Snakes
A snake is a scaled, limbless, elongate, carnivorous reptile. Snakes make up the entire suborder Serpentes, alongside lizards in the order Squamata (Latin: "scaled"). The 2,900 species of snakes spread across every continent except Antarctica. They range in size from the tiny thread snake to pythons and anacondas over 10 meters long. In order to accommodate snakes' narrow bodies, paired organs (such as kidneys) appear one in front of the other instead of side by side. A snake has only one lung functional for breathing. Snakes lack external ears. All snakes are strict carnivores.
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Of the twenty biological families of snakes, two are venomous with some venomous members found in a third family. Although the majority of venomous snakes are small innocuous creatures, many are capable of causing painful injury or death to humans. Venom in snakes, however is more for killing and subduing prey than it is for self-defense.
Snakes may have evolved from a lizard which adapted to burrowing during the Cretaceous period. The diversity of modern snakes appeared during the Paleocene period. In Christianity, the serpent is often identified with the devil, as in the Biblical account of Adam and Eve.
Phylogeny of snakes is poorly known because snake skeletons are typically small and fragile, making fossilization uncommon. However 150 million year old specimens readily definable as snakes with lizard-like skeletal structures have been uncovered in South America and Africa. It has been agreed, on the basis of morphology, that snakes descended from lizard-like ancestors.
An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was 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 their bodies became more streamlined for burrowing. According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to combat subterranean conditions such as scratched corneas and dirt in the ears with snakes re-emerged onto the surface of the earth 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, have vestigial hind limbs: tiny, clawed digits known as anal spurs which are used to grasp during mating. Leptotyphlopidae and Typhlopidae are other examples where remnants of the pelvic girdle are still present, sometimes appearing as horny projections when visible. The frontal limbs in all snakes are non-existent 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. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and 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), making most of the snake's skeleton being composed 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.
An alternative hypothesis, based on morphology, suggests that the ancestors of snakes 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, although some of these features are shared with varanids. In recent years, genetic studies have indicated that snakes are not as closely related to monitor lizards as it was once believed, and 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, correlating with the adaptive radiation of mammals following the extinction of the dinosaurs. There are over 2,900 species of snakes ranging as far northward as the Arctic Circle in Scandanavia and southward through Australia and Tasmania. Snakes can be found on every continent with the exception of Antarctica dwelling in the sea and as high as 16,000 feet (4900m)in the Himalayan Mountains of Asia. There are numerous islands from which snakes are conspicuously absent such as Ireland, Iceland, and New Zealand.
All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails 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, allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself. The African Egg-eating Snake has flexible jaws adapted for eating eggs much larger than the diameter of its head.
After eating, snakes become torpid while the process of digestion takes place. Digestion is an intense 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. Being ectothermic or cold blooded, the surrounding temperature plays a large role in a snakes digestion. 30 degrees celsius is the ideal temperature for snakes to digest their food. 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. 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 die from trying to swallow an animal that is too big.
The skin of a snake is covered in scales. Contrary to the popular notion of snakes being slimy because of possible confusion of snakes with worms, snakeskin has a smooth, dry texture. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. Snake's eyelids are transparent "spectacle" scales which remain permanently closed, also known as brille.
The shedding of scales is called ecdysis, or, in normal usage moulting or sloughing. In the case of snakes, the complete outer layer of skin is shed in one layer. Snake scales are not discrete but extensions of the epidermis hence they are not shed separately, but are ejected as a complete contiguous outer layer of skin during each moult, akin to a sock being turned inside out. Moulting serves a number of functions - firstly, the old and worn skin is replaced, secondly, it helps get rid of parasites such as mites and ticks. Renewal of the skin by moulting is supposed to allow growth in some animals such as insects, however this view has been disputed in the case of snakes.
Moulting is repeated periodically throughout a snake's life. Before a moult, the snake stops eating and often hides or moves to a safe place. Just prior to shedding, the skin becomes dull and dry looking and the eyes become cloudy or blue-colored. The inner surface of the old outer skin liquefies. This causes the old outer skin to separate from the new inner skin. After a few days, the eyes clear and the snake "crawls" out of its old skin. The old skin breaks near the mouth and the snake wriggles out aided by rubbing against rough surfaces. In many cases the cast skin peels backward over the body from head to tail, in one piece like an old sock. A new, larger, and brighter layer of skin has formed underneath.
An older snake may shed its skin only once or twice a year, but a younger, still-growing snake, may shed up to four times a year. The discarded skin gives a perfect imprint of the scale pattern and it is usually possible to identify the snake if this discard is reasonably complete and intact. Although the primary purpose of shedding is for the snake's growth; it 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.
The shape and number of scales on the head, back and belly are characteristic to family, genus and species. Scales have a nomenclature analogous to the position on the body. 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.
Scalation counts are also used to tell the sex of a snake when the species is not readily sexually dimorphic. A probe is inserted into the cloaca until it can go no further. The probe is marked at the point where it stops, removed, and compared to the subcaudal depth by laying it alongside the scales. The scalation count determines whether the snake is a male or female as hemipenes of a male will probe to a different depth (usually shorter) than the cloaca of a female.
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. Most pythons coil around their egg-clutches after they have laid them and remain with the eggs until they hatch. The female python will not leave the eggs, except to occassionally bask in the sun or drink water and will generate heat to incubate the eggs by shivering.
Some species of snake 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 boa constrictor and 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 most often associated with colder environments, as the retention of the young within the female.
Snakes do not 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 are 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 more of a hazard to humans.
In some cultures, the consumption of snakes is acceptable, or even considered a delicacy, prized for its alleged pharmaceutical effect of warming the heart. Western cultures document the consumption of snakes under extreme circumstances of hunger. Cooked rattlesnake meat is an exception, which is commonly consumed in the Western United States and referred to as "Prarie Chicken". In Asian countries such as Thailand,Indonesia, and Cambodia, 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. One example of this is the Habu snake sometimes placed in the Okinawan liquor Awamori also known as "Habu Sake".
In Islam, 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 Life. The snake returns in Exodus when Moses, as a sign of God's power, turns his staff 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. Christianity believes this to be a 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). Elsewhere Jesus instructed his disciples to be as shrewd as snakes and as innocent as doves (Matthew 10:16). 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.
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