Bone fossils show how snakes haveadapted to their surroundings, enabling them to become successful predators in variousecosystems. Over time, the snake skull has evolved and become more flexible, which makes it possible to swallow prey whole.[7] Snakes also lost their legs over time and developed elongated bodies, which helped them move more efficiently.
TheSquamata, ancestors of snakes and lizards, are amonophyletic group - descended from a single ancestor. It is a sister group to theTuatara.
Judged by their fossil record, squamates existed in theMesozoic era, but they had a minor place in the landecology. Three of the six lines are recorded first in the UpperJurassic, the others in theCretaceous. Probably all, certainly the lizards, arose earlier in the Jurassic.[8] TheMosasaurs of theUpper Cretaceous were by far the most successful of all the lizards, becoming the top predator in theirecosystem.
Although snakes and lizards look very different, neither is a properclade. Snakes did descend from early lizards, not once, but many different times.
Scientists have a fossil of a primitive snake from theLower Cretaceousepoch. It lived about 113 million years ago.[16] 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 wasadapted toburrowing. 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".[17] The snake came from theCrato Formation inBrazil, and lay in a private collection for many years. It was re-discovered in a museum atSolnhofen,Bavaria.
Most snakes live on the ground, and in the trees. Others live in the water, and a few live under thesoil. Like other reptiles, snakes areectotherms. They control their body temperature by moving in and out of the direct sunshine. That is why they are rare in cold places.[19]
Most snakes are nonvenomous. Those that havevenom 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.
Many species haveskulls with more joints than the skulls of theirlizard ancestors. This allows the snakes to swallowprey much larger than their heads. The bones of the head and jaws can move apart to let large prey move into their body. Thethroat,stomach andintestines can also expand. In this way, a thin-looking snake canswallow anddigest a larger animal.
Their long, slender body has some special features.[18] They have overlappingscales which protect them, and help them move and climb trees. The scales have colours which may becamouflage orwarning colours.
In their narrow bodies, snakes' paired organs (such askidneys) appear one in front of the other instead of side by side. Most have only one workinglung. Some species have kept apelvic girdle with a pair ofvestigial claws on either side of thecloaca. They have no eyelids or external ears. They canhiss, but otherwise make no vocal sounds.[verification needed]
Snakes need to shed their skin regularly while they grow. This is calledmoulting.
Snakes shed their skin by rubbing their head against something rough and hard, like a piece of wood or arock. This causes theskin, which is already stretched, tosplit 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.
All snakes are carnivorous; they eat otheranimals. Some arevenomous; they injectvenom along grooves in their teeth. Other snakes are constrictors. Constrictors are not venomous, so they squeeze their prey to death. All snakesswallow theirfood whole, and they cannot chew.[25]
Because snakes arecold-blooded, they do not have to eat as regularly asmammals. 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 lowerjaw; its two halves are not rigidly attached. Neither are many otherjoints in theirskulls. This enables snakes to open their mouths wide enough to swallow their prey whole, even if the prey is larger indiameter than the snake itself.[26]
A water snake using lateral undulation to move through water
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.[27][28]
Lateral undulation is a snake's only way of moving inwater, though snakes commonly use it on land.[28] In this mode, the body of the snake alternately bends to the left and right, resulting in a series of rearward-moving "waves".[27]
While this movement appears rapid, snakes have rarely been seen moving faster than two body-lengths per second, often much less.[29] A snake moving this way burns as manycaloriespermetre as a running lizard does.[30]
Terrestrial lateral undulation is the most common mode of moving for most snake species.[27] In this mode, the posteriorly moving waves push against contact points in theenvironment, such asrocks,twigs, and irregularities in thesoil.[27] The movement is just the same over sand and in water. The movement makes a force against the ground, sand, or water. This gives forwardthrust.[31]
The speed of the movement depends upon thedensity of push-points in the environment.[29] 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 densevegetation, small openings,sand, and water all quite well.[31]
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.[32] They achieve thrust by pushing their body against the water.
Studies show that thepattern ofmuscle activation is somewhat different in lateral undulation on land compared to in water.[33] 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).[27]
A Mojaverattlesnake (Crotalus scutulatus) sidewinding
This is most often used by colubroid snakes (colubrids,elapids, andvipers). They use it in environments where there is nothing firm to push against, like a slickmud flat or asand 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.[34][35] 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.[34]
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 1⁄3 of the cost for a lizard or normal snake to move the same distance.[30]
When push-points are absent, but the space is too narrow for sidewinding (like in tunnels), snakes rely onconcertina moving.[27][35] 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.[34] The front portion then flexes and forms ananchor 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.[30] 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.
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.[36] 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.[36] The ribs of the snake do not move in this mode of moving.
Largepythons,boas, andvipers use this method when stalking prey across open ground, because it makes the snake's movements more subtle and harder for prey to detect.[34]
The movement of snakes in trees has only recently been studied.[37] While ontree branches, snakes use several modes of moving depending on species andbark texture.[37] In general, snakes will use a modified form of concertina moving on smooth branches, but will laterally undulate if contact points are available.[37] Snakes move faster on small branches and when contact points are present. This differs fromlimbed animals, which do better on large branches with little 'clutter'.[37]
Gliding snakes (Chrysopelea) insoutheast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees.[34][38][39] These snakes can perform a controlled glide for hundreds of feet depending upon launchaltitude and can even turn in midair.[34][38]
↑Vidal, Nicolas & S. Blair Hedges 2009. The molecular evolutionary tree of lizards, snakes, and amphisbaenians.Comptes rendus biologies332, (2) 129-139.[1]Archived 2013-10-30 at theWayback Machine
↑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 biology13, (1) 93.
↑Wiens, John J.et al 2012. Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species.Biology letters8, (6) 1043-1046.
↑29.029.1Hekrotte, Carlton (1967). "Relations of body temperature, size, and crawling speed of the Common Garter Snake,Thamnophis s. sirtalis".Copeia.23 (4):759–763.doi:10.2307/1441886.JSTOR1441886.
↑Gray, J; Lissman (1953). "Undulatory propulsion".Quarterly Journal of Microscopical Science.94:551–578.
↑Jayne, B.C. (1988). "Muscular mechanisms of snake locomotion: an electromyographic study of lateral undulation of the Florida banded water snake (Nerodia fasciata) and the yellow rat snake (Elaphe obsoleta)".Journal of Morphology.197 (2):159–181.doi:10.1002/jmor.1051970204.PMID3184194.S2CID25729192.
↑37.037.137.237.3Astley, H.C.; Jayne, B.C. (2007). "Effects of perch diameter and incline on the kinematics, performance and modes of arboreal locomotion of corn snakes (Elaphe guttata)".Journal of Experimental Biology.210 (Pt 21):3862–3872.doi:10.1242/jeb.009050.PMID17951427.S2CID18129284.
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