Monday, 7 September 2009

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. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. In order 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 lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.

Living snakes are found on every continent except Antarctica and most islands. Fifteen families are currently recognized comprising 456 genera and over 2,900 species.[1][2] They range in size from the tiny, 10 cm long thread snake to pythons and anacondas of up to 7.6 metres (25 ft) in length. The recently discovered fossil Titanoboa was 13 metres (43 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the Cretaceous period (c 150 Ma). The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma).

Most species are non-venomous and those that have venom use it primarily to kill and subdue prey rather than self-defense. Some possess venom potent enough to cause painful injury or death to humans. Those which are non-venomous either swallow prey alive or kill it via constriction.

Contents

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Etymology

The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. German Schnake "ring snake", Swedish snok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl, creep", which also gave sneak as well as Sanskrit nāgá "snake".[3] The word ousted adder as it went on to narrow in meaning, though in Old English næddre was the general word for snake.[4] The other term, serpent, is a french word, ultimately from Indo-European *serp- "to creep"[5] which also gave Greek érpein (ερπω) "to crawl".

Evolution

Modern snakes
Scolecophidia

Leptotyphlopidae


Anomalepididae


Typhlopidae



Alethinophidia

Anilius

Core Alethinophidia
Uropeltidae

Cylindrophis


Anomochilus


Uropeltinae



Macrostomata
Pythonidae

Pythoninae


Xenopeltis


Loxocemus


Caenophidia

Colubroidea


Acrochordidae


Boidae

Erycinae


Boinae


Calabaria



Ungaliophiinae



Tropidophiinae




Note that the tree only indicates relationships not evolutionary branching times.[6]

The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, making fossilization uncommon. However 150 million-year-old specimens, readily identifiable as snakes, yet with lizard-like skeletal structures, have been uncovered in South America and Africa.[7]:11 There is consensus, on the basis of comparative anatomy, that snakes descended from lizards.[7]:11[8] Fossil evidence suggests that snakes may have evolved from burrowing lizards, such as the varanids or a similar group during the Cretaceous Period.[9] An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial.[10] One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic.[11] Subterranean forms evolved bodies that were streamlined for burrowing and lost their limbs.[11] According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with fossorial difficulies such as scratched corneas and dirt in the ears.[9][11] Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lack a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.[12]

Fossil of Archaeophis proavus

Primitive groups among the modern snakes, pythons and boas, have vestigial hind limbs; tiny, clawed digits known as anal spurs which are used to grasp during mating.[7]:11[12] Leptotyphlopidae and Typhlopidae are other groups where remnants of the pelvic girdle are present, sometimes appearing as horny projections when visible. The frontal limbs are non-existent in all snakes and this loss is associated with the evolution of the Hox genes controlling limb morphogenesis. 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. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and as a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis and one to three neck vertebrae), making most of the snake's skeleton being composed of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only two to ten lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species. An alternative hypothesis, based on morphology, suggests that the ancestors of snakes were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizards.[8] Under this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water-loss through osmosis), while the external ears were lost through disuse in an aquatic environment, ultimately leading to an animal similar in appearance to sea snakes of today. In the Late Cretaceous, snakes re-colonized land to appear as they are today. Fossil snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial Najash rionegrina. Similar skull structure; reduced/absent limbs; and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, although some of these features are shared with varanids. In recent years, genetic studies have indicated that snakes are not as closely related to monitor lizards as it was once believed, and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, there is more evidence linking mosasaurs to snakes than to varanids. Fragmentary remains that have been found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may eventually refute either hypothesis.

The great diversity of modern snakes appeared in the Paleocene, correlating with the adaptive radiation of mammals following the extinction of the non-avian dinosaurs. One of the more common groups today, the colubrids, became particularly diverse due to their preying on rodents, a mammal group that has been particularly successful. There are over 2,900 species of snakes ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia and Tasmania.[8] Snakes can be found on every continent (with the exception of Antarctica), dwelling in the sea, and as high as 16,000 feet (4,900 m)in the Himalayan Mountains of Asia.[8][13]:143 There are numerous islands from which snakes are conspicuously absent such as Ireland, Iceland, and New Zealand.[13]:143

Taxonomy

All modern snakes are grouped within the suborder Serpentes in Linnean taxonomy, part of the order Squamata, though their precise placement within squamates is controversial.[1] There are two infraorders of Serpentes: Alethinophidia and Scolecophidia.[1] This separation is based on morphological characteristics and mitochondrial DNA sequence similarity. Alethinophidia is sometimes split into Henophidia and Caenophidia, with the latter consisting of "Colubroid" snakes (colubrids, vipers, elapids, hydrophiids, and attractaspids) and acrochordids, while the other alethinophidian families comprise Henophidia[14]. While not extant today, the Madtsoiidae, a family of giant, primitive, python-like snakes, was around until 50,000 years ago in Australia, represented by genera such as Wonambi.

There are numerous debates in the systematics within the group. For instance, many sources classify Boidae and Pythonidae as one family, while some keep the Elapidae and Hydrophiidae, separate for practical reasons despite their extremely close relation.

Recent molecular studies support the monophyly of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (Cylindrophis, Anomochilus, uropeltines), macrostomatans, booids, boids, pythonids and caenophidians.[6]

Families

Infraorder Alethinophidia 15 families
Family[1] Taxon author[1] Genera[1] Species[1] Common name Geographic range[15]
Acrochordidae Bonaparte, 1831 1 3 Wart snakes Western India and Sri Lanka through tropical Southeast Asia to the Philippines, south through the Indonesian/Malaysian island group to Timor, east through New Guinea to the northern coast of Australia to Mussau Island, the Bismark Archipelago and Guadalcanal Island in the Solomon Islands.
Aniliidae Stejneger, 1907 1 1 False coral snake Tropical South America.
Anomochilidae Cundall, Wallach, 1993 1 2 Dwarf pipe snakes West Malaysia and on the Indonesian island of Sumatra.
Atractaspididae Günther, 1858 12 64 Burrowing asps Africa and the Middle East.[7][16][17]
Boidae Gray, 1825 8 43 Boas Northern, Central and South America, the Caribbean, southeastern Europe and Asia Minor, Northern, Central and East Africa, Madagascar and Reunion Island, the Arabian Peninsula, Central and southwestern Asia, India and Sri Lanka, the Moluccas and New Guinea through to Melanesia and Samoa.
Bolyeriidae Hoffstetter, 1946 2 2 Splitjaw snakes Mauritius.
Colubridae Oppel, 1811 304[2] 1938[2] Typical snakes Widespread on all continents, except Antarctica.[18]
Cylindrophiidae Fitzinger, 1843 1 8 Asian pipe snakes Sri Lanka east through Myanmar, Thailand, Cambodia, Vietnam and the Malay Archipelago to as far east as Aru Islands off the southwestern coast of New Guinea. Also found in southern China (Fujian, Hong Kong and on Hainan Island) and in Laos.
Elapidae Boie, 1827 61 235 Elapids On land, worldwide in tropical and subtropical regions, except in Europe. Sea snakes occur in the Indian Ocean and the Pacific.[19]
Loxocemidae Cope, 1861 1 1 Mexican burrowing snake Along the Pacific versant from Mexico south to Costa Rica.
Pythonidae Fitzinger, 1826 8 26 Pythons Subsaharan Africa, peninsular India, Myanmar, southern China, Southeast Asia and from the Philippines southeast through Indonesia to New Guinea and Australia.
Tropidophiidae Brongersma, 1951 4 22 Dwarf boas From southern Mexico and Central America, south to northwestern South America in Colombia, (Amazonian) Ecuador and Peru, as well as in northwestern and southeastern Brazil. Also found in the West Indies.
Uropeltidae Müller, 1832 8 47 Shield-tailed snakes Southern India and Sri Lanka.
Viperidae Oppel, 1811 32 224 Vipers The Americas, Africa and Eurasia.
Xenopeltidae Bonaparte, 1845 1 2 Sunbeam snakes Southeast Asia from the Andaman and Nicobar Islands, east through Myanmar to southern China, Thailand, Laos, Cambodia, Vietnam, the Malay Peninsula and the East Indies to Sulawesi, as well as the Philippines.


Infraorder Scolecophidia 3 families
Family[1] Taxon author[1] Genera[1] Species[1] Common name Geographic range[15]
Anomalepidae Taylor, 1939 4 15 Primitive blind snakes From southern Central America to northwestern South America. Disjunct populations in northeastern and southeastern South America.
Leptotyphlopidae Stejneger, 1892 2 87 Slender blind snakes Africa, western Asia from Turkey to northwestern India, on Socotra Island, from the southwestern United States south through Mexico and Central America to South America, though not in the high Andes. In Pacific South America they occur as far south as southern coastal Peru, and on the Atlantic side as far as Uruguay and Argentina. In the Caribbean they are found on the Bahamas, Hispaniola and the Lesser Antilles.
Typhlopidae Merrem, 1820 6 203 Typical blind snakes Most tropical and many subtropical regions around the world, particularly in Africa, Madagascar, Asia, islands in the Pacific, tropical America and in southeastern Europe.

Skeleton

The skeleton of most snakes consists solely of the skull, hyoid, vertebral column, and ribs, though henophidian snakes retain vestiges of the pelvis and rear limbs. The skull of the snake consists of a solid and complete braincase, 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 only by a flexible ligament at the anterior tips, allowing them to separate widely, while the posterior end of the lower jaw bones articulate with a quadrate bone, allowing further mobility. The bones of the mandible and quadrate bones can also pick up ground borne vibrations.[20] The hyoid is a small bone located posterior and ventral to the skull, in the 'neck' region, which serves as an attachment for muscles of the snake's tongue, as it does in all other tetrapods.

The vertebral column consists of anywhere between 200 to over 400 vertebrae. Tail vertebrae are comparatively few in number (often less than 20% of the total) and lack ribs, while body vertebrae each have two ribs articulating with them. The vertebrae have projections that allow for strong muscle attachment enabling locomotion without limbs. Autotomy of the tail, a feature found in some lizards is absent in most snakes.[21] Caudal autotomy in snakes is rare and is intervertebral unlike that seen in lizards which is intravertebral, that is, the break happens along a predefined fracture plane present on a vertebra.[22][23]

An adult Barbados threadsnake, Leptotyphlops carlae, on an American quarter dollar

Size comparison

The now extinct Titanoboa cerrejonensis snakes found were between 12 and 15 meters (39 and 49 ft) in length. In comparison, the largest extant snakes are the reticulated python, which measures about 9 meters (30 ft) long, and the anaconda, which measures about 7.5 meters (25 ft) long[24] and is considered the heaviest snake on Earth. At the other end of the scale, the smallest extant snake is Leptotyphlops carlae with a length of about 10 centimeters (4 in).[25] However, most snakes are fairly small animals, approximately 3 feet in length[26].

Skin

Main article: Snake scales

The skin of a snake is covered in scales. Contrary to the popular notion of snakes being slimy because of possible confusion of snakes with worms, snakeskin has a smooth, dry texture. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. The eyelids of a snake are transparent "spectacle" scales which remain permanently closed, also known as brille.

A line diagram from G.A. Boulenger's Fauna of British India (1890) illustrating the terminology of shields on the head of a snake

The shedding of scales is called ecdysis, or, in normal usage moulting or sloughing. In the case of snakes, the complete outer layer of skin is shed in one layer.[27] Snake scales are not discrete but extensions of the epidermis hence they are not shed separately, but are ejected as a complete contiguous outer layer of skin during each moult, akin to a sock being turned inside out.[28]

The shape and number of scales on the head, back and belly can be characteristic are often used for taxonomic purposes. Scales are named mainly according to their positions on the body. In "advanced" (Caenophidian) snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection. Snake's eyes are covered by their clear scales rather than movable eyelids; therefore, their eyes are always open.[citation needed]

Moulting

Moulting serves a number of functions. Firstly, the old and worn skin is replaced; secondly, it helps get rid of parasites such as mites and ticks. Renewal of the skin by moulting is supposed to allow growth in some animals such as insects, however this view has been disputed in the case of snakes.[28][29]

A snake shedding its skin

Moulting is repeated periodically throughout a snake's life. Before a moult, 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 eyes become cloudy or blue-colored. The inner surface of the old outer skin liquefies. This causes the old outer skin to separate from the new inner skin. After a few days, the eyes clear and the snake "crawls" out of its old skin. The old skin breaks near the mouth and the snake wriggles out aided by rubbing against rough surfaces. In many cases the cast skin peels backward over the body from head to tail, in one piece like an old sock. A new, larger, and brighter layer of skin has formed underneath.[28][30]

An older snake may shed its skin only once or twice a year, but a younger, still-growing snake, may shed up to four times a year.[30] The discarded skin gives a perfect imprint of the scale pattern and it is usually possible to identify the snake if this discard is reasonably complete and intact.[28] This periodic renewal has led to the snake being a symbol of healing and medicine, as pictured in the Rod of Asclepius.[31]


Perception

Thermographic image of a snake eating a mouse

Eyesight

Snake vision varies greatly, from only being able to distinguish light from dark to keen eyesight, but the main trend is that their vision is adequate although not sharp, and allows them to track movements.[32] Generally, vision is best in arboreal snakes and weakest in burrowing snakes. Some snakes, such as the Asian vine snake (genus Ahaetulla), have binocular vision, with both eyes capable of focusing on the same point. Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched.

Smell

Snakes use smell to track their prey. It smells by using its forked tongue to collect airborne particles then passing them to the Jacobson's organ or the Vomeronasal organ in the mouth for examination.[33] The fork in the tongue gives the snake a sort of directional sense of smell and taste simultaneously.[33] The snake keeps its tongue constantly in motion, sampling particles from the air, ground, and water analyzing the chemicals found and determining the presence of prey or predators in its local environment.[33]

Vibration sensitivity

The part of the body which is in direct contact with the surface of the ground is very sensitive to vibration, thus a snake is able to sense other animals approaching through detecting faint vibrations in the air and on the ground.[33]

Infrared sensitivity

Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves between the nostril and eye, although some have labial pits on their upper lip just below the nostrils (common in pythons) which allow them to "see" the radiated heat.[33] Infrared sensitivity helps snakes locate nearby prey, especially warm-blooded mammals.

Feeding and diet

Snake eating a rodent
Carpet python constricting and consuming a chicken.

All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insects.[7][2][8][34] Because snakes cannot bite or tear their food to pieces, a snake must swallow its prey whole. The body size of a snake has a major influence on its eating habits. 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.

African Egg-eating snake

The snake's jaw is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull (see snake skull), allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself,[34] as snakes do not chew. For example, the African Egg-eating Snake has flexible jaws adapted for eating eggs much larger than the diameter of its head.[7]:81 This snake has no teeth, but does have bony protrusions on the inside edge of its spine which are used to aid in breaking the shells of the eggs it eats.[7]:81

While the majority of snakes eat a variety of prey animals, there is some specialization by some species. King cobras and the Australian Bandy-bandy consume other snakes. Pareas iwesakii and other snail-eating Colubrids of subfamily Pareatinae have more teeth on the right side of their mouths than on the left, as the shells of their prey usually spiral clockwise[7]:184[35]

Some snakes have a venomous bite, which they use to kill their prey before eating it.[34][36] Other snakes kill their prey by constriction.[34] Still others swallow their prey whole and alive.[7]:81[34]

After eating, snakes become dormant while the process of digestion takes place.[37] Digestion is an intense activity, especially after consumption of very large prey. In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy, and the digestive system is 'up-regulated' to full capacity within 48 hours of prey consumption. Being cold-blooded (ectothermic), the surrounding temperature plays a large role in a snake's digestion. 30℃ is the ideal temperature for snakes to digest their food. So much metabolic energy is involved in a snake's digestion that in Crotalus durissus, the Mexican rattlesnake, an increase of body temperature to as much as 1.2℃ above the surrounding environment has been observed.[38] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey in order to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient, with the snake's digestive enzymes dissolving and absorbing everything but the prey's hair and claws, which are excreted along with waste.

Internal organs

1: esophagus 2: trachea 3:tracheal lungs 4: rudimentary left lung 4: right lung 6: heart 7: liver 8 stomach 9: air sac 10: gallbladder 11: pancreas 12: spleen 13: intestine 14: testicles 15: kidneys
Anatomy of a snake. 1 esophagus, 2 trachea, 3 tracheal lungs, 4 rudimentary left lung, 5 right lung, 6 heart, 7 liver, 8 stomach, 9 air sac, 10 gallbladder, 11 pancreas, 12 spleen, 13 intestine, 14 testicles, 15 kidneys.

The snake's heart is encased in a sac, called the pericardium, located at the bifurcation of the bronchi. The heart is able to move around, however, owing to the lack of a diaphragm. This adjustment protects the heart from potential damage when large ingested prey is passed through the esophagus. The spleen is attached to the gall bladder and pancreas and filters the blood. The thymus gland is located in fatty tissue above the heart and is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is also 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.[39]

The vestigial left lung is often small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin.[39] In the majority of species, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion which does not function in gas exchange.[39] This 'saccular lung' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species.[39] Many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other.[39] Snakes have no lymph nodes.[39]

Locomotion

The lack of limbs does not impede the movement of snakes, and 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, and transitions between modes are abrupt.[40][41]

Lateral undulation

See also: Lateral undulation

Lateral undulation is the sole mode of aquatic locomotion, and the most common mode of terrestrial locomotion.[41] In this mode, the body of the snake alternately flexes to the left and right, resulting in a series of rearward-moving 'waves'.[40] While this movement appears rapid, snakes have been documented moving faster than two body-lengths per second, often much less.[42] This mode of movement is similar to running in lizards of the same mass.[43]

Terrestrial lateral undulation

Banded sea snake, Laticauda sp.

Terrestrial lateral undulation is the most common mode of terrestrial locomotion for most snake species.[40] In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, irregularities in the soil, etc.[40] 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.[44] 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.[42] 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.[44]

Aquatic

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.[45] 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 vs terrestrial lateral undulation, which justifies calling them separate modes.[46] All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the pattern, i.e. moving backwards via forward-traveling waves.[40]

A Mojave rattlesnake (Crotalus scutulatus) sidewinding

Sidewinding

When compared, the skeletons of snakes are radically different from those of most other reptiles (such as the turtle, right), being made up almost entirely of an extended ribcage.
See also: Sidewinding

Most often employed by colubroid snakes (colubrids, elapids, and vipers) when the snake must move in an environment which lacks any irregularities to push against (and which therefore renders 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.[47][48] 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.[47] 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 or snake to move the same distance.[43] Contrary to popular beliefs, there is no evidence that sidewinding is associated with hot sand.[47]

Concertina locomotion

See also: Concertina movement

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.[40][48] 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.[47] 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.[43] 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.

Rectilinear locomotion

See also: Rectilinear locomotion

The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one in which the snake does not need to bend its body laterally, though it may do so when turning.[49] 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.[49] The ribs of the snake do not move in this mode of locomotion and this method is most often used by large pythons, boas, and vipers when stalking prey across open ground as the snake's movements are subtle and harder to detect by their prey in this manner.[47]

Other

The movement of snakes in arboreal habitats has only recently been studied.[50] While on tree branches, snakes use several modes of locomotion depending on species and bark texture.[50] In general, snakes will use a modified form of concertina locomotion on smooth branches, but will laterally undulate if contact points are available[50]. 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'[50].

Gliding snakes (Chrysopelea) of Southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees.[47][51][52] These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in mid-air.[47][51]

Reproduction

Although a wide range of reproductive modes are used by snakes, all snakes employ internal fertilization, accomplished by means of paired, forked hemipenes, which are stored inverted in the male's tail.[53] The hemipenes are often grooved, hooked, or spined in order to grip the walls of the female's cloaca.[53]

Most species of snake lay eggs, and most of those species abandon them shortly after laying; however, individual species such as the King cobra actually construct nests and stay in the vicinity of the hatchlings after incubation.[53] Most pythons coil around their egg-clutches after they have laid them and remain with the eggs until they hatch.[54] The female python will not leave the eggs, except to occasionally bask in the sun or drink water and will generate heat to incubate the eggs by shivering.[54]

Some species of snake are ovoviviparous and retain the eggs within their bodies until they are almost ready to hatch.[55][56] Recently, it has been confirmed that several species of snake are fully viviparous, such as the boa constrictor and green anaconda, nourishing their young through a placenta as well as a yolk sac, which is highly unusual among reptiles, or anything else outside of placental mammals.[55][56] Retention of eggs and live birth are most often associated with colder environments, as the retention of the young within the female.[53][56]

Venom

See also: Snake venom
Vipera berus, one fang in glove with a small venom stain, the other still in place

Cobras, vipers, and closely related species use venom to immobilize or kill their prey. The venom is modified saliva, delivered through fangs.[7]:243 The fangs of 'advanced' venomous snakes like viperids and elapids are hollow in order to inject venom more effectively, while the fangs of rear-fanged snakes such as the Boomslang merely have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey specific, its role in self-defense is secondary.[7]:243 Venom, like all salivary secretions, is a pre-digestant which initiates the breakdown of food into soluble compounds allowing for proper digestion and even "non-venomous" snake bites (like any animal bite) will cause tissue damage.[7]:209

Certain birds, mammals, and other snakes such as kingsnakes that prey on venomous snakes have developed resistance and even immunity to certain venom.[7]:243 Venomous snakes include three families of snakes and do not constitute a formal classification group used in taxonomy. The term poisonous snake is mostly incorrect—poison is inhaled or ingested whereas venom is injected.[57] There are, however, two exceptions—Rhabdophis sequesters toxins from the toads it eats then secretes them from nuchal glands to ward off predators, and a small population of garter snakes in Oregon retains enough toxin in their liver from the newts they eat to be effectively poisonous to local small predators such as crows and foxes.[58]

Snake venoms are complex mixtures of proteins and are stored in poison glands at the back of the head.[58] In all venomous snakes these glands open through ducts into grooved or hollow teeth in the upper jaw.[7]:243[57] These proteins can potentially be a mix of neurotoxins (which attack the nervous system), hemotoxins (which attack the circulatory system), cytotoxins, bungarotoxins and many other toxins that affect the body in different ways.[57] Almost all snake venom contains hyaluronidase, an enzyme that ensures rapid diffusion of the venom.[7]:243

Venomous snakes that use hemotoxins usually have the fangs that secrete the venom in the front of their mouths, making it easier for them to inject the venom into their victims.[57] Some snakes that use neurotoxins, such as the mangrove snake, have their fangs located in the back of their mouths, with the fangs curled backwards.[59] This makes it both difficult for the snake to use its venom and for scientists to milk them.[57] Elapid snakes, however, such as cobras and kraits are proteroglyphous, possessing hollow fangs which cannot be erected toward the front of their mouths and cannot "stab" like a viper, they must actually bite the victim.[7]:242

It has recently been suggested that all snakes may be venomous to a certain degree, the harmless snakes having weak venom and no fangs.[60]. Most snakes that are considered non-venomous would still be considered harmless under this theory, because under most cases the snakes have no way of delivering much or any venom, certainly not enough to kill a human. Also under this theory, snakes may have evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard may have also derived, as well as the monitor lizards and now extinct mosasaurs. They share this venom clade with various other saurian species.

Venomous snakes are classified in two taxonomic families:

There is a third family containing the opistoglyphous (rear-fanged) snakes as well as the majority of other snake species:

Interactions with humans

Most common symptoms of any kind of snake bite poisoning.[61] [62] [63] Furthermore, there is vast variation in symptoms between bites from different types of snakes.[61]

Snake bite

Main article: Snakebite
Although not venomous, a Green tree python (Morelia viridis) can deliver a bite

Snakes do not ordinarily prey on humans and most will not attack humans unless the snake is startled or injured, preferring instead to avoid contact. With the exception of large constrictors, non-venomous snakes are not a threat to humans. The bite of non-venomous snakes is usually harmless because their teeth are designed for grabbing and holding, rather than tearing or inflicting a deep puncture wound. Although the possibility of an infection and tissue damage is present in the bite of a non-venomous snake, venomous snakes present far greater hazard to humans.[7]:209

Documented deaths resulting from snake bites are uncommon. Non-fatal 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. Although Australia is home to the largest number of venomous snakes in the world,[citation needed] it only has one fatal snake bite per year on average. In India, 250,000 snakebites are recorded in a single year with as many as 50,000 recorded initial deaths.[64]

The treatment for a snakebite is as variable as the bite itself. The most common and effective method is through antivenom, 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 pit vipers, with the exception of the coral snake. To produce antivenin, a mixture of the venoms of the different species of rattlesnakes, 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 and freeze-dried. It is reconstituted with sterile water and becomes antivenin. For this reason, people who are allergic to horses cannot be treated using antivenin. Antivenin for the more dangerous species (such as mambas, taipans, and cobras) is made in a similar manner in India, South Africa, and Australia with the exception being that those antivenins are species-specific.

An Indian cobra in a basket with a snake charmer. These snakes are perhaps the most common subjects of snake charmings.

Snake charmers

Main article: Snake charming

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 that contains a snake that he seemingly charms by playing tunes from his flutelike musical instrument, to which the snake responds.[65] Snakes lack external ears, and though they do have internal ears, they show no tendency to be influenced by music.[65]

The Wildlife Protection Act of 1972 in India technically proscribes snake charming on grounds of reducing animal cruelty. Other snake charmers also have a snake and mongoose show, where both the animals have a mock fight; however, this is not very common, as the snakes, as well as the mongooses, 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.[65]

Snake trapping

The tribals of "Irulas" from Andhra Pradesh and Tamil Nadu in India have been hunter-gatherers in the hot dry plains forests and have practiced this art for generations. They have a vast knowledge of snakes in the field. Irulas 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 on snake-skin industry in India and protection of all snakes under the Indian Wildlife (Protection) Act 1972, 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 antivenin, biomedical research and for other medicinal products.[66] 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 wranglers. Modern day snake trapping involves a herpetologist using a long stick with a "V" shaped end. Some like Bill Haast, Austin Stevens, and Jeff Corwin prefer to catch them using bare hands.

Human consumption

A "海豹蛇" ("sea-leopard snake", supposedly Enhydris bocourti) occupies a place of honor among the live delicacies waiting to meet their eaters outside of a Guangzhou restaurant

While not commonly thought of as a dietary item by most cultures, in some cultures, the consumption of snakes is acceptable, or even considered a delicacy, prized for its alleged pharmaceutical effect of warming the heart. Snake soup of Cantonese cuisine is consumed by local people in autumn, to warm up their body. Western cultures document the consumption of snakes under extreme circumstances of hunger.[67] Cooked rattlesnake meat is an exception, which is commonly consumed in parts of the Midwestern United States. In Asian countries such as China, Taiwan, Thailand, Indonesia, Vietnam and Cambodia, drinking the blood of snakes, particularly the cobra, is believed to increase sexual virility.[68] The blood is drained while the cobra is still alive when possible, and is usually mixed with some form of liquor to improve the taste.[68]

In some Asian countries, the use of snakes in alcohol is also accepted. In such cases, the body of a snake or several snakes is left to steep in a jar or container of liquor. It is claimed that this makes the liquor stronger (as well as more expensive). One example of this is the Habu snake sometimes placed in the Okinawan liquor Awamori also known as "Habu Sake".[69]

As pets

In the Western world some snakes, especially docile species such as the ball python and corn snake, are kept as pets. To supply this demand a captive breeding industry has developed. Snakes bred in captivity tend to make better pets and are considered preferable to wild caught specimens.[70] Snakes can be very low maintenance pets, especially in comparison to more traditional species. They require minimal space, as most common species do not exceed five feet in length. Pet snakes can be fed relatively infrequently, usually once every five to fourteen days. Certain snakes have a life span of more than forty years if given proper care.

Symbolism

Medusa by 16th Century Italian artist Caravaggio
Lilith with a snake, (1892), by John Collier (1892).
Rod of Asclepius, in which the snakes, through ecdysis, symbolize healing.
Main article: Serpent (symbolism)

In Egyptian history, the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was worshipped as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra).

In Greek mythology snakes are often associated with deadly and dangerous antagonists, but this is not to say that snakes are symbolic of evil; in fact, snakes are a chthonic symbol, roughly translated as 'earthbound'. The nine-headed Lernaean Hydra that Hercules defeated and the three Gorgon sisters are children of Gaia, the earth.[71] Medusa was one of the three Gorgon sisters who Perseus defeated.[71] Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze.[71] After killing her, Perseus gave her head to Athena who fixed it to her shield called the Aegis.[71] The Titans are also depicted in art with snakes instead of legs and feet for the same reason—they are children of Gaia and Ouranos (Uranus), so they are bound to the earth.

Three medical symbols involving snakes that are still used today are Bowl of Hygieia, symbolizing pharmacy, and the Caduceus and Rod of Asclepius, which are symbols denoting medicine in general.[31]

India is often called the land of snakes and is steeped in tradition regarding snakes.[72] Snakes are worshipped as gods even today with many women pouring milk on snake pits (despite snakes' aversion for milk).[72] The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping on a seven-headed snake or within the coils of a serpent.[73] 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 Nag Panchami each year on which day snakes are venerated and prayed to. See also Nāga.

In Christianity and Judaism, the snake makes its infamous appearance in the first book (Genesis 3:1) of the Bible when a serpent appears before the first couple Adam and Eve as an agent of the devil and tempts them with the forbidden fruit from the Tree of Knowledge. The snake returns in Exodus when Moses, as a sign of God's power, turns his staff into a snake and when Moses made 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 Satan in the Book of Revelation:"And he laid hold on the dragon the old serpent, which is the devil and Satan, and bound him for a thousand years." (Revelation 20:2)

The Ouroboros is a symbol that is associated with many different religions and customs, and is also claimed to be related to Alchemy. The Ouroboros or Oroboros is a snake eating its own tail in a clock-wise direction (from the head to the tail) in the shape of a circle, representing manifestation of one's own life and rebirth, leading to immortality.

The snake is one of the 12 celestial animals of Chinese Zodiac, in the Chinese calendar.

Many ancient Peruvian cultures worshipped nature.[74] They placed emphasis on animals and often depicted snakes in their art.[75]

In religion

A snake associated with Saint Simeon Stylites

Snakes are a part of Hindu worship. A festival Nag Panchami is celebrated every year on snakes. Most images of Lord Shiva depict snake around his neck. Puranas have various stories associated with Snakes. In the Puranas, Shesha is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of Vishnu from all his mouths. He is sometimes referred to as "Ananta-Shesha" which means "Endless Shesha". Other notable snakes in Hinduism are Ananta, Vasuki, Taxak, Karkotaka and Pingala. The term Nāga is used to refer to entities which take the form of large snakes in Hinduism and Buddhism.

Snakes have also been widely revered, such as in ancient Greece, where the serpent was seen as a healer, and Asclepius carried two intertwined on his wand, a symbol seen today on many ambulances. In Judaism, the snake of brass is also a symbol of healing, of one's life being saved from imminent death (Book of Numbers 26:6–9). In Christianity, Christ's redemptive work is compared to saving one's life through beholding the Serpent of brass (Gospel of John 3:14). Snake handlers use snakes as an integral part of church worship in order to exhibit their faith in divine protection. However, more commonly in Christianity, the serpent has been seen as a representative of evil and sly plotting, which can be seen in the description in Genesis chapter 3 of a snake in the Garden of Eden tempting Eve. Saint Patrick is reputed to have expelled all snakes from Ireland while Christianising the country in the 5th century, thus explaining the absence of snakes there.

In Neo-Paganism and Wicca, the snake is seen as a symbol of wisdom and knowledge.

See also

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Further reading

External links

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