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Serpentes (Ophidia)


 

Origins of snakes

Serpentes or 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.

The phylogeny of snakes is poorly known. Snake skeletons are fragile and unlikely to fossilize. However, it is generally assumed that snakes derived from lizard-like ancestors.

The likeliest ancestors are thought to be mosasaurs - extinct aquatic reptiles from the Cretaceous - which in turn are thought to have derived from varanid lizards. Features such as lack of eyelids and lack of external ears are thought to have evolved to combat marine conditions, leading to a snake similar in appearance to sea snakes of today. In the Late Cretaceous, snakes would reemerge from the ocean onto land as they are today. The best fossil snake remains, from Late Cretateous marine sediments, support this theory. 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.

A secondary hypothesis is that snakes directly evolved from varinids similar to the earless monitor of Borneo. Like this monitor, theoretical ancestors would have been land-burrowers. As these ancestors became more subterranean, they lost their limbs and became more streamlined for burrowing. Features such as lack of eyelids and external ears, according to this theory, evolved to combat subterranean conditions. In the Late Cretaceous, snakes would, as with the marine origin theory, re-emerge onto land as they are today.

Fragmentary remains that have been found from the Early Cretaceous may refute either theory, though. Additionally, genetic tests have been performed that show no close affinity to varanids and thus to mosasaurs either. These tests did not show to which lizard family snakes more closely align, either, allowing for more debate.

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Characteristics of snakes

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. 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.

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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. Contrary to the popular myth, at no point do they "unhinge" their jaws (disarticulate their mandibular joints).

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 Crotalus durissus, the Mexican rattlesnake, may actually raise its body temperature as much as 6 degrees above the surrounding environment. 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.

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, most snakes are non-venomous or have venom that is not harmful to humans.

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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.

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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.

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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.

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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.

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Author: Koen Stein
Last updated: 21/11/2005
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