View source for Biology:Snake

{{Short description|Limbless, scaly, elongate reptile}}
{{pp-semi|small=yes}}
{{Automatic taxobox
| name = Snakes
| fossil_range = <br/>[[Earth:Late Cretaceous|Late Cretaceous]] – [[Earth:Holocene|Present]],<ref name=snake_origin_ecology/> {{Fossil range|94|0|earliest=112|latest=|PS=}}
<imagemap>
File:Snakes Diversity.jpg|250px

rect 0 0 325 225 Horned rattlesnake
rect 0 120 325 450 [[Biology:Southern hognose snake|Southern hognose snake]]
rect 0 240 325 700 [[Biology:Blue krait|Blue krait]]
rect 0 380 325 900 [[Biology:Emerald tree boa|Emerald tree boa]]
rect 0 500 325 1100 Sri Lanka cat snake
rect 0 760 325 1400 Ringneck snake

rect 143 0 660 225 Striped house snake
rect 143 100 660 460 Blunthead tree snake
rect 143 200 660 700 [[Biology:Corn snake|Corn snake]]
rect 143 300 660 920 [[Biology:Indian cobra|Indian cobra]]
rect 143 400 660 1150 [[Biology:Grass snake|Grass snake]]
rect 143 500 660 1500 [[Biology:Pacific gopher snake|Pacific gopher snake]]

rect 286 0 1100 225 Green vine snake
rect 286 100 1100 470 [[Biology:Coral snake|Coral snake]]
rect 286 200 1100 690 [[Biology:Green tree python|Green tree python]]
rect 286 300 1100 930 Spiny bush viper
rect 286 400 1100 1150 False coral snake
rect 286 500 1100 1500 Puffing snake

</imagemap>
| taxon = Serpentes
| authority = [[Biography:Carl Linnaeus|Linnaeus]], [[Biology:10th edition of Systema Naturae|1758]]
| range_map = World distribution of snakes.svg
| range_map_caption = <small>Approximate world distribution of snakes, all species</small>
| subdivision_ranks = Infraorders
| subdivision =
* [[Biology:Alethinophidia|Alethinophidia]] <small>Nopcsa, 1923</small>
* [[Biology:Scolecophidia|Scolecophidia]] <small>Cope, 1864</small>
}}

'''Snakes''' are elongated, [[Biology:Limbless vertebrate|limbless]], [[Biology:Carnivore|carnivorous]] [[Biology:Reptile|reptile]]s of the suborder '''Serpentes''' {{IPAc-en|s|3r|'|p|E|n|t|iː|z}}.<ref>{{cite journal | vauthors = Reeder TW, Townsend TM, Mulcahy DG, Noonan BP, Wood PL, Sites JW, Wiens JJ | title = Integrated analyses resolve conflicts over squamate reptile phylogeny and reveal unexpected placements for fossil taxa | journal = PLOS ONE | volume = 10 | issue = 3 | pages = e0118199 | year = 2015 | pmid = 25803280 | pmc = 4372529 | doi = 10.1371/journal.pone.0118199 | bibcode = 2015PLoSO..1018199R | doi-access = free }}</ref> Like all other [[Biology:Squamata|squamates]], snakes are ectothermic, [[Biology:Amniote|amniote]] [[Biology:Vertebrate|vertebrate]]s covered in overlapping [[Biology:Scale (zoology)|scales]]. Many species of snakes have [[Biology:Skull|skull]]s with several more joints than their [[Biology:Lizard|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 [[Medicine:Lung|lung]]. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the [[Biology:Cloaca|cloaca]]. Lizards have evolved elongate bodies without limbs or with greatly reduced limbs about twenty-five times independently via [[Biology:Convergent evolution|convergent evolution]], leading to many lineages of [[Biology:Legless lizard|legless lizard]]s.<ref>{{cite journal | vauthors = Wiens JJ, Brandley MC, Reeder TW | title = Why does a trait evolve multiple times within a clade? Repeated evolution of snakelike body form in squamate reptiles | journal = Evolution; International Journal of Organic Evolution | volume = 60 | issue = 1 | pages = 123–41 | date = January 2006 | pmid = 16568638 | doi = 10.1554/05-328.1 | s2cid = 17688691 }}</ref> These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal (see [[Biology:Amphisbaenia|Amphisbaenia]], [[Biology:Dibamidae|Dibamidae]], and [[Biology:Pygopodidae|Pygopodidae]]).

Living snakes are found on every continent except Antarctica, and on most smaller land masses; exceptions include some large islands, such as Ireland, Iceland, Greenland, the [[Earth:Hawaiian Islands|Hawaiian archipelago]], and the islands of New Zealand, as well as many small islands of the Atlantic and central Pacific oceans.<ref name=Bauchot>{{cite book|editor=Roland Bauchot|title=Snakes: A Natural History|url=https://archive.org/details/snakesnaturalhis0000bauc|url-access=registration|year=1994|publisher=Sterling Publishing Co., Inc.|location=New York|isbn=978-1-4027-3181-5|page=[https://archive.org/details/snakesnaturalhis0000bauc/page/220 220]}}</ref> Additionally, [[Biology:Hydrophiinae|sea snakes]] are widespread throughout the Indian and Pacific oceans. More than twenty families are currently recognized, comprising about 520 [[Biology:Genus|genera]] and about 3,900 [[Biology:Species|species]].<ref name="NRDB-Cs">{{cite web|title=Search results for Higher taxa: snake|url=https://reptile-database.reptarium.cz/advanced_search?taxon=snake&submit=Search|url-status=live|archive-url=|archive-date=|access-date=7 March 2021|website=reptile-database.org}}</ref> They range in size from the tiny, {{convert|10.4|cm|in|abbr=on|adj=mid|-long}} [[Biology:Barbados threadsnake|Barbados threadsnake]]<ref name="zootaxa"/> to the reticulated python of {{convert|6.95|m|ft|sp=us}} in length.<ref name="SunBear">{{cite journal|author=Fredriksson, G. M.|title=Predation on Sun Bears by Reticulated Python in East Kalimantan, Indonesian Borneo|journal=Raffles Bulletin of Zoology|volume=53|issue=1|year=2005|pages=165–168|url=http://dare.uva.nl/document/161117|url-status=live|archive-url=https://web.archive.org/web/20140709001708/http://dare.uva.nl/document/161117|archive-date=2014-07-09}}</ref> The fossil species ''Titanoboa cerrejonensis'' was {{convert|12.8|m|ft|sp=us}} long.<ref name="head"/> Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during the [[Earth:Jurassic|Jurassic]] period, with the earliest known fossils dating to between 143 and 167&nbsp;Ma ago.<ref>{{cite web | last = Perkins | first = Sid | name-list-style = vanc | date = 27 January 2015 | title = Fossils of oldest known snakes unearthed | url = http://news.sciencemag.org/paleontology/2015/01/fossils-oldest-known-snakes-unearthed | website = news.sciencemag.org | access-date = 29 January 2015 | url-status = live | archive-url = https://web.archive.org/web/20150130043614/http://news.sciencemag.org/paleontology/2015/01/fossils-oldest-known-snakes-unearthed | archive-date = 30 January 2015 }}</ref><ref>{{cite journal | vauthors = Caldwell MW, Nydam RL, Palci A, Apesteguía S | title = The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution | journal = Nature Communications | volume = 6 | pages = 5996 | date = January 2015 | pmid = 25625704 | doi = 10.1038/ncomms6996 | number = 5996 | bibcode = 2015NatCo...6.5996C | doi-access = free }}</ref> The diversity of modern snakes appeared during the [[Earth:Paleocene|Paleocene]] epoch ({{circa}}&nbsp;66 to 56&nbsp;Ma ago, after the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]]). The oldest preserved descriptions of snakes can be found in the Brooklyn Papyrus.

Most species of snake are nonvenomous and those that have [[Biology:Venom|venom]] use it primarily to kill and subdue prey rather than for self-defense. Some possess venom that is potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by [[Biology:Constriction|constriction]].

==Etymology==
The English word ''snake'' comes from [[Social:Old English|Old English]] {{lang|ang|snaca}}, itself from Proto-Germanic {{lang|gem-x-proto|*snak-an-}} (cf. Germanic {{lang|gem|Schnake}} 'ring snake', Swedish {{lang|sv|snok}} 'grass snake'), from [[Social:Proto-Indo-European language|Proto-Indo-European]] root {{lang|ine-x-proto|*(s)nēg-o-}} 'to crawl to creep', which also gave ''sneak'' as well as [[Social:Sanskrit|Sanskrit]] {{transl|sa|nāgá}} 'snake'.<ref>Proto-IE: {{lang|ine-x-proto|*(s)nēg-o-}}, Meaning: snake, Old Indian: nāgá- m. 'snake', Germanic: {{lang|gem|*snēk-a-}} m., {{lang|gem|*snak-an-}} m., {{lang|gem|*snak-ō}} f.; {{lang|gem|*snak-a-}} vb., Russ. meaning: {{lang|ru|жаба}} ({{lang|ru|змея}}), References: WP ({{lang|de|Vergleichendes Wörterbuch der indogermanischen Sprachen}}) II 697 f.</ref> The word ousted ''adder'', as ''adder'' went on to narrow in meaning, though in Old English {{lang|ang|næddre}} was the general word for snake.<ref>{{cite web|url=https://www.etymonline.com/word/snake |title=snake (n.) |website=etymonline.com |archive-url=https://web.archive.org/web/20100719112253/http://www.etymonline.com/index.php?term=snake |archive-date=19 July 2010 |url-status=live |access-date=22 September 2009}}</ref> The other term, ''serpent'', is from French, ultimately from Indo-European {{lang|ine-x-proto|*serp-}} 'to creep',<ref name="serpent">{{cite web | work=Merriam-Webster Online Dictionary | title=Definition of serpent | url=http://www.merriam-webster.com/dictionary/serpent | access-date=12 October 2006 | url-status=live | archive-url=https://web.archive.org/web/20071017210917/http://www.merriam-webster.com/dictionary/serpent | archive-date=17 October 2007 }}</ref> which also gave [[Social:Ancient Greek|Ancient Greek]] {{lang|grc|ἕρπω}} ({{transl|grc|hérpō}}) 'I crawl'.

==Evolution==
{{Cladogram|title=A phylogenetic overview of modern snakes.
|clades={{clade|style=font-size:88.888888%;line-height:100%
 |label1=&nbsp;&nbsp;&nbsp;
 |1={{clade
  |label1=[[Biology:Scolecophidia|Scolecophidia]]
  |1={{clade
   |1=[[Biology:Leptotyphlopidae|Leptotyphlopidae]]
   |label2=&nbsp;
   |2={{clade
    |1=[[Biology:Anomalepididae|Anomalepididae]]
    |2=[[Biology:Typhlopidae|Typhlopidae]]
   }}
  }}
  |label2=[[Biology:Alethinophidia|Alethinophidia]]
  |2={{clade
  |label1=[[Biology:Amerophidia|Amerophidia]]
  |1={{clade
   |1=''[[Biology:Anilius|Anilius]]''
   |2=[[Biology:Tropidophiidae|Tropidophiidae]]
    }}
   |label2=Afrophidia
   |2={{clade
    |label1=[[Biology:Uropeltoidea|Uropeltoidea]]
    |1={{clade
     |1=[[Biology:Uropeltidae|Uropeltidae]]
     |label2=&nbsp;
     |2={{clade
      |1=''[[Biology:Anomochilus|Anomochilus]]''
      |2=''[[Biology:Cylindrophis|Cylindrophis]]''
     }}
    }}
    |label2=Macrostomata
    |2={{clade
     |label1=[[Biology:Pythonoidea|Pythonoidea]]
     |1={{clade
      |1=[[Biology:Pythonidae|Pythonidae]]
      |2=''[[Biology:Xenopeltis|Xenopeltis]]''
      |3=''[[Biology:Loxocemus|Loxocemus]]''
     }}
     |label2=[[Biology:Caenophidia|Caenophidia]]
     |2={{clade
      |1=[[Biology:Acrochordidae|Acrochordidae]]
      |2=[[Biology:Xenodermidae|Xenodermidae]]
      |3=[[Biology:Pareidae|Pareidae]]
      |4=[[Biology:Viperidae|Viperidae]]
      |5=[[Biology:Homalopsidae|Homalopsidae]]
      |6=[[Biology:Lamprophiidae|Lamprophiidae]]
      |7=[[Biology:Elapidae|Elapidae]]
      |8=[[Biology:Colubridae|Colubridae]]
     }}
     |label3=[[Biology:Booidea|Booidea]]
     |3={{clade
       |1=[[Biology:Boidae|Boidae]]
       |2=[[Biology:Erycinae|Erycinae]]
       |3=''[[Biology:Calabaria|Calabaria]]''
       |4=[[Biology:Ungaliophiinae|Ungaliophiinae]]
       |5=''[[Biology:Sanzinia|Sanzinia]]''
       |6=''[[Biology:Candoia|Candoia]]''
      }}
    }}
   }}
  }}
 }}
}}
|caption=Note: the tree only indicates relationships, not evolutionary branching times.<ref name=Lee/>
}}
The fossil record of snakes is relatively poor because snake [[Biology:Skeleton|skeleton]]s are typically small and fragile making [[Biology:Fossil|fossil]]ization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the [[Earth:Cretaceous|Cretaceous]] period.<ref>Durand, J.F. (2004). "The origin of snakes". Geoscience Africa 2004. Abstract Volume, University of the Witwatersrand, Johannesburg, South Africa, pp. 187.</ref> The earliest known true snake fossils (members of the crown group Serpentes) come from the marine [[Biology:Simoliophiidae|simoliophiid]]s, the oldest of which is the [[Earth:Late Cretaceous|Late Cretaceous]] ([[Earth:Cenomanian|Cenomanian]] age) ''Haasiophis terrasanctus'',<ref name=snake_origin_ecology>{{cite journal | vauthors = Hsiang AY, Field DJ, Webster TH, Behlke AD, Davis MB, Racicot RA, Gauthier JA | title = The origin of snakes: revealing the ecology, behavior, and evolutionary history of early snakes using genomics, phenomics, and the fossil record | journal = BMC Evolutionary Biology | volume = 15 | pages = 87 | date = May 2015 | pmid = 25989795 | pmc = 4438441 | doi = 10.1186/s12862-015-0358-5 }}</ref> dated to between 112 and 94 million years old.<ref name=timetree>Vidal, N., Rage, J.-C., Couloux, A. and Hedges, S.B. (2009). "Snakes (Serpentes)". Pp. 390–397 in Hedges, S. B. and Kumar, S. (eds.), ''The Timetree of Life''. Oxford University Press.</ref>

Based on comparative anatomy, there is consensus that snakes descended from [[Biology:Lizard|lizard]]s.<ref name="Meh87">Mehrtens JM. 1987. ''Living Snakes of the World in Color''. New York: Sterling Publishers. 480 pp. {{ISBN|0-8069-6460-X}}.</ref>{{Rp|11}}<ref name="Sanchez">{{cite web |last = Sanchez |first = Alejandro | name-list-style = vanc |title = Diapsids III: Snakes |work = Father Sanchez's Web Site of West Indian Natural History |url = http://www.kingsnake.com/westindian/metazoa12.html |access-date = 2007-11-26 |url-status = live |archive-url = https://web.archive.org/web/20071127163408/http://www.kingsnake.com/westindian/metazoa12.html |archive-date = 2007-11-27 }}
</ref> [[Biology:Pythonidae|Python]]s and [[Biology:Boidae|boa]]s—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as anal spurs, which are used to grasp during mating.<ref name="Meh87"/>{{Rp|11}}<ref name="legs"/> The families [[Biology:Leptotyphlopidae|Leptotyphlopidae]] and [[Biology:Typhlopidae|Typhlopidae]] also possess remnants of the pelvic girdle, appearing as horny projections when visible.

Front limbs are nonexistent in all known snakes. This is caused by the evolution of their [[Biology:Hox gene|Hox gene]]s, controlling limb [[Biology:Morphogenesis|morphogenesis]]. The axial skeleton of the snakes’ common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant. 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). In other words, most of a snake's skeleton is an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very reduced in number (only 2–10 lumbar and pelvic vertebrae are present), while only a short tail remains of the caudal vertebrae. However, the tail is still long enough to be of important use in many species, and is modified in some aquatic and tree-dwelling species.

Many modern snake groups originated during the [[Earth:Paleocene|Paleocene]], alongside the adaptive radiation of mammals following the extinction of (non-avian) [[Biology:Dinosaur|dinosaur]]s. The expansion of grasslands in North America also led to an explosive radiation among snakes.<ref name="Fossil Snakes of North America">{{cite book|last1=Holman|first1=J. Alan | name-list-style = vanc |title=Fossil Snakes of North America|date=2000|publisher=Indiana University Press|location=Bloomington, IN|isbn=978-0253337214|pages=284–323|edition=First}}</ref>  Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of vipers and elapids in North America and the significant diversification of [[Biology:Colubridae|Colubridae]] (including the origin of many modern genera such as [[Biology:Nerodia|Nerodia]], [[Biology:Lampropeltis|Lampropeltis]], [[Biology:Pituophis|Pituophis]], and [[Biology:Pantherophis|Pantherophis]]).<ref name="Fossil Snakes of North America"/>

===Origins===
There is fossil evidence to suggest that snakes may have evolved from burrowing lizards, during the [[Earth:Cretaceous|Cretaceous Period]].<ref name="EB">{{Cite book |last =Mc Dowell |first =Samuel | name-list-style = vanc |title =The evolution of the tongue of snakes and its bearing on snake origins |journal =Evolutionary Biology |year =1972 |volume=6 |pages = 191–273 |doi=10.1007/978-1-4684-9063-3_8|isbn =978-1-4684-9065-7 }}</ref> An early fossil snake relative, ''Najash rionegrina'', was a two-legged burrowing animal with a [[Biology:Sacrum|sacrum]], and was fully [[Biology:Terrestrial animal|terrestrial]].<ref name=najash>
{{cite journal | vauthors = Apesteguía S, Zaher H | title = A Cretaceous terrestrial snake with robust hindlimbs and a sacrum | journal = Nature | volume = 440 | issue = 7087 | pages = 1037–40 | date = April 2006 | pmid = 16625194 | doi = 10.1038/nature04413 | url = http://www.nature.com/nature/journal/v440/n7087/edsumm/e060420-11.html | url-status = live | archive-url = https://web.archive.org/web/20071218045427/http://www.nature.com/nature/journal/v440/n7087/edsumm/e060420-11.html | bibcode = 2006Natur.440.1037A | s2cid = 4417196 | archive-date = 2007-12-18 }}</ref> One extant analog of these putative ancestors is the earless monitor ''[[Biology:Lanthanotidae|Lanthanotus]]'' of Borneo (though it also is semiaquatic).<ref name="M2">{{Cite journal |last =Mertens | first =Robert | name-list-style = vanc |title =Lanthanotus: an important lizard in evolution |journal =Sarawak Museum Journal |year =1961 |volume=10 |pages = 320–322 }}</ref> Subterranean species evolved bodies streamlined for burrowing, and eventually lost their limbs.<ref name="M2"/> According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with [[Biology:Fossorial|fossorial]] difficulties, such as scratched [[Medicine:Cornea|cornea]]s and dirt in the ears.<ref name="EB"/><ref name="M2"/> Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection to the vertebrae. These include fossil species like ''[[Biology:Haasiophis|Haasiophis]]'', ''[[Biology:Pachyrhachis|Pachyrhachis]]'' and ''[[Biology:Eupodophis|Eupodophis]]'', which are slightly older than ''[[Biology:Najash|Najash]]''.<ref name="legs">{{cite web |title = New Fossil Snake With Legs |work = UNEP WCMC Database |publisher = American Association For The Advancement Of Science |location = Washington, D.C. |url = http://www.wildlifenews.co.uk/articles2000/march/march2500a.htm |access-date = 2007-11-29 |url-status = dead |archive-url = https://web.archive.org/web/20071225225126/http://www.wildlifenews.co.uk/articles2000/march/march2500a.htm |archive-date = 2007-12-25 }}
</ref>

This hypothesis was strengthened in 2015 by the discovery of a 113-million-year-old fossil of a four-legged snake in Brazil that has been named ''[[Biology:Tetrapodophis|Tetrapodophis amplectus]]''. It has many snake-like features, is adapted for burrowing and its stomach indicates that it was preying on other animals.<ref>{{cite web|title=Four-legged snake ancestor 'dug burrows'|url=https://www.bbc.co.uk/news/science-environment-33621491|first=Webb|last=Jonathan| name-list-style = vanc |date=24 July 2014|publisher=BBC Science & Environment|access-date=Jul 24, 2015|url-status=live|archive-url=https://web.archive.org/web/20150726051600/http://www.bbc.co.uk/news/science-environment-33621491|archive-date=26 July 2015}}</ref> It is currently uncertain if ''Tetrapodophis'' is a snake or another species, in the [[Biology:Squamata|squamate]] order, as a snake-like body has independently evolved at least 26 times. ''Tetrapodophis'' does not have distinctive snake features in its spine and skull.<ref>{{cite web|title = A Fossil Snake With Four Legs|url = http://phenomena.nationalgeographic.com/2015/07/23/a-fossil-snake-with-four-legs/|access-date = 2015-07-24|first = Ed|last = Yong| name-list-style = vanc |url-status = live|archive-url = https://web.archive.org/web/20150723221358/http://phenomena.nationalgeographic.com/2015/07/23/a-fossil-snake-with-four-legs/|archive-date = 2015-07-23|date = 2015-07-23}}</ref><ref>{{cite journal | vauthors = Martill DM, Tischlinger H, Longrich NR | title = EVOLUTION. A four-legged snake from the Early Cretaceous of Gondwana | journal = Science | volume = 349 | issue = 6246 | pages = 416–9 | date = July 2015 | pmid = 26206932 | doi = 10.1126/science.aaa9208 | url = https://researchportal.port.ac.uk/portal/en/publications/a-fourlegged-snake-from-the-early-cretaceous-of-gondwana(b6583c89-da01-4b9a-9f1e-dc142b9331ff).html | bibcode = 2015Sci...349..416M | s2cid = 25822461 }}</ref>

An alternative hypothesis, based on [[Biology:Morphology|morphology]], suggests the ancestors of snakes were related to [[Biology:Mosasaur|mosasaur]]s—extinct aquatic reptiles from the [[Earth:Cretaceous|Cretaceous]]—forming the clade [[Biology:Pythonomorpha|Pythonomorpha]].<ref name="Sanchez"/> According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment. This ultimately led to an animal similar to today's sea snakes. In the Late [[Earth:Cretaceous|Cretaceous]], snakes recolonized land, and continued to diversify into today's snakes. Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial ''Najash rionegrina''. Similar skull structure, reduced or absent limbs, and other anatomical features found in both mosasaurs and snakes lead to a positive [[Biology:Cladistics|cladistical]] correlation, although some of these features are shared with varanids.{{citation needed|date=June 2016}}

<gallery>
File:Tetrapodophis amplectus 3483.jpg|''[[Biology:Tetrapodophis|Tetrapodophis]]''
File:Pachyrhachis problematicus 45.JPG|''Eupodophis descouensi''
File:Eupodophis descouensi Holotype.jpg|''Eupodophis descouensi''
File:Eupodophis descouensi Holotype hind leg.jpg|''[[Biology:Eupodophis|Eupodophis]] descouensi'' hind leg
</gallery>

Genetic studies in recent years have indicated snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented remains found from the [[Earth:Jurassic|Jurassic]] and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis.<ref>{{cite journal | vauthors = Vidal N, Hedges SB | title = Molecular evidence for a terrestrial origin of snakes | journal = Proceedings. Biological Sciences | volume = 271 Suppl 4 | issue = suppl_4 | pages = S226-9 | date = May 2004 | pmid = 15252991 | pmc = 1810015 | doi = 10.1098/rsbl.2003.0151 }}</ref><ref>{{cite journal | vauthors = Caldwell MW, Nydam RL, Palci A, Apesteguía S | title = The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution | journal = Nature Communications | volume = 6 | issue = 1 | pages = 5996 | date = January 2015 | pmid = 25625704 | doi = 10.1038/ncomms6996 | bibcode = 2015NatCo...6.5996C | doi-access = free }}</ref>

In 2016, two studies reported that limb loss in snakes is associated with DNA mutations in the Zone of Polarizing Activity Regulatory Sequence (ZRS), a regulatory region of the [[Biology:Sonic hedgehog|sonic hedgehog]] gene which is critically required for limb development. More advanced snakes have no remnants of limbs, but basal snakes such as pythons and boas do have traces of highly reduced, vestigial hind limbs. Python embryos even have fully developed hind limb buds, but their later development is stopped by the DNA mutations in the ZRS.<ref>{{Cite news|url=https://www.theatlantic.com/science/archive/2016/10/what-a-legless-mouse-tells-us-about-snake-evolution/504779/|title=What a Legless Mouse Tells Us About Snake Evolution|newspaper=The Atlantic|access-date=2016-10-25|url-status=live|archive-url=https://web.archive.org/web/20161024222452/http://www.theatlantic.com/science/archive/2016/10/what-a-legless-mouse-tells-us-about-snake-evolution/504779/|archive-date=2016-10-24}}</ref><ref>{{Cite news|url=http://www.livescience.com/56573-mutation-caused-snakes-to-lose-legs.html|title=Snakes Used to Have Legs and Arms … Until These Mutations Happened|newspaper=Live Science|access-date=2016-10-22|url-status=live|archive-url=https://web.archive.org/web/20161022150634/http://www.livescience.com/56573-mutation-caused-snakes-to-lose-legs.html|archive-date=2016-10-22}}</ref><ref>{{cite journal | vauthors = Leal F, Cohn MJ | title = Loss and Re-emergence of Legs in Snakes by Modular Evolution of Sonic hedgehog and HOXD Enhancers | journal = Current Biology | volume = 26 | issue = 21 | pages = 2966–2973 | date = November 2016 | pmid = 27773569 | doi = 10.1016/j.cub.2016.09.020 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Kvon EZ, Kamneva OK, Melo US, Barozzi I, Osterwalder M, Mannion BJ, Tissières V, Pickle CS, Plajzer-Frick I, Lee EA, Kato M, Garvin TH, Akiyama JA, Afzal V, Lopez-Rios J, Rubin EM, Dickel DE, Pennacchio LA, Visel A | display-authors = 6 | title = Progressive Loss of Function in a Limb Enhancer during Snake Evolution | journal = Cell | volume = 167 | issue = 3 | pages = 633–642.e11 | date = October 2016 | pmid = 27768887 | pmc = 5484524 | doi = 10.1016/j.cell.2016.09.028 | url = }}</ref>

==Distribution==
[[File:World distribution of snakes.svg|thumb|upright=2.0|Approximate world distribution of snakes]]

There are about 3,900 species of snakes,<ref>{{Cite web|title=THE REPTILE DATABASE|url=http://www.reptile-database.org/|access-date=2021-03-06|website=www.reptile-database.org}}</ref> ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia.<ref name="Sanchez"/> Snakes can be found on every continent except Antarctica, as well as in the sea, and as high as {{convert|16000|ft|m}} in the Himalayan Mountains of Asia.<ref name="Sanchez"/><ref name="Con91">{{cite book |last1 =Conant |first1 =Roger |last2 =Collins |first2 =Joseph | name-list-style = vanc |title =A Field Guide to Reptiles and Amphibians Eastern/Central North America |publisher =Houghton Mifflin Company |year =1991 |location =Boston |isbn =978-0-395-58389-0 |url-access =registration |url =https://archive.org/details/fieldguidetorept00cona }}</ref>{{Rp|143}} There are numerous islands from which snakes are absent, such as Ireland, Iceland, and [[Earth:New Zealand|New Zealand]]<ref name=Bauchot/><ref name="Con91"/> (although New Zealand's waters are infrequently visited by the yellow-bellied sea snake and the banded sea krait).<ref name="AucklandMuseum">{{cite web |url=http://www.aucklandmuseum.com/349/natural-history-questions##2 |title=Natural History Questions |author1=Natural History Information Centre |author2=Auckland War Memorial Museum |work=Auckland War Memorial Museum {{pipe}} Tamaki Paenga Hira |publisher=Auckland War Memorial Museum |location=[[Place:Auckland|Auckland]], New Zealand |at=Q. Are there any snakes in New Zealand? |access-date=26 April 2012 |url-status=live |archive-url=https://web.archive.org/web/20120712124349/http://www.aucklandmuseum.com/349/natural-history-questions |archive-date=12 July 2012 }}</ref>

==Taxonomy==
All modern snakes are grouped within the [[Biology:Class|suborder]] '''Serpentes''' in Linnean taxonomy, part of the [[Biology:Class|order]] [[Biology:Squamata|Squamata]], though their precise placement within squamates remains controversial.<ref name="ITIS">{{ITIS|id=174118|taxon=Serpentes|access-date=4 April 2017}}</ref>

The two infraorders of Serpentes are: [[Biology:Alethinophidia|Alethinophidia]] and [[Biology:Scolecophidia|Scolecophidia]].<ref name="ITIS"/> This separation is based on morphological characteristics and [[Biology:Mitochondrial DNA|mitochondrial DNA]] sequence similarity. Alethinophidia is sometimes split into [[Biology:Henophidia|Henophidia]] and [[Biology:Caenophidia|Caenophidia]], with the latter consisting of "colubroid" snakes ([[Biology:Colubrids|colubrids]], [[Biology:Viperidae|vipers]], elapids, [[Biology:Hydrophiinae|hydrophiids]], and [[Biology:Atractaspidinae|atractaspids]]) and acrochordids, while the other alethinophidian families comprise Henophidia.<ref name="Pou92">{{cite book | vauthors = Pough FH |orig-year=1992 |title= Herpetology: Third Edition |publisher= Pearson Prentice Hall |year=2002 |isbn=978-0-13-100849-6  }}</ref> While not extant today, the [[Biology:Madtsoiidae|Madtsoiidae]], a family of giant, primitive, python-like snakes, was around until 50,000 years ago in Australia, represented by genera such as ''Wonambi''.

There are numerous debates in the systematics within the group. For instance, many sources classify [[Biology:Boidae|Boidae]] and [[Biology:Pythonidae|Pythonidae]] as one family, while some keep the [[Biology:Elapidae|Elapidae]] and Hydrophiidae (sea snakes) separate for practical reasons despite their extremely close relation.

Recent molecular studies support the [[Biology:Monophyly|monophyly]] of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (''Cylindrophis'', ''Anomochilus'', uropeltines), macrostomatans, booids, boids, pythonids and caenophidians.<ref name=Lee>{{cite journal |first1=Michael S. Y. |last1=Lee | first2 = Andrew F. | last2 = Hugall | first3 = Robin | last3 = Lawson | first4 = John D. | last4 = Scanlon | name-list-style = vanc |title=Phylogeny of snakes (Serpentes): combining morphological and molecular data in likelihood, Bayesian and parsimony analyses |journal=Systematics and Biodiversity |volume=5 |issue=4 |pages=371–389 |doi=10.1017/S1477200007002290 |year=2007|hdl=2440/44258 |s2cid=85912034 |hdl-access=free }}</ref>

===Families===
{|class="wikitable"
|-
| colspan="100%"  style="text-align:center; background:#bbf;"|Infraorder '''[[Biology:Alethinophidia|Alethinophidia]]''' 19 families
|-
! style="background:#f0f0f0;"|Family<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Taxon author<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Genera<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Species<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Common name
! style="background:#f0f0f0;"|Geographic range<ref name="McD99">McDiarmid RW, Campbell JA, Touré T. 1999. Snake Species of the World: A Taxonomic and Geographic Reference, vol. 1. Herpetologists' League. 511 pp. {{ISBN|1-893777-00-6}} (series). {{ISBN|1-893777-01-4}} (volume).</ref>
|-
|[[Biology:Acrochordidae|Acrochordidae]]
|Bonaparte, 1831
| style="text-align:center;"|1
| style="text-align:center;"|3
|Wart snakes
|style="width:40%"|Western India and Sri Lanka through tropical Southeast Asia to the Philippines, south through the Indonesian/Malaysian island group to Timor, east through New Guinea to the northern coast of Australia to Mussau Island, the Bismarck Archipelago and Guadalcanal Island in the Solomon Islands.
|-
|[[Biology:Aniliidae|Aniliidae]]
|Stejneger, 1907
| style="text-align:center;"|1
| style="text-align:center;"|1
|False coral snake
|Tropical South America.
|-
|[[Biology:Anomochilidae|Anomochilidae]]
|Cundall, Wallach, 1993
| style="text-align:center;"|1
| style="text-align:center;"|3
|Dwarf pipe snakes
|West Malaysia and on the Indonesian island of Sumatra.
|-
|[[Biology:Boidae|Boidae]]
|Gray, 1825
| style="text-align:center;"|14
| style="text-align:center;"|61
|Boas
|Northern, Central and South America, the Caribbean, southeastern Europe and Asia Minor, Northern, Central and East Africa, Madagascar and Reunion Island, the Arabian Peninsula, Central and southwestern Asia, India and Sri Lanka, the Moluccas and New Guinea through to Melanesia and Samoa.
|-
|[[Biology:Bolyeriidae|Bolyeriidae]]
|Hoffstetter, 1946
| style="text-align:center;"|2
| style="text-align:center;"|2
|Splitjaw snakes
|[[Earth:Mauritius|Mauritius]].
|-
|[[Biology:Colubridae|Colubridae]]
|Oppel, 1811
| style="text-align:center;"|258<ref name="NRDB-Cs"/>
| style="text-align:center;"|1866<ref name="NRDB-Cs"/>
|Typical snakes
|Widespread on all continents, except Antarctica.<ref name="Spa04">Spawls S, Howell K, Drewes R, Ashe J. 2004. A Field Guide To The Reptiles Of East Africa. London: A & C Black Publishers Ltd. 543 pp. {{ISBN|0-7136-6817-2}}.</ref>
|-
|[[Biology:Cylindrophiidae|Cylindrophiidae]]
|Fitzinger, 1843
| style="text-align:center;"|1
| style="text-align:center;"|14
|Asian pipe snakes
|Sri Lanka east through Myanmar, Thailand, Cambodia, Vietnam and the Malay Archipelago to as far east as Aru Islands off the southwestern coast of New Guinea. Also found in southern China (Fujian, Hong Kong and on Hainan Island) and in Laos.
|-
|[[Biology:Elapidae|Elapidae]]
|Boie, 1827
| style="text-align:center;"|55
| style="text-align:center;"|359
|Elapids
|On land, worldwide in tropical and subtropical regions, except in Europe. Sea snakes occur in the Indian Ocean and the Pacific.<ref name="NRDB-E">{{NRDB family |page=elapidae.php |family=Elapidae |date=3 December |year=2008}}</ref>
|-
|[[Biology:Homalopsidae|Homalopsidae]]
|Bonaparte, 1845
| style="text-align:center;"|28
| style="text-align:center;"|53
|Homalopsids
|Southeastern Asia and northern Australia.
|-
|[[Biology:Lamprophiidae|Lamprophiidae]]
|Fitzinger, 1843
| style="text-align:center;"|60
| style="text-align:center;"|314
|Lamprophiids (includes former Atractaspididae as well as 6 other subfamilies formerly considered colubrids)
|Africa, southern Europe, and western-central Asia; two species into southeastern Asia.
|-
|[[Biology:Loxocemidae|Loxocemidae]]
|Cope, 1861
| style="text-align:center;"|1
| style="text-align:center;"|1
|Mexican burrowing snake
|Along the Pacific versant from Mexico south to Costa Rica.
|-
|[[Biology:Pareidae|Pareidae]]
|Romer, 1956
| style="text-align:center;"|3
| style="text-align:center;"|20
|Snail-eating snakes
|Southeast Asia and islands on the Sunda Shelf (Sumatra, Borneo, Java, and their surrounding smaller islands).
|-
|[[Biology:Pythonidae|Pythonidae]]
|Fitzinger, 1826
| style="text-align:center;"|8
| style="text-align:center;"|40
|Pythons
|Subsaharan Africa, India, Myanmar, southern China, Southeast Asia and from the Philippines southeast through Indonesia to New Guinea and Australia.
|-
|[[Biology:Tropidophiidae|Tropidophiidae]]
|Brongersma, 1951
| style="text-align:center;"|2
| style="text-align:center;"|34
|Dwarf boas
|West Indies; also Panama and northwestern South America, as well as in northwestern and southeastern Brazil.
|-
|[[Biology:Uropeltidae|Uropeltidae]]
|Müller, 1832
| style="text-align:center;"|8
| style="text-align:center;"|55
|Shield-tailed snakes
|Southern India and Sri Lanka.
|-
|[[Biology:Viperidae|Viperidae]]
|Oppel, 1811
| style="text-align:center;"|35
| style="text-align:center;"|341
|Vipers
|The Americas, Africa, and Eurasia east to Wallace's Line.
|-
|[[Biology:Xenodermidae|Xenodermidae]]
|Cope, 1900
| style="text-align:center;"|6
| style="text-align:center;"|18
|Dragon & odd-scaled snakes
|Southern and southeastern Asia, and islands on the Sunda Shelf (Sumatra, Borneo, Java, and their surrounding smaller islands).
|-
|[[Biology:Xenopeltidae|Xenopeltidae]]
|Bonaparte, 1845
| style="text-align:center;"|1
| style="text-align:center;"|2
|Sunbeam snakes
|Southeast Asia from the [[Earth:Andaman Islands|Andaman]] and [[Earth:Nicobar Islands|Nicobar Islands]], east through Myanmar to southern China, Thailand, Laos, Cambodia, Vietnam, the Malay Peninsula and the East Indies to Sulawesi, as well as the Philippines.
|-
|[[Biology:Xenophidiidae|Xenophidiidae]]
|Wallach & Günther, 1998
| style="text-align:center;"|1
| style="text-align:center;"|2
|Spine-jawed snakes
|Borneo & peninsular Malaysia.
|}
<br />
{|class="wikitable"
|-
| colspan="100%"  style="text-align:center; background:#bbf;"|Infraorder '''[[Biology:Scolecophidia|Scolecophidia]]''' 5 families
|-
! style="background:#f0f0f0;"|Family<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Taxon author<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Genera<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Species<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Common name
! style="background:#f0f0f0;"|Geographic range<ref name="McD99"/>
|-
|Anomalepidae
|Taylor, 1939
| style="text-align:center;"|4
| style="text-align:center;"|18
|Primitive blind snakes
|style="width:40%"|From southern Central America to northwestern South America. Disjunct populations in northeastern and southeastern South America.
|-
|[[Biology:Gerrhopilidae|Gerrhopilidae]]
|style="width:15%"|Vidal, Wynn, Donnellan and Hedges 2010
| style="text-align:center;"|2
| style="text-align:center;"|18
|Indo-Malayan blindsnakes
|Southern & southeastern Asia, including Sri Lanka, the Philippines, and New Guinea.
|-
|[[Biology:Leptotyphlopidae|Leptotyphlopidae]]
|Stejneger, 1892
| style="text-align:center;"|13
| style="text-align:center;"|139
|Slender blind snakes
|Africa, western Asia from Turkey to northwestern India, on Socotra Island, from the southwestern United States south through Mexico and Central to South America, though not in the high [[Place:Andes|Andes]]. In Pacific South America they occur as far south as southern coastal Peru, and on the Atlantic side as far as Uruguay and Argentina. In the Caribbean they are found on the Bahamas, Hispaniola and the Lesser Antilles.
|-
|[[Biology:Typhlopidae|Typhlopidae]]
|Merrem, 1820
| style="text-align:center;"|18
| style="text-align:center;"|266
|Typical blind snakes
|Most tropical and many subtropical regions around the world, particularly in Africa, Madagascar, Asia, islands in the Pacific, tropical America and in southeastern Europe.
|-
|[[Biology:Xenotyphlopidae|Xenotyphlopidae]]
|Vidal, Vences, Branch and Hedges 2010
| style="text-align:center;"|1
| style="text-align:center;"|1
|Round-nosed blindsnake
|Northern Madagascar.
|}

===Legless lizards===
{{Main|Biology:Legless lizard}}
While snakes are limbless reptiles, evolved from (and grouped with) lizards, there are many other species of lizards that have lost their limbs independently but which superficially look similar to snakes. These include the slowworm and glass snake.

Other [[Biology:Limbless vertebrate|serpentine tetrapods]] that are unrelated to snakes include [[Biology:Caecilian|caecilians]] (amphibians), [[Biology:Amphisbaenia|amphisbaenians]] (near-lizard squamates), and the extinct [[Biology:Aistopoda|aistopods]] (amphibians).

==Biology==
[[File:Leptotyphlops carlae.jpg|thumb|upright=0.9|An adult Barbados threadsnake, ''Leptotyphlops carlae'', on an American quarter dollar ]]

===Size===
The now [[Biology:Extinction|extinct]] ''[[Biology:Titanoboa|Titanoboa cerrejonensis]]'' was {{convert|12.8|m|ft|abbr=on}} in length.<ref name="head">{{cite journal | vauthors = Head JJ, Bloch JI, Hastings AK, Bourque JR, Cadena EA, Herrera FA, Polly PD, Jaramillo CA | display-authors = 6 | title = Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures | journal = Nature | volume = 457 | issue = 7230 | pages = 715–7 | date = February 2009 | pmid = 19194448 | doi = 10.1038/nature07671 | bibcode = 2009Natur.457..715H | s2cid = 4381423 }}</ref> By comparison, the largest extant snakes are the reticulated python, measuring about {{convert|6.95|m|abbr=on}} long,<ref name="SunBear"/> and the [[Biology:Green anaconda|green anaconda]], which measures about {{convert|5.21|m|abbr=on}} long and is considered the heaviest snake on Earth at {{convert|97.5|kg|lb|abbr=on}}.<ref name="Riv00">{{cite thesis |last=Rivas |first=Jesús Antonio |name-list-style=vanc |year=2000 |title=The life history of the green anaconda (''Eunectes murinus''), with emphasis on its reproductive Biology |degree=[[Social:Doctor of Philosophy|Ph.D.]] |publisher=[[Organization:University of Tennessee|University of Tennessee]] |url=http://www.anacondas.org/diss/disser.pdf |url-status=dead |archive-url=https://web.archive.org/web/20160303202240/http://www.anacondas.org/diss/disser.pdf |archive-date=2016-03-03 |access-date=2014-12-12 }}</ref>

At the other end of the scale, the smallest extant snake is ''[[Biology:Barbados threadsnake|Leptotyphlops carlae]]'', with a length of about {{convert|10.4|cm|in|abbr=on}}.<ref name="zootaxa">{{cite journal |url=http://www.mapress.com/zootaxa/2008/f/zt01841p030.pdf |title=At the lower size limit in snakes: two new species of threadsnakes (Squamata: Leptotyphlopidae: Leptotyphlops) from the Lesser Antilles | first = S. Blair | last = Hedges | name-list-style = vanc |journal=Zootaxa |volume=1841 |pages=1–30 |date=August 4, 2008 |access-date=2008-08-04 |url-status=live |archive-url=https://web.archive.org/web/20080813023833/http://www.mapress.com/zootaxa/2008/f/zt01841p030.pdf |archive-date=August 13, 2008 |doi=10.11646/zootaxa.1841.1.1 |doi-access=free }}</ref> Most snakes are fairly small animals, approximately {{convert|1|m|ft|abbr=on}} in length.<ref>{{cite journal | vauthors = Boback SM, Guyer C | title = Empirical evidence for an optimal body size in snakes | journal = Evolution; International Journal of Organic Evolution | volume = 57 | issue = 2 | pages = 345–51 | date = February 2003 | pmid = 12683530 | doi = 10.1554/0014-3820(2003)057[0345:EEFAOB]2.0.CO;2 }}</ref>

===Perception===
[[File:wiki snake eats mouse.jpg|thumb|left|Thermographic image of a snake eating a mouse]]
Pit vipers, pythons, and some boas have [[Physics:Infrared sensing in snakes|infrared-sensitive receptors]] in deep grooves on the snout, allowing them to "see" the radiated heat of warm-blooded prey. In pit vipers, the grooves are located between the nostril and the eye in a large "pit" on each side of the head. Other infrared-sensitive snakes have multiple, smaller labial pits lining the upper lip, just below the nostrils.<ref name="Cogger91_180"/>

A snake tracks its prey using smell, collecting airborne particles with its [[Biology:Forked tongue|forked tongue]], then passing them to the [[Biology:Vomeronasal organ|vomeronasal organ]] or ''Jacobson's organ'' in the mouth for examination.<ref name="Cogger91_180">Cogger (1991), p. 180.</ref> The fork in the tongue provides a sort of directional sense of smell and taste simultaneously.<ref name="Cogger91_180"/> The snake's tongue is constantly in motion, sampling particles from the air, ground, and water, analyzing the chemicals found, and determining the presence of prey or predators in the local environment. In water-dwelling snakes, such as the [[Biology:Anaconda|anaconda]], the tongue functions efficiently underwater.<ref name="Cogger91_180"/>

[[File:Ptyas gab fbi.png|thumb|A line diagram from ''The Fauna of British India'' by G. A. Boulenger (1890), illustrating the terminology of shields on the head of a snake]]
The underside of a snake is very sensitive to vibration, allowing the snake to detect approaching animals by sensing faint vibrations in the ground.<ref name="Cogger91_180"/>

Snake vision varies greatly between species. Some have keen eyesight and others are only able to distinguish light from dark, but the important trend is that a snake's visual perception is adequate enough to track movements.<ref>{{cite web |url=http://www.petplace.com/reptiles/reptile-senses-understanding-their-world/page1.aspx |title=Reptile Senses: Understanding Their World |website=Petplace.com |date=2015-05-18 |access-date=2016-01-09 |url-status=live |archive-url=https://web.archive.org/web/20150219062040/http://www.petplace.com/reptiles/reptile-senses-understanding-their-world/page1.aspx |archive-date=2015-02-19 }}</ref> Generally, vision is best in tree-dwelling snakes and weakest in burrowing snakes. Some have [[Binocular vision|binocular vision]], where both eyes are capable of focusing on the same point, an example of this being the [[Biology:Ahaetulla|Asian vine snake]]. Most snakes focus by moving the [[Biology:Lens (anatomy)|lens]] back and forth in relation to the [[Biology:Retina|retina]]. Diurnal snakes have round pupils and many nocturnal snakes have slit pupils. Most species possess three visual pigments and are probably able to see two primary colors in daylight. It has been concluded that the last common ancestors of all snakes had [[Physics:Ultraviolet|UV]]-sensitive vision, but most snakes that depend on their eyesight to hunt in daylight have evolved lenses that act like sunglasses for filtering out the UV-light, which probably also sharpens their vision by improving the [[Contrast (vision)|contrast]].<ref>{{cite web|url=https://www.sciencedaily.com/releases/2016/08/160816182620.htm |title=Snake eyes: New insights into visual adaptations |date=16 August 2016 |website=ScienceDaily}}</ref><ref>{{cite journal|url=https://academic.oup.com/mbe/article/33/10/2483/2925599 |title=Visual Pigments, Ocular Filters and the Evolution of Snake Vision |last1=Simões |first1=Bruno F. |last2=Sampaio |first2=Filipa L. |last3=Douglas |first3=Ronald H. |last4=Kodandaramaiah |first4=Ullasa |last5=Casewell |first5=Nicholas R. |last6=Harrison |first6=Robert A. |last7=Hart |first7=Nathan S. |last8=Partridge |first8=Julian C. |last9=Hunt |first9=David M. |last10=Gower |first10=David J. |display-authors=1 |journal=[[Biology:Molecular Biology and Evolution|Molecular Biology and Evolution]] |publisher=Oxford University Press |volume=33 |issue=10 |date=October 2016 |pages=2483–2495 |doi=10.1093/molbev/msw148|pmid=27535583 |doi-access=free }}</ref>

===Skin===
The skin of a snake is covered in [[Biology:Scale (anatomy)|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, allowing them to grip surfaces. The body scales may be smooth, keeled, or granular. The eyelids of a snake are transparent "spectacle" scales, also known as brille, which remain permanently closed.

The shedding of scales is called ''[[Biology:Ecdysis|ecdysis]]'' (or in normal usage, ''molting'' or ''sloughing''). Snakes shed the complete outer layer of skin in one piece.<ref name = "Smith1_30">Smith, Malcolm A. ''The Fauna of British India, Including Ceylon and Burma''. Vol I, Loricata and Testudines. p. 30.</ref> Snake scales are not discrete, but extensions of the [[Biology:Epidermis|epidermis]]—hence they are not shed separately but as a complete outer layer during each molt, akin to a sock being turned inside out.<ref name="RSSlimy">{{cite web|url=http://www.szgdocent.org/resource/rr/c-slimy.htm |title=Are Snakes Slimy? |website=szgdocent.org |archive-url=https://web.archive.org/web/20060805131135/http://www.szgdocent.org/resource/rr/c-slimy.htm |archive-date=5 August 2006 |url-status=live}}</ref>

Snakes have a wide diversity of skin coloration patterns which are often related to behavior, such as the tendency to have to flee from predators. Snakes that are at a high risk of predation tend to be plain, or have longitudinal stripes, providing few reference points to predators, thus allowing the snake to escape without being noticed. Plain snakes usually adopt active hunting strategies, as their pattern allows them to send little information to prey about motion. Blotched snakes usually use ambush-based strategies, likely because it helps them blend into an environment with irregularly shaped objects, like sticks or rocks. Spotted patterning can similarly help snakes to blend into their environment.<ref name="AllenBaddeley2013">{{cite journal|last1=Allen|first1=William L.|last2=Baddeley|first2=Roland|last3=Scott-Samuel|first3=Nicholas E.|last4=Cuthill|first4=Innes C. | name-list-style = vanc |title=The evolution and function of pattern diversity in snakes|journal=Behavioral Ecology|volume=24|issue=5|year=2013|pages=1237–1250|issn=1465-7279|doi=10.1093/beheco/art058|doi-access=free}}</ref>

The shape and number of scales on the head, back, and belly are often characteristic and used for taxonomic purposes. Scales are named mainly according to their positions on the body. In "advanced" ([[Biology:Caenophidia|Caenophidia]]n) snakes, the broad belly scales and rows of dorsal scales correspond to the [[Biology:Vertebra|vertebra]]e, allowing these to be counted without the need for [[Biology:Dissection|dissection]].

====Molting====
[[File:Nerodia sipedon shedding.JPG|thumb|left|A [[Biology:Common watersnake|common watersnake]] shedding its skin]]
Molting (or "ecdysis") serves a number of purposes. Firstly, the old and worn skin is replaced, and secondly, it helps get rid of parasites such as [[Biology:Mite|mite]]s and [[Biology:Tick|tick]]s. Renewal of the skin by molting supposedly allows growth in some animals such as insects, but this has been disputed in the case of snakes.<ref name="RSSlimy"/><ref name="ZooPax3">{{cite web |url=http://whozoo.org/ZooPax/ZPScales_3.htm |title=ZooPax: A Matter of Scale: Part III |website=Whozoo.org |access-date=2016-01-09 |url-status=live |archive-url=https://web.archive.org/web/20160116034245/http://whozoo.org/ZooPax/ZPScales_3.htm |archive-date=2016-01-16 }}</ref>

Molting occurs periodically throughout the life of a snake. Before each molt, the snake stops eating and often hides or moves to a safe place. Just before shedding, the skin becomes dull and dry looking and the snake's eyes turn cloudy or blue-colored. The inner surface of the old skin liquefies, causing it to separate from the new skin beneath it. After a few days, the eyes become clear and the snake "crawls" out of its old skin, which splits close to the snake's mouth. The snake rubs its body against rough surfaces to aid in the shedding of its old skin. In many cases, the cast skin peels backward over the body from head to tail in one piece, like pulling a sock off inside-out, revealing a new, larger, brighter layer of skin which has formed underneath.<ref name="RSSlimy"/><ref name="GenSnakeInfo">{{cite web|url=http://www.sdgfp.info/Wildlife/Snakes/SnakeInfo.htm |title=General Snake Information |website=sdgfp.info |archive-url=https://web.archive.org/web/20071125210255/http://www.sdgfp.info/Wildlife/Snakes/SnakeInfo.htm |archive-date=25 November 2007 |url-status=dead}}</ref>

A young snake that is still growing may shed its skin up to four times a year, but an older snake may shed only once or twice a year.<ref name = "GenSnakeInfo"/> The discarded skin carries a perfect imprint of the scale pattern, so it is usually possible to identify the snake from the cast skin if it is reasonably intact.<ref name="RSSlimy"/> This periodic renewal has led to the snake being a symbol of [[Biology:Healing|healing]] and [[Medicine:Medicine|medicine]], as pictured in the [[Rod of Asclepius]].<ref name=AIM>{{cite journal | vauthors = Wilcox RA, Whitham EM | title = The symbol of modern medicine: why one snake is more than two | journal = Annals of Internal Medicine | volume = 138 | issue = 8 | pages = 673–7 | date = April 2003 | pmid = 12693891 | doi = 10.7326/0003-4819-138-8-200304150-00016 | citeseerx = 10.1.1.731.8485 | s2cid = 19125435 }}</ref>

Scale counts can sometimes be used to identify the sex of a snake when the species is not distinctly [[Biology:Sexual dimorphism|sexually dimorphic]]. A probe is fully inserted into the [[Biology:Cloaca|cloaca]], marked at the point where it stops, then removed and measured against the subcaudal scales.<ref name="Rosenfeld_11">Rosenfeld (1989), p. 11.</ref> The scalation count determines whether the snake is a male or female, as the [[Biology:Hemipenis|hemipenes]] of a male will probe to a different depth (usually longer) than the cloaca of a female.<ref name="Rosenfeld_11"/>{{clarify|reason=Is it really scale counts or are the scales merely used to measure the probe penetration?|date=July 2016}}

===Skeleton===
[[File:Snake Skeletons.jpg|thumb|left|The skeletons of snakes are radically different from those of most other reptiles (as compared with the [[Biology:Turtle|turtle]] here, for example), consisting almost entirely of an extended ribcage.]]
The skeleton of most snakes consists solely of the skull, hyoid, vertebral column, and ribs, though [[Biology:Henophidia|henophidian]] snakes retain vestiges of the pelvis and rear limbs.

The skull consists of a solid and complete [[Medicine:Neurocranium|neurocranium]], to which many of the other bones are only loosely attached, particularly the highly mobile jaw bones, which facilitate manipulation and ingestion of large prey items. The left and right sides of the lower jaw are joined together only by a flexible ligament at the anterior tips, allowing them to separate widely, and the posterior end of the lower jaw bones articulate with a [[Biology:Quadrate bone|quadrate bone]], allowing further mobility. The mandible and quadrate bones can pick up ground-borne vibrations;<ref>{{cite journal | vauthors = Hartline PH | title = Physiological basis for detection of sound and vibration in snakes | journal = The Journal of Experimental Biology | volume = 54 | issue = 2 | pages = 349–71 | date = April 1971 | doi = 10.1242/jeb.54.2.349 | pmid = 5553415 | url = http://jeb.biologists.org/cgi/reprint/54/2/349.pdf | url-status = live | archive-url = https://web.archive.org/web/20081217012157/http://jeb.biologists.org/cgi/reprint/54/2/349.pdf | archive-date = 2008-12-17 }}</ref> because the sides of the lower jaw can move independently of one another, a snake resting its jaw on a surface has sensitive stereo [[Physics:Hearing|auditory perception]], used for detecting the position of prey. The jaw–quadrate–[[Biology:Stapes|stapes]] pathway is capable of detecting vibrations on the [[Angstrom|angstrom]] scale, despite the absence of an outer ear and the lack of an [[Impedance matching|impedance matching]] mechanism—provided by the [[Medicine:Ossicles|ossicles]] in other vertebrates—for receiving vibrations from the air.<ref>{{cite journal | vauthors = Friedel P, Young BA, van Hemmen JL | title = Auditory localization of ground-borne vibrations in snakes | journal = Physical Review Letters | volume = 100 | issue = 4 | pages = 048701 | date = February 2008 | pmid = 18352341 | doi = 10.1103/physrevlett.100.048701 | bibcode = 2008PhRvL.100d8701F }}</ref><ref>{{cite web|url=http://www.physorg.com/news122123444.html|title=Desert Snake Hears Mouse Footsteps with its Jaw|date=2008-02-13| first = Lisa | last = Zyga | name-list-style = vanc |publisher=PhysOrg|url-status=live|archive-url=https://web.archive.org/web/20111010152337/http://www.physorg.com/news122123444.html|archive-date=2011-10-10}}</ref>

The hyoid is a small bone located posterior and ventral to the skull, in the 'neck' region, which serves as an attachment for the muscles of the snake's tongue, as it does in all other [[Biology:Tetrapod|tetrapod]]s.

The vertebral column consists of between 200 and 400 vertebrae, or sometimes more. The body vertebrae each have two ribs articulating with them. The tail vertebrae are comparatively few in number (often less than 20% of the total) and lack ribs. The vertebrae have projections that allow for strong muscle attachment, enabling locomotion without limbs.

Caudal [[Biology:Autotomy|autotomy]] (self-amputation of the tail), a feature found in some lizards, is absent in most snakes.<ref>Cogger, H 1993 Fauna of Australia. Vol. 2A Amphibia and Reptilia. Australian Biological Resources Studies, Canberra.</ref> In the rare cases where it does exist in snakes, caudal autotomy is intervertebral (meaning the separation of adjacent vertebrae), unlike that in lizards, which is intravertebral, i.e. the break happens along a predefined fracture plane present on a vertebra.<ref>{{cite journal | vauthors = Arnold EN | doi=10.1080/00222938400770131 | title=Evolutionary aspects of tail shedding in lizards and their relatives | journal=Journal of Natural History | year=1984 | volume=18 | issue=1 | pages=127–169 }}</ref><ref>{{cite journal | vauthors = Ananjeva NB, Orlov NL | year = 1994 | title = Caudal autotomy in Colubrid snake ''Xenochrophis piscator'' from Vietnam | journal = Russian Journal of Herpetology | volume = 1 | issue = 2 }}</ref>

In some snakes, most notably boas and pythons, there are vestiges of the hindlimbs in the form of a pair of pelvic spurs. These small, claw-like protrusions on each side of the cloaca are the external portion of the vestigial hindlimb skeleton, which includes the remains of an ilium and femur.

Snakes are [[Biology:Polyphyodont|polyphyodont]]s with teeth that are continuously replaced.<ref>{{cite journal | vauthors = Gaete M, Tucker AS | title = Organized emergence of multiple-generations of teeth in snakes is dysregulated by activation of Wnt/beta-catenin signalling | journal = PLOS ONE | volume = 8 | issue = 9 | pages = e74484 | year = 2013 | pmid = 24019968 | pmc = 3760860 | doi = 10.1371/journal.pone.0074484 | bibcode = 2013PLoSO...874484G | doi-access = free }}</ref>

===Internal organs===
Snakes and other reptiles have a three-chambered heart that controls the [[Biology:Circulatory system|circulatory system]] via the left and right atrium, and one ventricle.<ref>{{cite journal | vauthors = Jensen B, Moorman AF, Wang T | title = Structure and function of the hearts of lizards and snakes | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 89 | issue = 2 | pages = 302–36 | date = May 2014 | pmid = 23998743 | doi = 10.1111/brv.12056 | s2cid = 20035062 }}</ref> Internally, the ventricle is divided into three interconnected cavities: the cavum arteriosum, the cavum pulmonale, and the cavum venosum.<ref>{{cite journal |last1=Burggren |first1=Warren W. | name-list-style = vanc |title=Form and Function in Reptilian Circulations |journal=Integrative and Comparative Biology |date=1 February 1987 |volume=27 |issue=1 |pages=5–19 |doi=10.1093/icb/27.1.5 |language=en |issn=1540-7063|doi-access=free }}</ref> The cavum venosum receives deoxygenated [[Biology:Blood|blood]] from the right atrium and the cavum arteriosum receives oxygenated blood from the left atrium. Located beneath the cavum venosum is the cavum pulmonale, which pumps blood to the pulmonary trunk.<ref>{{cite journal |last1=Mathur |first1=Prahlad | name-list-style = vanc |title=The anatomy of the reptilian heart. Part I. Varanus monitor (Linn.) |journal=Proc. Ind. Acad. Sci. |date=1944 |volume=Sect. B 20 |pages=1–29 |url=https://www.ias.ac.in/article/fulltext/secb/020/01/0001-0029 |access-date=10 May 2019}}</ref>

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, owing to the lack of a diaphragm; this adjustment protects the heart from potential damage when large ingested prey is passed through the [[Biology:Esophagus|esophagus]]. The [[Biology:Spleen|spleen]] is attached to the gall bladder and pancreas and filters the blood. The [[Biology:Thymus|thymus]], located in fatty tissue above the heart, is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is 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.<ref name="Mader"/>

The vestigial left [[Medicine:Lung|lung]] is often small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin.<ref name="Mader">{{Cite journal | last = Mader | first = Douglas | name-list-style = vanc | title = Reptilian Anatomy | journal = Reptiles | volume = 3 | issue = 2 | pages = 84–93 | date = June 1995 }}</ref> 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.<ref name="Mader"/> This 'saccular lung' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species.<ref name="Mader"/> Many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, one located ahead of the other.<ref name="Mader"/>

Snakes have no [[Biology:Lymph node|lymph node]]s.<ref name="Mader"/>

===Venom===
[[File:Red milk snake.JPG|thumb|Innocuous [[Biology:Milk snake|milk snake]]s are often mistaken for coral snakes whose venom is deadly to humans.]]

Cobras, vipers, and closely related species use [[Biology:Venom|venom]] to immobilize, injure, or kill their prey. The venom is modified [[Biology:Saliva|saliva]], delivered through fangs.<ref name="Meh87"/>{{Rp|243}} The fangs of 'advanced' venomous snakes like viperids and elapids are hollow, allowing venom to be injected more effectively, and the fangs of rear-fanged snakes such as the boomslang simply have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey-specific, and their role in self-defense is secondary.<ref name="Meh87"/>{{Rp|243}}

Venom, like all salivary secretions, is a predigestant that initiates the breakdown of food into soluble compounds, facilitating proper digestion. Even nonvenomous snakebites (like any animal bite) cause tissue damage.<ref name="Meh87"/>{{Rp|209}}

Certain birds, mammals, and other snakes (such as [[Biology:Kingsnake|kingsnake]]s) that prey on venomous snakes have developed resistance and even immunity to certain venoms.<ref name="Meh87"/>{{Rp|243}} Venomous snakes include three families of snakes, and do not constitute a formal [[Biology:Taxonomy|taxonomic classification]] group.

The [[Social:Colloquialism|colloquial]] term "poisonous snake" is generally an incorrect label for snakes. A poison is inhaled or ingested, whereas venom produced by snakes is injected into its victim via fangs.<ref name="Freiberg84_125"/> There are, however, two exceptions: ''[[Biology:Rhabdophis|Rhabdophis]]'' sequesters toxins from the toads it eats, then secretes them from nuchal glands to ward off predators; and a small unusual population of [[Biology:Garter snake|garter snake]]s in the US state of Oregon retains enough toxins in their livers from ingested [[Biology:Newt|newt]]s to be effectively poisonous to small local predators (such as [[Biology:Crow|crow]]s and [[Biology:Fox|fox]]es).<ref name="Freiberg84_123">Freiberg (1984), p. 123.</ref>

Snake venoms are complex mixtures of [[Biology:Protein|protein]]s, and are stored in venom glands at the back of the head.<ref name="Freiberg84_123"/> In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw.<ref name="Meh87"/>{{Rp|243}}<ref name="Freiberg84_125">Freiberg (1984), p. 125.</ref> The proteins can potentially be a mix of [[Chemistry:Neurotoxin|neurotoxin]]s (which attack the nervous system), [[Medicine:Hemotoxin|hemotoxin]]s (which attack the circulatory system), cytotoxins, [[Biology:Bungarotoxin|bungarotoxin]]s, and many other toxins that affect the body in different ways.<ref name="Freiberg84_125"/> Almost all snake venom contains ''[[Biology:Hyaluronidase|hyaluronidase]]'', an enzyme that ensures rapid diffusion of the venom.<ref name="Meh87"/>{{Rp|243}}

Venomous snakes that use hemotoxins usually have fangs in the front of their mouths, making it easier for them to inject the venom into their victims.<ref name="Freiberg84_125"/> Some snakes that use neurotoxins (such as the [[Biology:Boiga dendrophila|mangrove snake]]) have fangs in the back of their mouths, with the fangs curled backwards.<ref name="Freiberg84_126">Freiberg (1984), p. 126.</ref> This makes it difficult both for the snake to use its venom and for scientists to milk them.<ref name="Freiberg84_125"/> Elapids, however, such as cobras and kraits are ''proteroglyphous''—they possess hollow fangs that cannot be erected toward the front of their mouths, and cannot "stab" like a viper. They must actually bite the victim.<ref name="Meh87"/>{{Rp|242}}

It has been suggested that all snakes may be venomous to a certain degree, with harmless snakes having weak venom and no fangs.<ref name="Fry_2006_earlyevolution">{{cite journal | vauthors = Fry BG, Vidal N, Norman JA, Vonk FJ, Scheib H, Ramjan SF, Kuruppu S, Fung K, Hedges SB, Richardson MK, Hodgson WC, Ignjatovic V, Summerhayes R, Kochva E | display-authors = 6 | title = Early evolution of the venom system in lizards and snakes | journal = Nature | volume = 439 | issue = 7076 | pages = 584–8 | date = February 2006 | pmid = 16292255 | doi = 10.1038/nature04328 | bibcode = 2006Natur.439..584F | s2cid = 4386245 }}</ref> According to this theory, most snakes that are labelled "nonvenomous" would be considered harmless because they either lack a venom delivery method or are incapable of delivering enough to endanger a human. The theory postulates that snakes may have evolved from a common lizard ancestor that was venomous, and also that venomous lizards like the [[Biology:Gila monster|gila monster]], beaded lizard, [[Biology:Monitor lizard|monitor lizard]]s, and the now-extinct mosasaurs, may have derived from this same common ancestor. They share this "venom clade" with various other [[Biology:Sauria|sauria]]n species.

Venomous snakes are classified in two taxonomic families:

* '''[[Biology:Elapidae|Elapid]]s''' – cobras including [[Biology:King cobra|king cobra]]s, [[Biology:Bungarus|kraits]], [[Biology:Mamba|mamba]]s, [[Biology:Austrelaps|Australian copperheads]], [[Biology:Hydrophiinae|sea snake]]s, and [[Biology:Coral snake|coral snake]]s.<ref name="Freiberg84_126"/>
* '''[[Biology:Viperidae|Viperids]]''' – vipers, rattlesnakes, [[Biology:Agkistrodon contortrix|copperheads]]/[[Biology:Agkistrodon piscivorus|cottonmouths]], and [[Biology:Lachesis (genus)|bushmasters]].<ref name="Freiberg84_126"/>

There is a third family containing the ''opistoglyphous'' (rear-fanged) snakes (as well as the majority of other snake species):

* '''[[Biology:Colubridae|Colubrid]]s''' – [[Biology:Boomslang|boomslang]]s, tree snakes, [[Biology:Ahaetulla|vine snakes]], [[Biology:Boiga|cat snakes]], although not all colubrids are venomous.<ref name="Meh87"/>{{Rp|209}}<ref name="Freiberg84_126"/>

===Reproduction===
Although a wide range of reproductive modes are used by snakes, all employ internal fertilization. This is accomplished by means of paired, forked [[Biology:Hemipenis|hemipenes]], which are stored, inverted, in the male's tail.<ref name="Capula89_117">Capula (1989), p. 117.</ref> The hemipenes are often grooved, hooked, or spined—designed to grip the walls of the female's [[Biology:Cloaca|cloaca]].<ref name="AldridgeSever2016">{{cite book| first1 = Robert D. | last1 = Aldridge | first2 = David M. | last2 = Sever | name-list-style = vanc |title=Reproductive Biology and Phylogeny of Snakes|url=https://books.google.com/books?id=u-3RBQAAQBAJ |date=19 April 2016|publisher=CRC Press|isbn=978-1-4398-5833-2}}</ref><ref name="Capula89_117"/>

Most species of snakes lay eggs which they abandon shortly after laying. However, a few species (such as the king cobra) construct nests and stay in the vicinity of the hatchlings after incubation.<ref name="Capula89_117"/> Most pythons coil around their egg-clutches and remain with them until they hatch.<ref name="Cogger91_186">Cogger (1991), p. 186.</ref> A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even "shiver" to generate heat to incubate the eggs.<ref name="Cogger91_186"/>

Some species of snake are ovoviviparous and retain the eggs within their bodies until they are almost ready to hatch.<ref name="Capula89_118">Capula (1989), p. 118.</ref><ref name="Cogger91_182">Cogger (1991), p. 182.</ref> Several species of snake, such as the [[Biology:Boa constrictor|boa constrictor]] and green anaconda, are fully viviparous, nourishing their young through a [[Biology:Placenta|placenta]] as well as a yolk sac; this is highly unusual among reptiles, and normally found in requiem sharks or placental mammals.<ref name="Capula89_118"/><ref name="Cogger91_182"/> Retention of eggs and live birth are most often associated with colder environments.<ref name="Capula89_117"/><ref name="Cogger91_182"/>

[[File:Coast Garter Snake.jpg|thumb|right|The garter snake has been studied for sexual selection.]]
[[Biology:Sexual selection|Sexual selection]] in snakes is demonstrated by the 3,000 species that each use different tactics in acquiring mates.<ref name="two">{{cite journal |doi=10.1016/j.anbehav.2003.05.007 |title=Courtship tactics in garter snakes: How do a male's morphology and behaviour influence his mating success? |year=2004 |last1=Shine |first1=Richard |last2=Langkilde |first2=Tracy |last3=Mason |first3=Robert T | name-list-style = vanc |journal=Animal Behaviour |volume=67 |issue=3 |pages=477–83|s2cid=4830666 }}</ref> Ritual combat between males for the females they want to [[Biology:Mating|mate]] with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and force it downward. It is common for neck-biting to occur while the snakes are entwined.<ref name="three">{{cite journal |doi=10.1016/j.anbehav.2004.03.012 |title=Genetic evidence for sexual selection in black ratsnakes, ''Elaphe obsoleta'' |year=2005 |last1=Blouin-Demers |first1=Gabriel |last2=Gibbs |first2=H. Lisle |last3=Weatherhead |first3=Patrick J. | name-list-style = vanc |journal=Animal Behaviour |volume=69 |issue=1 |pages=225–34|s2cid=3907523 }}</ref>

===Facultative parthenogenesis===

[[Biology:Parthenogenesis|Parthenogenesis]] is a natural form of reproduction in which growth and development of embryos occur without fertilization. ''Agkistrodon contortrix'' (copperhead) and ''Agkistrodon piscivorus'' (cottonmouth) can reproduce by [[Biology:Parthenogenesis#Facultative|facultative parthenogenesis]], meaning that they are capable of switching from a [[Biology:Sexual reproduction|sexual]] mode of reproduction to an asexual mode.<ref name=Booth2012>{{cite journal | vauthors = Booth W, Smith CF, Eskridge PH, Hoss SK, Mendelson JR, Schuett GW | title = Facultative parthenogenesis discovered in wild vertebrates | journal = Biology Letters | volume = 8 | issue = 6 | pages = 983–5 | date = December 2012 | pmid = 22977071 | pmc = 3497136 | doi = 10.1098/rsbl.2012.0666 }}</ref> The most likely type of parthenogenesis to occur is [[Biology:Parthenogenesis#Automictic|automixis]] with terminal fusion, a process in which two terminal products from the same [[Biology:Meiosis|meiosis]] fuse to form a diploid [[Biology:Zygote|zygote]]. This process leads to genome-wide [[Biology:Zygosity#Homozygous|homozygosity]], expression of deleterious recessive [[Biology:Allele|allele]]s, and often to developmental abnormalities. Both captive-born and wild-born copperheads and cottonmouths appear to be capable of this form of parthenogenesis.<ref name=Booth2012 />

Reproduction in [[Biology:Squamata|squamate]] reptiles is almost exclusively sexual. Males ordinarily have a ZZ pair of sex-determining chromosomes, and females a ZW pair. However, the Colombian Rainbow boa (''[[Biology:Epicrates maurus|Epicrates maurus]]'') can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny.<ref name="pmid21868391">{{cite journal | vauthors = Booth W, Million L, Reynolds RG, Burghardt GM, Vargo EL, Schal C, Tzika AC, Schuett GW | display-authors = 6 | title = Consecutive virgin births in the new world boid snake, the Colombian rainbow Boa, Epicrates maurus | journal = The Journal of Heredity | volume = 102 | issue = 6 | pages = 759–63 | year = 2011 | pmid = 21868391 | doi = 10.1093/jhered/esr080 | doi-access = free }}</ref> The WW females are likely produced by terminal automixis.

==Behavior==

===Winter dormancy===
In regions where winters are too cold for snakes to tolerate while remaining active, local species will enter a period of [[Biology:Dormancy#Brumation|brumation]]. Unlike [[Biology:Dormancy#Hibernation|hibernation]], in which the dormant mammals are actually asleep, brumating reptiles are awake but inactive. Individual snakes may brumate in burrows, under rock piles, or inside fallen trees, or large numbers of snakes may clump together in [[Biology:Hibernaculum (zoology)|hibernacula]].

===Feeding and diet===
[[File:Eierschlange frisst Zwergwachtelei.jpg|thumb|upright|African egg-eating snake eating an egg]]
All snakes are strictly carnivorous, [[Earth:Predation|preying]] on small animals including lizards, frogs, other snakes, small mammals, birds, eggs, fish, snails, worms, and insects.<ref name="Meh87"/>{{Ref|81}}<ref name="Sanchez"/><ref name="Bebler79_581">Behler (1979) p. 581</ref> Snakes cannot bite or tear their food to pieces so must swallow their prey whole. The eating habits of a snake are largely influenced by body size; 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.

The snake's [[Medicine:Jaw|jaw]] is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, they have an extremely flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in the skull, which allow the snake to open its mouth wide enough to swallow prey whole, even if it is larger in diameter than the snake itself.<ref name="Bebler79_581"/> For example, the [[Biology:Dasypeltis|African egg-eating snake]] has flexible jaws adapted for eating eggs much larger than the diameter of its head.<ref name="Meh87"/>{{Rp|81}} This snake has no teeth, but does have bony protrusions on the inside edge of its [[Biology:Vertebral column|spine]], which it uses to break the shell when eating eggs.<ref name="Meh87"/>{{Rp|81}}
[[File:Carpet snake.jpg|thumb|upright|left|Carpet python constricting and consuming a chicken]]
The majority of snakes eat a variety of prey animals, but there is some specialization in certain species. [[Biology:King cobra|King cobra]]s and the Australian bandy-bandy consume other snakes. Species of the family [[Biology:Pareidae|Pareidae]] have more teeth on the right side of their mouths than on the left, as they mostly prey on snails and the shells usually spiral clockwise.<ref name="Meh87"/>{{Rp|184}}<ref>{{cite journal | vauthors = Hoso M, Asami T, Hori M | title = Right-handed snakes: convergent evolution of asymmetry for functional specialization | journal = Biology Letters | volume = 3 | issue = 2 | pages = 169–72 | date = April 2007 | pmid = 17307721 | pmc = 2375934 | doi = 10.1098/rsbl.2006.0600 }}</ref><ref>{{cite journal | vauthors = Pyron RA, Burbrink FT, Wiens JJ | title = A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes | journal = BMC Evolutionary Biology | volume = 13 | pages = 93 | date = April 2013 | pmid = 23627680 | pmc = 3682911 | doi = 10.1186/1471-2148-13-93 }}</ref>

Some snakes have a venomous bite, which they use to kill their prey before eating it.<ref name="Bebler79_581"/><ref>Freiberg (1984), pp. 125–127.</ref> Other snakes kill their prey by [[Biology:Constriction|constriction]],<ref name="Bebler79_581"/> while some swallow their prey when it is still alive.<ref name="Meh87"/>{{Rp|81}}<ref name="Bebler79_581"/>

[[File:PikiWiki Israel 37648 Nature and Colors.jpg|thumb|upright|''[[Biology:Dolichophis jugularis|Dolichophis jugularis]]'' preying on a sheltopusik ]]
After eating, snakes become dormant to allow the process of [[Biology:Digestion|digestion]] to take place;<ref name="Rosenfeld_11"/> this is an intense activity, especially after consumption of large prey. In species that feed only sporadically, the entire [[Chemistry:Intestine|intestine]] enters a reduced state between meals to conserve energy. The digestive system is then 'up-regulated' to full capacity within 48&nbsp;hours of prey consumption. Being ectothermic ("cold-blooded"), the surrounding temperature plays an important role in the digestion process. The ideal temperature for snakes to digest food is {{convert|30|°C|°F}}. There is a huge amount of [[Biology:Metabolism|metabolic]] energy involved in a snake's digestion, for example the surface body temperature of the South American rattlesnake (''[[Biology:Crotalus durissus|Crotalus durissus]]'')  increases by as much as {{convert|1.2|C-change|sigfig=2}} during the digestive process.<ref>{{cite journal | vauthors = Tattersall GJ, Milsom WK, Abe AS, Brito SP, Andrade DV | title = The thermogenesis of digestion in rattlesnakes | journal = The Journal of Experimental Biology | volume = 207 | issue = Pt 4 | pages = 579–85 | date = February 2004 | pmid = 14718501 | doi = 10.1242/jeb.00790 | doi-access = free }}</ref> If a snake is disturbed after having eaten recently, it will often [[Medicine:Vomiting|regurgitate]] its prey to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient; the snake's digestive [[Biology:Enzymes|enzymes]] dissolve and absorb everything but the prey's hair (or feathers) and claws, which are excreted along with [[Chemistry:Uric acid|waste]].

=== Hooding and spitting ===
Hooding (expansion of the neck area) is a visual deterrent, mostly seen in cobras (elapids), and is primarily controlled by rib muscles.<ref>{{Cite journal|last=Young|first=BA|date=2010|title=The functional morphology of hooding in cobras|url=https://journals.biologists.com/jeb/article/213/9/1521/10270/The-functional-morphology-of-hooding-in-cobras|journal=Journal of Experimental Biology|volume=213|issue=9|pages=1521–1528|doi=10.1242/jeb.034447|pmid=20400637|doi-access=free}}</ref> Hooding can be accompanied by spitting venom towards the threatening object,<ref>{{Cite journal|last=Young|first=BA|date=2004|title=The buccal buckle: the functional morphology of venom spitting in cobras|url=https://journals.biologists.com/jeb/article/207/20/3483/14911/The-buccal-buckle-the-functional-morphology-of|journal=Journal of Experimental Biology|volume=207|issue=20|pages=3483–3494|doi=10.1242/jeb.01170|pmid=15339944|doi-access=free}}</ref> and producing a specialized sound; hissing. Studies on captive cobras showed that 13 to 22% of the body length is raised during hooding.<ref>{{Cite journal|date=2016|title=The defensive behaviour of Naja oxiana, with comments on the visual displays of cobras|url=https://www.thebhs.org/publications/the-herpetological-bulletin/issue-number-138-winter-2016/960-04-the-defensive-behaviour-of-i-naja-oxiana-i-with-comments-on-the-visual-displays-of-cobras/file|journal=The Herpetological Bulletin|archive-url=|lay-url=https://www.thebhs.org/publications/the-herpetological-bulletin/issue-number-138-winter-2016/960-04-the-defensive-behaviour-of-i-naja-oxiana-i-with-comments-on-the-visual-displays-of-cobras}}</ref> 

===Locomotion===
The lack of limbs does not impede the movement of snakes. They have developed several different modes of locomotion to deal with particular environments. Unlike the gaits of limbed animals, which form a continuum, each mode of snake locomotion is discrete and distinct from the others; transitions between modes are abrupt.<ref name="Cogger91_175">Cogger(1991), p. 175.</ref><ref name = "Gray">{{cite journal | vauthors = Gray J | title = The mechanism of locomotion in snakes | journal = The Journal of Experimental Biology | volume = 23 | issue = 2 | pages = 101–20 | date = December 1946 | doi = 10.1242/jeb.23.2.101 | pmid = 20281580 }}</ref>

====Lateral undulation====
{{Main|Physics:Undulatory locomotion|Biology:Hydrophiinae}}
[[File:Foot prints of Snake.jpg|thumb|Crawling prints of a snake]]
Lateral undulation is the sole mode of aquatic locomotion, and the most common mode of terrestrial locomotion.<ref name = "Gray"/> In this mode, the body of the snake alternately flexes to the left and right, resulting in a series of rearward-moving "waves".<ref name="Cogger91_175"/> While this movement appears rapid, snakes have rarely been documented moving faster than two body-lengths per second, often much less.<ref name = "Hekrotte">{{Cite journal | last = Hekrotte | first = Carlton | name-list-style = vanc | title = Relations of Body Temperature, Size, and Crawling Speed of the Common Garter Snake, Thamnophis s. sirtalis | journal = Copeia | year = 1967 | volume = 23 | issue = 4 | pages = 759–763 | doi = 10.2307/1441886 | jstor = 1441886 }}</ref> This mode of movement has the same net cost of transport (calories burned per meter moved) as running in lizards of the same mass.<ref name = "Walton">{{cite journal | vauthors = Walton M, Jayne BC, Bennet AF | title = The energetic cost of limbless locomotion | journal = Science | volume = 249 | issue = 4968 | pages = 524–7 | date = August 1990 | pmid = 17735283 | doi = 10.1126/science.249.4968.524 | bibcode = 1990Sci...249..524W | s2cid = 17065200 }}</ref>

Terrestrial lateral undulation is the most common mode of terrestrial locomotion for most snake species.<ref name="Cogger91_175"/> In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, irregularities in the soil, etc.<ref name="Cogger91_175"/> Each of these environmental objects, in turn, generates a reaction force directed forward and towards the midline of the snake, resulting in forward thrust while the lateral components cancel out.<ref name = "Gray_lissman"/> The speed of this movement depends upon the density of push-points in the environment, with a medium density of about 8{{clarify|What does this refer to? What scale?|date=June 2016}} along the snake's length being ideal.<ref name = "Hekrotte"/> The wave speed is precisely the same as the snake speed, and as a result, every point on the snake's body follows the path of the point ahead of it, allowing snakes to move through very dense vegetation and small openings.<ref name="Gray_lissman">{{Cite journal | vauthors = Gray J, Lissmann HW | title = Kinetics of locomotion of the grass snake | journal = Journal of Experimental Biology | year = 1950 | volume = 26 | issue = 4 | pages = 354–367 | doi = 10.1242/jeb.26.4.354 | url = http://jeb.biologists.org/cgi/content/abstract/26/4/354 | url-status = live | archive-url = https://web.archive.org/web/20080709022026/http://jeb.biologists.org/cgi/content/abstract/26/4/354 | archive-date = 2008-07-09 }}</ref>

When swimming, the waves become larger as they move down the snake's body, and the wave travels backwards faster than the snake moves forwards.<ref name = "Gray2">{{Cite journal | vauthors = Gray J | title = Undulatory propulsion | journal = Quarterly Journal of Microscopical Science | year = 1953 | volume = 94 | pages = 551–578 }}</ref> Thrust is generated by pushing their body against the water, resulting in the observed slip. In spite of overall similarities, studies show that the pattern of muscle activation is different in aquatic versus terrestrial lateral undulation, which justifies calling them separate modes.<ref name = "Jayne1">{{cite journal | vauthors = Jayne BC | title = 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 = Journal of Morphology | volume = 197 | issue = 2 | pages = 159–81 | date = August 1988 | pmid = 3184194 | doi = 10.1002/jmor.1051970204 | s2cid = 25729192 }}</ref> 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).<ref name="Cogger91_175"/>

====Sidewinding====
{{Main|Physics:Sidewinding}}
[[File:Neonate sidewinder sidewinding with tracks unlabeled.jpg|thumb|right|A neonate sidewinder rattlesnake (''[[Biology:Crotalus cerastes|Crotalus cerastes]]'') sidewinding]]
Most often employed by colubroid snakes ([[Biology:Colubrids|colubrids]], elapids, and [[Biology:Viperidae|vipers]]) when the snake must move in an environment that lacks irregularities to push against (rendering lateral undulation impossible), such as 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, resulting in a peculiar "rolling" motion.<ref name="Cogger91_177">Cogger(1991), p. 177.</ref><ref name = "Jayne2">{{Cite journal | vauthors = Jayne BC | title = Kinematics of terrestrial snake locomotion | journal = Copeia | year = 1986 | pages = 915–927 | doi = 10.2307/1445288 | volume = 1986 | issue = 4 | jstor = 1445288 }}</ref> This mode of locomotion overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping.<ref name="Cogger91_177"/> 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 locomotion has very low caloric cost, less than ⅓ of the cost for a lizard to move the same distance.<ref name="Walton"/> Contrary to popular belief, there is no evidence that sidewinding is associated with the sand being hot.<ref name="Cogger91_177"/>

====Concertina====
{{Main|Biology:Concertina movement}}
When push-points are absent, but there is not enough space to use sidewinding because of lateral constraints, such as in tunnels, snakes rely on concertina locomotion.<ref name="Cogger91_175"/><ref name = "Jayne2"/> In this mode, the snake braces the posterior portion of its body against the tunnel wall while the front of the snake extends and straightens.<ref name="Cogger91_177"/> The front portion then flexes and forms an anchor point, and the posterior is straightened and pulled forwards. This mode of locomotion is slow and very demanding, up to seven times the cost of laterally undulating over the same distance.<ref name="Walton"/> This high cost is due to the repeated stops and starts of portions of the body as well as the necessity of using active muscular effort to brace against the tunnel walls.

====Arboreal====
[[File:Golden tree snake.jpg|thumb|left|Golden tree snake climbing a flower]]
The movement of snakes in arboreal habitats has only recently been studied.<ref name = "Astley">{{cite journal | vauthors = Astley HC, Jayne BC | title = Effects of perch diameter and incline on the kinematics, performance and modes of arboreal locomotion of corn snakes (Elaphe guttata) | journal = The Journal of Experimental Biology | volume = 210 | issue = Pt 21 | pages = 3862–72 | date = November 2007 | pmid = 17951427 | doi = 10.1242/jeb.009050 | doi-access = free }}</ref> While on tree branches, snakes use several modes of locomotion depending on species and bark texture.<ref name="Astley"/> In general, snakes will use a modified form of concertina locomotion on smooth branches, but will laterally undulate if contact points are available.<ref name="Astley"/> Snakes move faster on small branches and when contact points are present, in contrast to limbed animals, which do better on large branches with little 'clutter'.<ref name="Astley"/>

Gliding snakes (''[[Biology:Chrysopelea|Chrysopelea]]'') of Southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees.<ref name="Cogger91_177"/><ref name="Freiberg84_135">Freiberg (1984), p. 135.</ref><ref>{{cite journal | vauthors = Socha JJ | title = Gliding flight in the paradise tree snake | journal = Nature | volume = 418 | issue = 6898 | pages = 603–4 | date = August 2002 | pmid = 12167849 | doi = 10.1038/418603a | bibcode = 2002Natur.418..603S | s2cid = 4424131 }}</ref> These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair.<ref name="Cogger91_177"/><ref name="Freiberg84_135"/>

====Rectilinear====
{{Main|Biology:Rectilinear locomotion}}
The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning.<ref name="Cogger91_176">Cogger (1991), p. 176.</ref> 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.<ref name="Cogger91_176"/> The ribs of the snake do not move in this mode of locomotion and this method is most often used by large [[Biology:Pythonidae|python]]s, [[Biology:Boidae|boa]]s, and [[Biology:Viperidae|viper]]s when stalking prey across open ground as the snake's movements are subtle and harder to detect by their prey in this manner.<ref name="Cogger91_177"/>

==Interactions with humans==
[[File:Snake bite symptoms.png|thumb|Most common symptoms of any kind of snake bite envenomation.<ref name=MedlinePlus/><ref>{{cite web |url=http://www.health-care-clinic.org/diseases/snakebite.html |title=Snake Bite First Aid – Snakebite |website=Health-care-clinic.org |access-date=2016-01-09 |url-status=live |archive-url=https://web.archive.org/web/20160116034305/http://www.health-care-clinic.org/diseases/snakebite.html |archive-date=2016-01-16 }}</ref> Furthermore, there is vast variation in symptoms between bites from different types of snakes.<ref name=MedlinePlus>{{cite web|url=https://medlineplus.gov/ency/article/000031.htm |title=Snake bites |website=MedlinePlus.gov |archive-url=https://web.archive.org/web/20101204102457/http://www.nlm.nih.gov/medlineplus/ency/article/000031.htm |archive-date=4 December 2010 |url-status=live |access-date=2010-03-09}} from Tintinalli JE, Kelen GD, Stapcynski JS, eds. ''Emergency Medicine: A Comprehensive Study Guide''. 6th ed. New York, NY: McGraw Hill; 2004. Update Date: 2/27/2008. Updated by: Stephen C. Acosta, MD, Department of Emergency Medicine, Portland VA Medical Center, Portland, OR. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.</ref>]]

===Bite===
{{Main|Medicine:Snakebite}}
[[File:ViperaBerusFang.JPG|thumb|left|''[[Biology:Vipera berus|Vipera berus]]'', one fang in glove with a small venom stain, the other still in place]]
Snakes do not ordinarily prey on humans. Unless startled or injured, most snakes prefer to avoid contact and will not attack humans. With the exception of large constrictors, nonvenomous snakes are not a threat to humans. The bite of a nonvenomous snake is usually harmless; their teeth are not adapted for tearing or inflicting a deep puncture wound, but rather grabbing and holding. Although the possibility of infection and tissue damage is present in the bite of a nonvenomous snake, venomous snakes present far greater hazard to humans.<ref name="Meh87"/>{{Rp|209}} The [[Organization:World Health Organization|World Health Organisation]] (WHO) lists [[Medicine:Snakebite|snakebite]] under the "other neglected conditions" category.<ref>{{cite web|last1=WHO|title=The 17 neglected tropical diseases|url=https://www.who.int/neglected_diseases/diseases/en/|website=WHO|publisher=WHO|access-date=24 October 2014|url-status=live|archive-url=https://web.archive.org/web/20140222035302/http://www.who.int/neglected_diseases/diseases/en/|archive-date=22 February 2014}}</ref>

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 {{wipe|India}}, 250,000 snakebites are recorded in a single year, with as many as 50,000 recorded initial deaths.<ref name="Sinha">{{Cite news |last = Sinha |first = Kounteya | name-list-style = vanc |title = No more the land of snake charmers... |newspaper = The Times of India |date = 25 July 2006 |url = http://timesofindia.indiatimes.com/articleshow/1803026.cms |url-status     = live |archive-url = https://web.archive.org/web/20110811133738/http://articles.timesofindia.indiatimes.com/2006-07-25/india/27815969_1_snake-bites-russell-s-viper-krait-cobra |archive-date = 11 August 2011 }}</ref> The WHO estimates that on the order of 100 000 people die each year as a result of snake bites, and around three times as many amputations and other permanent disabilities are caused by snakebites annually.<ref>{{Cite web|url=https://www.who.int/mediacentre/factsheets/fs337/en/|title=Snakebite envenoming|website=World Health Organization|language=en-GB|access-date=2017-10-27|url-status=live|archive-url=https://web.archive.org/web/20170418105431/http://www.who.int/mediacentre/factsheets/fs337/en/|archive-date=2017-04-18}}</ref>

The treatment for a snakebite is as variable as the bite itself. The most common and effective method is through [[Medicine:Antivenom|antivenom]] (or antivenin), a serum made from the venom of the snake. Some antivenom is species-specific (monovalent) while some is made for use with multiple species in mind (polyvalent). In the United States for example, all species of venomous snakes are [[Biology:Pit viper|pit viper]]s, with the exception of the [[Biology:Coral snake|coral snake]]. To produce antivenom, a mixture of the venoms of the different species of rattlesnakes, copperheads, and cottonmouths is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted from the immunized horse. The serum is separated and further purified and freeze-dried. It is reconstituted with sterile water and becomes antivenom. For this reason, people who are allergic to horses are more likely to suffer an allergic reaction to antivenom.<ref>{{cite journal | vauthors = Dubinsky I | title = Rattlesnake bite in a patient with horse allergy and von Willebrand's disease: case report | journal = Canadian Family Physician | volume = 42 | pages = 2207–11 | date = November 1996 | pmid = 8939322 | pmc = 2146932 }}</ref> Antivenom for the more dangerous species (such as [[Biology:Mamba|mamba]]s, [[Biology:Taipan|taipan]]s, and cobras) is made in a similar manner in India, South Africa, and Australia, although these antivenoms are species-specific.

===Snake charmers===
[[File:Snake in basket.jpg|thumb|upright|left|The Indian cobra is the most common subject of snake charmings.]] 
In some parts of the world, especially in India, snake charming is a roadside show performed by a charmer. In such a show, the snake charmer carries a basket containing a snake that he seemingly charms by playing tunes with his flutelike musical instrument, to which the snake responds.<ref name="Bagla"/> The snake is in fact responding to the movement of the flute, not the sound it makes, as snakes lack external ears (though they do have internal ears).<ref name="Bagla">{{cite news |last = Bagla |first = Pallava | name-list-style = vanc |title = India's Snake Charmers Fade, Blaming Eco-Laws, TV |date = April 23, 2002 |work = National Geographic News |url = http://news.nationalgeographic.com/news/2002/04/0417_020423_snakecharm_2.html |access-date  = 2007-11-26 |url-status = live |archive-url  = https://web.archive.org/web/20071218213538/http://news.nationalgeographic.com/news/2002/04/0417_020423_snakecharm_2.html |archive-date = December 18, 2007 }}</ref>

The Wildlife Protection Act of 1972 in India technically prohibits snake charming on the grounds of reducing animal cruelty. Other types of snake charmers use a snake and mongoose show, where the two animals have a mock fight; however, this is not very common, as the animals may be seriously injured or killed. Snake charming as a profession is dying out in India because of competition from modern forms of entertainment and environment laws proscribing the practice. Many Indians have never seen snake charming and it is becoming a folktale of the past.<ref name="Bagla"/><ref>{{Cite news|last=Harding|first=Luke| name-list-style = vanc |url=https://www.theguardian.com/world/2002/apr/02/worlddispatch.lukeharding|title=Snake tricks lose their charm|date=2 April 2002|work=The Guardian|access-date=16 April 2020}}</ref><ref>{{Cite news|last=Chandra|first=Sharmila| name-list-style = vanc |url=https://www.indiatoday.in/magazine/indiascope/story/19891231-indias-snake-charmers-sway-on-the-edge-of-extinction-816887-1989-12-31|title=India's snake-charmers sway on the edge of extinction|date=12 November 2013|work=India Today|access-date=16 April 2020}}</ref><ref>{{cite book|url=https://books.google.com/books?id=BumyQJ14n8sC&pg=PA482 |title=International Wildlife Encyclopedia |last1=Burton |last2=Burton |first1=Maurice |first2=Robert |year=2002 |volume=4 |edition=3rd |section=Snake charmer's bluff|page=482 |isbn=9780761472704 |archive-url=https://web.archive.org/web/20160818204631/https://books.google.com/books?id=BumyQJ14n8sC&pg=PA482 |archive-date=2016-08-18 |url-status=live}}</ref>

===Trapping===
The ''Irulas'' tribe of Andhra Pradesh and Tamil Nadu in India have been hunter-gatherers in the hot, dry plains forests, and have practiced the art of snake catching for generations. They have a vast knowledge of snakes in the field. They generally catch the snakes with the help of a simple stick. Earlier, the ''Irulas'' caught thousands of snakes for the snake-skin industry. After the complete ban of the snake-skin industry in India and protection of all snakes under the Indian Wildlife (Protection) Act 1972, they formed the Irula Snake Catcher's Cooperative and switched to catching snakes for removal of venom, releasing them in the wild after four extractions. The venom so collected is used for producing life-saving antivenom, biomedical research and for other medicinal products.<ref name="Whitaker et al.">Whitaker, Romulus & Captain, Ashok. ''Snakes of India: The Field Guide''. (2004) pp 11 to 13.</ref> The ''Irulas'' are also known to eat some of the snakes they catch and are very useful in rat extermination in the villages.

Despite the existence of snake charmers, there have also been professional snake catchers or wranglers. Modern-day snake trapping involves a herpetologist using a long stick with a V- shaped end. Some television show hosts, like Bill Haast, Austin Stevens, Steve Irwin, and Jeff Corwin, prefer to catch them using bare hands.
<!-- NOTE: Austin Stevens only catches nonvenomous snakes with his bare hands. He uses snake tongs otherwise. -->
<!--
At least one tribe uses a specialized form of snake catching as a rite of passage to manhood.{{Citation needed|date=February 2007}} The young man of interest will wrap his leg heavily in some type of cloth all the way to the inseam. He will then stick his leg in a burrow containing a large python, typically a reticulated python. After the snake swallows most of his leg, several other members of the tribe will pull him out of the hole along with the snake. The snake is then killed and the man's leg removed from the snake. These snakes can be over {{convert|7|m|ft|abbr=off|sp=us}} long, and it is possible for the man to have his leg dislocated. The scent of a prey animal may be used to help convince the snake to swallow the leg. Snakes have a single-tract digestive system, but the digestion process is actually much slower.

Commented this paragraph out till the tribe is identified and a reference given.
-->

===Consumption===
[[File:Guangzhou-snakes-at-restaurant-0457.jpg|thumb|A "海豹蛇" ("sea-leopard snake", supposedly ''[[Biology:Enhydris|Enhydris]] bocourti'') occupies a place of honor among the live delicacies on display outside a Guangzhou restaurant.]]
Although snakes are not commonly thought of as food, their consumption is acceptable in some cultures and may even be considered a delicacy. Snake soup is popular in Cantonese cuisine, consumed by locals in the autumn to warm their bodies. Western cultures document the consumption of snakes only under extreme circumstances of hunger,<ref>{{cite journal| vauthors = Irvine FR |year=1954|title=Snakes as food for man|journal=British Journal of Herpetology|volume=1|issue=10| pages=183–189 }}</ref> with the exception of cooked rattlesnake meat, which is commonly consumed in {{wipe|Texas}}<ref>{{cite news |last1=Shepherd |first1=Katie| name-list-style = vanc  |title=John Cornyn criticized Chinese for eating snakes. He forgot about the rattlesnake roundups back in Texas. |url=https://www.washingtonpost.com/nation/2020/03/19/coronavirus-china-cornyn-blame/ |access-date=March 19, 2020 |work=The Washington Post |date=March 19, 2020}}</ref> and parts of the Midwestern United States.
[[File:Snake meat.jpg|thumb|left|Snake meat, in a Taipei restaurant]]
In [[Earth:Asia|Asia]]n countries such as China, Taiwan, Thailand, Indonesia, Vietnam, and Cambodia, drinking the blood of a snake—particularly the cobra—is believed to increase sexual virility.<ref name="Flynn">{{cite news |last = Flynn |first = Eugene | name-list-style = vanc |title  = Flynn of the Orient Meets the Cobra |date = April 23, 2002 |publisher = Fabulous Travel |url = http://www.fabuloustravel.com/gourmet/travel/cobrasblood/cobra.html |access-date  = 2007-11-26 |url-status = dead |archive-url = https://web.archive.org/web/20071117191253/http://www.fabuloustravel.com/gourmet/travel/cobrasblood/cobra.html |archive-date = November 17, 2007}}</ref> When possible, the blood is drained while the cobra is still alive, and it is usually mixed with some form of [[Chemistry:Liquor|liquor]] to improve the taste.<ref name="Flynn"/>

The use of snakes in alcohol is accepted in some Asian countries. In such cases, one or more snakes are left to steep in a jar or container of liquor, as this is claimed to make the liquor stronger (as well as more expensive). One example of this is the Habu snake, which is sometimes placed in the Okinawan liqueur ''Habushu'' (ブ酒), also known as "Habu Sake".<ref name="Allen">
{{cite news
 |last        = Allen
 |first       = David
 |title       = Okinawa's potent habu sake packs healthy punch, poisonous snake
 |date        = July 22, 2001
 |work        = Stars and Stripes
 |url         = http://ww2.pstripes.osd.mil/01/mag/sm072201c.html
 |access-date  = 2007-11-26
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20071128045728/http://ww2.pstripes.osd.mil/01/mag/sm072201c.html
 |archive-date = November 28, 2007
}}</ref>

[[Social:Snake wine|Snake wine]] (蛇酒) is an alcoholic beverage produced by infusing whole snakes in rice wine or grain alcohol. First recorded as being consumed in China during the Western Zhou dynasty, this drink is considered an important curative and is believed to reinvigorate a person according to [[Medicine:Traditional Chinese medicine|traditional Chinese medicine]].<ref name=sw1>{{cite web|title=蛇酒的泡制与药用(The production and medicinal qualities of snake wine)|date=2007-04-09|url=http://www.cn939.com/tcm-article-read-4694.html|website=CN939.com|url-status=live|archive-url=https://web.archive.org/web/20110706150614/http://www.cn939.com/tcm-article-read-4694.html|archive-date=2011-07-06}}</ref>

===Pets===
In the Western world, some snakes are kept as pets, especially docile species such as the ball python and [[Biology:Corn snake|corn snake]]. To meet the demand, a captive breeding industry has developed. Snakes bred in captivity are considered preferable to specimens caught in the wild and tend to make better pets.<ref>{{cite book |last1 = Ernest |first1 = Carl | first2 = George R. | last2 = Zug | first3 = Molly Dwyer | last3 = Griffin | name-list-style = vanc |title = Snakes in Question: The Smithsonian Answer Book |publisher = Smithsonian Books |year = 1996 |location = Washington, D.C. |page = [https://archive.org/details/snakesinquestion00erns/page/203 203] |isbn = 978-1-56098-648-5 |url = https://archive.org/details/snakesinquestion00erns/page/203 }}</ref> Compared with more traditional types of companion animal, snakes can be very low-maintenance pets; they require minimal space, as most common species do not exceed {{convert|5|ft|m}} in length, and can be fed relatively infrequently—usually once every five to 14 days. Certain snakes have a lifespan of more than 40 years if given proper care.

===Symbolism===
{{Main|Unsolved:Serpent (symbolism)}}
[[File:Golden Uraes Cobra Tutankhamun's Throne.jpg|thumb|left|The reverse side of the throne of Pharaoh Tutankhamun with four golden uraeus cobra figures. Gold with lapis lazuli; Valley of the Kings, Thebes (1347–37 BCE).]]
[[File:Longane, Sicily.JPG|thumb|upright=0.65|Snakes composing a bronze kerykeion from the mythical Longanus river in Sicily]]
[[File:Salvage Scrap propaganda poster crop2.jpg|thumb|upright|left|Imperial Japan depicted as an evil snake in a WWII propaganda poster]]
[[File:Brazilian Expeditionary Forces insignia (smoking snake).svg|thumb|upright|left|"The Smoking Snake", insignia of the Brazilian Expeditionary Force in WWII]]
[[File:Kyyjärvi.vaakuna.svg|thumb|upright|left|A common adder in the coat of arms of the Kyyjärvi municipality]]

In [[Place:Mesopotamia|ancient Mesopotamia]], [[Religion:Nirah|Nirah]], the messenger god of [[Religion:Ištaran|Ištaran]], was represented as a serpent on ''kudurrus'', or boundary stones.<ref name="BlackGreen1992">{{cite book|last1=Black|first1=Jeremy|first2=Anthony|last2=Green | name-list-style = vanc |title=Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary|location=Austin, Texas|publisher=University of Texas Press|year=1992|isbn=978-0714117058|pages=166–168}}</ref> Representations of two intertwined serpents are common in Sumerian art and Neo-Sumerian artwork<ref name="BlackGreen1992"/> and still appear sporadically on cylinder seals and amulets until as late as the thirteenth century BC.<ref name="BlackGreen1992"/> The horned viper (''[[Biology:Cerastes cerastes|Cerastes cerastes]]'') appears in Kassite and [[History:Neo-Assyrian Empire|Neo-Assyrian]] kudurrus<ref name="BlackGreen1992"/> and is invoked in [[Place:Assyria|Assyria]]n texts as a magical protective entity.<ref name="BlackGreen1992"/> A dragon-like creature with horns, the body and neck of a snake, the forelegs of a lion, and the hind-legs of a bird appears in Mesopotamian art from the Akkadian Period until the Hellenistic Period (323 BC–31 BC).<ref name="BlackGreen1992"/> This creature, known in [[Social:Akkadian language|Akkadian]] as the ''[[Unsolved:Mušḫuššu|mušḫuššu]]'', meaning "furious serpent", was used as a symbol for particular deities and also as a general protective emblem.<ref name="BlackGreen1992"/> It seems to have originally been the attendant of the Underworld god [[Religion:Ninazu|Ninazu]],<ref name="BlackGreen1992"/> but later became the attendant to the Hurrian storm-god [[Religion:Tishpak|Tishpak]], as well as, later, Ninazu's son [[Religion:Ningishzida|Ningishzida]], the Babylonian national god [[Religion:Marduk|Marduk]], the scribal god [[Religion:Nabu|Nabu]], and the Assyrian national god Ashur.<ref name="BlackGreen1992"/>

In Egyptian history, the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was worshipped as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra).{{citation needed|date=April 2018}} The [[Ouroboros|ouroboros]] was a well-known ancient Egyptian symbol of a serpent swallowing its own tail.<ref name="Hornung">{{cite book|last=Hornung|first=Erik|date=2001|title=The Secret Lore of Egypt: Its Impact on the West|url=https://books.google.com/books?id=SB_y56Vlz5kC&q=Ouroboros+dragon+ancient+Egypt&pg=PA75|location=Ithaca, New York and London, England|publisher=Cornell University Press|isbn=978-0-8014-3847-9|pages=13, 44}}</ref> The precursor to the ouroboros was the "Many-Faced",<ref name="Hornung"/> a serpent with five heads, who, according to the [[Religion:Amduat|Amduat]], the oldest surviving [[Religion:Book of the Dead|Book of the Afterlife]], was said to coil around the corpse of the sun god Ra protectively.<ref name="Hornung"/> The earliest surviving depiction of a "true" ouroboros comes from the gilded shrines in the tomb of Tutankhamun.<ref name="Hornung"/> In the early centuries AD, the ouroboros was adopted as a symbol by [[Philosophy:Gnosticism|Gnostic]] Christians<ref name="Hornung"/> and chapter 136 of the ''Pistis Sophia'', an early Gnostic text, describes "a great dragon whose tail is in its mouth".<ref name="Hornung"/> In medieval alchemy, the ouroboros became a typical western dragon with wings, legs, and a tail.<ref name="Hornung"/>

In the [[Religion:Bible|Bible]], King Nahash of Ammon, whose name means "Snake", is depicted very negatively, as a particularly cruel and despicable enemy of the ancient Hebrews.

[[File:Medusa by Carvaggio.jpg|thumb|upright|''Medusa'' (1597) by the Italian artist Caravaggio ]]
The ancient Greeks used the [[Social:Gorgoneion|Gorgoneion]], a depiction of a hideous face with serpents for hair, as an [[Social:Apotropaic magic|apotropaic symbol]] to ward off evil.<ref>{{cite journal|last=Phinney|first=Edward, Jr.|date=1971|title=Perseus' Battle with the Gorgons|journal=Transactions and Proceedings of the American Philological Association|volume=102|pages=445–463|doi=10.2307/2935950|jstor=2935950}}</ref> In a [[Religion:Greek mythology|Greek myth]] described by Pseudo-Apollodorus in his ''Bibliotheca'', [[Unsolved:Medusa|Medusa]] was a [[Unsolved:Gorgon|Gorgon]] with serpents for hair whose gaze turned all those who looked at her to stone and was slain by the hero [[Social:Perseus|Perseus]].<ref>{{cite book|last=Kinsley|first=David|date=1989|title=The Goddesses' Mirror: Visions of the Divine from East and West|location=Albany, New York|publisher=New York State University Press|isbn=978-0-88706-836-2|url=https://books.google.com/books?id=82oFlfs3MpwC&q=Athena+pallas+goddess&pg=PA142|page=151}}</ref><ref>{{cite book|last=Deacy|first=Susan| title=Athena|location=New York City, New York and London, England|publisher=Routledge|date=2008|isbn=978-0-415-30066-7|url=https://books.google.com/books?id=kIiCAgAAQBAJ&q=Athena+and+Ares+Darmon&pg=PA163}}</ref><ref>Pseudo-Apollodorus, ''Bibliotheca'' 2.37, 38, 39</ref> In the Roman poet Ovid's ''Metamorphoses'', [[Unsolved:Medusa|Medusa]] is said to have once been a beautiful priestess of Athena, whom Athena turned into a serpent-haired monster after she was raped by the god [[Religion:Poseidon|Poseidon]] in Athena's temple.<ref>{{cite journal|last=Seelig|first=Beth J.|date=August 2002|title=The Rape of Medusa in the Temple of Athena: Aspects of Triangulation in the Girl|journal=The International Journal of Psychoanalysis|volume=83|issue=4|pages=895–911|doi=10.1516/3NLL-UG13-TP2J-927M|pmid=12204171|s2cid=28961886}}</ref> In another myth referenced by the [[Place:Boeotia|Boeotia]]n poet [[Biography:Hesiod|Hesiod]] and described in detail by Pseudo-Apollodorus, the hero [[Religion:Heracles|Heracles]] is said to have slain the [[Unsolved:Lernaean Hydra|Lernaean Hydra]],<ref name="West2007">{{cite book|last=West|first=Martin Litchfield|author-link=Martin Litchfield West|title=Indo-European Poetry and Myth|url=https://archive.org/details/indoeuropeanpoet00west|url-access=limited|date=2007|publisher=Oxford University Press|location=Oxford, England|isbn=978-0-19-928075-9|page=[https://archive.org/details/indoeuropeanpoet00west/page/n272 258]}}</ref><ref name="Ogden2013">{{cite book|last=Ogden|first=Daniel|date=2013|title=''Drakon'': Dragon Myth and Serpent Cult in the Ancient Greek and Roman Worlds|url=https://books.google.com/books?id=FQ2pAK9luwkC&q=ancient+Greek+dragons|location=Oxford, England|publisher=Oxford University Press|isbn=978-0-19-955732-5|pages=28–29}}</ref> a multiple-headed serpent which dwelt in the swamps of Lerna.<ref name="West2007"/><ref name="Ogden2013"/>

The legendary account of the foundation of Thebes mentioned a monster snake guarding the spring from which the new settlement was to draw its water. In fighting and killing the snake, the companions of the founder Cadmus all perished – leading to the term "Cadmean victory" (i.e. a victory involving one's own ruin).{{citation needed|date=July 2016}}

[[File:Rod of asclepius.jpg|thumb|upright|[[Rod of Asclepius|Rod of Asclepius]], in which the snake, through ecdysis, symbolizes healing]]
Three medical symbols involving snakes that are still used today are [[Bowl of Hygieia]], symbolizing pharmacy, and the [[Caduceus]] and [[Rod of Asclepius]], which are symbols denoting medicine in general.<ref name=AIM/>

One of the etymologies proposed for the common female first name ''Linda'' is that it might derive from Old German ''Lindi'' or ''Linda'', meaning a serpent.

India is often called the land of snakes and is steeped in tradition regarding snakes.<ref name="Deane_61"/> Snakes are worshipped as gods even today with many women pouring milk on snake pits (despite snakes' aversion for milk).<ref name="Deane_61">Deane (1833). p. 61.</ref> The cobra is seen on the neck of [[Religion:Shiva|Shiva]] and [[Religion:Vishnu|Vishnu]] is depicted often as sleeping on a seven-headed snake or within the coils of a serpent.<ref>Deane (1833). pp. 62–64.</ref> There are also several temples in India solely for cobras sometimes called ''Nagraj'' (King of Snakes) and it is believed that snakes are symbols of fertility. There is a Hindu festival called Nag Panchami each year on which day snakes are venerated and prayed to. See also ''[[Unsolved:Nāga|Nāga]]''.{{citation needed|date=July 2016}}

In India there is another mythology about snakes. Commonly known in [[Social:Hindi|Hindi]] as "Ichchhadhari" snakes. Such snakes can take the form of any living creature, but prefer human form. These mythical snakes possess a valuable gem called "Mani", which is more brilliant than diamond. There are many stories in India about greedy people trying to possess this gem and ending up getting killed.{{citation needed|date=July 2016}}

The [[Unsolved:Snake (zodiac)|snake]] is one of the 12 celestial animals of [[Unsolved:Chinese zodiac|Chinese zodiac]], in the Chinese calendar.<ref>{{cite web | work=timeanddate.com | title=The Chinese Calendar | url=https://www.timeanddate.com/calendar/about-chinese.html | access-date=1 June 2017 | url-status=live | archive-url=https://web.archive.org/web/20170815132857/https://www.timeanddate.com/calendar/about-chinese.html | archive-date=15 August 2017 }}</ref>

Many ancient Peruvian cultures worshipped nature.<ref>{{cite book | last=Benson |first=  Elizabeth | title= The Mochica: A Culture of Peru |location=London | publisher=Thames and Hudson |year= 1972 | isbn= 978-0-500-72001-1 }}</ref> They emphasized animals and often depicted snakes in their art.<ref>{{cite book | last1=Berrin |first1= Katherine |last2= Larco Museum | name-list-style = vanc |title=The Spirit of Ancient Peru: Treasures from the Museo Arqueológico Rafael Larco Herrera | location= New York |publisher= Thames and Hudson |year= 1997 |isbn=978-0-500-01802-6 |title-link= Larco Museum }}</ref>

===Religion===
Snakes are used in [[Religion:Hinduism|Hinduism]] as a part of ritual worship.<ref>{{cite web|url=https://timesofindia.indiatimes.com/city/goa/hindus-unite-to-worship-the-snake-god-today/articleshow/9474357.cms |title=Hindus unite to worship the snake god today |date=2011-08-04 |last=Kerkar |first=Rajendra P. |work=The Times of India |access-date=2021-03-03}}</ref> In the annual Nag Panchami festival, participants worship either live cobras or images of [[Unsolved:Nāga|Nāga]]s. Lord [[Religion:Shiva|Shiva]] is depicted in most images with a snake coiled around his neck.<ref>{{cite web|url=https://www.timesnownews.com/spiritual/religion/article/what-is-the-significance-of-the-snake-around-lord-shivas-neck/468331 |title=What is the significance of the snake around Lord Shiva's neck? |date=2019-08-22 |last=Iyer |first=Gayathri |website=TimesNowNews.com |access-date=2021-03-02}}</ref> [[Religion:Puranas|Puranic]] literature includes various stories associated with snakes, for example [[Religion:Shesha|Shesha]] is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of [[Religion:Vishnu|Vishnu]] from all his mouths. Other notable snakes in Hinduism are Vasuki, Takshaka, Karkotaka, and [[Pingala]]. The term ''Nāga'' is used to refer to entities that take the form of large snakes in Hinduism and [[Religion:Buddhism|Buddhism]].{{citation needed|date=May 2021}}

Snakes have been widely revered in many cultures, such as in {{wipe|Ancient Greece}} where the serpent was seen as a healer.<ref>{{cite book|url=https://www.sciencedirect.com/science/article/pii/B9780128153390000172 |via=[[ScienceDirect]] |access-date=2021-03-03 |editor-last=Wexler |editor-first=Philip |title=History of Toxicology and Environmental Health Series: Toxicology in Antiquity |edition=Second |contribution=Chapter 17 – Asclepius and the Snake as Toxicological Symbols in Ancient Greece and Rome |last1=Tsoucalas |first1=Gregory |last2=Androutsos |first2=George |year=2019 |pages=257–265 |publisher=Elsevier Inc. |isbn=978-0-12-815339-0}}</ref> [[Religion:Asclepius|Asclepius]] carried a serpent wound around his wand, a symbol seen today on many ambulances.<ref>{{cite web|url=https://www.mediband.com/gb/blog/medical-emergency-symbol-origin/ |title=The Origin of the Medical Emergency Symbol |date=2019-02-26 |website=Mediband.com |access-date=2021-02-26}}</ref> In [[Religion:Judaism|Judaism]], the snake of [[Chemistry:Brass|brass]] is also a symbol of healing, of one's life being saved from imminent death.<ref>{{bibleverse|Numbers|21:6–21:9|NAB}}</ref>

In religious terms, the snake and [[Biology:Jaguar|jaguar]] were arguably the most important animals in ancient [[Place:Mesoamerica|Mesoamerica]].<ref>{{cite web|url=https://www.manzanillosun.com/the-mesoamerican-serpent/ |title=The Mesoamerican Serpent |date=2019-08-01 |last=Vickery |first=Kirby |work=Manzanillo Sun |access-date=2021-03-03}}</ref> "In states of ecstasy, lords dance a serpent dance; great descending snakes adorn and support buildings from [[Astronomy:Chichen Itza|Chichen Itza]] to Tenochtitlan, and the [[Social:Nahuatl|Nahuatl]] word ''coatl'' meaning serpent or twin, forms part of primary deities such as [[Religion:Mixcoatl|Mixcoatl]], [[Unsolved:Quetzalcoatl|Quetzalcoatl]], and Coatlicue."<ref>''The Gods and Symbols of Ancient Mexico and the Maya''. Miller, Mary 1993 Thames & Hudson. London {{ISBN|978-0-500-27928-1}}</ref> In the [[Astronomy:Maya calendar|Maya]] and Aztec calendars, the fifth day of the week was known as Snake Day.

In some parts of [[Religion:Christianity|Christianity]], the redemptive work of [[Biography:Jesus|Jesus Christ]] is compared to saving one's life through beholding the [[Unsolved:Nehushtan|Nehushtan]] (serpent of brass).<ref>{{bibleverse|John|3:14|NAB}}</ref> Snake handlers use snakes as an integral part of church worship, to demonstrate their faith in divine protection. However, more commonly in Christianity, the serpent has been depicted as a representative of [[Philosophy:Evil|evil]] and sly plotting, as seen in the description in [[Religion:Book of Genesis|Genesis]] of a snake tempting Eve in the [[Religion:Garden of Eden|Garden of Eden]].<ref name=genesis/> Saint Patrick is purported to have expelled all snakes from Ireland while converting the country to Christianity in the 5th century, thus explaining the absence of snakes there.<ref>{{cite web|url=https://www.irishcentral.com/roots/saint-patrick-banish-snakes-irleand |title=Did St. Patrick really banish all the snakes from Ireland? |date=2019-02-24 |last=Drew |first=April |website=IrishCentral.com |access-date=2021-03-02}}</ref>

In Christianity and Judaism, the snake makes its infamous appearance in the first book of the Bible when a serpent appears before [[Religion:Adam and Eve|Adam and Eve]] and tempts them with the [[Religion:Forbidden fruit|forbidden fruit]] from the Tree of Knowledge.<ref name=genesis>{{bibleverse|Genesis|3:1|NAB}}</ref> The snake returns in the Book of Exodus when [[Biography:Moses|Moses]] turns his staff into a snake as a sign of God's power, and later when he makes the 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. The serpent makes its final appearance symbolizing [[Biography:Satan|Satan]] in the [[Religion:Book of Revelation|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."<ref>{{bibleverse|Revelation|20:2|NAB}}</ref>

In Neo-Paganism and [[Religion:Wicca|Wicca]], the snake is seen as a symbol of wisdom and knowledge.{{citation needed|date=May 2021}}
[[File:Mixco Viejo ballcourt marker.jpg|thumb|left|upright=0.85|Ballcourt marker from the Postclassic site of Mixco Viejo in Guatemala. This sculpture depicts Kukulkan, jaws agape, with the head of a human warrior emerging from his maw.<ref>{{cite book |last1 = Sharer | first1 = Robert J. | first2 = Loa P. | last2 = Traxler | name-list-style = vanc |year=2006 |title=The Ancient Maya |edition=6th (fully revised) |location=Stanford, California |publisher=Stanford University Press |isbn=978-0-8047-4817-9 |page=[https://archive.org/details/ancientmaya0006shar/page/619 619] |oclc=57577446 |url=https://archive.org/details/ancientmaya0006shar/page/619 }}</ref>]]

===Medicine===
Several compounds from snake venoms are being researched as potential treatments or preventatives for pain, cancers, arthritis, stroke, heart disease, hemophilia, and hypertension, and to control bleeding (e.g. during surgery).<ref>{{cite journal | vauthors = Vyas VK, Brahmbhatt K, Bhatt H, Parmar U | title = Therapeutic potential of snake venom in cancer therapy: current perspectives | journal = Asian Pacific Journal of Tropical Biomedicine | volume = 3 | issue = 2 | pages = 156–62 | date = February 2013 | pmid = 23593597 | pmc = 3627178 | doi = 10.1016/S2221-1691(13)60042-8 }}</ref><ref>{{cite journal|last1=Holland|first1=Jennifer S.| name-list-style = vanc |title=The Bite That Heals|journal=National Geographic|date=Feb 2013|url=https://www.nationalgeographic.com/magazine/2013/02/venom/}}</ref><ref>{{cite book|last1=Wilcox|first1=Christie| name-list-style = vanc |title=Venomous|date=2016|publisher=Scientific American|isbn=978-0374283377|url=http://christiewilcox.com/venomous.html}}</ref>

== See also ==
{{div col|colwidth=30em}}
* Limbless vertebrates
* List of Serpentes families
* List of snakes
* Ophiology
* Snakebot
* Snake skeleton
* Spinal osteoarthropathy (reptile disease)
* ''[[The New Encyclopedia of Snakes]]''
* The Snakes of Europe and Snakes of Europe Wikibooks:Snakes of Europe, a wikibook
* Venomous snake
{{div col end}}
{{Clear}}

== References ==
{{reflist}}

== Further reading ==
{{Refbegin|30em}}
* {{cite book |last1 = Behler |first1 = John L. |last2 = King |first2 = F. Wayne | name-list-style = vanc |title = The Audubon Society Field Guide to Reptiles and Amphibians of North America |publisher = Alfred A. Knopf |location = New York |page = [https://archive.org/details/audubonsocietyfi00behl/page/581 581] |year = 1979 |isbn = 978-0-394-50824-5 |url = https://archive.org/details/audubonsocietyfi00behl/page/581 }}
* {{cite book |last =Bullfinch |first =Thomas | name-list-style = vanc |title =Bullfinch's Complete Mythology |publisher =Chancellor Press |year =2000 |location =London |page =679 |url =http://etext.library.adelaide.edu.au/b/bulfinch/thomas/ |isbn =978-0-7537-0381-6 |url-status =dead |archive-url =https://web.archive.org/web/20090209004721/http://etext.library.adelaide.edu.au/b/bulfinch/thomas/ |archive-date =2009-02-09 }}
* {{cite book |last= Capula |first= Massimo |author2= Behler | name-list-style = vanc |title= Simon & Schuster's Guide to Reptiles and Amphibians of the World |year= 1989 |publisher= Simon & Schuster |location= New York |isbn= 978-0-671-69098-4 |url= https://archive.org/details/simonschustersgu00capu }}
* {{cite book |last= Coborn |first= John | name-list-style = vanc |title=The Atlas of Snakes of the World |year= 1991 |location=New Jersey |publisher = TFH Publications |isbn= 978-0-86622-749-0}}
* {{cite book |last1 = Cogger |first1 = Harold | name-list-style = vanc |last2 = Zweifel |first2 = Richard |title = Reptiles & Amphibians |publisher = Weldon Owen |location = Sydney |year = 1992 |isbn = 978-0-8317-2786-4 |url = https://archive.org/details/reptilesamphibia00coggrich }}
* {{cite book |last1 =Conant |first1 =Roger |last2 =Collins |first2 =Joseph | name-list-style = vanc |title =A Field Guide to Reptiles and Amphibians Eastern/Central North America |publisher =Houghton Mifflin Company |year =1991 |location =Boston |isbn =978-0-395-58389-0 |url-access =registration |url =https://archive.org/details/fieldguidetorept00cona }}
* {{cite book |last =Deane |first =John |title =The Worship of the Serpent |publisher =Kessinger Publishing |year =1833 |location =Whitefish, Montana |page =412 |url =http://www.sacred-texts.com/etc/wos/index.htm |isbn =978-1-56459-898-1}}
* {{cite book |last =Ditmars |first =Raymond L |title = Poisonous Snakes of the United States: How to Distinguish Them |publisher =E. R. Sanborn |year =1906 |location =New York |page =11}}
* {{cite book |last =Ditmars |first =Raymond L |title = Snakes of the World |url =https://archive.org/details/snakesofworld00ditm |url-access =registration |publisher =Macmillan |year =1931 |location =New York |page =[https://archive.org/details/snakesofworld00ditm/page/11 11] |isbn = 978-0-02-531730-7 }}
* {{cite book |last =Ditmars |first =Raymond L | name-list-style = vanc | title = Reptiles of the World: The Crocodilians, Lizards, Snakes, Turtles and Tortoises of the Eastern and Western Hemispheres |publisher =Macmillan |year =1933 |location =New York |page =321}}
* {{cite book |last1 =Ditmars |first1 =Raymond L | first2 = W. | last2 = Bridges | name-list-style = vanc  |title = Snake-Hunters' Holiday |publisher =D. Appleton and Company |year =1935 |location =New York |page =309}}
* {{cite book |last =Ditmars |first =Raymond L | name-list-style = vanc |title = A Field Book of North American Snakes |publisher =Doubleday, Doran & Co |year =1939 |location =Garden City, New York |page =305}}
* {{cite book |last1= Freiberg |first1= Dr. Marcos |last2= Walls |first2= Jerry | name-list-style = vanc |title= The World of Venomous Animals |year= 1984 |publisher= TFH Publications |location= New Jersey |isbn= 978-0-87666-567-1 |url= https://archive.org/details/worldofvenomousa00marc }}
* {{cite book |last1 =Gibbons |first1 =J. Whitfield |last2 =Gibbons |first2 =Whit | name-list-style = vanc |title =Their Blood Runs Cold: Adventures With Reptiles and Amphibians |url =https://archive.org/details/theirbloodrunsco0000gibb |url-access =registration |publisher =University of Alabama Press |year =1983 |location =Alabama |page =[https://archive.org/details/theirbloodrunsco0000gibb/page/164 164] |isbn=978-0-8173-0135-4 }}
* {{cite book |last =Mattison |first =Chris | name-list-style = vanc |title =The New Encyclopedia of Snakes |publisher =Princeton University Press |year =2007 |location =New Jersey |page =272 |isbn=978-0-691-13295-2 }}
* {{cite book | vauthors = McDiarmid RW, Campbell JA, Touré T |year= 1999 |title= Snake Species of the World: A Taxonomic and Geographic Reference |volume= 1 |publisher= Herpetologists' League |page=511 |isbn= 978-1-893777-00-2 }}
* {{cite book |last= Mehrtens |first= John| name-list-style = vanc |title= Living Snakes of the World in Color |url= https://archive.org/details/livingsnakesofwo00mehr |url-access= registration |year= 1987 |publisher= Sterling |location= New York |isbn= 978-0-8069-6461-4}}
* {{cite journal | last1= Nóbrega Alves | first1= RôMulo Romeu | last2= Silva Vieira | first2= Washington Luiz | last3= Santana | first3= Gindomar Gomes | name-list-style = vanc | title= Reptiles used in traditional folk medicine: conservation implications | journal= Biodiversity and Conservation |volume= 17 |issue=8 |pages= 2037–2049 | year= 2008 | doi = 10.1007/s10531-007-9305-0 | s2cid= 42500066 }}
* {{cite book | first = Romulus | last = Whitaker | name-list-style = vanc |title=நம்மை சுட்ரியுள்ள பாம்புகள் (Snakes around us, Tamil) |publisher=National Book Trust |year=1996 |isbn=978-81-237-1905-4 |}}
* {{cite book |last = Rosenfeld |first = Arthur | name-list-style = vanc |title = Exotic Pets |url = https://archive.org/details/exoticpets0000rose |url-access = limited |publisher = Simon & Schuster |location= New York |year= 1989 |page=[https://archive.org/details/exoticpets0000rose/page/293 293] |isbn = 978-0-671-47654-0}}
* {{cite book |last1 =Spawls |first1=Steven |last2 =Branch |first2= Bill | name-list-style = vanc |year= 1995 |title= The Dangerous Snakes of Africa |publisher=Ralph Curtis Publishing |location=Sanibel Island, Florida |page= 192 |isbn = 978-0-88359-029-4}}
{{Refend}}

== External links ==
{{commons and category|Serpentes}}
{{Americana Poster|Serpents}}
* {{cite web |url=https://www.biodiversitylibrary.org/name/Serpentes |title=Bibliography for "Serpentes" |publisher=Biodiversity Heritage Library }}
* {{ITIS |id=174118 |taxon=Serpentes}}
* {{cite web |url=http://enature.com/fieldguides/view_default.asp?curGroupID=7&shapeID=1060 |title=US Snakes |publisher=eNature |url-status=dead |archive-url=https://web.archive.org/web/20080315050142/http://www.enature.com/fieldguides/view_default.asp?curGroupID=7&shapeID=1060 |archive-date=2008-03-15 }}
* {{cite web |url=http://www.naturemagics.com/stock-photo/thumbnails.php?album=7 |title=Snakes of the Indian Subcontinent |publisher=Naturemagics Kerala Photo Gallery }}
* {{cite web |url=http://artedi.nrm.se/nrmherps/qbrowse.php?FormData=scientificName |title=Herpetology Database |publisher=Swedish Museum of Natural History}}
*[https://www.bbc.co.uk/nature/life/Snake BBC Nature:] Snake news, and video clips from BBC programmes past and present.
*[http://snakesarelong.blogspot.com/2013/05/basics-of-snake-taxonomy.html Basics of snake taxonomy] at Life is Short but Snakes are Long


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{{Sourceattribution|Snake|1}}