Editing Reptile (section)

==Morphology and physiology{{anchor|Systems}}==

===Circulation===
[[File:wiki varano.jpg|thumb|Thermographic image of [[monitor lizard]]s]]

All [[Lepidosauria|lepidosaurs]] and [[turtle]]s have a three-chambered [[heart]] consisting of two [[atrium (heart)|atria]], one variably partitioned [[ventricle (heart)|ventricle]], and two aortas that lead to the [[systemic circulation]]. The degree of mixing of [[oxygen]]ated and 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 a [[fitness (biology)|fitness]] advantage.<ref>{{cite journal |last=Hicks |first=James |year=2002 |title=The Physiological and Evolutionary Significance of Cardiovascular Shunting Patterns in Reptiles |journal=News in Physiological Sciences |volume=17 |issue=6 |pages=241–245 |pmid=12433978|doi=10.1152/nips.01397.2002 |s2cid=20040550 }}</ref>

[[File:Bisected Iguana Heart Image.png|thumb|Juvenile [[iguana]] [[heart]] bisected through the ventricle, bisecting the left and right atrium]]
For example,&nbsp;[[iguana]]&nbsp;hearts, like the majority of the&nbsp;[[Squamata|squamate]]&nbsp;hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.<ref>{{Cite news|url=http://veterinarycalendar.dvm360.com/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings?id=&pageID=1&sk=&date=|title=Reptilian cardiovascular anatomy and physiology: evaluation and monitoring (Proceedings)|last=DABVP|first=Ryan S. De Voe DVM MSpVM DACZM|work=dvm360.com|access-date=2017-04-22|archive-date=2018-11-06|archive-url=https://web.archive.org/web/20181106205150/http://veterinarycalendar.dvm360.com/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings?id=&pageID=1&sk=&date=|url-status=dead}}</ref> The main structures of the heart are the&nbsp;[[sinus venosus]], the pacemaker, the&nbsp;[[Atrium (heart)|left atrium]], the&nbsp;[[Atrium (heart)|right atrium]], the&nbsp;[[Heart valve|atrioventricular valve]], the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge,&nbsp;[[pulmonary vein]]s, and paired&nbsp;[[aortic arches]].<ref>{{Cite news|url=http://reptile-parrots.com/forums/showthread.php/825-Iguana-Internal-Body-Parts|title=Iguana Internal Body Parts|work=Reptile & Parrots Forum|access-date=2017-04-22|language=en|archive-date=2017-04-22|archive-url=https://web.archive.org/web/20170422123904/http://reptile-parrots.com/forums/showthread.php/825-Iguana-Internal-Body-Parts|url-status=dead}}</ref>

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 during [[cardiac cycle|ventricular diastole]] and completely divides it during [[systole (medicine)|ventricular systole]]. Because of this ridge, some of these [[squamata|squamates]] are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.<ref>{{cite journal | last=Wang | first=Tobias |author2=Altimiras, Jordi |author3=Klein, Wilfried |author4= Axelsson, Michael | title=Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures | journal=The Journal of Experimental Biology | year=2003 | volume=206 | pages=4242–4245 | doi=10.1242/jeb.00681 | pmid=14581594 | issue=Pt 23| doi-access=free | bibcode=2003JExpB.206.4241W }}</ref>

[[Crocodilia]]ns have an anatomically four-chambered heart, similar to [[bird]]s, but also have two systemic aortas and are therefore capable of bypassing their [[pulmonary circulation]].<ref>{{cite journal |last=Axelsson |first=Michael |author2=Craig E. Franklin |year=1997 |title=From anatomy to angioscopy: 164 years of crocodilian cardiovascular research, recent advances, and speculations |journal=Comparative Biochemistry and Physiology A |volume=188 |issue=1 |pages=51–62 |doi=10.1016/S0300-9629(96)00255-1}}</ref> In turtles, the ventricle is not perfectly divided, so a mix of aerated and nonaerated blood can occur.<ref>{{cite web |last1=Zug |first1=George R. |title=Reptile |url=https://www.britannica.com/animal/reptile. Accessed 30 September 2024. |website=Encyclopedia Britannica |publisher=Britannica |access-date=30 September 2024}}</ref>

===Metabolism===
[[File:Homeothermy-poikilothermy.png|thumb|right|Sustained energy output ([[joule]]s) 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 of [[Ectotherm|cold-bloodedness]] (i.e. some mix of [[poikilotherm]]y, [[ectotherm]]y, and [[bradymetabolism]]) 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 than [[bird]]s and [[mammal]]s, reptilian biochemistry requires [[enzyme]]s capable of maintaining efficiency over a greater range of temperatures than in the case for [[warm-blooded]] animals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals; for many lizards, it falls in the {{convert|24|–|35|C|F}} range,<ref>Huey, R.B. & Bennett, A.F. (1987):Phylogenetic studies of coadaptation: Preferred temperatures versus optimal performance temperatures of lizards. ''Evolution'' No. 4, vol 5: pp. 1098–1115 [http://faculty.washington.edu/hueyrb/pdfs/Coadaptation_Evolution.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20220911080152/http://faculty.washington.edu/hueyrb/pdfs/Coadaptation_Evolution.pdf |date=2022-09-11 }}</ref> while extreme heat-adapted species, like the American [[desert iguana]] ''Dipsosaurus dorsalis'', can have optimal physiological temperatures in the mammalian range, between {{convert|35|and|40|C|F}}.<ref>Huey, R.B. (1982): Temperature, physiology, and the ecology of reptiles. Side 25–91. In Gans, C. & Pough, F.H. (red), ''Biology of the Reptili'' No. 12, Physiology (C). Academic Press, London.[http://faculty.washington.edu/hueyrb/pdfs/temp_phys_ecol_rep_1982.pdf artikkel] {{Webarchive|url=https://web.archive.org/web/20220419174304/http://faculty.washington.edu/hueyrb/pdfs/temp_phys_ecol_rep_1982.pdf |date=2022-04-19 }}</ref> 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, like [[leatherback turtle]]s, 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 a [[warm-blooded]] metabolism.<ref>Spotila J.R. & Standora, E.A. (1985) Environmental constraints on the thermal energetics of sea turtles. ''Copeia'' 3: 694–702</ref> This form of homeothermy is called [[gigantothermy]]; it has been suggested as having been common in large [[dinosaur]]s and other extinct large-bodied reptiles.<ref>Paladino, F.V.; Spotila, J.R. & Dodson, P. (1999): A blueprint for giants: modeling the physiology of large dinosaurs. ''The Complete Dinosaur''. Bloomington, Indiana University Press. pp. 491–504. {{ISBN|978-0-253-21313-6}}.</ref><ref>{{cite journal | last1 = Spotila | first1 = J.R. | last2 = O'Connor | first2 = M.P. | last3 = Dodson | first3 = P. | last4 = Paladino | first4 = F.V. | year = 1991 | title = Hot and cold running dinosaurs: body size, metabolism and migration | journal = Modern Geology | volume = 16 | pages = 203–227 }}</ref>

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.<ref>Campbell, N.A. & Reece, J.B. (2006): Outlines & Highlights for Essential Biology. ''Academic Internet Publishers''. 396 pp. {{ISBN|978-0-8053-7473-5}}</ref> A crocodile needs from a tenth to a fifth of the food necessary for a [[lion]] of the same weight and can live half a year without eating.<ref name=Garnett>{{cite journal|last=Garnett|first=S. T.|year= 2009|title=Metabolism and survival of fasting Estuarine crocodiles|journal=Journal of Zoology|number=208|volume=4|pages=493–502|doi=10.1111/j.1469-7998.1986.tb01518.x}}</ref> Lower food requirements and adaptive metabolisms allow reptiles to dominate the animal life in regions where net [[calorie]] availability 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.<ref>{{cite book |author1=Willmer, P. |author2=Stone, G. |author3=Johnston, I.A. |year=2000 |title=Environmental Physiology of Animals |publisher=Blackwell Science |place=London, UK  |isbn=978-0-632-03517-5}}</ref> Higher energetic capacity might have been responsible for the evolution of [[warm-blooded]]ness in birds and mammals.<ref>{{cite journal | last1 = Bennett | first1 = A. | last2 = Ruben | first2 = J. | year = 1979 | title = Endothermy and Activity in Vertebrates | journal = [[Science (journal)|Science]] | volume = 206 | issue = 4419 | pages = 649–654 | doi = 10.1126/science.493968 | pmid = 493968 | url = http://compphys.bio.uci.edu/bennett/pubs/30.pdf | bibcode = 1979Sci...206..649B | citeseerx = 10.1.1.551.4016 | access-date = 2017-10-27 | archive-date = 2017-08-11 | archive-url = https://web.archive.org/web/20170811152427/http://compphys.bio.uci.edu/bennett/pubs/30.pdf | url-status = dead }}</ref> However, investigation of correlations between active capacity and [[Thermoregulation|thermophysiology]] show a weak relationship.<ref name="CGF1">{{cite journal | author = Farmer, C.G. | year = 2000 | title = Parental care: The key to understanding endothermy and other convergent features in birds and mammals | journal = American Naturalist | volume = 155 | issue = 3 | pages = 326–334 | pmid = 10718729 | s2cid = 17932602 | doi = 10.1086/303323 | bibcode = 2000ANat..155..326F }}</ref> Most extant reptiles are carnivores with a sit-and-wait feeding strategy; whether reptiles are cold blooded due to their ecology <!-- 'or because their metabolism is a result of their ecology' : As this it is just a restatement of 'reptiles are cold blooded due to their ecology', essentially it was saying whether A was because of B or A was because of B, which is stupid! --> is not clear. Energetic studies on some reptiles have shown active capacities equal to or greater than similar sized warm-blooded animals.<ref>{{cite journal | last1 = Hicks | first1 = J.W.  | last2 = Farmer | first2 = C.G. | year = 1999 | title = Gas exchange potential in reptilian lungs: Implications for the dinosaur-avian connection | journal = Respiratory Physiology | volume = 117 | issue = 2–3 | pages = 73–83 | pmid=10563436 | doi = 10.1016/S0034-5687(99)00060-2}}</ref>

===Respiratory system===
[[File:X-ray video of a female American alligator (Alligator mississippiensis) while breathing - pone.0004497.s009.ogv|right|thumb|X-ray [[fluoroscopy]] videos of a female American alligator showing contraction of the lungs while breathing]]
All reptiles breathe using [[lung]]s. Aquatic [[turtle]]s have developed more permeable skin, and some species have modified their [[cloaca]] to increase the area for [[gas exchange]].<ref>{{cite book | last=Orenstein | first=Ronald | title=Turtles, Tortoises & Terrapins: Survivors in Armor | publisher=Firefly Books | year=2001 | isbn=978-1-55209-605-5 | url-access=registration | url=https://archive.org/details/turtlestortoises0000oren }}</ref> Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In [[squamata|squamates]], the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this [[Carrier's constraint|constraint]], 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, employ [[buccal pumping]] as 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. [[Tupinambis|Tegu lizards]] are known to possess a proto-[[thoracic diaphragm|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.<ref>{{cite journal | last=Klein | first=Wilfied |author2=Abe, Augusto |author3=Andrade, Denis |author4= Perry, Steven | title=Structure of the posthepatic septum and its influence on visceral topology in the tegu lizard, ''Tupinambis merianae'' (Teidae: Reptilia) | journal=Journal of Morphology | volume=258 | issue=2 | year=2003 | pages=151–157 | doi=10.1002/jmor.10136 | pmid=14518009| s2cid=9901649 }}</ref>

[[Crocodilia]]ns actually 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 "[[Liver|hepatic]] [[piston]]". The [[Bronchus|airways]] form 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,<ref>{{cite journal|last=Farmer|first=CG|author2=Sanders, K |title=Unidirectional airflow in the lungs of alligators|journal=Science|year=2010|volume=327|issue=5963|pages=338–340|doi=10.1126/science.1180219|pmid=20075253|bibcode=2010Sci...327..338F|s2cid=206522844}}</ref> monitor lizards<ref>{{cite journal | last1 = Schachner | first1 = E.R. | last2 = Cieri | first2 = R.L. | last3 = Butler | first3 = J.P. | last4 = Farmer | first4 = C.G. | year = 2013 | title = Unidirectional pulmonary airflow patterns in the savannah monitor lizard | doi = 10.1038/nature12871 | journal = [[Nature (journal)|Nature]] | pmid = 24336209| volume=506 | issue = 7488 | pages=367–370| bibcode = 2014Natur.506..367S | s2cid = 4456381 | url = http://nrs.harvard.edu/urn-3:HUL.InstRepos:32631102 | url-access = subscription }}</ref> and iguanas.<ref>{{cite journal |author1=Cieri, Robert L. |author2=Craven, Brent A. |author3=Schachner, Emma R. |author4=Farmer, C.G. |year=2014 |title=New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs |journal=[[Proceedings of the National Academy of Sciences]] |volume=111 |issue=48 |pages=17218–17223 |pmid=25404314 |pmc=4260542 |doi=10.1073/pnas.1405088111 |bibcode=2014PNAS..11117218C |doi-access=free }}{{open access}}</ref>

Most reptiles lack a [[secondary 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 (family [[Skink|Scincidae]]) 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.<ref>{{Cite journal|last1=Chiodini|first1=Rodrick J.|last2=Sundberg|first2=John P.|last3=Czikowsky|first3=Joyce A.|date=January 1982|editor-last=Timmins|editor-first=Patricia|title=Gross anatomy of snakes.|url=https://www.researchgate.net/publication/241830127|journal=Veterinary Medicine/Small Animal Clinician|via=ResearchGate}}</ref>

====Turtles and tortoises====
[[File:Tortue de Floride Amiens.jpg|thumb|[[Red-eared slider]] taking a gulp of air]]

How [[turtle]]s breathe 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 turtles [[Breathing|breathe]]. The varied results indicate that turtles have found a variety of solutions to this problem.

The difficulty is that most [[turtle shell]]s are 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 (''[[Indian flapshell turtle|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 a [[thoracic diaphragm|diaphragm]]), 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).<ref>{{cite journal |first1=Tyler R. |last1=Lyson |first2=Emma R. |last2=Schachner |first3=Jennifer |last3=Botha-Brink |first4=Torsten M. |last4=Scheyer |first5=Markus |last5=Lambertz |first6=G.S. |last6=Bever |first7=Bruce S. |last7=Rubidge |first8=Kevin |last8=de Queiroz |year=2014|title=Origin of the unique ventilatory apparatus of turtles |journal=Nature Communications |volume=5 |number=5211 |page=5211 |doi=10.1038/ncomms6211 |doi-access=free |pmid=25376734 |bibcode=2014NatCo...5.5211L |url=http://www.zora.uzh.ch/id/eprint/100716/7/LysonEtAl_NatCommun2014_Vol5_OriginVentilationApparatusTurtles_Supplem_s1.pdf}}{{open access}}</ref>

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.<ref name=Landberg>{{cite journal | last=Landberg | first=Tobias |author2=Mailhot, Jeffrey |author3=Brainerd, Elizabeth | title=Lung ventilation during treadmill locomotion in a terrestrial turtle, ''Terrapene carolina'' | journal=Journal of Experimental Biology | volume=206 | issue=19 | year=2003 | pages=3391–3404| doi=10.1242/jeb.00553| pmid=12939371| doi-access=free | bibcode=2003JExpB.206.3391L }}</ref> 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.<ref name=Landberg/>

===Sound production===
Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited to [[wikt:hiss|hissing]], which is produced merely by forcing air though a partly closed [[glottis]] and 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 the [[larynx]] or glottis. Some [[gecko]]s and turtles possess true [[vocal cord]]s, which have [[elastin]]-rich connective tissue.<ref>{{cite journal |author1=Russell, Anthony P. |author2=Bauer, Aaron M. |year=2020 |title=Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus |journal=The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology |volume=304 |issue=7 |pages=1478–1528 |doi=10.1002/ar.24553 |pmid=33099849 |s2cid=225069598|doi-access=free }}</ref><ref>{{cite book |author1=Capshaw, Grace |author2=Willis, Katie L. |author3=Han, Dawei |author4=Bierman, Hilary S. |year=2020 |section=Reptile sound production and perception |pages=101–118 |editor1=Rosenfeld, Cheryl S. |editor2=Hoffmann, Frauke |title=Neuroendocrine Regulation of Animal Vocalization |publisher=Academic Press |isbn=978-0128151600}}</ref>

====Hearing in snakes====
Hearing in humans relies on 3&nbsp;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 a [[Tympanum (anatomy)|tympanum]] but have an inner ear structure with [[cochlea]]s directly connected to their jawbone.<ref>{{cite journal |last1=Christensen |first1=Christian Bech |last2=Christensen-Dalsgaard |first2=Jakob |last3=Brandt |first3=Christian |last4=Madsen |first4=Peter Teglberg |date=2012-01-15 |title=Hearing with an atympanic ear: Good vibration and poor sound-pressure detection in the royal python,''Python regius'' |journal=Journal of Experimental Biology |volume=215 |issue=2 |pages=331–342 |doi=10.1242/jeb.062539 |pmid=22189777 |s2cid=11909208 |issn=1477-9145|doi-access=free |bibcode=2012JExpB.215..331C }}</ref> 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 of [[mechanoreceptor]]s, 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.<ref>{{Cite journal |last=YOUNG |first=BRUCE A. |title=A Review of Sound Production and Hearing in Snakes, with a Discussion of Intraspecific Acoustic Communication in Snakes |date=1997 |url=https://www.jstor.org/stable/44149431 |journal=Journal of the Pennsylvania Academy of Science |volume=71 |issue=1 |pages=39–46 |jstor=44149431 |issn=1044-6753}}</ref>

===Skin===
[[File:Lacertae skin.jpg|thumb|Skin of a [[sand lizard]], showing [[Squamata|squamate reptiles]] iconic [[scale (zoology)|scales]]]]

Reptilian skin is covered in a horny [[Epidermis (zoology)|epidermis]], 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 thick [[dermis|dermal]] layer that produces [[leather]] in mammals.<ref>Hildebran, M. & Goslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & sons inc, New York. 635 pp. {{ISBN|978-0-471-29505-1}}</ref>
Exposed parts of reptiles are protected by [[reptile scales|scales]] or [[scutes]], sometimes with a bony base ([[osteoderm]]s), forming [[armour (zoology)|armor]]. In [[lepidosaur]]s, such as lizards and snakes, the whole skin is covered in overlapping [[epidermis (zoology)|epidermal]] scales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurs.{{citation needed|date=August 2015}} The scales found in turtles and crocodiles are of [[dermis|dermal]], rather than epidermal, origin and are properly termed scutes.{{citation needed|date=August 2015}} 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 called [[ecdysis]] which 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.<ref>{{cite book|last1=Paterson|first1=Sue|title=Skin Diseases of Exotic Pets|date=December 17, 2007|publisher=Blackwell Science, Ltd.|isbn=9780470752432|pages=74–79}}</ref> 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. [[Proteolysis|Proteolytic]] enzymes and [[Lymph|lymphatic fluid]] is secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.<ref name="Dermatological Diseases in Lizards">{{cite journal|last1=Hellebuyck|first1=Tom|last2=Pasmans|first2=Frank|last3=Haesbrouck|first3=Freddy|last4=Martel|first4=An|title=Dermatological Diseases in Lizards|journal=The Veterinary Journal|date=July 2012|volume=193|issue=1|pages=38–45|doi=10.1016/j.tvjl.2012.02.001|pmid=22417690}}</ref> Snakes will shed from the head to the tail while lizards shed in a "patchy pattern".<ref name="Dermatological Diseases in Lizards" /> [[Dysecdysis]], a common skin disease in snakes and lizards, will occur when ecdysis, or shedding, fails.<ref name="Veterinary Nursing of Exotic Pets">{{cite book|last1=Girling|first1=Simon|title=Veterinary Nursing of Exotic Pets|date=June 26, 2013|publisher=Blackwell Publishing, Ltd.|isbn=9781118782941|edition=2}}</ref> There are numerous reasons why shedding fails and can be related to inadequate humidity and temperature, nutritional deficiencies, dehydration and traumatic injuries.<ref name="Dermatological Diseases in Lizards" /> 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.<ref name="Veterinary Nursing of Exotic Pets" />

===Excretion===
[[Excretion]] is performed mainly by two small [[kidney]]s. In diapsids, [[uric acid]] is the main [[nitrogen]]ous waste product; turtles, like [[mammal]]s, excrete mainly [[urea]]. Unlike the [[mammalian kidney|kidneys of mammals]] and birds, [[Kidney (vertebrates)#Reptile kidney|reptile kidneys]] are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure called a [[loop of Henle]], which is present in the [[nephron]]s of birds and mammals. Because of this, many reptiles use the [[colon (anatomy)|colon]] to aid in the [[reabsorption]] of water. Some are also able to take up water stored in the [[Urinary bladder|bladder]]. Excess salts are also excreted by nasal and lingual [[salt gland]]s in some reptiles.

In all reptiles, the urinogenital ducts and the [[rectum]] both empty into an organ called a [[cloaca]]. 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 the [[monitor lizard]], the [[legless lizard]]s. It is absent in the snakes, alligators, and crocodiles.<ref>{{cite book |author=Rand, Herbert W. |year=1950 |title=The Chordates |publisher=Balkiston |url=https://archive.org/stream/chordates00rand/#page/276/mode/1up/search/bladder}}</ref>

Many turtles and lizards have proportionally very large bladders. [[Charles Darwin]] noted that the [[Galapagos tortoise]] had a bladder which could store up to 20% of its body weight.<ref>{{cite book |author=Bentley, P.J. |date=14 March 2013 |title=Endocrines and Osmoregulation: A comparative account in vertebrates |publisher=Springer Science & Business Media |isbn=978-3-662-05014-9 |url=https://books.google.com/books?id=U0D3BwAAQBAJ&pg=PA143}}</ref> Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.<ref>{{cite conference |last=Paré|first=Jean |date=11 January 2006 |title=Reptile basics: Clinical anatomy&nbsp;101 |conference=North American Veterinary Conference |volume=20 |pages=1657–1660 |url=http://www.ivis.org/proceedings/navc/2006/SAE/600.pdf?LA=1}}</ref>{{rp|143}} Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid in [[osmoregulation]].<ref>{{Cite journal |last1=Davis |first1=Jon R. |last2=de&nbsp;Nardo |first2=Dale F. |date=2007-04-15 |title=The urinary bladder as a physiological reservoir that moderates dehydration in a large desert lizard, the Gila monster ''Heloderma suspectum'' |journal=Journal of Experimental Biology |language=en |volume=210 |issue=8 |pages=1472–1480 |doi=10.1242/jeb.003061 |issn=0022-0949 |pmid=17401130 |doi-access=free|bibcode=2007JExpB.210.1472D }}</ref>

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.<ref>{{Cite journal|last1=Wyneken|first1=Jeanette|last2=Witherington|first2=Dawn|date=February 2015|title=Urogenital System|url=http://www.ivis.org/advances/wyneken/16.pdf?LA|journal=Anatomy of Sea Turtles|volume=1|pages=153–165}}</ref> 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 have [[Bladder stone (animal)|calculi]].<ref>{{Cite book|title=Reptile Medicine and Surgery|last1=Divers|first1=Stephen J.|last2=Mader|first2=Douglas R.|publisher=Elsevier Health Sciences|year=2005|isbn=9781416064770|location=Amsterdam|pages=481, 597|url=https://books.google.com/books?id=7Ai4BKhi0VUC}}</ref>

===Digestion===
[[File:PikiWiki Israel 37648 Nature and Colors.jpg|thumb|right|A colubrid snake, ''[[Dolichophis jugularis]]'', eating a [[sheltopusik|legless lizard]], ''Pseudopus apodus''. Most reptiles are carnivorous, and many primarily eat other reptiles and small mammals.]]
<!--[[File:Watersnake.JPG|thumb|Watersnake ''[[Malpolon monspessulanus]]'' eating a lizard.]]-->
[[File:PSM V53 D226 Silicious pebbles from the stomach of a plesiosaur.jpg|thumb|right|[[Gastrolith]]s from a [[plesiosaur]]]]

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. [[Digestion]] is slower than in [[mammals]], reflecting their lower resting [[metabolism]] and their inability to divide and [[masticate]] their food.<ref>{{cite book |author=Karasov, W.H. |year=1986 |chapter=Nutrient requirement and the design and function of guts in fish, reptiles and mammals |editor=Dejours, P. |editor2=Bolis, L. |editor3=Taylor, C.R. |editor4=Weibel, E.R. |title=Comparative Physiology: Life in water and on land |publisher=Liviana Press/Springer Verlag |isbn=978-0-387-96515-4 |pages=181–191 |chapter-url=https://books.google.com/books?id=zuT5z5cPWhcC&dq=reptiles+carnivory+gastric&pg=PA181 |access-date=November 1, 2012}}</ref> Their [[poikilotherm]] metabolism 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.<ref name=Garnett/>

While modern reptiles are predominantly carnivorous, during the early history of reptiles several groups produced some herbivorous [[megafauna]]: in the [[Paleozoic]], the [[pareiasaur]]s; and in the [[Mesozoic]] several lines of [[dinosaur]]s.<ref name=Sahney-Benton-Ferry-2010/> Today, [[turtle]]s are the only predominantly herbivorous reptile group, but several lines of [[Agamidae|agamas]] and [[Iguanidae|iguanas]] have evolved to live wholly or partly on plants.<ref name=herbivory>{{cite book |last=King |first=Gillian |year=1996 |title=Reptiles and Herbivory |edition=1 |publisher=Chapman & Hall |location=London, UK |isbn=978-0-412-46110-1}}</ref>

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 called [[gastrolith]]s) to aid in digestion: The rocks are washed around in the stomach, helping to grind up plant matter.<ref name=herbivory/> Fossil gastroliths have been found associated with both [[ornithopod]]s and [[sauropods]], though whether they actually functioned as a gastric mill in the latter is disputed.<ref>{{cite journal|last=Cerda|first=Ignacio A. |date=1 June 2008|title=Gastroliths in an ornithopod dinosaur|journal=Acta Palaeontologica Polonica |volume=53|issue=2|pages=351–355|doi=10.4202/app.2008.0213|doi-access=free}}</ref><ref>{{cite journal |author1=Wings, O. |author2=Sander, P.M. |title=No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostriches |journal=Proceedings of the Royal Society B: Biological Sciences |date=7 March 2007 |volume=274 |issue=1610 |pages=635–640 |doi=10.1098/rspb.2006.3763 |pmc=2197205  |pmid=17254987}}{{open access}}</ref> [[Salt water crocodile]]s also use gastroliths as [[Sailing ballast|ballast]], stabilizing them in the water or helping them to dive.<ref>{{cite journal|last=Henderson|first=Donald M.|title=Effects of stomach stones on the buoyancy and equilibrium of a floating crocodilian: a computational analysis|journal=Canadian Journal of Zoology|date=1 August 2003|volume=81|issue=8|pages=1346–1357|doi=10.1139/z03-122|bibcode=2003CaJZ...81.1346H }}</ref> A dual function as both stabilizing ballast and digestion aid has been suggested for gastroliths found in [[plesiosaur]]s.<ref>{{cite journal|last=McHenry|first=C.R.|title=Bottom-Feeding Plesiosaurs|journal=Science|date=7 October 2005|volume=310|issue=5745|pages=75|doi=10.1126/science.1117241|pmid=16210529|s2cid=28832109}}{{open access}}</ref>

===Nerves===
The reptilian nervous system contains the same basic part of the [[amphibian]] brain, but the reptile [[cerebrum]] and [[cerebellum]] are slightly larger. Most typical sense organs are well developed with certain exceptions, most notably the [[snake]]'s lack of external ears (middle and inner ears are present). There are twelve pairs of [[cranial nerves]].<ref>{{cite web |url=http://www.curator.org/legacyvmnh/weboflife/kingdom/p_chordata/ClassReptilia/reptiles.htm |title=de beste bron van informatie over cultural institution. Deze website is te koop! |publisher=Curator.org |access-date=March 16, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20090917002815/http://www.curator.org/legacyvmnh/weboflife/kingdom/p_chordata/ClassReptilia/reptiles.htm |archive-date=September 17, 2009 }}</ref> Due to their short cochlea, reptiles use [[electrical tuning]] to expand their range of audible frequencies.

===Vision===
Most reptiles are [[Diurnality|diurnal]] animals. The vision is typically adapted to daylight conditions, with color vision and more advanced visual [[depth perception]] than 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 of [[Photoreceptor cell|photoreceptor]]: [[rod cell|rods]], single [[Cone cell|cones]] and double cones, which gives them sharp color vision and enables them to see [[ultraviolet]] wavelengths.<ref name=Brames>{{Cite journal |journal=Iguana: Conservation, Natural History, and Husbandry of Reptiles |last=Brames |first=Henry |title=Aspects of Light and Reptile Immunity |url=https://www.academia.edu/6822325 |publisher=International Reptile Conservation Foundation |volume=14 |issue=1 |year=2007 |pages=19–23}}</ref> The lepidosaurs appear to have lost the [[duplex retina]] and only have a single class of receptor that is cone-like or rod-like depending on whether the species is diurnal or nocturnal.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S2352154619300257 | doi=10.1016/j.cobeha.2019.10.009 | title=The evolutionary ecology of bird and reptile photoreceptor spectral sensitivities | journal=Current Opinion in Behavioral Sciences | series=Visual perception | date=December 2019 | volume=30 | pages=223–227 | last1=Osorio | first1=Daniel | url-access=subscription }}</ref> In many burrowing species, such as [[blind snake]]s, vision is reduced.

Many [[Lepidosauria|lepidosaurs]] have a photosensory organ on the top of their heads called the [[parietal eye]], which are also called [[third eye]], [[pineal eye]] or [[pineal 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.<ref name=Brames/>

Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of [[Loreal pit|pits]] sensitive to [[infrared]] radiation (heat). Such heat-sensitive pits are particularly well developed in the [[pit vipers]], but are also found in [[boidae|boas]] and [[pythonidae|pythons]]. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.{{efn|
"The copperhead is a pit viper and, like others pit vipers, it has heat-sensitive pit organs on each side of its head between the eye and the nostril. These pits detect objects that are warmer than the environment and enable copperheads to locate nocturnal, mammalian prey."<ref>{{cite web |title=Northern copperhead |date=25 April 2016 |website=Smithsonian's National Zoo & Conservation Biology Institute |url=https://nationalzoo.si.edu/animals/northern-copperhead |access-date=12 February 2020}}</ref>
}}

Most reptiles, as well as birds, possess a [[nictitating 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.<ref>{{cite encyclopedia |title=Nictitating membrane |series=Anatomy |encyclopedia=Encyclopædia Britannica |lang=en |url=https://www.britannica.com/science/nictitating-membrane |access-date=2020-02-20}}</ref> However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called the [[brille]], spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.<ref>{{cite news |title=Catalina Island Conservancy |website=www.catalinaconservancy.org |url=https://www.catalinaconservancy.org/index.php?s=news&p=article_149 |access-date=2020-02-20}}</ref>

===Reproduction===
[[File:Crocodile Egg Diagram.svg|thumb|Crocodilian egg diagram<br/>
(1)&nbsp;eggshell, (2)&nbsp;yolk sac, (3)&nbsp;yolk (nutrients), (4)&nbsp;vessels, (5)&nbsp;[[amnion]], (6)&nbsp;[[chorion]], (7)&nbsp;air space, (8)&nbsp;[[allantois]], (9)&nbsp;albumin (egg white), (10)&nbsp;amniotic sac, (11)&nbsp;crocodile embryo, (12)&nbsp;amniotic fluid]]
[[File:Hemidactylus_frenatus_mating,_ventral_view.jpg|thumb|[[Common house gecko]]s mating, ventral view with [[hemipenis]] inserted in the [[cloaca]]]]
[[File:Trachylepis maculilabris mating.jpg|thumb|Most reptiles reproduce sexually, for example this ''Trachylepis maculilabris'' [[skink]]]]
[[File:Tortoise-Hatchling.jpg|thumb|Reptiles have [[amniote|amniotic]] eggs with hard or leathery shells, requiring [[internal fertilization]] when mating.]]

Reptiles generally [[sexual reproduction|reproduce sexually]],<ref>{{cite journal |title=Male reproductive behaviour of ''Naja oxiana'' {{small|(Eichwald, 1831)}} in captivity, with a case of unilateral hemipenile prolapse |year=2018 |journal=Herpetology Notes |url=https://www.researchgate.net/publication/335270872}}</ref> though some are capable of [[asexual reproduction]]. All reproductive activity occurs through the [[cloaca]], the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have [[copulatory organ]]s, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median [[penis]], while squamates, including snakes and lizards, possess a pair of [[hemipenis|hemipenes]], 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.<ref>{{cite book |author=Lutz, Dick |year=2005 |title=Tuatara: A living fossil |place=Salem, OR |publisher=DIMI Press |isbn=978-0-931625-43-5}}</ref>

Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An [[amnion]]&nbsp;(5), [[chorion]]&nbsp;(6), and [[allantois]]&nbsp;(8) are present during [[embryo]]nic life. The eggshell&nbsp;(1) protects the crocodile embryo&nbsp;(11) and keeps it from drying out, but it is flexible to allow gas exchange. The chorion&nbsp;(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&nbsp;(9) further protects the embryo and serves as a reservoir for water and protein. The allantois&nbsp;(8) is a sac that collects the metabolic waste produced by the embryo. The amniotic sac&nbsp;(10) contains amniotic fluid&nbsp;(12) which protects and cushions the embryo. The amnion&nbsp;(5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac&nbsp;(2) surrounding the yolk&nbsp;(3) contains protein and fat rich nutrients that are absorbed by the embryo via vessels&nbsp;(4) that allow the embryo to grow and metabolize. The air space&nbsp;(7) provides the embryo with oxygen while it is hatching. This ensures that the embryo will not suffocate while it is hatching. There are no [[larva]]l stages of development. [[Viviparity]] and [[ovoviviparity]] have 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 mammalian [[placenta]]. The earliest documented case of viviparity in reptiles is the Early [[Permian]] [[mesosaur]]s,<ref name=PFML12>{{cite journal |author1=Piñeiro, G. |author2=Ferigolo, J. |author3=Meneghel, M. |author4=Laurin, M. |year=2012 |title=The oldest known amniotic embryos suggest viviparity in mesosaurs |journal=Historical Biology |volume=24 |issue=6 |pages=620–630 |s2cid=59475679 |doi=10.1080/08912963.2012.662230 |bibcode=2012HBio...24..620P }}</ref> 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 as [[mosasaur]]s, [[ichthyosaur]]s, and [[Sauropterygia]], a group that includes [[pachypleurosaur]]s and [[Plesiosauria]].<ref name=S12/>

Asexual reproduction has been identified in [[squamata|squamates]] in 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, called [[parthenogenesis]], occurs in several species of [[gecko]], and is particularly widespread in the [[teiidae|teiids]] (especially ''Aspidocelis'') and [[lacertidae|lacertids]] (''[[Lacerta (genus)|Lacerta]]''). In captivity, [[Komodo dragon]]s (Varanidae) have reproduced by [[parthenogenesis]].

Parthenogenetic species are suspected to occur among [[chameleon]]s, [[agamidae|agamids]], [[night lizard|xantusiids]], and [[typhlopidae|typhlopids]].

Some reptiles exhibit [[temperature-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.<ref>{{cite book |title=FireFly Encyclopedia of Reptiles and Amphibians |year=2008 |publisher=Firefly Books Ltd |location=Richmond Hill, Ontario |isbn=978-1-55407-366-5 |pages=117–118 }}</ref> To date, there has been no confirmation of whether TDSD occurs in snakes.<ref>{{cite book |last1=Chadwick |first1=Derek |last2=Goode |first2=Jamie |year=2002 |title=The Genetics and Biology of Sex ... |publisher=John Wiley & Sons |isbn=978-0-470-84346-8 |url=https://books.google.com/books?id=lc5Bg-hmsBYC&q=temperature%20dependent%20sex%20determination%20snake&pg=PA101 |via=Google Books |access-date=March 16, 2010}}</ref>

===Longevity===

Giant [[tortoise]]s are among the longest-lived vertebrate animals (over 100 years by some estimates) and have been used as a model for studying [[longevity]].<ref name="Quesada2019">Quesada V, Freitas-Rodríguez S, Miller J, Pérez-Silva JG, Jiang ZF, Tapia W, Santiago-Fernández O, Campos-Iglesias D, Kuderna LFK, Quinzin M, Álvarez MG, Carrero D, Beheregaray LB, Gibbs JP, Chiari Y, Glaberman S, Ciofi C, Araujo-Voces M, Mayoral P, Arango JR, Tamargo-Gómez I, Roiz-Valle D, Pascual-Torner M, Evans BR, Edwards DL, Garrick RC, Russello MA, Poulakakis N, Gaughran SJ, Rueda DO, Bretones G, Marquès-Bonet T, White KP, Caccone A, López-Otín C. Giant tortoise genomes provide insights into longevity and age-related disease. Nat Ecol Evol. 2019 Jan;3(1):87-95. doi: 10.1038/s41559-018-0733-x. Epub 2018 Dec 3. PMID 30510174; PMCID: PMC6314442</ref> DNA analysis of the [[genome]]s of [[Lonesome George]], the iconic last member of ''[[Pinta Island tortoise|Chelonoidis abingdonii]]'', and the [[Aldabra giant tortoise]] ''Aldabrachelys gigantea'' led to the detection of lineage-specific variants affecting [[DNA repair]] genes that might contribute to our understanding of increased lifespan.<ref name = Quesada2019/>