Editing Lizard (section)

==Physiology==
===Locomotion===
[[File:Gecko foot on glass.JPG|thumb|upright|Adhesive pads enable [[gecko]]s to climb vertically.]]
Aside from [[legless lizards]], most lizards are quadrupedal and move using [[gait]]s with alternating movement of the right and left limbs with substantial body bending. This body bending prevents significant respiration during movement, limiting their endurance, in a mechanism called [[Carrier's constraint]]. Several species can run bipedally,<ref>{{cite journal|url=http://jeb.biologists.org/content/202/9/1047|title=Comparative three-dimensional kinematics of the hindlimb for high-speed bipedal and quadrupedal locomotion of lizards|first1=D. J.|last1=Irschick|first2=B. C.|last2=Jayne|date=1 May 1999|journal=Journal of Experimental Biology|volume=202|issue=9|pages=1047–1065|doi=10.1242/jeb.202.9.1047|via=jeb.biologists.org|pmid=10101105|bibcode=1999JExpB.202.1047I |access-date=6 July 2017|archive-date=31 December 2020|archive-url=https://web.archive.org/web/20201231195836/https://jeb.biologists.org/content/202/9/1047|url-status=live}}</ref> and a few can prop themselves up on their hindlimbs and tail while stationary. Several small species such as those in the genus ''[[Draco (genus)|Draco]]''  can glide: some can attain a distance of {{convert|60|m|ft|abbr=off}}, losing {{convert|10|m|ft|abbr=off}} in height.<ref>[[Ross Piper|Piper, Ross]] (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'', [[Greenwood Press (publisher)|Greenwood Press]].</ref> Some species, like geckos and chameleons, adhere to vertical surfaces including glass and ceilings.<ref name=Spinner2014/> Some species, like the [[common basilisk]], can run across water.<ref>Pianka and Vitt, 23–24</ref>

===Senses===

Lizards make use of their [[sense]]s of [[sight]], [[touch]], [[olfaction]] and [[hearing]] like other [[vertebrate]]s. The balance of these varies with the habitat of different species; for instance, skinks that live largely covered by loose soil rely heavily on olfaction and touch, while geckos depend largely on acute vision for their ability to hunt and to evaluate the distance to their prey before striking. Monitor lizards have acute vision, hearing, and olfactory senses. Some lizards make unusual use of their sense organs: chameleons can steer their eyes in different directions, sometimes providing non-overlapping fields of view, such as forwards and backwards at once. Lizards lack external ears, having instead a circular opening in which the tympanic membrane (eardrum) can be seen. Many species rely on hearing for early warning of predators, and flee at the slightest sound.<ref name=Wilson2012>{{cite book |last=Wilson |first=Steve |title=Australian Lizards: A Natural History |url=https://books.google.com/books?id=NXg0N_vm2W4C&pg=PA65 |year=2012 |publisher=Csiro Publishing |isbn=978-0-643-10642-0 |pages=65–74}}</ref>
[[File:Nile Monitor (Varanus niloticus) (16545024096).jpg|thumb|right|Nile monitor using its tongue for smell]]
As in snakes and many mammals, all lizards have a specialised olfactory system, the [[vomeronasal organ]], used to detect [[pheromone]]s. Monitor lizards transfer scent from the tip of their tongue to the organ; the tongue is used only for this information-gathering purpose, and is not involved in manipulating food.<ref>{{cite journal |author=Frasnelli, J.|display-authors=etal|title=The vomeronasal organ is not involved in the perception of endogenous odors |journal=Hum. Brain Mapp. |volume=32 |issue=3 |pages=450–60 |date=2011 |pmid=20578170 |pmc=3607301 |doi=10.1002/hbm.21035}}</ref><ref name=Wilson2012/>
[[File:Bearded Dragon Skeleton.jpg|thumb|Skeleton of bearded dragon ([[pogona]] sp.) on display at the [[Museum of Osteology]].]]
Some lizards, particularly iguanas, have retained a photosensory organ on the top of their heads called the [[parietal eye]], a [[Basal (phylogenetics)|basal]] ("primitive") feature also present in the [[tuatara]]. This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps them detect predators stalking it from above.<ref>{{citation | last = Brames | first = Henry | title = Aspects of Light and Reptile Immunity | journal = Iguana: Conservation, Natural History, and Husbandry of Reptiles | volume = 14 | issue = 1 | year = 2007 | pages = 19–23 | url = http://www.ircf.org/downloads/wwdigitalmembers/Iguana_14-1web.pdf }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

===Venom===
[[File:AZ Gila Monster 02.jpg|thumb|left|Some lizards including the [[Gila monster]] are [[venom]]ous.]]
{{further|Evolution of snake venom}}
Until 2006 it was thought that the [[Gila monster]] and the [[Mexican beaded lizard]] were the only venomous lizards. However, several species of monitor lizards, including the [[Komodo dragon]], produce powerful venom in their oral [[gland]]s. [[Varanus varius|Lace monitor]] venom, for instance, causes swift loss of consciousness and extensive bleeding through its pharmacological effects, both lowering [[blood pressure]] and preventing [[blood clotting]]. Nine classes of [[toxin]] known from snakes are produced by lizards. The range of actions provides the potential for new [[medicinal drug]]s based on lizard venom [[protein]]s.<ref name=Fry2005>{{cite journal |last1=Fry |first1=Bryan G.|display-authors=etal|title=Early evolution of the venom system in lizards and snakes |journal=Nature |date=16 November 2005 |volume=439 |issue=7076 |pages=584–588 |doi=10.1038/nature04328|pmid=16292255 |bibcode=2006Natur.439..584F |s2cid=4386245}}</ref><ref name=Casey2013>{{cite magazine |last1=Casey |first1=Constance |title=Don't Call It a Monster |url=http://www.slate.com/articles/health_and_science/science/2013/04/gila_monster_revolting_creature_the_large_venomous_lizard_of_the_u_s_southwest.html |magazine=Slate |date=26 April 2013 |access-date=5 July 2017 |archive-date=10 October 2018 |archive-url=https://web.archive.org/web/20181010001503/http://www.slate.com/articles/health_and_science/science/2013/04/gila_monster_revolting_creature_the_large_venomous_lizard_of_the_u_s_southwest.html |url-status=live }}</ref>

Genes associated with venom toxins have been found in the salivary glands of a wide range of lizards, including species traditionally thought of as non-venomous, such as iguanas and bearded dragons. This suggests that [[Evolution of snake venom|these genes evolved in the common ancestor]] of lizards and [[snake]]s, some 200 million years ago (forming a single [[clade]], the [[Toxicofera]]<!--seems a reasonable overlink-->).<ref name=Fry2005/> However, most of these putative venom genes were "housekeeping genes" found in all cells and tissues, including skin and cloacal scent glands. The genes in question may thus be evolutionary precursors of venom genes.<ref>{{cite journal |last1=Hargreaves |first1=Adam D. |display-authors=etal |title=Testing the Toxicofera: Comparative transcriptomics casts doubt on the single, early evolution of the reptile venom system |journal=Toxicon |date=2014 |volume=92 |pages=140–156 |doi=10.1016/j.toxicon.2014.10.004 |pmid=25449103 |bibcode=2014Txcn...92..140H |url=https://research.bangor.ac.uk/portal/en/researchoutputs/testing-the-toxicofera-comparative-reptile-transcriptomics-casts-doubt-on-the-single-early-evolution-of-the-reptile-venom-system(5e889426-c4ec-4324-81e8-6df67558bd3b).html |hdl=2160/26793 |hdl-access=free |access-date=2019-08-19 |archive-date=2020-11-06 |archive-url=https://web.archive.org/web/20201106092216/https://research.bangor.ac.uk/portal/en/researchoutputs/testing-the-toxicofera-comparative-reptile-transcriptomics-casts-doubt-on-the-single-early-evolution-of-the-reptile-venom-system(5e889426-c4ec-4324-81e8-6df67558bd3b).html |url-status=live }}</ref>

===Respiration===
Recent studies (2013 and 2014) on the lung anatomy of the [[savannah monitor]] and [[green iguana]] found them to have a unidirectional airflow system, which involves the air moving in a loop through the lungs when breathing. This was previously thought to only exist in the [[archosaurs]] ([[crocodilian]]s and [[bird]]s). This may be evidence that unidirectional airflow is an ancestral trait in [[diapsid]]s.<ref>{{cite journal |author1=Schachner, Emma R. |author2=Cieri, Robert L. |author3=Butler, James P. |author4=Farmer, C. G. |year=2014 |title=Unidirectional pulmonary airflow patterns in the savannah monitor lizard |journal=Nature |volume=506 |issue=7488 |pages=367–370 |doi=10.1038/nature12871|pmid=24336209 |bibcode=2014Natur.506..367S |s2cid=4456381 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:32631102 |url-access=subscription }}</ref><ref>{{cite journal |author1=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=PNAS |volume=111 |issue=48 |pages=17218–17223 |doi=10.1073/pnas.1405088111|pmid=25404314 |pmc=4260542 |bibcode=2014PNAS..11117218C |doi-access=free }}</ref>

===Reproduction and life cycle===
[[File:Trachylepis maculilabris mating.jpg|thumb|250px|''Trachylepis maculilabris'' [[skinks]] mating]]
As with all amniotes, lizards rely on internal fertilisation and copulation involves the male inserting one of his [[Hemipenis|hemipene]]s into the female's [[cloaca]].<ref>Pianka and Vitt, pp. 108.</ref> Female lizards also have
[[Hemipenis#Hemiclitoris|hemiclitorises]], a doubled
clitoris. The majority of species are [[oviparous]] (egg laying). The female deposits the eggs in a protective structure like a nest or crevice or simply on the ground.<ref name="Pianka-Vitt115"/> Depending on the species, clutch size can vary from 4–5 percent of the females body weight to 40–50 percent and clutches range from one or a few large eggs to dozens of small ones.<ref>Pianka and Vitt, pp. 110–111.</ref> 
[[File:EasternFenceLizard Egg.png|thumb|200px|left|Two pictures of an [[eastern fence lizard]] egg layered onto one image.]]
In most lizards, the eggs have leathery shells to allow for the exchange of water, although more arid-living species have calcified shells to retain water. Inside the eggs, the embryos use nutrients from the [[yolk]]. Parental care is uncommon and the female usually abandons the eggs after laying them. [[Egg incubation|Brooding]] and protection of eggs do occur in some species. The female [[prairie skink]] uses respiratory water loss to maintain the humidity of the eggs which facilitates embryonic development. In [[lace monitor]]s, the young hatch close to 300 days, and the female returns to help them escape the termite mound where the eggs were laid.<ref name="Pianka-Vitt115">Pianka and Vitt, pp. 115–116.</ref>

Around 20 percent of lizard species reproduce via [[viviparity]] (live birth). This is particularly common in Anguimorphs. Viviparous species give birth to relatively developed young which look like miniature adults. Embryos are nourished via a [[placenta]]-like structure.<ref>Pianka and Vitt, pp. 117–118.</ref> A minority of lizards have [[parthenogenesis]] (reproduction from unfertilised eggs). These species consist of all females who reproduce asexually with no need for males. This is known to occur in various species of [[whiptail lizard]]s.<ref name="Pianka-Vitt119">Pianka and Vitt, pp. 119.</ref> Parthenogenesis was also recorded in species that normally reproduce sexually. A captive female Komodo dragon produced a clutch of eggs, despite being separated from males for over two years.<ref>{{cite news |author=Morales, Alex |publisher=[[Bloomberg Television]] |url=https://www.bloomberg.com/apps/news?pid=20601082&sid=apLYpeppu8ag&refer=canada |title=Komodo Dragons, World's Largest Lizards, Have Virgin Births |access-date=28 March 2008 |date=20 December 2006 |archive-date=8 October 2007 |archive-url=https://web.archive.org/web/20071008112514/http://www.bloomberg.com/apps/news?pid=20601082 |url-status=live }}</ref>

Sex determination in lizards can be [[Temperature-dependent sex determination|temperature-dependent]]. The temperature of the eggs' micro-environment can determine the sex of the hatched young: low temperature incubation produces more females while higher temperatures produce more males. However, some lizards have [[sex chromosomes]] and both male [[Heterogametic sex|heterogamety]] (XY and XXY) and female heterogamety (ZW) occur.<ref name="Pianka-Vitt119"/>

===Aging===

A significant component of [[ageing|aging]] in the painted dragon lizard ''[[Ctenophorus pictus]]'' is fading breeding colors.<ref name = Olsson2012>Olsson M, Tobler M, Healey M, Perrin C, Wilson M. A significant component of ageing (DNA damage) is reflected in fading breeding colors: an experimental test using innate antioxidant mimetics in painted dragon lizards. Evolution. 2012 Aug;66(8):2475-83. doi: 10.1111/j.1558-5646.2012.01617.x. Epub 2012 Apr 9. PMID 22834746</ref>  By manipulating [[superoxide]] levels (using a [[superoxide dismutase]] mimetic) it was shown that this fading coloration is likely due to gradual loss with lizard age of an innate capacity for [[antioxidant|antioxidation]] due to increasing [[DNA damage (naturally occurring)|DNA damage]].<ref name = Olsson2012/>