Editing Snake (section)

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

===Size===
The now [[Extinction|extinct]] ''[[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 (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 taxon|extant]] snakes are the [[reticulated python]], measuring about {{convert|6.95|m|abbr=on}} long,<ref name="SunBear"/> and the [[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=[[Doctor of Philosophy|Ph.D.]] |publisher=[[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=March 3, 2016 |access-date=December 12, 2014}}</ref>

At the other end of the scale, the smallest extant snake is ''[[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=August 4, 2008 |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 |s2cid=198156987}}</ref>

=== Perception ===
The sensory systems of snakes, particularly those of the Crotalidae family, commonly known as pit vipers, are among the most specialized in the animal kingdom. Pit vipers, which include rattlesnakes and related species, possess all the sensory organs found in other snakes, as well as additional adaptations. These include specialized [[infrared sensing in snakes|infrared-sensitive receptors]], known as pits, located on either side of the head between the nostrils and eyes. These pits, which resemble an additional pair of nostrils, are highly developed and allow pit vipers to detect minute temperature changes. Each pit consists of two cavities: a larger outer cavity positioned just behind and below the nostril, and a smaller inner cavity. These cavities are connected internally by a membrane containing nerves highly sensitive to thermal variations. The forward-facing pits create a combined field of detection, enabling pit vipers to distinguish objects from their surroundings and accurately judge distances. The sensitivity of these pits allows them to detect temperature differences as small as one-third of a degree Fahrenheit. Other infrared-sensitive snakes, such as those in the Boidae family, possess multiple smaller labial pits along the upper lip, just below the nostrils.{{sfn|Cogger|Zweifel|1992|p=180}}

Snakes rely heavily on their sense of smell to track prey. They collect particles from the air, ground, or water using their [[forked tongue]], which are then transferred to the [[vomeronasal organ]] (also known as Jacobson's organ) in the mouth for analysis.{{sfn|Cogger|Zweifel|1992|p=180}} The forked structure of the tongue provides directional information of smell which helps locate prey or predators. In aquatic species, such as the [[anaconda]], the tongue functions efficiently underwater.{{sfn|Cogger|Zweifel|1992|p=180}} When the tongue is retracted, the forked tips are pressed into the cavities of the Jacobson's organ, enabling a combined taste-smell analysis that provides the snake with detailed information about its environment.{{sfn|Campbell|Shaw|1974}}{{page needed|date=April 2024}}{{sfn|Cogger|Zweifel|1992|p=180}}

[[File:Ptyas gab fbi.png|thumb|A line diagram from ''[[The Fauna of British India, Including Ceylon and Burma|The Fauna of British India]]'' by [[George Albert Boulenger|G. A. Boulenger]] (1890), illustrating the terminology of shields on the head of a snake]] Until the mid-20th century, it was widely believed that snakes were unable to hear.{{sfn|Campbell|Shaw|1974|p=19}}<ref name="ReferenceA">{{Cite journal |title=Sound garden: How snakes respond to airborne and groundborne sounds |date=2023 |doi=10.1371/journal.pone.0281285 |doi-access=free |last1=Zdenek |first1=Christina N. |last2=Staples |first2=Timothy |last3=Hay |first3=Chris |last4=Bourke |first4=Lachlan N. |last5=Candusso |first5=Damian |journal=PLOS ONE |volume=18 |issue=2 |pages=e0281285 |pmid=36787306 |pmc=9928108 |bibcode=2023PLoSO..1881285Z }}</ref> However, snakes possess two distinct auditory systems. One system, the somatic system, involves the transmission of vibrations through ventral skin receptors to the spine. The other system involves vibrations transmitted through the snake's elongated lung to the brain via cranial nerves. Snakes exhibit high sensitivity to vibrations, allowing them to detect even subtle sounds, such as soft speech, in quiet environments.{{sfn|Campbell|Shaw|1974|p=19}}{{Sfn|Cogger|Zweifel|1992|p=180}}<ref name="ReferenceA"/>

Snake vision varies significantly among species. While some snakes have keen eyesight, others can only distinguish light from dark. However, most snakes possess visual acuity sufficient to track movement.<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=May 18, 2015 |access-date=January 9, 2016 |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=February 19, 2015}}</ref> Arboreal snakes generally have better vision than burrowing species. Some snakes, such as the [[Ahaetulla|Asian vine snake]], possess [[binocular vision]], enabling both eyes to focus on the same point. Most snakes focus by moving the [[Lens (anatomy)|lens]] back and forth relative to the [[retina]]. Diurnal snakes typically have round pupils, while many nocturnal species have slit pupils. Most snakes possess three visual pigments, allowing them to perceive two primary colors in daylight. Certain species, such as the [[Hydrophis cyanocinctus|annulated sea snake]] and members of the genus [[Helicops (snake)|Helicops]], have regained significant color vision as an adaptation to their aquatic environments.<ref>{{Cite web |url=https://neurosciencenews.com/color-vision-snake-genetics-23618/ |title=Sea Snakes Regained Color Vision via Rare Genetic Evolution |date=12 July 2023 |access-date=28 September 2023 |archive-date=28 September 2023 |archive-url=https://web.archive.org/web/20230928181052/https://neurosciencenews.com/color-vision-snake-genetics-23618/ |url-status=live }}</ref><ref>{{Cite web |url=https://www.newscientist.com/article/2382064-some-sea-snakes-have-re-evolved-the-ability-to-see-more-colours/ |title=Some sea snakes have re-evolved the ability to see more colours |access-date=28 September 2023 |archive-date=28 September 2023 |archive-url=https://web.archive.org/web/20230928182513/https://www.newscientist.com/article/2382064-some-sea-snakes-have-re-evolved-the-ability-to-see-more-colours/ |url-status=live }}</ref> Research suggests that the last common ancestor of all snakes had [[Ultraviolet|UV]]-sensitive vision. However, many diurnal snakes have evolved lenses that filter out UV light, likely improving [[Contrast (vision)|contrast]] and sharpening their vision.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2016/08/160816182620.htm |title=Snake eyes: New insights into visual adaptations |date=August 16, 2016 |website=[[ScienceDaily]] |access-date=18 December 2019 |archive-date=18 December 2019 |archive-url=https://web.archive.org/web/20191218105018/https://www.sciencedaily.com/releases/2016/08/160816182620.htm |url-status=live }}</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=[[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 |access-date=1 March 2021 |archive-date=23 March 2021 |archive-url=https://web.archive.org/web/20210323015254/https://academic.oup.com/mbe/article/33/10/2483/2925599 |url-status=live |hdl=1983/2008257f-b043-4206-a44b-39ebbdd1bea8 |hdl-access=free }}</ref>

=== Skin ===
{{Main|Snake scale}}
The skin of a snake is covered in [[Scale (anatomy)|scales]]. Contrary to the popular notion of snakes being slimy (because of possible confusion of snakes with [[worm]]s), [[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 scales|keeled]], or granular. The eyelids of a snake are transparent "spectacle" scales, also known as [[brille]], which remain permanently closed.{{Citation needed|date=November 2024}}

For a snake, the skin has been modified to its specialized form of locomotion. Between the inner layer and the outer layer lies the dermis, which contains all the pigments and cells that make up the snake's distinguishing pattern and color. The epidermis, or outer layer, is formed of a substance called [[keratin]], which in mammals is the same basic material that forms nails, claws, and hair. The snake's epidermis of keratin provides it with the armor it needs to protect its internal organs and reduce friction as it passes over rocks. Parts of this keratin armor are rougher than others. The less restricted portion overlaps the front of the scale beneath it. Between them lies a folded back connecting material, also of keratin, also part of the epidermis. This folded back material gives as the snake undulates or eats things bigger than the circumference of its body.{{sfn|Campbell|Shaw|1974}}{{page needed|date=April 2024}}

The shedding of scales is called ''[[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 [[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 (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" ([[Caenophidia]]n) snakes, the broad belly scales and rows of [[dorsal scale]]s correspond to the [[vertebra]]e, allowing these to be counted without the need for [[dissection]].{{Citation needed|date=November 2024}}

==== Molting ====
[[File:Nerodia sipedon shedding.JPG|thumb|left|A [[common watersnake]] shedding its skin]]
[[Molting]] (or "ecdysis") serves a number of purposes - it allows old, worn skin to be replaced and can be synced to mating cycles, as with other animals. Molting occurs periodically throughout the life of a snake. Before each molt, the snake regulates its diet and seeks defensible shelter. Just before shedding, the skin becomes grey and the snake's eyes turn silvery. The inner surface of the old skin liquefies, causing it to separate from the new skin beneath it. After a few days, the eyes clear and the snake reaches out of its old skin, which splits. The snake rubs its body against rough surfaces to aid in the shedding of its old skin. In many cases, the castaway skin peels backward over the body from head to tail in one piece, like taking the dust jacket off a book, 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=November 25, 2007 |url-status=dead}}</ref> Renewal of the skin by molting supposedly increases the mass of some animals such as insects, but in the case of snakes this has been disputed.<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=January 9, 2016 |url-status=live |archive-url=https://web.archive.org/web/20160116034245/http://whozoo.org/ZooPax/ZPScales_3.htm |archive-date=January 16, 2016}}</ref> Shedding skin can release pheromones and revitalize color and patterns of the skin to increase attraction of mates.<ref>{{Cite journal |last1=Bauwens |first1=Dirk |last2=Van Damme |first2=Raoul |last3=Verheyen |first3=Rudolf F. |date=1989 |title=Synchronization of Spring Molting with the Onset of Mating Behavior in Male Lizards, Lacerta vivipara |url=https://www.jstor.org/stable/1564326 |journal=[[Journal of Herpetology]] |volume=23 |issue=1 |pages=89–91 |doi=10.2307/1564326 |jstor=1564326 |issn=0022-1511 |access-date=29 April 2022 |url-access=subscription }}</ref>

[[File:Shed skin of a snake.jpg|thumb|Shed skin of a snake]]

Snakes may shed four of five times a year, depending on the weather conditions, food supply, age of the snake, and other factors.{{sfn|Campbell|Shaw|1974}}{{page needed|date=April 2024}}<ref name = "GenSnakeInfo"/> It is theoretically possible to identify the snake from its cast skin if it is reasonably intact.<ref name="RSSlimy"/> Mythological associations of snakes with symbols of [[healing]] and [[medicine]], as pictured in the [[Rod of Asclepius]], are derivative of molting.<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>

One can attempt to identify the sex of a snake when the species is not distinctly [[Sexual dimorphism|sexually dimorphic]] by counting scales. The [[cloaca]] is probed and measured against the [[subcaudal scales]].<ref name="Rosenfeld_11">Rosenfeld (1989), p. 11.</ref> Counting scales determines whether a snake is a male or female, as the [[Hemipenis|hemipenes]] of a male being probed is usually longer.<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 ===
{{main|Snake skeleton}}
[[File:Reticulated python (Python reticulatus) skull 1 (cropped).jpg|thumb|[[Reticulated python]] skull, showing jaw movements when swallowing]]
The skull of a snake differs from a lizards in several ways. Snakes have more flexible jaws, that is, instead of a juncture at the upper and lower jaw, the snake's jaws are connected by a bone hinge that is called the [[quadrate bone]]. Between the two halves of the lower jaw at the chin there is an elastic ligament that allows for a separation. This allows the snake to swallow food larger in proportion to their size and go longer without it, since snakes ingest relatively more in one feeding.{{sfn|Campbell|Shaw|1974|p=11}} Because the sides of the lower jaw can move independently of one another, a snake resting its jaw on a surface has stereo [[Hearing|auditory perception]], used for detecting the position of prey. The jaw–quadrate–[[stapes]] pathway is capable of detecting vibrations on the [[angstrom]] scale, despite the absence of an outer ear and the lack of an [[impedance matching]] mechanism—provided by the [[ossicles]] in other vertebrates.<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=February 13, 2008 |first=Lisa |last=Zyga |name-list-style=vanc |publisher=[[Phys.org]] |url-status=live |archive-url=https://web.archive.org/web/20111010152337/http://www.physorg.com/news122123444.html |archive-date=October 10, 2011}}</ref> In a snake's skull the brain is well protected. As brain tissues could be damaged through the palate, this protection is especially valuable. The solid and complete [[neurocranium]] of snakes is closed at the front.{{sfn|Campbell|Shaw|1974}}{{page needed|date=April 2024}}<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 |bibcode=1971JExpB..54..349H |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=December 17, 2008}}</ref>

[[File:Snake Skeletons.jpg|thumb|The skeletons of snakes are radically different from those of most other reptiles (as compared with the [[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 [[henophidia]]n snakes retain vestiges of the pelvis and rear limbs.  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 [[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.{{Citation needed|date=November 2024}}

Caudal [[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|bibcode=1984JNatH..18..127A }}</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 spur]]s. 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.{{Citation needed|date=November 2024}}

Snakes are [[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 ===
{{snake anatomy imagemap}}
Snakes and other non-[[archosaur]] ([[crocodilia]]ns, [[dinosaur]]s + [[bird]]s and allies) reptiles have a three-chambered heart that controls the [[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 [[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=May 10, 2019 |archive-date=10 May 2019 |archive-url=https://web.archive.org/web/20190510203803/https://www.ias.ac.in/article/fulltext/secb/020/01/0001-0029 |url-status=live }}</ref>

The snake's heart is encased in a sac, called the ''[[pericardium]]'', located at the [[:wiktionary:bifurcation|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 [[esophagus]]. The [[spleen]] is attached to the [[gall bladder]] and [[pancreas]] and filters the blood. The [[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 circulatory system of a snake is basically like those of any other vertebrae. However, snakes do not regulate internally the temperature of their blood. Called [[Ectotherm|cold-blooded]], snakes actually have blood that is responsive to the varying temperature of the immediate environment. Snakes can regulate blood temperature by moving. Too long in direct sunlight, the snakes' blood is heated by beyond tolerance. Left in the ice or snow, the snake may freeze. In temperate zones with pronounced seasonal changes, snakes denning together have adapted to the onslaught of winter.{{sfn|Campbell|Shaw|1974}}{{page needed|date=April 2024}}

The [[vestige|vestigial]] left [[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"/>

The snake with its particular arrangement of organs may achieve a greater efficiency.{{compared to?|date=April 2024}} For example, the lung encloses at the part nearest the head and throat an oxygen intake organ, while the other half is used for air reserve. The esophagus-stomach-intestine arrangement is a straight line. It ends where intestinal, urinary, and reproductive tracts open, in a chamber called the cloaca.{{sfn|Campbell|Shaw|1974}}{{page needed|date=April 2024}}

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

=== Venom ===
{{See also|Snake venom|Venomous snake|#Bite}}{{Multi image
| image1            = Eastern Milksnake (Lampropeltis triangulum)1.jpg
| image2            = Micrurus pyrrhocryptus 1127095.jpg
| total_width       = 250
| direction         = vertical
| footer            = Innocuous [[milk snakes]] (top) are often mistaken for [[coral snakes]] (bottom) whose venom is deadly to humans.
}}
Cobras, vipers, and closely related species use [[venom]] to immobilize, injure, or kill their prey. The venom is modified [[saliva]], delivered through [[fangs]].<ref name="Meh87" /><ref name=":3">{{Cite journal |last1=Oliveira |first1=Ana L. |last2=Viegas |first2=Matilde F. |last3=da Silva |first3=Saulo L. |last4=Soares |first4=Andreimar M. |last5=Ramos |first5=Maria J. |last6=Fernandes |first6=Pedro A. |date=July 2022 |title=The chemistry of snake venom and its medicinal potential |journal=Nature Reviews Chemistry |language=en |volume=6 |issue=7 |pages=451–469 |doi=10.1038/s41570-022-00393-7 |issn=2397-3358 |pmc=9185726 |pmid=35702592}}</ref>{{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 [[Snake skeleton#Opisthoglyph|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" /><ref name=":3" />{{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"/><ref name=":3" />{{Rp|209}}
[[File:Snake fang types (cropped).jpg|thumb|Skulls from left to right: Nonvenomous (''[[Pseustes]] sp.''), rear-fanged (''[[Toxicodryas blandingii]]''), elapid (''[[Micropechis|Micropechis ikaheca]]''), viperid (''[[Eastern diamondback rattlesnake|Crotalus adamanteus]]''), venomous lizard (''[[Gila monster|Heloderma suspectum]]''). [[Maxilla]] in red.]]
Certain birds, mammals, and other snakes (such as [[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 [[Family (biology)|families]] of snakes, and do not constitute a formal [[Taxonomy (biology)|taxonomic classification]] group.{{Citation needed|date=November 2024}}

The [[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.{{sfn|Freiberg|Walls|1984|p=125}} There are, however, two exceptions: ''[[Rhabdophis]]'' sequesters toxins from the toads it eats, then secretes them from nuchal glands to ward off predators; and a small unusual population of [[garter snake]]s in the US state of [[Oregon]] retains enough toxins in their livers from ingested [[newt]]s to be effectively poisonous to small local predators (such as [[crow]]s and [[fox]]es).{{sfn|Freiberg|Walls|1984|p=123}}

Snake venoms are complex mixtures of [[protein]]s,<ref name=":3" /> and are stored in [[Venom gland|venom glands]] at the back of the head.{{sfn|Freiberg|Walls|1984|p=123}} In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw.<ref name="Meh87"/>{{Rp|243}}{{sfn|Freiberg|Walls|1984|p=125}} The proteins can potentially be a mix of [[neurotoxin]]s (which attack the nervous system), [[hemotoxin]]s (which attack the circulatory system), [[cytotoxin]]s (which attack the cells directly), [[bungarotoxin]]s (related to neurotoxins, but also directly affect muscle tissue), and many other toxins that affect the body in different ways.{{sfn|Freiberg|Walls|1984|p=125}}<ref name=":3" /> Almost all snake venom contains ''[[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=":3" />{{sfn|Freiberg|Walls|1984|p=125}} Some snakes that use neurotoxins (such as the [[Boiga dendrophila|mangrove snake]]) have fangs in the back of their mouths, with the fangs curled backwards.{{sfn|Freiberg|Walls|1984|p=126}} This makes it difficult both for the snake to use its venom and for scientists to milk them.{{sfn|Freiberg|Walls|1984|p=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 (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 [[gila monster]], [[beaded 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 [[sauria]]n species.{{Citation needed|date=November 2024}}

Venomous snakes are classified in two taxonomic families:

* '''[[Elapidae|Elapid]]s''' – [[cobra]]s including [[king cobra]]s, [[Bungarus|kraits]], [[mamba]]s, [[Austrelaps|Australian copperheads]], [[Hydrophiinae|sea snake]]s, and [[coral snake]]s.{{sfn|Freiberg|Walls|1984|p=126}}
* '''[[Viperidae|Viperids]]''' – vipers, [[rattlesnake]]s, [[Agkistrodon contortrix|copperheads]]/[[Agkistrodon piscivorus|cottonmouths]], and [[Lachesis (genus)|bushmasters]].{{sfn|Freiberg|Walls|1984|p=126}}

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

* '''[[Colubridae|Colubrid]]s''' – [[boomslang]]s, [[tree snake]]s, [[Ahaetulla|vine snakes]], [[Boiga|cat snakes]], although not all colubrids are venomous.<ref name="Meh87"/>{{Rp|209}}{{sfn|Freiberg|Walls|1984|p=126}}

=== Reproduction ===
{{See also|Sexual selection in scaled reptiles}}
[[File:Boa and python.JPG|thumb|''[[Boa imperator]]'' (left) and an [[Albinism|albino]] ''[[Indian python|Python molurus]]'' (right). The former gives birth to live young, while the latter lays eggs.]]
Although a wide range of reproductive modes are used by snakes, all employ [[internal fertilization]]. This is accomplished by means of paired, forked [[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 [[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=April 19, 2016 |publisher=[[CRC Press]] |isbn=978-1-4398-5833-2 |via=[[Google Books]]}}</ref><ref name="Capula89_117"/> The [[Hemipenis#Hemiclitoris|clitoris of the female snake]] consists of two structures located between the cloaca and the scent glands.<ref name="hemic">{{cite journal |last1=Fowell |first1=Megan J. |last2=Sanders |first2=Kate L. |last3=Brennan |first3=Patricia L. R. |last4=Crowe-Riddell |first4=Jenna M. |title=First evidence of hemiclitores in snakes |journal=[[Proceedings of the Royal Society B]] |date=December 21, 2022 |volume=289 |issue=1989 |doi=10.1098/rspb.2022.1702 |pmid=36515117 |pmc=9748774}}</ref>

Most species of snakes lay [[egg (biology)|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.{{sfn|Cogger|Zweifel|1992|p=186}} 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.{{sfn|Cogger|Zweifel|1992|p=186}}

Some species of snake are [[Ovoviviparity|ovoviviparous]] and retain the eggs within their bodies until they are almost ready to hatch.<ref name="Capula89_118">Capula (1989), p. 118.</ref>{{sfn|Cogger|Zweifel|1992|p=182}} Several species of snake, such as the [[boa constrictor]] and green anaconda, are fully [[Viviparity|viviparous]], nourishing their young through a [[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"/>{{sfn|Cogger|Zweifel|1992|p=182}} Retention of eggs and live birth are most often associated with colder environments.<ref name="Capula89_117"/>{{sfn|Cogger|Zweifel|1992|p=182}}

[[File:Coast Garter Snake.jpg|thumb|right|The [[garter snake]] has been studied for 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 (journal)|Animal Behaviour]] |volume=67 |issue=3 |pages=477–83 |s2cid=4830666}}</ref> Ritual combat between males for the females they want to [[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 (journal)|Animal Behaviour]] |volume=69 |issue=1 |pages=225–34 |s2cid=3907523}}</ref>

=== Facultative 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 [[Parthenogenesis#Facultative|facultative parthenogenesis]], meaning that they are capable of switching from a [[Sexual reproduction|sexual]] mode of reproduction to an [[Asexual reproduction|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 [[Parthenogenesis#Automictic|automixis]] with terminal fusion, a process in which two terminal products from the same [[meiosis]] fuse to form a diploid [[zygote]]. This process leads to genome-wide [[Zygosity#Homozygous|homozygosity]], expression of deleterious recessive [[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 [[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 (''[[Epicrates maurus]]'') can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny.<ref name="pmid21868391">{{cite journal |author6-link=Coby Schal |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.{{Citation needed|date=November 2024}}

=== Embryonic development ===
[[File:Mouse_and_Snake_Embryos.jpg|thumb|right|[[Mouse]] embryo 12 day post [[Fertilisation|fertilization]] side by side with [[corn snake]] embryo 2 days post ovo-positioning.<ref name=":2" />]]
Snake embryonic development initially follows similar steps as any vertebrate [[embryo]]. The snake embryo begins as a [[zygote]], undergoes rapid cell division, forms a [[germinal disc]], also called a blastodisc, then undergoes [[gastrulation]], [[neurulation]], and [[organogenesis]].<ref name=":0">{{cite journal |last1=Zehr |first1=David R. |date=20 July 1962 |title=Stages in the Normal Development of the Common Garter Snake, Thamnophis sirtalis sirtalis |journal=[[Copeia]] |volume=1962 |issue=2 |pages=322–329 |doi=10.2307/1440898 |jstor=1440898}}</ref> Cell division and proliferation continues until an early snake embryo develops and the typical body shape of a snake can be observed.<ref name=":0" /> Multiple features differentiate the embryologic development of snakes from other vertebrates, two significant factors being the elongation of the body and the lack of limb development.{{Citation needed|date=November 2024}}
[[File:Comparative_somite_creation.jpg|thumb|Diagram illustrating differential [[somite]] size due to difference in [[somitogenesis]] clock oscillation<ref name=":2" />]]
The elongation in snake body is accompanied by a significant increase in [[vertebra]] count (mice have 60 vertebrae, whereas snakes may have over 300).<ref name=":2">{{cite journal |last1=Woltering |first1=Joost M. |title=From Lizard to Snake; Behind the Evolution of an Extreme Body Plan |journal=[[Current Genomics]] |year=2012 |volume=13 |issue=4 |pages=289–299 |doi=10.2174/138920212800793302 |pmid=23204918 |pmc=3394116}}</ref> This increase in vertebrae is due to an increase in [[somite]]s during embryogenesis, leading to an increased number of vertebrae which develop.<ref name=":2" /> Somites are formed at the [[Paraxial mesoderm|presomitic mesoderm]] due to a set of oscillatory genes that direct the [[Clock and wavefront model|somitogenesis clock]]. The snake somitogenesis clock operates at a frequency 4 times that of a mouse (after correction for developmental time), creating more somites, and therefore creating more vertebrae.<ref name=":2" /> This difference in clock speed is believed to be caused by differences in [[LFNG|''Lunatic fringe'' gene]] expression, a gene involved in the somitogenesis clock.<ref>{{Cite journal |last1=Gomez |first1=Céline |last2=Özbudak |first2=Ertuğrul M. |last3=Wunderlich |first3=Joshua |last4=Baumann |first4=Diana |last5=Lewis |first5=Julian |last6=Pourquié |first6=Olivier |date=July 17, 2008 |title=Control of segment number in vertebrate embryos |url=https://www.nature.com/articles/nature07020 |journal=[[Nature (journal)|Nature]] |language=en |volume=454 |issue=7202 |pages=335–339 |doi=10.1038/nature07020 |pmid=18563087 |bibcode=2008Natur.454..335G |s2cid=4373389 |issn=0028-0836 |access-date=30 April 2022 |archive-date=26 March 2023 |archive-url=https://web.archive.org/web/20230326063229/https://www.nature.com/articles/nature07020 |url-status=live |url-access=subscription }}</ref>

There is ample literature focusing on the limb development/lack of development in snake embryos and the gene expression associated with the different stages. In [[Henophidia|basal snakes]], such as the python, embryos in early development exhibit a hind [[limb bud]] that develops with some cartilage and a cartilaginous pelvic element, however this degenerates before hatching.<ref>{{cite journal |last1=Boughner |first1=Julia C. |last2=Buchtová |first2=Marcela |last3=Fu |first3=Katherine |last4=Diewert |first4=Virginia |last5=Hallgrímsson |first5=Benedikt |last6=Richman |first6=Joy M. |title=Embryonic development of Python sebae – I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs |journal=Zoology |date=June 2007 |volume=110 |issue=3 |pages=212–230 |doi=10.1016/j.zool.2007.01.005 |pmid=17499493 |bibcode=2007Zool..110..212B }}</ref> This presence of vestigial development suggests that some snakes are still undergoing hind limb reduction before they are eliminated.<ref name=":1">{{cite journal |last1=Leal |first1=Francisca |last2=Cohn |first2=Martin J. |date=January 2018 |title=Developmental, genetic, and genomic insights into the evolutionary loss of limbs in snakes |journal=[[Genesis (journal)|Genesis]] |volume=56 |issue=1 |pages=e23077 |doi=10.1002/dvg.23077 |pmid=29095557 |s2cid=4510082}}</ref> There is no evidence in basal snakes of forelimb rudiments and no examples of snake forelimb bud initiation in embryo, so little is known regarding the loss of this trait.<ref name=":1" /> Recent studies suggest that hind limb reduction could be due to mutations in enhancers for the [[Sonic hedgehog|SSH]] gene,<ref name=":1" /> however other studies suggested that mutations within the [[Hox gene|Hox Genes]] or their enhancers could contribute to snake limblessness.<ref name=":2" /> Since multiple studies have found evidence suggesting different genes played a role in the loss of limbs in snakes, it is likely that multiple gene mutations had an additive effect leading to limb loss in snakes<ref>{{Cite journal |last1=Kvon |first1=Evgeny Z. |last2=Kamneva |first2=Olga K. |last3=Melo |first3=Uirá S. |last4=Barozzi |first4=Iros |last5=Osterwalder |first5=Marco |last6=Mannion |first6=Brandon J. |last7=Tissières |first7=Virginie |last8=Pickle |first8=Catherine S. |last9=Plajzer-Frick |first9=Ingrid |last10=Lee |first10=Elizabeth A. |last11=Kato |first11=Momoe |date=October 2016 |title=Progressive Loss of Function in a Limb Enhancer during Snake Evolution |journal=[[Cell (journal)|Cell]] |language=en |volume=167 |issue=3 |pages=633–642.e11 |doi=10.1016/j.cell.2016.09.028 |pmc=5484524 |pmid=27768887}}</ref>