruel and despicable enemy of the ancient Hebrews. The ancient Greeks used the , a depiction of a hideous face with serpents for hair, as an to ward off evil. In a described by Pseudo-Apollodorus in his ', was a with serpents for hair whose gaze turned all those who looked at her to stone and was slain by the hero . In the Roman poet 's ', is said to have once been a beautiful priestess of , whom Athena turned into a serpent-haired monster after she was raped by the god in Athena's temple. In another myth referenced by the n poet and described in detail by Pseudo-Apollodorus, the hero is said to have slain the , a multiple-headed serpent which dwelt in the swamps of . The legendary account of the foundation of mentioned a monster snake guarding the spring from which the new settlement was to draw its water. In fighting and killing the snake, the companions of the founder all perished – leading to the term "" (i.e. a victory involving one's own ruin). Three medical symbols involving snakes that are still used today are , symbolizing pharmacy, and the and , which are symbols denoting medicine in general. One of the etymologies proposed for the common female first name ' is that it might derive from Old German ''Lindi'' or ''Linda'', meaning a serpent. India is often called the land of snakes and is steeped in tradition regarding snakes. Snakes are worshipped as gods even today with many women pouring milk on snake pits (despite snakes' aversion for milk).Deane (1833). p. 61. The cobra is seen on the neck of and is depicted often as sleeping on a seven-headed snake or within the coils of a serpent. There are also several temples in India solely for cobras sometimes called ''Nagraj'' (King of Snakes) and it is believed that snakes are symbols of fertility. There is a Hindu festival called each year on which day snakes are venerated and prayed to. See also '. In India there is another mythology about snakes. Commonly known in as "" snakes. Such snakes can take the form of any living creature, but prefer human form. These mythical snakes possess a valuable gem called "Mani", which is more brilliant than diamond. There are many stories in India about greedy people trying to possess this gem and ending up getting killed. The is one of the 12 celestial animals of , in the . Many ancient Peruvian cultures worshipped nature. They emphasized animals and often depicted snakes in their art.


Religion

Snakes are used in as a part of ritual worship. In the annual festival, participants worship either live cobras or images of s. Lord is depicted in most images with a snake coiled around his neck. literature includes various stories associated with snakes, for example is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of from all his mouths. Other notable snakes in Hinduism are , , , and . The term ''Nāga'' is used to refer to entities that take the form of large snakes in Hinduism and . Snakes have been widely revered in many cultures, such as in where the serpent was seen as a healer. carried a serpent wound around his wand, a symbol seen today on many ambulances. In , the snake of is also a symbol of healing, of one's life being saved from imminent death. In religious terms, the snake and were arguably the most important animals in ancient . "In states of ecstasy, lords dance a serpent dance; great descending snakes adorn and support buildings from to , and the word ''coatl'' meaning serpent or twin, forms part of primary deities such as , , and ." In the and s, the fifth day of the week was known as Snake Day. In some parts of , the redemptive work of is compared to saving one's life through beholding the (serpent of brass). use snakes as an integral part of church worship, to demonstrate their faith in divine protection. However, more commonly in Christianity, the serpent has been depicted as a representative of and sly plotting, as seen in the description in of a snake tempting in the . is purported to have expelled all snakes from Ireland while converting the country to Christianity in the 5th century, thus explaining the absence of snakes there. In Christianity and Judaism, the snake makes its infamous appearance in the first book of the Bible when a serpent appears before and tempts them with the from the . The snake returns in the when turns his staff into a snake as a sign of God's power, and later when he makes the Nehushtan, a bronze snake on a pole that when looked at cured the people of bites from the snakes that plagued them in the desert. The serpent makes its final appearance symbolizing in the : "And he laid hold on the dragon the old serpent, which is the devil and Satan, and bound him for a thousand years." In and , the snake is seen as a symbol of wisdom and knowledge.


Medicine

Several compounds from snake venoms are being researched as potential treatments or preventatives for pain, cancers, arthritis, stroke, heart disease, hemophilia, and hypertension, and to control bleeding (e.g. during surgery).


See also

* * * * * * * (reptile disease) * ' * and Snakes of Europe , a wikibook * * *


References


Further reading

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External links

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BBC Nature:
Snake news, and video clips from BBC programmes past and present.

at Life is Short but Snakes are Long {{Authority control Extant Cenomanian first appearances