Snake

Snakes, otherwise known as serpentes, are reptiles. They are part of the order Squamata. They are carnivores, with long narrow bodies and no legs. There are at least 20 families, about 500 genera and 3,400 species of snake.snake species list at the Reptile Database. Accessed 22 May 2012.

 Evolution 
The earliest known snake fossils are from the Middle Jurassic to Lower Cretaceous periods.Vidal N. et al. 2009. Snakes (Serpentes). In Hedges S.B. and Kumar S. (eds) The timetree of life. Oxford University Press, 390-397. Scientists think snakes evolved from lizard-like ancestors around 100 million years ago.  A wide range of snakes appeared during the Paleocene period (c 66 to 56 million years ago).

Bone fossils show how snakes have adapted to their surroundings, enabling them to become successful predators in various ecosystems. Over time, the snake skull has evolved and become more flexible, which makes it possible to swallow prey whole. Snakes also lost their legs over time and developed elongated bodies, which helped them move more efficiently. 

 Not a clade 
The Squamata, ancestors of snakes and lizards, are a monophyletic group - descended from a single ancestor. It is a sister group to the Tuatara. 

Judged by their fossil record, squamates existed in the Mesozoic era, but they had a minor place in the land ecology. Three of the six lines are recorded first in the Upper Jurassic, the others in the Cretaceous. Probably all, certainly the lizards, arose earlier in the Jurassic.Benton, Michael 1997. Vertebrate palaeontology. Chapman & Hall, London, 238. The Mosasaurs of the Upper Cretaceous were by far the most successful of all the lizards, becoming the top predator in their ecosystem.

Although snakes and lizards look very different, neither is a proper clade. Snakes did descend from early lizards, not once, but many different times.

There is a monophyletic clade within the Squamata. It is the Toxicofera. It includes all venomous snakes and lizards, and many related non-venomous species. The evidence for this is in recent molecular analyses.Vidal, Nicolas & S. Blair Hedges 2009. The molecular evolutionary tree of lizards, snakes, and amphisbaenians. Comptes rendus biologies 332, (2) 129-139.  Pyron R.A; Burbrink F.T. & Wiens J.J. 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC evolutionary biology 13, (1) 93.Wiens, John J. et al 2012. Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species. Biology letters 8, (6) 1043-1046.

 Fossil snakes 
Scientists have a fossil of a primitive snake from the Lower Cretaceous epoch. It lived about 113 million years ago.Martil D.M; Tischlinger H & Longrich N. 2015. A four-legged snake from the early Cretaceous of Gondwana. Science 349 (6246) 416–419. A four-legged snake from the early Cretaceous of Gondwana | Science It had rather small front and rear legs. Several other fossil snakes have been found with small rear legs, but this is the first one with all four legs. 

The snake, Tetrapodophis amplectus, lived on land and was adapted to burrowing. The researcher said there were "a lot of very advanced snake features, including its hooked teeth, flexible jaw and spine – and even snake-like scales. And there's the gut contents – it's swallowed another vertebrate. It was preying on other animals, which is a snake feature".Webb, Jonathan 2015. Four-legged snake ancestor 'dug burrows'. BBC News Science & Environment. Four-legged snake ancestor 'dug burrows' - BBC News The snake came from the Crato Formation in Brazil, and lay in a private collection for many years. It was re-discovered in a museum at Solnhofen, Bavaria.

 Habitat 
Most snake species live in the tropics. Few live beyond the Tropic of Cancer or Tropic of Capricorn. Only one species, the common viper (Vipera berus), lives beyond the Arctic Circle.

Most snakes live on the ground, and in the trees. Others live in the water, and a few live under the soil. Like other reptiles, snakes are ectotherms. They control their body temperature by moving in and out of the direct sunshine. That is why they are rare in cold places. 

 Venom 
Most snakes are nonvenomous. Those that have venom use it mainly to kill and subdue prey rather than for self-defense. Some have venom potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by squeezing.

There are three major families of snakes:

 Elapids are all venomous. They include cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes.
 Viperids are all venomous. They include vipers, rattlesnakes, copperheads/cottonmouths, and bushmasters.
 Some colubrids are venomous. They include "rear-fanged snakes" like boomslangs, tree snakes, vine snakes, and mangrove snakesMehrtens, John 1987. Living snakes of the world in color. New York: Sterling. 

 Anatomy 
Snakes range in size from the tiny, 10.4 cm (4 inch)-long thread snake to the reticulated python of  in length. The extinct snake Titanoboa was  long.

 Bones and joints 
Many species have skulls with more joints than the skulls of their lizard ancestors. This allows the snakes to swallow prey much larger than their heads. The bones of the head and jaws can move apart to let large prey move into their body. The throat, stomach and intestines can also expand. In this way, a thin-looking snake can swallow and digest a larger animal.

 Long body 
Their long, slender body has some special features. They have overlapping scales which protect them, and help them move and climb trees. The scales have colours which may be camouflage or warning colours.

In their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side. Most have only one working lung. Some species have kept a pelvic girdle with a pair of vestigial claws on either side of the cloaca. They have no eyelids or external ears. They can hiss, but otherwise make no vocal sounds.

 Shedding 
Snakes need to shed their skin regularly while they grow. This is called moulting. 

Snakes shed their skin by rubbing their head against something rough and hard, like a piece of wood or a rock. This causes the skin, which is already stretched, to split open. The snake keeps on rubbing its skin on various rough objects until the skin peels off from its head. This lets it crawl out, turning the skin inside out.

 Feeding 
All snakes are carnivorous; they eat other animals. Some are venomous; they inject venom along grooves in their teeth. Other snakes are constrictors. Constrictors are not venomous, so they squeeze their prey to death. All snakes swallow their food whole, and they cannot chew. 

Because snakes are cold-blooded, they do not have to eat as regularly as mammals. People who own pet snakes feed them as infrequently as once per month. Some snakes can go as long as six months without a good meal.

Snakes have a very flexible lower jaw; its two halves are not rigidly attached. Neither are many other joints in their skulls. This enables snakes to open their mouths wide enough to swallow their prey whole, even if the prey is larger in diameter than the snake itself.

 Movement 
Snakes can move even without arms and legs. They have developed several different ways of moving to deal with particular environments. Each type of snake movement is distinct from the others.Cogger(1991), p. 175.

 Lateral undulation 
Lateral undulation is a snake's only way of moving in water, though snakes commonly use it on land. In this mode, the body of the snake alternately bends to the left and right, resulting in a series of rearward-moving "waves". 

While this movement appears rapid, snakes have rarely been seen moving faster than two body-lengths per second, often much less. A snake moving this way burns as many calories per metre as a running lizard does.

 Terrestrial 
Terrestrial lateral undulation is the most common mode of moving for most snake species. In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, and irregularities in the soil. The movement is just the same over sand and in water. The movement makes a force against the ground, sand, or water. This gives forward thrust. 

The speed of the movement depends upon the density of push-points in the environment. The wave speed is the same as the snake's speed, and as a result, every point on the snake's body follows the path of the point ahead of it. Snakes move through dense vegetation, small openings, sand, and water all quite well.

 Aquatic 


Snakes move forward in water by moving their bodies in a wave-like motion. The waves become larger as they move down the snake's body, and the wave travels backwards faster than the snake moves forwards. They achieve thrust by pushing their body against the water. 

Studies show that the pattern of muscle activation is somewhat different in lateral undulation on land compared to in water.  All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the motion (moving backwards with forward-moving waves).

 Sidewinding 


This is most often used by colubroid snakes (colubrids, elapids, and vipers). They use it in environments where there is nothing firm to push against, like a slick mud flat or a sand dune. 

Sidewinding is a modified form of lateral undulation in which all of the body segments oriented in one direction remain in contact with the ground, while the other segments are lifted up. This results in a peculiar "rolling" motion.Cogger(1991), p. 177. This mode of moving overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping. 

The static nature of the contact points can be shown from the tracks of a sidewinding snake, which show each belly scale imprint, without any smearing. This mode of moving has very low caloric cost, less than  of the cost for a lizard or normal snake to move the same distance.

 Concertina 
When push-points are absent, but the space is too narrow for sidewinding (like in tunnels), snakes rely on concertina moving. In this mode, the snake braces the back part of its body against the tunnel wall while the front of the snake extends and straightens. The front portion then flexes and forms an anchor point, and the back part is straightened and pulled forwards. 

This mode of moving is slow and very demanding, needing up to seven times the energy of laterally undulating over the same distance. This high cost is due to the repeated stops and starts of portions of the body as well as the need to use the muscles to brace against the tunnel walls.

 Rectilinear 
The slowest mode of snake moving is rectilinear moving, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning.Cogger (1991), p. 176. In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them. Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin. The ribs of the snake do not move in this mode of moving.

Large pythons, boas, and vipers use this method when stalking prey across open ground, because it makes the snake's movements more subtle and harder for prey to detect.

 Other 
The movement of snakes in trees has only recently been studied. While on tree branches, snakes use several modes of moving depending on species and bark texture. In general, snakes will use a modified form of concertina moving on smooth branches, but will laterally undulate if contact points are available. Snakes move faster on small branches and when contact points are present. This differs from limbed animals, which do better on large branches with little 'clutter'.

Gliding snakes (Chrysopelea) in southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees.Freiberg (1984), p. 135. These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair.

 Related pages 
 Echidna (mythology)
 Ouroboros
 60 Most Popular ball python morphs 
 Top 100 Most Popular Non-Venomous snakes in the world 
 Cute Baby Snakes 

 References 

 Other websites 

 Snake (organism)  -Citizendium
 Snakes world  by Marie Curie

Category:Basic English 850 words

Category:Venomous animals
Category:Reptiles