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For other uses, see Snake (disambiguation).

Snakes
Temporal range: Cretaceous - Recent
File:Coralillo.jpg
A coral snake.
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Order:
Squamata
Suborder:
Serpentes

Linnaeus, 1758
Superfamilies and Families
Aniliidae
Anomochilidae
Boidae
Bolyeriidae
Cylindrophiidae
Loxocemidae
Pythonidae
Tropidophiidae
Uropeltidae
Xenopeltidae
Anomalepididae
Leptotyphlopidae
Typhlopidae
Acrochordidae
Atractaspididae
Colubridae
Elapidae
Hydrophiidae
Viperidae

Snakes (from Old English snaca, and ultimately from the Proto-Indo-European base snag- or sneg-, "to crawl"), also known as ophidians, are cold blooded legless reptiles closely related to lizards, which share the order Squamata. There are also several species of legless lizard which superficially resemble snakes, but are not otherwise related to them. A love of snakes is called ophiophilia, a fear of snakes is called ophidiophobia (or snakephobia). A specialist in snakes is an ophiologist.

An old synonym for snake is serpent (which comes from Old French, and ultimately from PIE *serp-, "to creep"); in modern usage this usually refers to a mythic or symbolic snake, and information about such creatures will be found under serpent (symbolism). This article deals with the biology of snakes.


Evolution

The phylogeny of snakes is poorly known due to the fact that 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 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.

Modern boas do have vestigal hind limbs, tiny, clawed digits known as anal spurs and used to grasp during mating.

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 Cretateous 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.

Feeding

Snake eating mouse

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.

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 which 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 Centigrade above the surrounding environment has been observed.[1] Because of this, a snake disturbed after having eaten recently will often regurgitate the 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 acids 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. While some particularly aggressive species exist, most will not attack humans unless startled or injured, preferring instead to avoid contact. In fact, the majority of snakes are either non-venomous or possess venom that is not harmful to humans.

Skin

The skin is covered in scales. Most snakes use specialized belly scales to move, gripping surfaces. The body scales may be smooth, keeled, or granular. Their eyelids are transparent "spectacle" scales which remain permanently closed. They shed their skin periodically. Unlike other reptiles, this is done in one piece, like pulling off a sock. It is thought that the primary purpose of this is to remove 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 the snake to become blind in the affected eye.

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.[1] Their sense of hearing is most sensitive to frequencies around 200–300Hz.

A snake smells by using its forked tongue to collect airborne particles then passing them to the Jacobson's 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 direct contact with the surface of the ground is very sensitive to vibration thus a snake able to sense a person approaching.

Internal organs

The left lung is very small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin, and 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. The most primitive snakes, including boas and pythons, have anal spurs, a pair of claws on either side of the cloaca which are used by the males for stimulation of females during mating.

Locomotion

Snakes utilize a variety of methods of movement which allows them substantial mobility in spite of their legless condition. All snakes are capable of lateral undulation, in which the body is flexed side-to-side, and the flexed areas propagate posteriorly, giving the overall shape of a posteriorly propagating sine wave. In addition, all snakes are capable of concertina movement. This method of movement can be used to both climb trees and move through small tunnels. In the case of trees, the branch is grasped by the posterior portion of the body, while the anterior portion is extended. The anterior portion then grasps the branch, and the posterior portion is pulled forward. In the case of tunnels, instead of grasping, the body loops are pressed against the tunnel walls to attain traction, but the motion is otherwise similar. Another common method of locomotion is rectilinear locomotion, in which the snake remains straight and propels itself via a caterpillar-like motion of its belly-muscles. This mode is usually only used by very large, heavy snakes, such as large pythons and vipers. The most complex and interesting mode is sidewinding, an undulatory motion used to move across slippery mud or loose sand.

Not all snakes dwell on land; sea snakes live in shallow tropical seas.

Studies of the motion and muscle activity of moving snakes have shed light on how each of these modes is achieved.

In terrestrial lateral undulation, posteriorly propagating unilateral waves of muscle contraction occur. The regions of muscle activity for each side extend from the most concave point on that side posteriorly to the most convex side. Thus, when a point on the snake's body is maximally flexed to the right, the right muscles activate, bending it back to the left until it's maximally right-convex, at which point the right side muscles turn off, and the left side muscles turn on. Speed is modulated primarily by alteration of frequency. Aquatic lateral undulation appears superficially similar, but the muscle activation pattern is different, with the regions of muscle activity being 'shifted' posteriorly to where they would be in terrestrial lateral undulation. The reasons for this difference are not fully understood.

Sidewinding, though it appears complex and confusing, is actually a simple modification of terrestrial lateral undulation. At the points of maximal flexion, the dorsalmost muscle group (traversospinalis group) activates, lifting that portion of the body over the ground, and resulting in other portions of the body remaining in static contact. This mode is used to cross slick surfaces such as mud flats and sand, and has nothing to do with thermoregulation, as is sometimes erroneously stated. Many species of snake, including species commonly kept as pets and which do not usually encounter deserts or mud flats, will sidewind when placed on a slick floor or tabletop and enticed to move fast.

Concertina locomotion and rectilinear locomotion are less well understood. Studies of muscle activity have only been done for tunnel concertina locomotion, which shows that the muscles are unilaterally active in static regions of bending in order to brace the snake against the tunnel walls. Rectilinear is believed to rely on different muscles from the other modes; while they all rely on the large epaxial muscles, rectilinear locomotion seems to rely upon the small costocutaneous muscles. However, this has not been verified experimentally, due to the difficulties in working with these small muscles.

"Flying" snakes

Several species of snake have the ability to glide, all being in the genus Chrysopelea. They are quite capable at it, able to travel as far as 137 metres through the air. They tend to make slithering motions to steer and help propel themselves along, propulsion being something unusual among the many gliding animals.

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 those most 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, 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.

Snake bites

Documented deaths resulting from snake bites are uncommon 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 (lightning kills more). See snakebites for more information, including prevention of snake bites and first aid treatment.

Venomous Snakes

While about a quarter of snakes are venomous, not all such species are dangerous to humans. See snake venom. The following groups of snakes can be aggressive and inflict dangerous, even potentially lethal bites. This list is incomplete.

Brown Snake

Snakes as pets

Many varieties of snakes are docile and suitable as exotic pets.

"Beginner snakes" typically grow less than 180cm. Certain snakes are preferred as pets because they readily take food items that are inexpensive and acquired at many pet stores (feeder fish, crickets, small rodents, etc.) Some owners train their snakes to eat pre-killed meals (especially larger ones such as guinea pigs and rabbits) in order to avoid possible struggles with live prey leading to injuries to the snake, or out of sheer sympathy for the prey.

F/T refers to frozen/thawed food items. Some species are more likely to eat F/T prey than others. Many people prefer feeding F/T mice, frogs, rats, and lizards because of the convenience. Some pet stores, such as PetSmart, only sell F/T feeders to avoid the perception of cruelty to the prey.

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. However, snakes' sense of hearing is not very sensitive to the range of the charmer's instrument, so they may not be able to hear the music at all. 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 neigbourhood 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 may be seriously injured or killed.

Snake trapping

Despite the existence of snake charmers, there have also been professional snake catchers. The tribals of "Irulas" from Andhra Pradesh and Tamil Nadu in India have been practicising 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. 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 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. The young man if 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 phython. 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 to snake to swallow the leg.

Human consumption of snakes

In some cultures, the consumption of snakes is acceptable[2] or even considered a delicacy[3], prized for its alleged pharmaceutical effect of warming the heart. Western cultures document the consumption of snake under extreme circumstances of hunger[4]. However, human consumption of snake meat, especially when eaten raw, may lead to dangerous parasitic infections in humans. Rattlesnake meat is eaten in the western United States somewhat commonly. 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.

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 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.

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.

In Christianity 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 turns his stick into a snake to devour the pharaoh's snake. Later he carves out a cross with a snake to cure the people in the desert.

Films

Classification

Order:Squamata


See also

Notes

  1. ^ "The thermogenesis of digestion in rattlesnakes". Journal of Experimental Biology 207. The Company of Biologists. 2004. pp. 579–585. Retrieved 2006-05-26. {{cite web}}: Unknown parameter |accessyear= ignored (|access-date= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

References

  • Romulus Whitaker (English edition); Tamil translation by O.Henry Francis (1996). நம்மை சுட்ரியுள்ள பாம்புகள் (Snakes around us, Tamil). National Book Trust. ISBN 8123719051.{{cite book}}: CS1 maint: multiple names: authors list (link)
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