Snake
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Wikipedia-Article "Snake"
- For other uses, see Snake (disambiguation).
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Snakes 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; 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.
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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.
Morphology further suggests that the likeliest 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. The best fossil snake remains are from Late Cretateous marine sediments, which supports this hypothesis. 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.
An alternative hypothesis is that snakes directly evolved from burrowing lizards, either varanids or some other group. 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). As with the marine origin hypothesis, snakes re-emerged onto the surface of the land much as they are today. Fragmentary remains that have been found from the Early Cretaceous may refute either hypothesis. Further insight will no doubt also come from more detailed genetic studies.
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
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 resulting in death by strangulation. 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.
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, and so much metabolic energy is involved that in Crotalus durissus, the Mexican rattlesnake, an increase of body temperature to as much as 6 degrees above the surrounding environment has been observed. 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 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. As snakes have no external ears, hearing is restricted to the sensing of vibrations, but this sense is extremely well developed. 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.
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. 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 the vestiges of limbs. The leg bones and remains of the pelvis are embedded within the body in these species.
Locomotion
Snakes utilize a variety of methods of movement which allow 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. This cycle may occur in several sections of the snake simultaneously. 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, a undulatory motion used to move across slippery mud or loose sand.
Not all snakes dwell on land; sea snakes live in shallow tropical seas.
The physical modelling of snake motion is done in The Motion Dynamics of Snakes and Worms. From the article:
- "Snakes are quite different internally than worms, since they have ribs attached to a flexible backbone. The worm motion described above is called "rectilinear progression" and can be done by a sliding its skin over its ribs. The more familiar horizontal motion of a snake is called "horizontal undulatory progression". This is simulated by sending compression waves down the body, as with the worm, but the springs on the left side are 180 degrees out of phase with those on the left side, i.e., one side is expanding while the other is compressing. This bends the snakes into waves. As with the worm, directional friction causes the snake to move forward. If the snake has a good grip on the ground then the body segments all follow the same path. If the snake has a poor grip on the ground then the segments move sideways, causing a "sidewinding" motion. The "sidewinding" motion occurs with real snakes when they don't have good contact with the ground, e.g., sand. The snake also partially lifts itself off the ground to reduce thermal contact and increase the pressure on the remaining part. This is simulated by adding a vertical sinusoidal flexing that is 90 degrees out of phase with the horizontal waves. This raises most of the snake off the ground. The wavelength used was the length of the snake divided by 1.4."
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 actually fully livebearing, nourishing their young through a placenta as well as a yolk sac. 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
There is little reason to fear death from snake bites. Only a quarter of snakes are venomous, and among the 7,000 Americans bitten by venomous snakes every year, fewer than fifteen die (lightning kills more). See snakebites for much more information, including prevention of snake bites and first aid treatment.
Snakes as pets
Many varieties of snakes are docile and suitable as exotic pets. The following snakes are often cited as being good "beginner snakes":
Lethal venomous snakes
While only a quarter of snakes are venomous, there are various species that are lethal to humans. The following groups of lethal snakes can be aggressive and their venom can kill a healthy adult if left untreated for several hours. This list is incomplete.
- Adder
- Asp
- Black snake
- Black mamba
- Boomslang
- Brown snake
- Bushmaster
- Cobra
- Coral snake
- Copperhead
- Cottonmouth
- Death adder
- Diamondback
- Fer-de-lance
- Fierce Snake
- Gaboon Viper
- King Cobra
- Krait
- Lancehead
- Mamba
- Philippine Cobra
- Philippine Spitting Cobra
- Pit Viper
- Rattlesnake
- Russell's Viper
- Saw-scaled Viper
- Sea snake
- Taipan
- Tiger snake
- Urutu
Snake charmers
In some parts of the world, especially in India, snake charming is a roadside show performed by a charmer. In this, the snake charmer carries a basket that contain a snake which he seemingly charms by playing tunes from his flute-like musical instrument, to which the snake responds. However, snakes are deaf, so they cannot hear the music of the charmer's instrument. 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.
Human consumption of snakes
In some cultures, the consumption of snakes is acceptable[1] or even considered a delicacy[2], prized for its alleged pharmaceutical effect of warming the heart. In Hong Kong and many parts of China, the bile of snakes mixed with alcohol is consumed as a form of aphrodisiac. Snake soup is popularly consumed during winter season. It is reported to taste like chicken[3]. 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.
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 including killing an opponent or for suicide as Cleopatra did.
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
- Anaconda (1997)
- Anacondas: The Hunt for the Blood Orchid (2004)
- Boa
- Cobra (1986)
- Harry Potter and the Chamber of Secrets (2002)
- The Jungle Book (1967)
- Python (2000)
- Raiders of the Lost Ark (1981)
Classification
Order:Squamata
- Suborder Serpentes
- Superfamily Typhlopoidea (Scolecophidia)
- Family Anomalepidae: dawn blind snakes
- Family Typhlopidae: blind snakes
- Family Leptotyphlopidae /Glauconiidae: slender blind snakes
- Superfamily Henophidia (Boidea)
- Family Aniliidae /Ilysiidae: pipe snakes
- Family Anomochilidae dwarf pipe snakes
- Family Boidae: boas
- Family Pythonidae: pythons
- Family Bolyeriidae: Round Island boas
- Family Cylindrophiidae: Asian pipe snakes
- Family Loxocemidae: Mexican burrowing pythons
- Family Tropidophiidae
- Family Ungaliophiidae: dwarf boas
- Family Uropeltidae: Shield-tail Snakes
- Family Xenopeltidae: Sunbeam snakes
- Family Acrochordidae: file snakes
- Superfamily Xenophidia (Colubroidea = Caenophidia)
- Family Atractaspididae: mole vipers
- Family Colubridae: colubrids (over half of all snake species, such as garter snake, rat snake, milk snake, king snake)
- Family Elapidae: cobras, kraits, coral snakes, Australian copperheads
- Family Hydrophiidae: sea snakes
- Family Viperidae: vipers and pit vipers (e.g. rattlesnake, American copperhead)
- Superfamily Typhlopoidea (Scolecophidia)
See also
References
- Romulus Whitaker (English edition); Tamil translation by O.Henry Francis (1996). நம்மை சுட்ரியுள்ள பாம்புகள் (Snakes around us, Tamil), National Book Trust. ISBN 8123719051.
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