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Friday, 8 July 2011

Snake

Snake


Snakes
Temporal range: Early Cretaceous – Recent,
112–0 Ma
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Coast garter snake,
Thamnophis elegans terrestris
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Superfamily: Varanoidea
(unranked): Pythonomorpha
Suborder: Serpentes
Linnaeus, 1758
Subgroups
  • Alethinophidia – Nopcsa, 1923
  • Scolecophidia – Cope, 1864
World range of snakes
(rough range of sea snakes in blue)
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.
Living snakes are found on every continent except Antarctica and on most islands. Fifteen families are currently recognized, comprising 456 genera and over 2,900 species. They range in size from the tiny, 10 cm-long thread snake to pythons and anacondas of up to 7.6 metres (25 ft) in length. The fossil species Titanoboa cerrejonensis was 15 metres (49 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the mid-Cretaceous period, and the earliest known fossils date to around 112 Ma ago. The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma ago).
Most species are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by constriction.

Etymology

Etymology

The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. German Schnake "ring snake," Swedish snok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl, creep," which also gave sneak as well as Sanskrit nāgá "snake."[3] The word ousted adder, as adder went on to narrow in meaning, though in Old English næddre was the general word for snake.[4] The other term, serpent, is from French, ultimately from Indo-European *serp- (to creep),[5] which also gave Greek érpo (ερπω) "I crawl."

Evolution

A phylogenetic overview of the extant groups
Modern snakes
Scolecophidia

Leptotyphlopidae

 

Anomalepididae


Typhlopidae



Alethinophidia

Anilius

Core Alethinophidia
Uropeltidae

Cylindrophis

 

Anomochilus


Uropeltinae



Macrostomata
Pythonidae

Pythoninae


Xenopeltis


Loxocemus


Caenophidia

Colubridae


Acrochordidae


Atractaspididae


Elapidae


Hydrophiidae


Viperidae


Boidae

Erycinae


Boinae


Calabaria



Ungaliophiinae



Tropidophiinae




Note: the tree only indicates relationships, not evolutionary branching times.[6]
The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, making fossilization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the Cretaceous period.[7] The earliest known snake fossils come from sites in Utah and Algeria, represented by the genera Coniophis and Lapparentophis, respectively. These fossil sites have been tentatively dated to the Albian or Cenomanian age of the late Cretaceous, between 112 and 94 Ma ago. However, an even older age has been suggested for one of the Algerian sites, which may be as old as the Aptian, 125-112 Ma ago.[8]

Internal organs

Internal organs

1: esophagus 2: trachea 3:tracheal lungs 4: rudimentary left lung 4: right lung 6: heart 7: liver 8 stomach 9: air sac 10: gallbladder 11: pancreas 12: spleen 13: intestine 14: testicles 15: kidneys
Anatomy of a snake. 1 esophagus, 2 trachea, 3 tracheal lungs, 4 rudimentary left lung, 5 right lung, 6 heart, 7 liver, 8 stomach, 9 air sac, 10 gallbladder, 11 pancreas, 12 spleen, 13 intestine, 14 testicles, 15 kidneys.
The snake's heart is encased in a sac, called the pericardium, located at the bifurcation of the bronchi. The heart is able to move around, however, owing to the lack of a diaphragm. This adjustment protects the heart from potential damage when large ingested prey is passed through the esophagus. The spleen is attached to the gall bladder and pancreas and filters the blood. The thymus gland is located in fatty tissue above the heart and is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is also unique for the presence of a renal portal system in which the blood from the snake's tail passes through the kidneys before returning to the heart.[26]
The vestigial left lung is often small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin.[26] In the majority of species, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion that does not function in gas exchange.[26] This 'saccular lung' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species.[26] Many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other.[26]
Snakes have no lymph nodes.[26]

Venom

Venom

Milk snakes are often mistaken for coral snakes, whose venom is deadly to humans.
Cobras, vipers, and closely related species use venom to immobilize or kill their prey. The venom is modified saliva, delivered through fangs.[9]:243 The fangs of 'advanced' venomous snakes like viperids and elapids are hollow 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, their role in self-defense is secondary.[9]:243
Venom, like all salivary secretions, is a predigestant that initiates the breakdown of food into soluble compounds, facilitating proper digestion. Even nonvenomous snake bites (like any animal bite) will cause tissue damage.[9]:209
Certain birds, mammals, and other snakes (such as kingsnakes) that prey on venomous snakes have developed resistance and even immunity to certain venoms.[9]: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.[37] 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 small local predators (such as crows and foxes).[38]

Behavior

Behavior

Feeding and diet

Snake eating a rodent.
Carpet python constricting and consuming a chicken.
All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insectsBecause snakes cannot bite or tear their food to pieces, they must swallow prey whole. The body size of a snake has a major influence on its eating habits. Smaller snakes eat smaller prey. Juvenile pythons might start out feeding on lizards or mice and graduate to small deer or antelope as an adult, for example.
African egg-eating snake.
The snake's jaw is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, snakes 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,[41] as snakes do not chew. For example, the African egg-eating snake has flexible jaws adapted for eating eggs much larger than the diameter of its head.[9]:81 This snake has no teeth, but does have bony protrusions on the inside edge of its spine, which it uses to break shells when it eats eggs.[9]:81
While the majority of snakes eat a variety of prey animals, there is some specialization by some species. King cobras and the Australian bandy-bandy consume other snakes. Pareas iwesakii and other snail-eating colubrids of subfamily Pareatinae have more teeth on the right side of their mouths than on the left, as the shells of their prey usually spiral clockwise[9]:184[42]
Some snakes have a venomous bite, which they use to kill their prey before eating it Other snakes kill their prey by constriction.[41] Still others swallow their prey whole and alive.[9]:81[41]
After eating, snakes become dormant while the process of digestion takes place.[44] Digestion is an intense activity, especially after consumption of large prey. In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy. The digestive system is then 'up-regulated' to full capacity within 48 hours of prey consumption. Being ectothermic (“cold-blooded”), the surrounding temperature plays a large role in snake digestion. The ideal temperature for snakes to digest is 30 °C (86 °F). So much metabolic energy is involved in a snake's digestion that in the Mexican rattlesnake (Crotalus durissus), surface body temperature increases by as much as 1.2 °C (2.2 °F) during the digestive process.[45] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient, with the snake's digestive enzymes dissolving and absorbing everything but the prey's hair (or feathers) and claws, which are excreted along with waste.

Interactions with humans

Interactions with humans

Most common symptoms of any kind of snake bite poisoning Furthermore, there is vast variation in symptoms between bites from different types of snakes.[63]

Bite

Vipera berus, one fang in glove with a small venom stain, the other still in place.
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, nonvenomous snakes are not a threat to humans. The bite of nonvenomous 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 nonvenomous snake, venomous snakes present far greater hazard to humans.[9]:209
Documented deaths resulting from snake bites are uncommon. Nonfatal 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. Australia averages only one fatal snake bite per year. In India, 250,000 snakebites are recorded in a single year, with as many as 50,000 recorded initial deaths.[66]