Monday 7 September 2009

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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