Reptile

This article is about the animal class. For other uses, seeReptile (disambiguation).

Reptiles,as commonly defined, are a group oftetrapodswith anectothermic('cold-blooded')metabolismandamniotic development.Living reptiles comprise fourorders:Testudines (turtles), Crocodilia (crocodilians),Squamata(lizardsandsnakes), andRhynchocephalia(thetuatara). As of May 2023, about 12,000 living species of reptiles are listed in theReptile Database.The study of the traditional reptile orders, customarily in combination with the study of modernamphibians,is calledherpetology.

Reptiles Temporal range:
Tuatara Saltwater crocodile Common box turtle Ladder snake
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sauropsida
Class:	Reptilia Laurenti,1768
Extant groups
Lepidosauria Rhynchocephalia(tuatara) Squamata(lizardsandsnakes) Archelosauria Testudines(turtles) Archosauria Crocodilia Dinosauria(dinosaurs) Aves(birds) Seetextfor extinct groups.

Reptiles have been subject to several conflictingtaxonomicdefinitions.InLinnaean taxonomy,reptiles are gathered together under theclassReptilia(/rɛpˈtɪliə/rep-TIL-ee-ə), which corresponds to common usage. Moderncladistic taxonomyregards that group asparaphyletic,sincegeneticandpaleontologicalevidence has determined thatbirds(class Aves), as members ofDinosauria,are more closely related to living crocodilians than to other reptiles, and are thus nested among reptiles from an evolutionary perspective. Many cladistic systems therefore redefine Reptilia as aclade(monophyleticgroup) including birds, though the precise definition of this clade varies between authors.Others prioritize the cladeSauropsida,which typically refers to allamniotesmore closely related to modern reptiles than tomammals.

The earliest known proto-reptiles originated from theCarboniferousperiod, having evolved from advancedreptiliomorphtetrapods which became increasingly adapted to life on dry land. The earliest knowneureptile( "true reptile" ) wasHylonomus,a small and superficially lizard-like animal which lived inNova Scotiaduring theBashkirianage of theLate Carboniferous,around318million years ago.Genetic and fossil data argues that the two largest lineages of reptiles,Archosauromorpha(crocodilians, birds, and kin) andLepidosauromorpha(lizards, and kin), diverged during thePermianperiod.In addition to the living reptiles, there are many diverse groups that are nowextinct,in some cases due tomass extinction events.In particular, theCretaceous–Paleogene extinction eventwiped out thepterosaurs,plesiosaurs,and all non-aviandinosaursalongside many species ofcrocodyliformsandsquamates(e.g.,mosasaurs). Modern non-bird reptiles inhabit all the continents except Antarctica.

Reptiles are tetrapodvertebrates,creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlikeamphibians,reptiles do not have an aquatic larval stage. Most reptiles areoviparous,although several species of squamates areviviparous,as were some extinct aquatic clades – the fetus develops within the mother, using a(non-mammalian) placentarather than contained in aneggshell.As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed theirfetusesthrough various forms of placenta analogous to those ofmammals,with some providing initial care for their hatchlings.Extantreptiles range in size from a tiny gecko,Sphaerodactylus ariasae,which can grow up to 17 mm (0.7 in) to thesaltwater crocodile,Crocodylus porosus,which can reach over 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).

Classification

See also:List of reptiles

Research history

See also:Skull roof

Reptiles, fromNouveau Larousse Illustré,1897–1904, notice the inclusion ofamphibians(below the crocodiles)

In the 13th century, the category ofreptilewas recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians, and worms", as recorded byBeauvaisin hisMirror of Nature. In the 18th century, the reptiles were, from the outset of classification, grouped with theamphibians.Linnaeus,working from species-poorSweden,where thecommon adderandgrass snakeare often found hunting in water, included all reptiles and amphibians inclass"III – Amphibia"in hisSystema Naturæ. The termsreptileandamphibianwere largely interchangeable,reptile(from Latinrepere,'to creep') being preferred by the French.J.N. Laurentiwas the first to formally use the termReptiliafor an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.Today, the two groups are still commonly treated under the single headingherpetology.

"Antediluvian monster", aMosasaurusdiscovered in aMaastrichtlimestone quarry, 1770 (contemporary engraving)

It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, andP.A. Latreilleerected the classBatracia(1825) for the latter, dividing thetetrapodsinto the four familiar classes of reptiles, amphibians, birds, and mammals.The British anatomistT.H. Huxleymade Latreille's definition popular and, together withRichard Owen,expanded Reptilia to include the various fossil "antediluvianmonsters ", includingdinosaursand the mammal-like (synapsid)Dicynodonhe helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at theRoyal College of Surgeonsin 1863, Huxley grouped the vertebrates intomammals,sauroids, and ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names ofSauropsidaandIchthyopsidafor the latter two groups.In 1866,Haeckeldemonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by theamniotic egg.

The termsSauropsida( "lizard faces" ) andTheropsida( "beast faces" ) were used again in 1916 byE.S. Goodrichto distinguish between lizards, birds, and their relatives on the one hand (Sauropsida) andmammalsand their extinct relatives (Theropsida) on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features, such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, Protosauria ( "first lizards" ) in which he included some animals today consideredreptile-like amphibians,as well as early reptiles.

In 1956,D.M.S. Watsonobserved that the first two groups diverged very early in reptilian history, so he divided Goodrich's Protosauria between them. He also reinterpreted Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida includedProcolophonia,Eosuchia,Millerosauria,Chelonia(turtles),Squamata(lizards and snakes),Rhynchocephalia,Crocodilia,"thecodonts"(paraphyleticbasalArchosauria), non-aviandinosaurs,pterosaurs,ichthyosaurs,andsauropterygians.

In the late 19th century, a number of definitions of Reptilia were offered. The biological traits listed byLydekkerin 1896, for example, include a singleoccipital condyle,a jaw joint formed by thequadrateandarticularbones, and certain characteristics of thevertebrae.The animals singled out by these formulations, theamniotesother than the mammals and the birds, are still those considered reptiles today.

The first reptiles had ananapsidtype ofskull roof,as seen in thePermiangenusCaptorhinus

The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position oftemporal fenestrae,openings in the sides of the skull behind the eyes. This classification was initiated byHenry Fairfield Osbornand elaborated and made popular byRomer's classicVertebrate Paleontology.Those four subclasses were:

• Anapsida– no fenestrae –cotylosaursandchelonia(turtlesand relatives)
• Synapsida– one low fenestra –pelycosaursandtherapsids(the 'mammal-like reptiles')
• Euryapsida– one high fenestra (above the postorbital and squamosal) –protorosaurs(small, early lizard-like reptiles) and the marinesauropterygiansandichthyosaurs,the latter calledParapsidain Osborn's work.
• Diapsida– two fenestrae – most reptiles, includinglizards,snakes,crocodilians,dinosaursandpterosaurs.

Phylogenetic classifications group the traditional "mammal-like reptiles", like thisVaranodon,with other synapsids, not with extant reptiles

The composition of Euryapsida was uncertain.Ichthyosaurswere, at times, considered to have arisen independently of the other euryapsids, and given the older name Parapsida. Parapsida was later discarded as a group for the most part (ichthyosaurs being classified asincertae sedisor with Euryapsida). However, four (or three if Euryapsida is merged into Diapsida) subclasses remained more or less universal for non-specialist work throughout the 20th century. It has largely been abandoned by recent researchers: In particular, the anapsid condition has been found to occur so variably among unrelated groups that it is not now considered a useful distinction.

Phylogenetics and modern definition

By the early 21st century, vertebrate paleontologists were beginning to adoptphylogenetictaxonomy, in which all groups are defined in such a way as to bemonophyletic;that is, groups which include all descendants of a particular ancestor. The reptiles as historically defined areparaphyletic,since they exclude both birds and mammals. These respectively evolved from dinosaurs and from early therapsids, both of which were traditionally called "reptiles".Birds are more closely related tocrocodiliansthan the latter are to the rest of extant reptiles.Colin Tudgewrote:

Mammals are aclade,and therefore thecladistsare happy to acknowledge the traditional taxonMammalia;and birds, too, are a clade, universally ascribed to the formal taxonAves.Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a section of the cladeAmniota:The section that is left after the Mammalia and Aves have been hived off. It cannot be defined bysynapomorphies,as is the proper way. Instead, it is defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptilia are 'non-avian, non-mammalian amniotes'.

Despite the early proposals for replacing the paraphyletic Reptilia with a monophyleticSauropsida,which includes birds, that term was never adopted widely or, when it was, was not applied consistently.

Bearded dragon (pogona) skeleton on display at theMuseum of Osteology

When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988,Jacques Gauthierproposed acladisticdefinition of Reptilia as a monophyletic node-basedcrown groupcontaining turtles, lizards and snakes, crocodilians, and birds, their common ancestor and all its descendants. While Gauthier's definition was close to the modern consensus, nonetheless, it became considered inadequate because the actual relationship of turtles to other reptiles was not yet well understood at this time.Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.Gauthier 1994 and Laurin and Reisz 1995's definition of Sauropsida defined the scope of the group as distinct and broader than that of Reptilia, encompassingMesosauridaeas well as Reptiliasensu stricto.

A variety of other definitions were proposed by other scientists in the years following Gauthier's paper. The first such new definition, which attempted to adhere to the standards of thePhyloCode,was published by Modesto and Anderson in 2004.Modesto and Anderson reviewed the many previous definitions and proposed a modified definition, which they intended to retain most traditional content of the group while keeping it stable and monophyletic. They defined Reptilia as all amniotes closer toLacerta agilisandCrocodylus niloticusthan toHomo sapiens.This stem-based definition is equivalent to the more common definition of Sauropsida, which Modesto and Anderson synonymized with Reptilia, since the latter is better known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds, as they are within the clade that includes both lizards and crocodiles.

Taxonomy

See also:List of reptilesandList of snakes

General classification of extinct and living reptiles, focusing on major groups.

• Reptilia/Sauropsida
  • †Parareptilia
  • Eureptilia
    • †Captorhinidae
    • Diapsida
      • †Araeoscelidia
      • Neodiapsida
        • †Drepanosauromorpha(placement uncertain)
        • †Younginiformes(paraphyletic)
        • †Ichthyosauromorpha(placement uncertain)
        • †Thalattosauria(placement uncertain)
        • Sauria
          • Lepidosauromorpha
            • Lepidosauriformes
              • Rhynchocephalia(tuatara)
              • Squamata(lizards and snakes)
          • †Choristodera(placement uncertain)
          • †Sauropterygia(placement uncertain)
          • Pantestudines(turtles and kin, placement uncertain)
          • Archosauromorpha
            • †Protorosauria(paraphyletic)
            • †Rhynchosauria
            • †Allokotosauria
            • Archosauriformes
              • †Phytosauria
              • Archosauria
                • Pseudosuchia
                  • Crocodilia(crocodilians)
                • Avemetatarsalia/Ornithodira
                  • †Pterosauria
                  • Dinosauria
                    • †Ornithischia
                    • Saurischia(including birds (Aves))

Phylogeny

Thecladogrampresented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013.Allgeneticstudies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha,though a few have recovered turtles as Lepidosauromorpha instead.The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.

Amniota

Synapsida(mammalsand their extinct relatives) Sauropsida / Reptilia †Parareptilia †Millerettidae unnamed †Eunotosaurus †Ankyramorpha †Lanthanosuchidae †Procolophonia †Pareiasauromorpha †Procolophonoidea Eureptilia †Captorhinidae Romeriida †Paleothyris Diapsida †Araeoscelidia Neodiapsida †Claudiosaurus †Younginiformes Crown Reptilia/ Pan-Lepidosauria/ †Kuehneosauridae Lepidosauria Rhynchocephalia(tuataraand their extinct relatives) Squamata(lizardsandsnakes) Lepidosauromorpha Archelosauria/ Pan-Testudines/ †Eosauropterygia †Placodontia †Sinosaurosphargis †Odontochelys Testudinata †Proganochelys Testudines(turtles) Pan-Archosauria †Choristodera Archosauromorphas. s. †Prolacertiformes †Rhynchosauria †Trilophosaurus Archosauriformes(crocodiles,birds,dinosaurs and extinct relatives) Pantestudines Archosauromorphas. l. Sauria (total group)

The position of turtles

The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.Molecular work has usually placed turtles within the diapsids. As of 2013, three turtle genomes have been sequenced.The results place turtles as asister cladeto thearchosaurs,the group that includes crocodiles, non-avian dinosaurs, and birds.However, in their comparative analysis of the timing oforganogenesis,Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade withinSauropsida,outside thesaurianclade altogether.

Evolutionary history

Main article:Evolution of reptiles

Origin of the reptiles

An early reptileHylonomus

Mesozoic scene showing typical reptilian megafauna:dinosaursincludingEuropasaurus holgeri,iguanodonts,andArchaeopteryx lithographicaperched on the foreground tree stump

The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the lateCarboniferousperiod, when the first reptiles evolved from advancedreptiliomorphs.

The oldest known animal that may have been anamnioteisCasineria(though it may have been atemnospondyl).A series of footprints from the fossil strata ofNova Scotiadated to315Mashow typical reptilian toes and imprints of scales.These tracks are attributed toHylonomus,the oldest unquestionable reptile known. It was a small, lizard-like animal, about 20 to 30 centimetres (7.9 to 11.8 in) long, with numerous sharp teeth indicating an insectivorous diet.Other examples includeWestlothiana(for the moment considered areptiliomorphrather than a trueamniote)andPaleothyris,both of similar build and presumably similar habit.

However,microsaurshave been at times considered true reptiles, so an earlier origin is possible.

Rise of the reptiles

The earliest amniotes, including stem-reptiles (those amniotes closer to modern reptiles than to mammals), were largely overshadowed by larger stem-tetrapods, such asCochleosaurus,and remained a small, inconspicuous part of the fauna until theCarboniferous Rainforest Collapse.This sudden collapse affected several large groups. Primitive tetrapods were particularly devastated, while stem-reptiles fared better, being ecologically adapted to the drier conditions that followed. Primitive tetrapods, like modern amphibians, need to return to water to lay eggs; in contrast, amniotes, like modern reptiles – whose eggs possess a shell that allows them to be laid on land – were better adapted to the new conditions. Amniotes acquired new niches at a faster rate than before the collapse and at a much faster rate than primitive tetrapods. They acquired new feeding strategies including herbivory and carnivory, previously only having been insectivores and piscivores.From this point forward, reptiles dominated communities and had a greater diversity than primitive tetrapods, setting the stage for the Mesozoic (known as the Age of Reptiles).One of the best known early stem-reptiles isMesosaurus,a genus from theEarly Permianthat had returned to water, feeding on fish.

A 2021 examination of reptile diversity in the Carboniferous and the Permian suggests a much higher degree of diversity than previously thought, comparable or even exceeding that of synapsids. Thus, the "First Age of Reptiles" was proposed.

Anapsids, synapsids, diapsids, and sauropsids

A = Anapsid,
B = Synapsid,
C = Diapsid

It was traditionally assumed that the first reptiles retained ananapsidskull inherited from their ancestors.This type of skull has askull roofwith only holes for the nostrils, eyes and apineal eye.The discoveries ofsynapsid-like openings (see below) in the skull roof of the skulls of several members ofParareptilia(the clade containing most of the amniotes traditionally referred to as "anapsids" ), includinglanthanosuchoids,millerettids,bolosaurids,somenycteroleterids,someprocolophonoidsand at least somemesosaursmade it more ambiguous and it is currently uncertain whether the ancestral amniote had an anapsid-like or synapsid-like skull.These animals are traditionally referred to as "anapsids", and form aparaphyleticbasic stock from which other groups evolved.Very shortly after the first amniotes appeared, a lineage calledSynapsidasplit off; this group was characterized by a temporal opening in the skull behind each eye giving room for the jaw muscle to move. These are the "mammal-like amniotes", or stem-mammals, that later gave rise to the truemammals.Soon after, another group evolved a similar trait, this time with a double opening behind each eye, earning them the nameDiapsida( "two arches" ).The function of the holes in these groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.

Turtles have been traditionally believed to be surviving parareptiles, on the basis of their anapsid skull structure, which was assumed to be primitive trait.The rationale for this classification has been disputed, with some arguing that turtles are diapsids that evolved anapsid skulls, improving their armor.Later morphologicalphylogeneticstudies with this in mind placed turtles firmly within Diapsida.Allmolecularstudies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extantarchosaurs.

Permian reptiles

With the close of theCarboniferous,the amniotes became the dominant tetrapod fauna. While primitive, terrestrialreptiliomorphsstill existed, the synapsid amniotes evolved the first truly terrestrialmegafauna(giant animals) in the form ofpelycosaurs,such asEdaphosaurusand the carnivorousDimetrodon.In the mid-Permian period, the climate became drier, resulting in a change of fauna: The pelycosaurs were replaced by thetherapsids.

The parareptiles, whose massiveskull roofshad no postorbital holes, continued and flourished throughout the Permian. Thepareiasaurianparareptiles reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).

Early in the period, the modern reptiles, orcrown-group reptiles,evolved and split into two main lineages: theArchosauromorpha(forebears ofturtles,crocodiles,anddinosaurs) and theLepidosauromorpha(predecessors of modernlizardsandtuataras). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.

Mesozoic reptiles

The close of the Permian saw the greatest mass extinction known (see thePermian–Triassic extinction event), an event prolonged by the combination of two or more distinct extinction pulses.Most of the earlier parareptile and synapsid megafauna disappeared, being replaced by the true reptiles, particularlyarchosauromorphs.These were characterized by elongated hind legs and an erect pose, the early forms looking somewhat like long-legged crocodiles. Thearchosaursbecame the dominant group during theTriassicperiod, though it took 30 million years before their diversity was as great as the animals that lived in the Permian.Archosaurs developed into the well-knowndinosaursandpterosaurs,as well as the ancestors ofcrocodiles.Since reptiles, firstrauisuchiansand then dinosaurs, dominated the Mesozoic era, the interval is popularly known as the "Age of Reptiles". The dinosaurs also developed smaller forms, including the feather-bearing smallertheropods.In theCretaceousperiod, these gave rise to the first truebirds.

Thesister groupto Archosauromorpha isLepidosauromorpha,containinglizardsandtuataras,as well as their fossil relatives. Lepidosauromorpha contained at least one major group of the Mesozoic sea reptiles: themosasaurs,which lived during theCretaceousperiod. The phylogenetic placement of other main groups of fossil sea reptiles – theichthyopterygians(includingichthyosaurs) and thesauropterygians,which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphsor to archosauromorphs,and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.

Cenozoic reptiles

Varanus priscuswas a giant carnivorousgoannalizard, perhaps as long as 7 metres and weighing up to 1,940 kilograms

Skeleton ofChampsosaurus,achoristodere,the latest surviving order of extinct reptiles. The last known choristoderes are known from theMiocene,around 11.3 million years ago

The close of theCretaceousperiod saw the demise of the Mesozoic era reptilian megafauna (see theCretaceous–Paleogene extinction event,also known as K-T extinction event). Of the largemarine reptiles,onlysea turtleswere left; and of the non-marine large reptiles, only the semi-aquaticcrocodilesand broadly similarchoristoderessurvived the extinction, with last members of the latter, the lizard-likeLazarussuchus,becoming extinct in theMiocene.Of the great host of dinosaurs dominating the Mesozoic, only the small beakedbirdssurvived. This dramatic extinction pattern at the end of the Mesozoic led into the Cenozoic. Mammals and birds filled the empty niches left behind by the reptilian megafauna and, while reptile diversification slowed, bird and mammal diversification took an exponential turn.However, reptiles were still important components of the megafauna, particularly in the form of large and gianttortoises.

After the extinction of most archosaur and marine reptile lines by the end of the Cretaceous, reptile diversification continued throughout the Cenozoic.Squamatestook a massive hit during the K–Pg event, only recovering ten million years after it,but they underwent a great radiation event once they recovered, and today squamates make up the majority of living reptiles (> 95%).Approximately 10,000 extant species of traditional reptiles are known, with birds adding about 10,000 more, almost twice the number of mammals, represented by about 5,700 living species (excludingdomesticatedspecies).

Species diversity of living reptiles (2013)

Reptile group	Described species	Percent of reptile species
Squamates	9193	96.3%
- Lizards	5634	59%
- Snakes	3378	35%
- Amphisbaenians	181	2%
Turtles	327	3.4%
Crocodilians	25	0.3%
Rhynchocephalians	1	0.01%
Total	9546	100%

Morphology and physiology

Circulation

Thermographic image ofmonitor lizards

Alllepidosaursandturtleshave a three-chamberedheartconsisting of twoatria,one variably partitionedventricle,and two aortas that lead to thesystemic circulation.The degree of mixing ofoxygenatedand deoxygenated blood in the three-chambered heart varies depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be afitnessadvantage.

Juvenileiguanaheartbisected through the ventricle, bisecting the left and right atrium

For example,iguanahearts, like the majority of thesquamateshearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.The main structures of the heart are thesinus venosus,the pacemaker, theleft atrium,theright atrium,theatrioventricular valve,the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge,pulmonary veins,and pairedaortic arches.

Some squamate species (e.g., pythons and monitor lizards) have three-chambered hearts that become functionally four-chambered hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle duringventricular diastoleand completely divides it duringventricular systole.Because of this ridge, some of thesesquamatesare capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.

Crocodilianshave an anatomically four-chambered heart, similar tobirds,but also have two systemic aortas and are therefore capable of bypassing theirpulmonary circulation.In turtles, the ventricle is not perfectly divided, so a mix of aerated and nonaerated blood can occur.

Metabolism

Sustained energy output (joules) of a typical reptile versus a similar size mammal as a function of core body temperature. The mammal has a much higher peak output, but can only function over a very narrow range of body temperature.

Modern non-avian reptiles exhibit some form ofcold-bloodedness(i.e. some mix ofpoikilothermy,ectothermy,andbradymetabolism) so that they have limited physiological means of keeping the body temperature constant and often rely on external sources of heat. Due to a less stable core temperature thanbirdsandmammals,reptilian biochemistry requiresenzymescapable of maintaining efficiency over a greater range of temperatures than in the case forwarm-bloodedanimals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals; for many lizards, it falls in the 24–35 °C (75–95 °F) range,while extreme heat-adapted species, like the Americandesert iguanaDipsosaurus dorsalis,can have optimal physiological temperatures in the mammalian range, between 35 and 40 °C (95 and 104 °F).While the optimum temperature is often encountered when the animal is active, the low basal metabolism makes body temperature drop rapidly when the animal is inactive.

As in all animals, reptilian muscle action produces heat. In large reptiles, likeleatherback turtles,the low surface-to-volume ratio allows this metabolically produced heat to keep the animals warmer than their environment even though they do not have awarm-bloodedmetabolism.This form of homeothermy is calledgigantothermy;it has been suggested as having been common in largedinosaursand other extinct large-bodied reptiles.

The benefit of a low resting metabolism is that it requires far less fuel to sustain bodily functions. By using temperature variations in their surroundings, or by remaining cold when they do not need to move, reptiles can save considerable amounts of energy compared to endothermic animals of the same size.A crocodile needs from a tenth to a fifth of the food necessary for alionof the same weight and can live half a year without eating.Lower food requirements and adaptive metabolisms allow reptiles to dominate the animal life in regions where netcalorieavailability is too low to sustain large-bodied mammals and birds.

It is generally assumed that reptiles are unable to produce the sustained high energy output necessary for long distance chases or flying.Higher energetic capacity might have been responsible for the evolution ofwarm-bloodednessin birds and mammals.However, investigation of correlations between active capacity andthermophysiologyshow a weak relationship.Most extant reptiles are carnivores with a sit-and-wait feeding strategy; whether reptiles are cold blooded due to their ecology is not clear. Energetic studies on some reptiles have shown active capacities equal to or greater than similar sized warm-blooded animals.

Respiratory system

X-rayfluoroscopyvideos of a female American alligator showing contraction of the lungs while breathing

All reptiles breathe usinglungs.Aquaticturtleshave developed more permeable skin, and some species have modified theircloacato increase the area forgas exchange.Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. Insquamates,the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of thisconstraint,most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employbuccal pumpingas a complement to their normal "axial breathing". This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time.Tegu lizardsare known to possess a proto-diaphragm,which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs.

Crocodiliansactually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepaticpiston".Theairwaysform a number of double tubular chambers within each lung. On inhalation and exhalation air moves through the airways in the same direction, thus creating a unidirectional airflow through the lungs. A similar system is found in birds,monitor lizardsand iguanas.

Most reptiles lack asecondary palate,meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains against damage by struggling prey). Skinks (familyScincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.

Turtles and tortoises

Red-eared slidertaking a gulp of air

Howturtlesbreathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how those turtlesbreathe.The varied results indicate that turtles have found a variety of solutions to this problem.

The difficulty is that mostturtle shellsare rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles, such as the Indian flapshell (Lissemys punctata), have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to adiaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction).

Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements.This is because they use their abdominal muscles to breathe during locomotion. The last species to have been studied is the red-eared slider, which also breathes during locomotion, but takes smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells.

Sound production

Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited tohissing,which is produced merely by forcing air though a partly closedglottisand is not considered to be a true vocalization. The ability to vocalize exists in crocodilians, some lizards and turtles; and typically involves vibrating fold-like structures in thelarynxor glottis. Somegeckosand turtles possess truevocal cords,which haveelastin-rich connective tissue.

Hearing in snakes

Hearing in humans relies on 3 parts of the ear; the outer ear that directs sound waves into the ear canal, the middle ear that transmits incoming sound waves to the inner ear, and the inner ear that helps in hearing and keeping their balance. Unlike humans and other mammals, snakes do not possess an outer ear, a middle ear, and atympanumbut have an inner ear structure withcochleasdirectly connected to their jawbone.They are able to feel the vibrations generated from the sound waves in their jaw as they move on the ground. This is done by the use ofmechanoreceptors,sensory nerves that run along the body of snakes directing the vibrations along the spinal nerves to the brain. Snakes have a sensitive auditory perception and can tell which direction sound being made is coming from so that they can sense the presence of prey or predator but it is still unclear how sensitive snakes are to sound waves traveling through the air.

Skin

Skin of asand lizard,showingsquamate reptilesiconicscales

Reptilian skin is covered in a hornyepidermis,making it watertight and enabling reptiles to live on dry land, in contrast to amphibians. Compared to mammalian skin, that of reptiles is rather thin and lacks the thickdermallayer that producesleatherin mammals. Exposed parts of reptiles are protected byscalesorscutes,sometimes with a bony base (osteoderms), formingarmor.Inlepidosaurs,such as lizards and snakes, the whole skin is covered in overlappingepidermalscales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurs.The scales found in turtles and crocodiles are ofdermal,rather than epidermal, origin and are properly termed scutes.In turtles, the body is hidden inside a hard shell composed of fused scutes.

Lacking a thick dermis, reptilian leather is not as strong as mammalian leather. It is used in leather-wares for decorative purposes for shoes, belts and handbags, particularly crocodile skin.

Shedding

Reptiles shed their skin through a process calledecdysiswhich occurs continuously throughout their lifetime. In particular, younger reptiles tend to shed once every five to six weeks while adults shed three to four times a year.Younger reptiles shed more because of their rapid growth rate. Once full size, the frequency of shedding drastically decreases. The process of ecdysis involves forming a new layer of skin under the old one.Proteolyticenzymes andlymphatic fluidis secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.Snakes will shed from the head to the tail while lizards shed in a "patchy pattern".Dysecdysis,a common skin disease in snakes and lizards, will occur when ecdysis, or shedding, fails.There are numerous reasons why shedding fails and can be related to inadequate humidity and temperature, nutritional deficiencies, dehydration and traumatic injuries.Nutritional deficiencies decrease proteolytic enzymes while dehydration reduces lymphatic fluids to separate the skin layers. Traumatic injuries on the other hand, form scars that will not allow new scales to form and disrupt the process of ecdysis.

Excretion

Excretionis performed mainly by two smallkidneys.In diapsids,uric acidis the mainnitrogenouswaste product; turtles, likemammals,excrete mainlyurea.Unlike thekidneys of mammalsand birds,reptile kidneysare unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure called aloop of Henle,which is present in thenephronsof birds and mammals. Because of this, many reptiles use thecolonto aid in thereabsorptionof water. Some are also able to take up water stored in thebladder.Excess salts are also excreted by nasal and lingualsalt glandsin some reptiles.

In all reptiles, the urinogenital ducts and therectumboth empty into an organ called acloaca.In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards, but is lacking in themonitor lizard,thelegless lizards.It is absent in the snakes, alligators, and crocodiles.

Many turtles and lizards have proportionally very large bladders.Charles Darwinnoted that theGalapagos tortoisehad a bladder which could store up to 20% of its body weight.Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid inosmoregulation.

Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity.Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to havecalculi.

Digestion

A colubrid snake,Dolichophis jugularis,eating alegless lizard,Pseudopus apodus.Most reptiles are carnivorous, and many primarily eat other reptiles and small mammals.

Gastrolithsfrom aplesiosaur

Most reptiles are insectivorous or carnivorous and have simple and comparatively short digestive tracts due to meat being fairly simple to break down and digest.Digestionis slower than inmammals,reflecting their lower restingmetabolismand their inability to divide andmasticatetheir food.Theirpoikilothermmetabolism has very low energy requirements, allowing large reptiles like crocodiles and large constrictors to live from a single large meal for months, digesting it slowly.

While modern reptiles are predominantly carnivorous, during the early history of reptiles several groups produced some herbivorousmegafauna:in thePaleozoic,thepareiasaurs;and in theMesozoicseveral lines ofdinosaurs.Today,turtlesare the only predominantly herbivorous reptile group, but several lines ofagamasandiguanashave evolved to live wholly or partly on plants.

Herbivorous reptiles face the same problems of mastication as herbivorous mammals but, lacking the complex teeth of mammals, many species swallow rocks and pebbles (so calledgastroliths) to aid in digestion: The rocks are washed around in the stomach, helping to grind up plant matter.Fossil gastroliths have been found associated with bothornithopodsandsauropods,though whether they actually functioned as a gastric mill in the latter is disputed.Salt water crocodilesalso use gastroliths asballast,stabilizing them in the water or helping them to dive.A dual function as both stabilizing ballast and digestion aid has been suggested for gastroliths found inplesiosaurs.

Nerves

The reptilian nervous system contains the same basic part of theamphibianbrain, but the reptilecerebrumandcerebellumare slightly larger. Most typical sense organs are well developed with certain exceptions, most notably thesnake's lack of external ears (middle and inner ears are present). There are twelve pairs ofcranial nerves.Due to their short cochlea, reptiles useelectrical tuningto expand their range of audible frequencies.

Vision

Most reptiles arediurnalanimals. The vision is typically adapted to daylight conditions, with color vision and more advanced visualdepth perceptionthan in amphibians and most mammals.

Reptiles usually have excellent vision, allowing them to detect shapes and motions at long distances. They often have poor vision in low-light conditions. Birds, crocodiles and turtles have three types ofphotoreceptor:rods,singleconesand double cones, which gives them sharp color vision and enables them to seeultravioletwavelengths.The lepidosaurs appear to have lost theduplex retinaand only have a single class of receptor that is cone-like or rod-like depending on whether the species is diurnal or nocturnal.In many burrowing species, such asblind snakes,vision is reduced.

Manylepidosaurshave a photosensory organ on the top of their heads called theparietal eye,which are also calledthird eye,pineal eyeorpineal gland.This "eye" does not work the same way as a normal eye does as it has only a rudimentary retina and lens and thus, cannot form images. It is, however, sensitive to changes in light and dark and can detect movement.

Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form ofpitssensitive toinfraredradiation (heat). Such heat-sensitive pits are particularly well developed in thepit vipers,but are also found inboasandpythons.These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.

Most reptiles, as well as birds, possess anictitating membrane,a translucent third eyelid which is drawn over the eye from the inner corner. In crocodilians, it protects its eyeball surface while allowing a degree of vision underwater.However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called thebrille,spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.

Reproduction

Crocodilian egg diagram
(1) eggshell, (2) yolk sac, (3) yolk (nutrients), (4) vessels, (5)amnion,(6)chorion,(7) air space, (8)allantois,(9) albumin (egg white), (10) amniotic sac, (11) crocodile embryo, (12) amniotic fluid

Common house geckosmating, ventral view withhemipenisinserted in thecloaca

Most reptiles reproduce sexually, for example thisTrachylepis maculilabrisskink

Reptiles haveamnioticeggs with hard or leathery shells, requiringinternal fertilizationwhen mating.

Reptiles generallyreproduce sexually,though some are capable ofasexual reproduction.All reproductive activity occurs through thecloaca,the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles havecopulatory organs,which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single medianpenis,while squamates, including snakes and lizards, possess a pair ofhemipenes,only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm.

Most reptiles lay amniotic eggs covered with leathery or calcareous shells. Anamnion(5),chorion(6), andallantois(8) are present duringembryoniclife. The eggshell (1) protects the crocodile embryo (11) and keeps it from drying out, but it is flexible to allow gas exchange. The chorion (6) aids in gas exchange between the inside and outside of the egg. It allows carbon dioxide to exit the egg and oxygen gas to enter the egg. The albumin (9) further protects the embryo and serves as a reservoir for water and protein. The allantois (8) is a sac that collects the metabolic waste produced by the embryo. The amniotic sac (10) contains amniotic fluid (12) which protects and cushions the embryo. The amnion (5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac (2) surrounding the yolk (3) contains protein and fat rich nutrients that are absorbed by the embryo via vessels (4) that allow the embryo to grow and metabolize. The air space (7) provides the embryo with oxygen while it is hatching. This ensures that the embryo will not suffocate while it is hatching. There are nolarvalstages of development.Viviparityandovoviviparityhave evolved in squamates and many extinct clades of reptiles. Among squamates, many species, including all boas and most vipers, use this mode of reproduction. The degree of viviparity varies; some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalianplacenta.The earliest documented case of viviparity in reptiles is the EarlyPermianmesosaurs,although some individuals or taxa in that clade may also have been oviparous because a putative isolated egg has also been found. Several groups of Mesozoic marine reptiles also exhibited viviparity, such asmosasaurs,ichthyosaurs,andSauropterygia,a group that includespachypleurosaursandPlesiosauria.

Asexual reproduction has been identified insquamatesin six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother. This form of asexual reproduction, calledparthenogenesis,occurs in several species ofgecko,and is particularly widespread in theteiids(especiallyAspidocelis) andlacertids(Lacerta). In captivity,Komodo dragons(Varanidae) have reproduced byparthenogenesis.

Parthenogenetic species are suspected to occur amongchameleons,agamids,xantusiids,andtyphlopids.

Some reptiles exhibittemperature-dependent sex determination(TDSD), in which the incubation temperature determines whether a particular egg hatches as male or female. TDSD is most common in turtles and crocodiles, but also occurs in lizards and tuatara.To date, there has been no confirmation of whether TDSD occurs in snakes.

Longevity

Gianttortoisesare among the longest-lived vertebrate animals (over 100 years by some estimates) and have been used as a model for studyinglongevity.DNA analysis of thegenomesofLonesome George,the iconic last member ofChelonoidis abingdonii,and theAldabra giant tortoiseAldabrachelys gigantealed to the detection of lineage-specific variants affectingDNA repairgenes that might contribute to our understanding of increased lifespan.

Cognition

See also:Animal cognition

Reptiles are generally considered less intelligent than mammals and birds.Thesize of their brain relative to their bodyis much less than that of mammals, theencephalization quotientbeing about one tenth of that of mammals,though larger reptiles can show more complex brain development. Larger lizards, like themonitors,are known to exhibit complex behavior, including cooperationand cognitive abilities allowing them to optimize theirforagingandterritorialityover time.Crocodiles have relatively larger brains and show a fairly complex social structure. TheKomodo dragonis even known to engage in play,as are turtles, which are also considered to be social creatures,and sometimes switch between monogamy and promiscuity in their sexual behavior.One study found thatwood turtleswere better thanwhite ratsat learning to navigate mazes.Another study found that giant tortoises are capable of learning throughoperant conditioning,visual discrimination and retained learned behaviors with long-term memory.Sea turtles have been regarded as having simple brains, but their flippers are used for a variety of foraging tasks (holding, bracing, corralling) in common with marine mammals.

There is evidence that reptiles aresentientand able to feel emotions includinganxietyandpleasure.

Defense mechanisms

Many small reptiles, such as snakes and lizards, that live on the ground or in the water are vulnerable to being preyed on by all kinds of carnivorous animals. Thus,avoidanceis the most common form of defense in reptiles.At the first sign of danger, most snakes and lizards crawl away into the undergrowth, and turtles and crocodiles will plunge into water and sink out of sight.

Camouflage and warning

A camouflagedPhelsuma deubiaon a palm frond

Reptiles tend to avoid confrontation throughcamouflage.Two major groups of reptile predators are birds and other reptiles, both of which have well-developed color vision. Thus the skins of many reptiles havecrypticcoloration of plain or mottled gray, green, and brown to allow them to blend into the background of their natural environment.Aided by the reptiles' capacity for remaining motionless for long periods, the camouflage of many snakes is so effective that people or domestic animals are most typically bitten because they accidentally step on them.

When camouflage fails to protect them,blue-tongued skinkswill try to ward off attackers by displaying their blue tongues, and thefrill-necked lizardwill display its brightly colored frill. These same displays are used in territorial disputes and during courtship.If danger arises so suddenly that flight is useless, crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy.Rattlesnakesrapidly vibrate the tip of the tail, which is composed of a series of nested, hollow beads to ward off approaching danger.

In contrast to the normal drab coloration of most reptiles, the lizards of the genusHeloderma(theGila monsterand thebeaded lizard) and many of thecoral snakeshave high-contrast warning coloration, warning potential predators they are venomous.A number of non-venomous North American snake species have colorful markings similar to those of the coral snake, an oft cited example ofBatesian mimicry.

Alternative defense in snakes

Further information:VenomandEvolution of snake venom

Camouflage does not always fool a predator. When caught out, snake species adopt different defensive tactics and use a complicated set of behaviors when attacked. Some species, like cobras or hognose snakes, first elevate their head and spread out the skin of their neck in an effort to look large and threatening. Failure of this strategy may lead to other measures practiced particularly by cobras, vipers, and closely related species, which usevenomto attack. The venom is modified saliva, delivered through fangs from avenom gland.Some non-venomous snakes, such as Americanhognose snakesor Europeangrass snake,play deadwhen in danger; some, including the grass snake, exude a foul-smelling liquid to deter attackers.

Defense in crocodilians

When acrocodilianis concerned about its safety, it will gape to expose the teeth and tongue. If this does not work, the crocodilian gets a little more agitated and typically begins to make hissing sounds. After this, the crocodilian will start to change its posture dramatically to make itself look more intimidating. The body is inflated to increase apparent size. If absolutely necessary, it may decide to attack an enemy.

AWhite-headed dwarf geckowith shed tail

Some species try to bite immediately. Some will use their heads assledgehammersand literally smash an opponent, some will rush or swim toward the threat from a distance, even chasing the opponent onto land or galloping after it.The main weapon in all crocodiles is the bite, which can generate very high bite force. Many species also possesscanine-like teeth. These are used primarily for seizing prey, but are also used in fighting and display.

Shedding and regenerating tails

Main article:Autotomy

Geckos,skinks,and some other lizards that are captured by the tail will shed part of the tail structure through a process calledautotomyand thus be able to flee. The detached tail will continue to thrash, creating a deceptive sense of continued struggle and distracting the predator's attention from the fleeing prey animal. The detached tails ofleopard geckoscan wiggle for up to 20 minutes. The tail grows back in most species, but some, like crested geckos, lose their tails for the rest of their lives.In many species the tails are of a separate and dramatically more intense color than the rest of the body so as to encourage potential predators to strike for the tail first. In theshingleback skinkand some species of geckos, the tail is short and broad and resembles the head, so that the predators may attack it rather than the more vulnerable front part.

Reptiles that are capable of shedding their tails can partiallyregeneratethem over a period of weeks. The new section will however contain cartilage rather than bone, and will never grow to the same length as the original tail. It is often also distinctly discolored compared to the rest of the body and may lack some of the external sculpting features seen in the original tail.

Relations with humans

Further information:Human uses of reptiles

In cultures and religions

Main article:Reptiles in culture

Painting of fighting "Laelaps"(nowDryptosaurus) byCharles R. Knight(1897)

Dinosaurs have been widely depicted in culture since the English palaeontologistRichard Owencoined the namedinosaurin 1842. As soon as 1854, theCrystal Palace Dinosaurswere on display to the public in south London.One dinosaur appeared in literature even earlier, asCharles Dickensplaced aMegalosaurusin the first chapter of his novelBleak Housein 1852. The dinosaurs featured in books, films, television programs, artwork, and other media have been used for both education and entertainment. The depictions range from the realistic, as in the televisiondocumentariesof the 1990s and first decade of the 21st century, to the fantastic, as in themonster moviesof the 1950s and 1960s.

The snake or serpent has played a powerfulsymbolic rolein different cultures. InEgyptian history,the Nile cobra adorned the crown of thepharaoh.It wasworshippedas one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra). InGreek mythology,snakes are associated with deadly antagonists, as achthonicsymbol, roughly translated asearthbound.The nine-headedLernaean HydrathatHerculesdefeated and the threeGorgonsisters are children ofGaia,the earth.Medusawas one of the three Gorgon sisters whoPerseusdefeated. Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze. After killing her, Perseus gave her head toAthenawho fixed it to her shield called theAegis.TheTitansare depicted in art with their legs replaced by bodies of snakes for the same reason: They are children of Gaia, so they are bound to the earth.In Hinduism,snakes are worshippedas gods, with many women pouring milk on snake pits. The cobra is seen on the neck ofShiva,whileVishnuis depicted often as sleeping on a seven-headed snake or within the coils of a serpent. There are temples in India solely for cobras sometimes calledNagraj(King of Snakes), and it is believed that snakes are symbols of fertility. In the annual Hindu festival ofNag Panchami,snakes are venerated and prayed to.In religious terms, the snake andjaguarare arguably the most important animals in ancientMesoamerica."In states of ecstasy, lords dance a serpent dance; great descending snakes adorn and support buildings fromChichen ItzatoTenochtitlan,and theNahuatlwordcoatlmeaning serpent or twin, forms part of primary deities such asMixcoatl,Quetzalcoatl,andCoatlicue."In Christianity and Judaism, a serpent appears in Genesis to temptAdam and Evewith theforbidden fruitfrom theTree of Knowledge of Good and Evil.

The turtle has a prominent position as a symbol of steadfastness and tranquility in religion, mythology, and folklore from around the world.A tortoise's longevity is suggested by its long lifespan and its shell, which was thought to protect it from any foe.In thecosmological mythsof several cultures aWorld Turtlecarries the world upon its back or supports the heavens.

Medicine

See also:Epidemiology of snakebites

TheRod of Asclepiussymbolizes medicine

Deaths fromsnakebitesare uncommon in many parts of the world, but are still counted in tens of thousands per year in India.Snakebite can be treated withantivenommade from the venom of the snake. To produce antivenom, a mixture of the venoms of different species of snake is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted; the serum is separated, purified and freeze-dried.Thecytotoxiceffect of snake venom is being researched as a potential treatment for cancers.

Lizards such as the Gila monster produce toxins with medical applications. Gila toxin reduces plasma glucose; the substance is now synthesised for use in the anti-diabetesdrugexenatide(Byetta).Another toxin from Gila monster saliva has been studied for use as an anti-Alzheimer'sdrug.

Geckos have also been used as medicine, especially in China.Turtles have been used in Chinese traditional medicine for thousands of years, with every part of the turtle believed to have medical benefits. There is a lack of scientific evidence that would correlate claimed medical benefits to turtle consumption. Growing demand for turtle meat has placed pressure on vulnerable wild populations of turtles.

Commercial farming

See also:Crocodile farm,Snake farm,andTurtle farming

Crocodiles are protected in many parts of the world, and arefarmed commercially.Their hides are tanned and used to make leather goods such as shoes andhandbags;crocodile meatis also considered a delicacy.The most commonly farmed species are the saltwater and Nile crocodiles. Farming has resulted in an increase in the saltwater crocodile population inAustralia,as eggs are usually harvested from the wild, so landowners have an incentive to conserve their habitat.Crocodile leatheris made into wallets, briefcases, purses, handbags, belts, hats, and shoes.Crocodile oilhas been used for various purposes.

Snakes are also farmed, primarily inEastandSoutheast Asia,and their production has become more intensive in the last decade.Snake farminghas been troubling for conservation in the past as it can lead tooverexploitationof wild snakes and their natural prey to supply the farms. However, farming snakes can limit the hunting of wild snakes, while reducing the slaughter of higher-order vertebrates like cows. The energy efficiency of snakes is higher than expected for carnivores, due to their ectothermy and low metabolism. Waste protein from the poultry and pig industries is used as feed in snake farms.Snake farms produce meat,snake skin,and antivenom.

Turtle farmingis another known but controversial practice. Turtles have been farmed for a variety of reasons, ranging from food to traditional medicine, the pet trade, and scientific conservation. Demand for turtle meat and medicinal products is one of the main threats to turtle conservation in Asia. Though commercial breeding would seem to insulate wild populations, it can stoke the demand for them and increase wild captures.Even the potentially appealing concept of raising turtles at a farm to release into the wild is questioned by some veterinarians who have had some experience with farm operations. They caution that this may introduce into the wild populations infectious diseases that occur on the farm, but have not (yet) been occurring in the wild.

Reptiles in captivity

Further information:Herpetoculture

Aherpetariumis azoological exhibitionspace for reptiles and amphibians.

In the Western world, some snakes (especially relatively docile species such as theball pythonandcorn snake) are sometimes kept as pets.Numerous species of lizard are kept aspets,includingbearded dragons,iguanas,anoles,andgeckos(such as the popularleopard geckoand the crested gecko).

Turtles and tortoises are increasingly popular pets, but keeping them can be challenging due to their particular requirements, such as temperature control, the need for UV light sources, and a varied diet. The long lifespans of turtles and especially tortoises mean they can potentially outlive their owners. Good hygiene and significant maintenance is necessary when keeping reptiles, due to the risks ofSalmonellaand other pathogens.Regular hand-washing after handling is an important measure to prevent infection.

See also

Main article:Outline of reptiles

• Amphibian and reptile tunnel
• List of reptiles
• Lists of reptiles by region
• Reptile Database
• Reptiles portal

Further reading

• Colbert, Edwin H.(1969).Evolution of the Vertebrates(2nd ed.). New York, NY: John Wiley and Sons Inc.ISBN978-0-471-16466-1.
• Duellman, William E., Berg, Barbara (1962),Type Specimens of Amphibians and Reptiles in the Museum of Natural History, the University of Kansas
• Landberg, Tobias; Mailhot, Jeffrey; Brainerd, Elizabeth (2003)."Lung ventilation during treadmill locomotion in a terrestrial turtle,Terrapene carolina".Journal of Experimental Biology.206(19): 3391–3404.doi:10.1242/jeb.00553.PMID12939371.
• Pianka, Eric; Vitt, Laurie (2003).Lizards: Windows to the evolution of diversity.University of California Press. pp.116–118.ISBN978-0-520-23401-7.
• Pough, Harvey; Janis, Christine; Heiser, John (2005).Vertebrate Life.Pearson Prentice Hall.ISBN978-0-13-145310-4.

External links

• Data related toReptiliaat Wikispecies
• Reptiliaat Wikibooks
• "Reptiles".Encyclopædia Britannica(11th ed.). 1911.
• "Reptile phylogeny".whozoo.org.Herpetology.
• Reptile images.1833.{{cite book}}:|website=ignored (help)
• "Sri Lanka wildlife information database".wildreach.com.
• "Biology of the reptilia".carlgans.org.— an online full text copy of a 22 volume 13,000 page summary of the state of reptile research.