Most lizards are quadrupedal, running with a strong side-to-side motion. Some lineages (known as "legless lizards") have secondarily lost their legs, and have long snake-like bodies. Some lizards, such as the forest-dwellingDraco, are able to glide. They are oftenterritorial, the males fighting off other males and signalling, often with bright colours, to attract mates and to intimidate rivals. Lizards are mainly carnivorous, often beingsit-and-wait predators; many smaller species eat insects, while the Komodo eats mammals as big aswater buffalo.
Lizards typically have rounded torsos, elevated heads on short necks, four limbs and long tails, although some are legless.[4] Lizards and snakes share a movablequadrate bone, distinguishing them from therhynchocephalians, which have more rigiddiapsidskulls.[5] Some lizards such as chameleons haveprehensile tails, assisting them in climbing among vegetation.[6]
As in other reptiles, the skin of lizards is covered in overlappingscales made ofkeratin. This provides protection from the environment and reduces water loss through evaporation. This adaptation enables lizards to thrive in some of the driest deserts on earth. The skin is tough and leathery, and is shed (sloughed) as the animal grows. Unlike snakes which shed the skin in a single piece, lizards slough their skin in several pieces. The scales may be modified into spines for display or protection, and some species have boneosteoderms underneath the scales.[6][7]
The dentitions of lizards reflect their wide range of diets, including carnivorous, insectivorous, omnivorous, herbivorous, nectivorous, and molluscivorous. Species typically have uniform teeth suited to their diet, but several species have variable teeth, such as cutting teeth in the front of the jaws and crushing teeth in the rear. Most species arepleurodont, though agamids and chameleons areacrodont.[8][6]
The tongue can be extended outside the mouth, and is often long. In the beaded lizards, whiptails and monitor lizards, the tongue is forked and used mainly or exclusively to sense the environment, continually flicking out to sample the environment, and back to transfer molecules to the vomeronasal organ responsible for chemosensation, analogous to but different from smell or taste. In geckos, the tongue is used to lick the eyes clean: they have no eyelids. Chameleons have very long sticky tongues which can be extended rapidly to catch their insect prey.[6]
Three lineages, thegeckos,anoles, andchameleons, havemodified the scales under their toes to form adhesive pads, highly prominent in the first two groups. The pads are composed of millions of tiny setae (hair-like structures) which fit closely to the substrate to adhere usingvan der Waals forces; no liquid adhesive is needed.[9] In addition, the toes of chameleons are divided into two opposed groups on each foot (zygodactyly), enabling them to perch on branches as birds do.[a][6]
Aside fromlegless lizards, most lizards are quadrupedal and move usinggaits 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 calledCarrier's constraint. Several species can run bipedally,[10] and a few can prop themselves up on their hindlimbs and tail while stationary. Several small species such as those in the genusDraco can glide: some can attain a distance of 60 metres (200 feet), losing 10 metres (33 feet) in height.[11] Some species, like geckos and chameleons, adhere to vertical surfaces including glass and ceilings.[9] Some species, like thecommon basilisk, can run across water.[12]
Senses
Lizards make use of theirsenses ofsight,touch,olfaction andhearing like othervertebrates. 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.[13]
Nile monitor using its tongue for smell
As in snakes and many mammals, all lizards have a specialised olfactory system, thevomeronasal organ, used to detectpheromones. 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.[14][13]
Some lizards, particularly iguanas, have retained a photosensory organ on the top of their heads called theparietal eye, abasal ("primitive") feature also present in thetuatara. 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.[15]
Until 2006 it was thought that theGila monster and theMexican beaded lizard were the only venomous lizards. However, several species of monitor lizards, including theKomodo dragon, produce powerful venom in their oralglands.Lace monitor venom, for instance, causes swift loss of consciousness and extensive bleeding through its pharmacological effects, both loweringblood pressure and preventingblood clotting. Nine classes oftoxin known from snakes are produced by lizards. The range of actions provides the potential for newmedicinal drugs based on lizard venomproteins.[16][17]
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 thatthese genes evolved in the common ancestor of lizards andsnakes, some 200 million years ago (forming a singleclade, theToxicofera).[16] 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.[18]
Respiration
Recent studies (2013 and 2014) on the lung anatomy of thesavannah monitor andgreen 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 thearchosaurs (crocodilians andbirds). This may be evidence that unidirectional airflow is an ancestral trait indiapsids.[19][20]
As with all amniotes, lizards rely on internal fertilisation and copulation involves the male inserting one of hishemipenes into the female'scloaca.[21] Female lizards also havehemiclitorises, a doubledclitoris. The majority of species areoviparous (egg laying). The female deposits the eggs in a protective structure like a nest or crevice or simply on the ground.[22] 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.[23]
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 theyolk. Parental care is uncommon and the female usually abandons the eggs after laying them.Brooding and protection of eggs do occur in some species. The femaleprairie skink uses respiratory water loss to maintain the humidity of the eggs which facilitates embryonic development. Inlace monitors, the young hatch close to 300 days, and the female returns to help them escape the termite mound where the eggs were laid.[22]
Around 20 percent of lizard species reproduce viaviviparity (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 aplacenta-like structure.[24] A minority of lizards haveparthenogenesis (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 ofwhiptail lizards.[25] 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.[26]
Sex determination in lizards can betemperature-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 havesex chromosomes and both maleheterogamety (XY and XXY) and female heterogamety (ZW) occur.[25]
Aging
A significant component ofaging in the painted dragon lizardCtenophorus pictus is fading breeding colors.[27] By manipulatingsuperoxide levels (using asuperoxide dismutase mimetic) it was shown that this fading coloration is likely due to gradual loss with lizard age of an innate capacity forantioxidation due to increasingDNA damage.[27]
Behaviour
Diurnality and thermoregulation
The majority of lizard species areactive during the day,[28] though some areactive at night, notably geckos. Asectotherms, lizards have a limited ability to regulate their body temperature, and must seek out and bask in sunlight to gain enough heat to become fully active.[29] Thermoregulation behavior can be beneficial in the short term for lizards as it allows the ability to buffer environmental variation and endure climate warming.[30]
In high altitudes, thePodarcis hispaniscus responds to higher temperature with a darker dorsal coloration to prevent UV-radiation and background matching. Their thermoregulatory mechanisms also allow the lizard to maintain their ideal body temperature for optimal mobility.[31]
Most social interactions among lizards are between breeding individuals.[28]Territoriality is common and is correlated with species that use sit-and-wait hunting strategies. Males establish and maintain territories that contain resources that attract females and which they defend from other males. Important resources include basking, feeding, and nesting sites as well as refuges from predators. The habitat of a species affects the structure of territories, for example, rock lizards have territories atop rocky outcrops.[32] Some species may aggregate in groups, enhancing vigilance and lessening the risk of predation for individuals, particularly for juveniles.[33]Agonistic behaviour typically occurs between sexually mature males over territory or mates and may involve displays, posturing, chasing, grappling and biting.[32]
Lizards signal both to attract mates and to intimidate rivals. Visual displays include body postures and inflation, push-ups, bright colours, mouth gapings and tail waggings. Maleanoles and iguanas havedewlaps or skin flaps which come in various sizes, colours and patterns and the expansion of the dewlap as well as head-bobs and body movements add to the visual signals.[34][6] Some species have deep blue dewlaps and communicate withultraviolet signals.[28]Blue-tongued skinks will flash their tongues as athreat display.[35] Chameleons are known to change their complex colour patterns when communicating, particularly during agonistic encounters. They tend to show brighter colours when displaying aggression[36] and darker colours when they submit or "give up".[37]
Several gecko species are brightly coloured; some species tilt their bodies to display their coloration. In certain species, brightly coloured males turn dull when not in the presence of rivals or females. While it is usually males that display, in some species females also use such communication. In thebronze anole, head-bobs are a common form of communication among females, the speed and frequency varying with age and territorial status. Chemical cues orpheromones are also important in communication. Males typically direct signals at rivals, while females direct them at potential mates. Lizards may be able to recognise individuals of the same species by their scent.[34]
Acoustic communication is less common in lizards.Hissing, a typical reptilian sound, is mostly produced by larger species as part of a threat display, accompanying gaping jaws. Some groups, particularly geckos, snake-lizards, and some iguanids, can produce more complex sounds and vocal apparatuses haveindependently evolved in different groups. These sounds are used for courtship, territorial defense and in distress, and include clicks, squeaks, barks and growls. The mating call of the maletokay geckois heard as "tokay-tokay!".[35][34][38] Tactile communication involves individuals rubbing against each other, either in courtship or in aggression.[34] Some chameleon species communicate with one another by vibrating the substrate that they are standing on, such as a tree branch or leaf.[39]
Defence
Lizards are normally quick and agile to easily outrun attackers.[40]
Ecology
Lizard in tree. Many species are tree-dwellingA lizard fromThar desert
Distribution and habitat
Lizards are found worldwide, excluding the far north and Antarctica, and some islands. They can be found in elevations from sea level to 5,000 m (16,000 ft). They prefer warmer, tropical climates but are adaptable and can live in all but the most extreme environments. Lizards also exploit a number of habitats; most primarily live on the ground, but others may live in rocks, on trees, underground and even in water.[40] The marine iguana is adapted for life in the sea.[6]
The majority of lizard species arepredatory and the most common prey items are small, terrestrial invertebrates, particularlyinsects.[6][41] Many species aresit-and-wait predators though others may be more active foragers.[42] Chameleons prey on numerous insect species, such asbeetles,grasshoppers and wingedtermites as well asspiders. They rely on persistence and ambush to capture these prey. An individual perches on a branch and stays perfectly still, with only its eyes moving. When an insect lands, the chameleon focuses its eyes on the target and slowly moves toward it before projecting its long sticky tongue which, when hauled back, brings the attached prey with it. Geckos feed oncrickets, beetles, termites andmoths.[6][41]
Termites are an important part of the diets of some species of Autarchoglossa, since, associal insects, they can be found in large numbers in one spot.Ants may form a prominent part of the diet of some lizards, particularly among the lacertas.[6][41]Horned lizards are also well known for specializing on ants. Due to their small size and indigestiblechitin, ants must be consumed in large amounts, and ant-eating lizards have larger stomachs than evenherbivorous ones.[43] Species of skink andalligator lizards eatsnails and their power jaws and molar-like teeth are adapted for breaking the shells.[6][41]
Young Komodo dragon feeding on awater buffalo carcassMarine iguana foraging under water at Galápagos Islands, Ecuador.
Larger species, such as monitor lizards, can feed on larger prey including fish, frogs, birds, mammals and other reptiles. Prey may be swallowed whole and torn into smaller pieces. Both bird and reptile eggs may also be consumed as well. Gila monsters and beaded lizards climb trees to reach both the eggs and young of birds. Despite being venomous, these species rely on their strong jaws to kill prey. Mammalian prey typically consists ofrodents andleporids; the Komodo dragon can kill prey as large aswater buffalo. Dragons are prolificscavengers, and a single decaying carcass can attract several from 2 km (1.2 mi) away. A 50 kg (110 lb) dragon is capable of consuming a 31 kg (68 lb) carcass in 17 minutes.[41]
Around 2 percent of lizard species, including many iguanids, are herbivores. Adults of these species eat plant parts like flowers, leaves, stems and fruit, while juveniles eat more insects. Plant parts can be hard to digest, and, as they get closer to adulthood, juvenile iguanas eat faeces from adults to acquire themicroflora necessary for their transition to a plant-based diet. Perhaps the most herbivorous species is the marine iguana which dives 15 m (49 ft) to forage foralgae,kelp and other marine plants. Some non-herbivorous species supplement their insect diet with fruit, which is easily digested.[6][41]
Antipredator adaptations
Thefrilled-neck lizard with fully extended frill. The frilled neck serves to make it look bigger than it actually is.
Lizards exploit a variety of differentcamouflage methods. Many lizards aredisruptively patterned. In some species, such asAegean wall lizards, individuals vary in colour, and select rocks which best match their own colour to minimise the risk of being detected by predators.[44] TheMoorish gecko is able tochange colour for camouflage: when a light-coloured gecko is placed on a dark surface, it darkens within an hour to match the environment.[45] Thechameleons in general use their ability to change their coloration for signalling rather than camouflage, but some species such asSmith's dwarf chameleon do use active colour change for camouflage purposes.[46]
Many lizards, includinggeckos andskinks, are capable of shedding their tails (autotomy). The detached tail, sometimes brilliantly coloured, continues to writhe after detaching, distracting the predator's attention from the fleeing prey. Lizards partiallyregenerate their tails over a period of weeks. Some 326 genes are involved in regenerating lizard tails.[48] The fish-scale geckoGeckolepis megalepis sheds patches of skin and scales if grabbed.[49]
Escape, playing dead, reflex bleeding
Many lizards attempt to escape from danger by running to a place of safety;[50][b] for example, wall lizards can run up walls and hide in holes or cracks.[9] Horned lizards adopt differing defences for specific predators. They mayplay dead to deceive a predator that has caught them; attempt to outrun therattlesnake, which does not pursue prey; but stay still, relying on their cryptic coloration, forMasticophis whip snakes which can catch even swift prey. If caught, some species such as thegreater short-horned lizard puff themselves up, making their bodies hard for a narrow-mouthed predator like a whip snake to swallow. Finally, horned lizards cansquirt blood atcat anddog predators from a pouch beneath its eyes, to a distance of about two metres (6.6 feet); the blood tastes foul to these attackers.[52]
The closest living relatives of lizards arerhynchocephalians, a once diverse order of reptiles, of which is there is now only one living species, thetuatara of New Zealand. Some reptiles from the Early and MiddleTriassic, likeSophineta andMegachirella, are suggested to bestem-group squamates, more closely related to modern lizards than rhynchocephalians, however, their position is disputed, with some studies recovering them as less closely related to squamates than rhynchocephalians are.[53] The oldest undisputed lizards date to the Middle Jurassic, from remains found In Europe, Asia and North Africa.[54] Lizard morphological and ecological diversity substantially increased over the course of theCretaceous.[55] In thePalaeogene, lizard body sizes in North America peaked during the middle of the period.[56]
The position of the lizards and otherSquamata among the reptiles was studied using fossil evidence by Rainer Schoch and Hans-Dieter Sues in 2015. Lizards form about 60% of the extant non-avian reptiles.[60]
Both the snakes and theAmphisbaenia (worm lizards) areclades deep within theSquamata (the smallest clade that contains all the lizards), so "lizard" isparaphyletic.[61]The cladogram is based on genomic analysis by Wiens and colleagues in 2012 and 2016.[62][63] Excluded taxa are shown in upper case on the cladogram.
In the 13th century, lizards were recognized in Europe as part of a broad category ofreptiles that consisted of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, […], assorted amphibians, and worms", as recorded byVincent of Beauvais in hisMirror of Nature.[64] The seventeenth century saw changes in this loose description. The nameSauria was coined byJames Macartney (1802);[65] it was the Latinisation of the French nameSauriens, coined byAlexandre Brongniart (1800) for an order of reptiles in the classification proposed by the author, containing lizards andcrocodilians,[66] later discovered not to be each other's closest relatives. Later authors used the term "Sauria" in a more restricted sense, i.e. as a synonym of Lacertilia, a suborder ofSquamata that includes all lizards but excludessnakes. This classification is rarely used today because Sauria so-defined is aparaphyletic group. It was defined as aclade byJacques Gauthier, Arnold G. Kluge and Timothy Rowe (1988) as the group containing the most recent common ancestor ofarchosaurs andlepidosaurs (the groups containing crocodiles and lizards, as per Mcartney's original definition) and all its descendants.[67] A different definition was formulated by Michael deBraga and Olivier Rieppel (1997), who defined Sauria as the clade containing the most recent common ancestor ofChoristodera,Archosauromorpha,Lepidosauromorpha and all their descendants.[68] However, these uses have not gained wide acceptance among specialists.
Most lizard species are harmless to humans. Only the largest lizard species, theKomodo dragon, which reaches 3.3 m (11 ft) in length and weighs up to 166 kg (366 lb), has been known to stalk, attack, and, on occasion, kill humans. An eight-year-old Indonesian boy died from blood loss after an attack in 2007.[71]
Green iguanas are eaten in Central America, where they are sometimes referred to as "chicken of the tree" after their habit of resting in trees and their supposedly chicken-like taste,[74] whilespiny-tailed lizards are eaten inAfrica. In North Africa,Uromastyx species are considereddhaab or 'fish of the desert' and eaten by nomadic tribes.[75]
Lizards such as the Gila monster produce toxins with medical applications. Gila toxin reduces plasma glucose; the substance is now synthesized for use in the anti-diabetes drugexenatide (Byetta).[17] Another toxin from Gila monster saliva has been studied for use as an anti-Alzheimer's drug.[76]
In culture
Lizards appear in myths and folktales around the world. InAustralian Aboriginal mythology, Tarrotarro, the lizard god, split the human race into male and female, and gave people the ability to express themselves in art. A lizard king named Mo'o features in Hawaii and other cultures in Polynesia. In the Amazon, the lizard is the king of beasts, while among the Bantu of Africa, the god UNkulunkulu sent a chameleon to tell humans they would live forever, but the chameleon was held up, and another lizard brought a different message, that the time of humanity was limited.[77] A popular legend inMaharashtra tells the tale of how acommon Indian monitor, with ropes attached, was used to scale the walls of the fort in theBattle of Sinhagad.[78]
Lizards in many cultures share the symbolism of snakes, especially as an emblem of resurrection. This may have derived from their regular molting. The motif of lizards on Christian candle holders probably alludes to the same symbolism. According to Jack Tresidder, in Egypt and the Classical world, they were beneficial emblems, linked with wisdom. In African, Aboriginal and Melanesian folklore they are linked to cultural heroes or ancestral figures.[79]
Notes
^Chameleon forefeet have groups composed of 3 inner and 2 outer digits; the hindfeet have groups of 2 inner and 3 outer digits.[6]
^The BBC's 2016Planet Earth II showed a sequence of newly-hatchedmarine iguanas running to the sea past a waiting crowd ofracer snakes. It was edited for dramatic effect but the sections were all genuine.[51]
References
^"The Reptile Database".Reptile-database.reptarium.cz.Archived from the original on 2022-06-13. Retrieved2022-06-13. Retrieved on 2022-06-13
^McDiarmid, Roy W. (2012). "Reptile Diversity and Natural History: An Overview". In McDiarmid, Roy W.; et al. (eds.).Reptile Biodiversity: Standard Methods for Inventory and Monitoring. University of California Press. p. 13.ISBN978-0-520-26671-1.
^abOlsson 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
^Lanham, E. J.; Bull. M. C. (2004). "Enhanced vigilance in groups inEgernia stokesii, a lizard with stable social aggregations".Journal of Zoology.263 (1):95–99.doi:10.1017/S0952836904004923.
^Cooper, William E. Jr. (2010). "Initiation of Escape Behavior by the Texas Horned Lizard (Phrynosoma cornutum)".Herpetologica.66 (1):23–30.doi:10.1655/08-075.1.S2CID84653226.
^Dash, Sean (2008).Prehistoric Monsters Revealed. United States: Workaholic Productions / History Channel. Archived fromthe original on 2016-01-27. RetrievedDecember 18, 2015.
^Zheng, Yuchi; Wiens, John J. (2016). "Combining phylogenomic and supermatrix approaches, and a time-calibrated phylogeny for squamate reptiles (lizards and snakes) based on 52 genes and 4162 species".Molecular Phylogenetics and Evolution.94 (Pt B):537–547.Bibcode:2016MolPE..94..537Z.doi:10.1016/j.ympev.2015.10.009.PMID26475614.
^Franklin-Brown, Mary (2012).Reading the world: encyclopedic writing in the scholastic age. Chicago London: The University of Chicago Press. p. 223;377.ISBN978-0-226-26070-9.
^James Macartney: Table IIIin: George Cuvier (1802) "Lectures on Comparative Anatomy" (translated by William Ross under the inspection of James Macartney). Vol I. London, Oriental Press, Wilson and Co.
^Alexandre Brongniart (1800) "Essai d'une classification naturelle des reptiles. 1ère partie: Etablissement des ordres." Bulletin de la Science. Société Philomathique de Paris 2 (35): 81–82
^Losos, Jonathan B. (1992). "The Evolution of Convergent Structure in Caribbean Anolis Communities".Systematic Biology.41 (4):403–420.doi:10.1093/sysbio/41.4.403.
Ditmars, Raymond L (1933).Reptiles of the World: The Crocodilians, Lizards, Snakes, Turtles and Tortoises of the Eastern and Western Hemispheres. New York: Macmillan. p. 321.
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