Snake

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A snake is a scaly, limbless, elongate reptile from the order Squamata. A literary word for snake is serpent (a Middle English word which comes from Old French, and ultimately from *serp-, "to creep"^[1]); in modern usage this usually refers to a mythic or symbolic snake, and information about such creatures can be found under serpent (symbolism). This article deals mostly with the biology of snakes.

[edit] Evolution

The phylogeny of snakes is poorly known because snake skeletons are typically small and fragile, making fossilization unlikely. It has however been generally agreed, on the basis of morphology, that snakes descended from lizard-like ancestors. Recent research based on genetics and biochemistry confirms this; snakes form a venom clade with several extant lizard families.

Recent fossil evidence suggests that snakes directly evolved from burrowing lizards, either varanids or some other group. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, fully terrestrial. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic. As these ancestors became more subterranean, they lost their limbs and became more streamlined for burrowing. Features such as the transparent, fused eyelids (brille) and loss of external ears, according to this hypothesis, evolved to combat subterranean conditions (scratched corneas, dirt in the ears). According to this hypothesis, snakes re-emerged onto the surface of the land much as they are today. Other primitive snakes are known to have possessed hindlimbs but lacked a direct connection of the pelvic bones to the vertebrae, including Haasiophis, Pachyrhachis and Eupodophis) which are slightly older than Najash'

Primitive groups among the modern snakes, pythons and boas, do have vestigial hind limbs, tiny, clawed digits known as anal spurs and used to grasp during mating. Leptotyphlopidae and Typhlopidae are other examples where remnants of the pelvic girdle are still present, in Leptotyphlopidae sometimes as horny projections or not visible at all. The frontal limbs in all snakes are gone because of the evolution of the Hox genes in this area. The axial skeleton of the snakes' common ancestor had like most other tetrapods the familiar regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. But the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution. 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 1-3 neck vertebrae), meaning most of the snake's skeleton is actually made up 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 2-10 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. Because the front (thoracic) limbs in tetrapods appear in the area between the neck and the thorax, a location that is now almost absent in snakes, there is simply no longer any room left where they can develop.

The alternative hypothesis, based on morphology, suggests that ancestors were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizards. 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 the land much like 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, though some features are also shared with varanids. Supposedly similar locomotion for both groups is also used as support for this hypothesis. Genetic studies have indicated that snakes are not especially related to monitor lizards, and (it has been claimed) 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, probably correlated with the adaptive radiation of mammals following the extinction of the dinosaurs.

[edit] Digestion and diet

All snakes are carnivorous, eating small animals including lizards and other snakes, rodents and other small mammals, birds, eggs or insects. Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by constriction. Still others swallow their prey whole and alive. 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^[2]. Most snakes are very easy to feed in captivity.

Snakes do not chew their food and 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. It is a common misconception that snakes actually dislocate their lower jaw to consume large prey.

After eating, snakes become torpid while the process of digestion takes place. Digestion is an intensive activity, especially after the 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. So much metabolic energy is involved in digestion that in Crotalus durissus, the Mexican rattlesnake, an increase of body temperature to as much as 14 degrees Celsius above the surrounding environment has been observed.^[3] 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. However, when undisturbed, the digestive process is highly efficient, dissolving and absorbing everything but hair and claws, which are excreted along with uric acid waste. Snakes have been known to occasionally die from trying to swallow an animal that is too big. Snake digestive fluids are unable to digest most plant matter, which passes through the digestive system mostly untouched.

Snakes do not normally prey on people, but there are instances of small children being eaten by large constrictors in the jungle.^{[citation needed]} While some particularly aggressive species exist, most will not attack humans unless startled or injured, preferring instead to avoid contact. The majority of snakes are either non-venomous or possess venom that is not harmful to humans.

As a general rule, snakes eat small vertebrates such as rodents, fish, lizards, birds, and frogs. There are exceptions to this, such as the natal green snake, which eats insects. Snakes generally specialise in a few food types (for example, royal pythons will generally eat mice and gerbils in the wild). However, they do not need to hunt every day. A big meal will keep some snakes content for a long time. Anacondas and pythons can live for a year after eating large prey.[1]

[edit] Skin

The skin is covered in scales. Many people are surprised to find that snakeskin has a smooth, dry texture, instead of a slimy texture as might be expected. Some people are afraid to touch them because they confuse snakes with worms. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. Their eyelids are transparent "spectacle" scales which remain permanently closed, called brille. They shed their skin periodically. Unlike other reptiles, this is done in one piece, like pulling off a sock, with the snake rubbing its nose against something rough, like a rock, for instance, creating a rip in the skin around the nose and the mouth until the skin is completely removed.[2] The primary purpose of shedding is to grow; shedding also removes external parasites. This periodic renewal has led to the snake being a symbol of healing and medicine, as pictured in the Rod of Asclepius. 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. If there is not enough humidity in the air while snakes are shedding their skin, it can be very dangerous for the snake, because the dry skin does not shed. Skin that remains attached to the snake can harbour diseases and parasites. A tail tip that is not removed can constrict as the snake grows, cutting off the blood supply to the end of the tail causing it to drop off. A retained spectacle can cause blindness in the affected eye.

[edit] Perception

While snake vision is unremarkable (generally being best in arboreal species and worst in burrowing species), it is able to detect movement. Some snakes, like the Asian vine snake, have binocular vision. In most snakes, the lens moves back and forth within the eyeball to focus. In addition to their eyes, some snakes (pit vipers, pythons, and some boas) have infrared-sensitive receptors in deep grooves between the nostril and eye which allow them to "see" the radiated heat.

Snakes have no external ears, but they do have a bone called the quadrate under the skin on either side of the head which focuses sound into the cochlea. [3] Their sense of hearing is most sensitive to frequencies around 200–300 Hz.

A snake 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. The fork in the tongue gives the snake a sort of directional sense of smell. 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.

[edit] Internal organs

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 left lung is very small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. To accommodate them all, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion which does not function in gas exchange. This 'saccular lung' may be used to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. Also, many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other. Snakes have no urinary bladder.

[edit] Locomotion

Snakes utilize a variety of methods of movement which allows them substantial mobility in both aquatic and terrestrial habitats in spite of their legless condition.

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'. 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. This contrasts with terrestrial lateral undulation, in which the wave speed is precisely the same and 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 though very dense vegetation and small openings. In aquatic lateral undulation, snakes generate forward thrust by pushing their body against the water, resulting in the observed slip, while in terrestrial lateral undulation, thrust is generated by pushing against irregularities in the substrate such as pebbles and grass, resuling in 'path following'. In spite of overall similaties, studies show that the pattern of muscle activation is substantially difference in aquatic vs terrestrial lateral undulation, which justifies calling them separate modes. All snakes can laterally undulate forwards (with backward-moving waves), but only sea snakes have been observed reversing the pattern, moving backwards via forward-traveling waves.

When the snake must move in an environment which lacks any irregulaties to push against, such as a slick mud flat or sand dune, colubroid snakes (colubrids, elapids, and vipers) usually employ sidewinding. Most common in short, stocky snakes, 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. Contrary to some sources, there is no evidence that sidewinding is associated with hot sand. Boas and pythons have never been observed sidewinding.

Both sidewinding and lateral undulation require substantial space, but some environments, such as tunnels, have very limited space. In tunnels, snakes use 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 is a very slow, energetically expensive mode of locomotion.

The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one in which the snake does not bend its body laterally. In this mode, the belly scales are lifted and pulled forwards 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. In spite of appearances, the ribs do not move in this mode of locomotion.

The movement of snakes in arboreal habitats has only recently been studied. Gliding snakes (Chrysopelea) of Southeast Asia launch themselves from branch tips, spread their ribs and laterally undulate as they glide between trees; these snakes are even capable of executing sharp turns in mid-air. While on the branches, snakes appear to use several modes of locomotion depending on species and bark texture, most of which are new to science.

[edit] Reproduction

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. Most snakes lay eggs, and of most those species abandon them shortly after laying; however, some species are ovoviviparous and retain the eggs within their bodies until they are almost ready to hatch. Recently, it has been confirmed that several species of snake are fully viviparous, such as the green anaconda, nourishing their young through a placenta as well as a yolk sac, highly unusual among reptiles, or indeed anything else outside of placental mammals. Retention of eggs and live birth are commonly, but not exclusively, associated with cold environments, as the retention of the young within the female allows her to control their temperature more effectively than if the developing young were in external eggs.

[edit] Venom

See also: Snake venom

A venomous snake is a snake that uses modified saliva, venom, delivered through fangs in its mouth, to immobilize or kill its prey. Venomous snakes include several families of snakes and do not constitute a formal classification group used in taxonomy. The term poisonous snake is mostly false - poison is inhaled or ingested whereas venom is injected. (There are, however, two examples - 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.) Snake venom can contain many different active agents, and 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. Snake venom is never a single type of toxin^{[citation needed]}.

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. Some snakes that use neurotoxins, such as the mildly venomous mangrove snake, may have their fangs located in the back of their mouths, with the fangs curled backwards. This makes it both difficult for the snake to use its venom and for scientists to milk them.^{[citation needed]}

It has recently been suggested that all snakes are in fact venomous to some degree (see Toxicofera for more information). Snakes all evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard also derived. The research suggests that snakes all have venom glands, even species thought totally harmless such as the Corn Snake, commonly kept as a pet. What differentiates 'venomous' from 'non-venomous' is the evolution of a venom delivery system, the most advanced being that of vipers, with fangs that are hinged to prevent self envenomation, curling out only when the snake strikes.

Venomous snakes are generally classified in three taxonomic families:

• Elapids - cobras including king cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes.
• Viperids - vipers, rattlesnakes, copperheads/cottonmouths, adders and bushmasters.
• Colubrids - boomslangs, tree snakes, vine snakes, mangrove snakes, and many others, though not all colubrids are venomous.

[edit] Interactions with humans

[edit] Snake bite

Main article: Snake bite

Documented deaths resulting from snake bites are uncommon (about 1/1000 in most areas of the world. Only about 450 species of snakes are venomous (with only about 250 that are able to kill a human), and among the 7,000 Americans bitten by venomous snakes every year, fewer than fifteen die. See snakebites for more information, including prevention of snake bites and first aid treatment.

[edit] Snake charmers

In some parts of the world, especially in India and Pakistan, snake charming is a roadside show performed by a charmer. In this, the snake charmer carries a basket that contains a snake which he seemingly charms by playing tunes from his flute-like musical instrument, to which the snake responds. Snakes lack external ears, though have internal ears. However, snakes show no tendency to be influenced by music.

Researchers have pointed out that many of these snake charmers are good sleight-of-hand artists. The snake moves corresponding to the flute movement and the vibrations from the tapping of the charmer's foot which is not noticed by the public. They rarely catch their snakes and the snakes are either nonvenomous or defanged cobras. Sometimes these people exploit the fear of snakes by releasing snakes into the neighbourhood and then offering to rid the residence of snakes. 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 now dissuaded^[clarify] in India as a contribution to forest and snake conservation. In fact in some places in India snake charming is banned by law.^{[citation needed]}

[edit] Snake trapping

Despite the existence of snake charmers, there have also been professional snake catchers or wranglers. The tribals of "Irulas" from Andhra Pradesh and Tamil Nadu in India have been practicing this art for generations. They generally don't use gimmicks and with the help of a simple stick catch the snakes from the fields or houses. They are also known to eat some of the snakes they catch and are very useful in rat extermination in the villages.^{[citation needed]} Their knowledge of snakes and their behaviour is uncanny. Modern day snake trapping involves a herpetologist using a long stick with a "V" shaped end. Some like Steve Irwin, Bill Hasst, Joel La Rocque, and Jeff Corwin prefer to catch them using bare hands. At least one tribe of natives uses a specialized form of snake catching as a rite of passage to manhood.^{[citation needed]} The young man of interest will wrap his leg heavily in some type of cloth all the way to the inseam. He will then stick his leg in a burrow containing a large python, typically a reticulated python. After the snake swallows most of his leg several other members of the tribe will pull him out of the hole along with the snake. The snake is then killed and the man's leg removed from the snake. These snakes can be over 20 ft long and it is possible for the man to have his leg dislocated. The scent of a prey animal may be used to help convince the snake to swallow the leg. Snakes have a single-track digestive system, but the digestion process actually takes longer.

[edit] Consumption of snakes

In some cultures, the consumption of snakes is acceptable,[4] or even considered a delicacy,[5] prized for its alleged pharmaceutical effect of warming the heart. Western cultures document the consumption of snakes under extreme circumstances of hunger.[6] However, human consumption of snake meat, especially when eaten raw, may lead to dangerous parasitic infections. Cooked rattlesnake meat is an exception, which is commonly consumed in the Western United States. In Asian countries, drinking the blood of snakes, particularly the cobra, is believed to increase sexual virility. 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.

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.

[edit] Symbolism

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. The 9 headed Hydra that Hercules defeated and the three Gorgon sisters are literary examples. Medusa was one of the three Gorgon sisters who Perseus defeated. Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze.

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.

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). The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping only on a 7 headed snake. 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 Nagpanchami each year on which day snakes are venerated and prayed to. See also Nāga.

In the popular Harry Potter books by J.K. Rowling, the Hogwarts' house Slytherin has the snake as its symbol.

In Christianity and Judaism the snake makes its infamous appearance in the first book (Genesis) of the Bible when a snake appears before the first couple Adam and Eve and tempts them with the forbidden fruit. It is also seen in Exodus when Moses, as a sign of God's power, turns his stick into a snake; snakes are similarly produced by the pharaoh's magic-practicing priests, but Moses's snake devours them. Later Moses made 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. Jesus instructed his disciples to be as shrewd as snakes and as innocent as doves.

According to the Bible, the snake is the symbol of Jesus Christ, in His quality of being the Redeemer: And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up: John 3:14

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.

Snake belongs to one of the 12 celestial animals of Chinese Zodiac, in the Chinese calendar.