Geckos are unique among lizards for their vocalizations, which differ from species to species. Most geckos in the family Gekkonidae use chirping or clicking sounds in their social interactions, tokay geckos (Gekko gecko) are known for their loud mating calls, and some other species are capable of making hissing noises when alarmed or threatened. They are the most species-rich group of lizards, with about 1,500 different species worldwide. The New Latingekko and English "gecko" stem from the Indonesian-Malayg?koq, which is imitative of sounds that some species make.
All geckos except species in the family Eublepharidae lack eyelids; instead, the outer surface of the eyeball has a transparent membrane, the cornea. They have a fixed lens within each iris that enlarges in darkness to let in more light.
Since they cannot blink, species without eyelids generally lick their own corneas when they need to clear them of dust and dirt, in order to keep them clean and moist.
Nocturnal species have excellent night vision; their color vision in low light is 350 times more sensitive than human color vision. The nocturnal geckos evolved from diurnal species, which had lost the eye rods. The gecko eye, therefore, modified its cones that increased in size into different types, both single and double. Three different photopigments have been retained and are sensitive to UV, blue, and green. They also use a multifocal optical system that allows them to generate a sharp image for at least two different depths. While most gecko species are nocturnal, some species are diurnal and active during the day, which has evolved multiple times independently.
Like most lizards, geckos can lose their tails in defense, a process called autotomy. Many species are well known for their specialised toe pads that enable them to climb smooth and vertical surfaces, and even cross indoor ceilings with ease. Geckos are well known to people who live in warm regions of the world, where several species make their home inside human habitations. These (for example the house gecko) become part of the indoor menagerie and are often welcomed, as they feed on insects, including moths and mosquitoes. Unlike most lizards, geckos are usually nocturnal.
The largest species, the kawekaweau, is only known from a single, stuffed specimen found in the basement of a museum in Marseille, France. This gecko was 60 cm (24 in) long and it was likely endemic to New Zealand, where it lived in native forests. It was probably wiped out along with much of the native fauna of these islands in the late 19th century, when new invasive species such as rats and stoats were introduced to the country during European colonization. The smallest gecko, the Jaragua sphaero, is a mere 1.6 cm (about half an inch) long and was discovered in 2001 on a small island off the coast of Hispaniola.
Like other reptiles, geckos are ectothermic, producing very little metabolic heat. Essentially, a gecko's body temperature is dependent on its environment. Also, to accomplish their main functions--such as locomotion, feeding, reproduction, etc.--geckos must have a relatively elevated temperature.
All geckos shed their skin at fairly regular intervals, with species differing in timing and method. Leopard geckos shed at about two- to four-week intervals. The presence of moisture aids in the shedding. When shedding begins, the gecko speeds the process by detaching the loose skin from its body and eating it.
For young geckos, shedding occurs more frequently, once a week, but when they are fully grown, they shed once every one to two months.
Close-up of the underside of a gecko's foot as it walks on vertical glass
Gecko on window pane
About 60% of gecko species have adhesive toe pads that allow them to adhere to most surfaces without the use of liquids or surface tension. Such pads have been gained and lost repeatedly over the course of gecko evolution. Adhesive toepads evolved independently in about 11 different gecko lineages and were lost in at least 9 lineages.
A recent study suggests that gecko adhesion is in fact mainly determined by electrostatic interaction (caused by contact electrification), not van der Waals or capillary forces.
The setae on the feet of geckos are also self-cleaning and usually remove any clogging dirt within a few steps.Teflon, which has very low surface energy, is more difficult for geckos to adhere to than many other surfaces.
Gecko adhesion is typically improved by higher humidity, even on hydrophobic surfaces, yet is reduced under conditions of complete immersion in water. The role of water in that system is under discussion, yet recent experiments agree that the presence of molecular water layers (water molecules carry a very large dipole moment) on the setae, as well as on the surface, increase the surface energy of both, therefore the energy gain in getting these surfaces in contact is enlarged, which results in an increased gecko adhesion force. Moreover, the elastic properties of the b-keratin change with water uptake.
Gecko toes seem to be "double jointed", but this is a misnomer and is properly called digital hyperextension. Gecko toes can hyperextend in the opposite direction from human fingers and toes. This allows them to overcome the van der Waals force by peeling their toes off surfaces from the tips inward. In essence, by this peeling action, the gecko separates spatula by spatula from the surface, so for each spatula separation, only some force necessary. (The process is similar to removing Scotch Tape from a surface.)
Geckos' toes operate well below their full attractive capabilities most of the time because the margin for error is great depending upon the surface roughness, and therefore the number of setae in contact with that surface.
Use of small van der Waals force requires very large surface areas; every square millimeter of a gecko's footpad contains about 14,000 hair-like setae. Each seta has a diameter of 5 ?m. Human hair varies from 18 to 180 ?m, so the cross-sectional area of a human hair is equivalent to 12 to 1300 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae. Each spatula is 0.2 ?m long (one five-millionth of a meter), or just below the wavelength of visible light.
The setae of a typical mature 70 g (2.5 oz) gecko would be capable of supporting a weight of 133 kg (293 lb): each spatula can exert an adhesive force of 5 to 25 nN. The exact value of the adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. Recent studies  have moreover shown that the component of the surface energy derived from long-range forces, such as van der Waals forces, depends on the material's structure below the outermost atomic layers (up to 100 nm beneath the surface); taking that into account, the adhesive strength can be inferred.
Apart from the setae, phospholipids--fatty substances produced naturally in their bodies--also come into play. These lipids lubricate the setae and allow the gecko to detach its foot before the next step.
The origin of gecko adhesion likely started as simple modifications to the epidermis on the underside of the toes. This was recently discovered in the genus Gonatodes from South America. Simple elaborations of the epidermal spinules into setae have enabled Gonatodes humeralis to climb smooth surfaces and sleep on smooth leaves.
Biomimetic technologies designed to mimic gecko adhesion could produce reusable self-cleaning dry adhesives with many applications. Development effort is being put into these technologies, but manufacturing synthetic setae is not a trivial material design task.
A Mossy Leaf-tailed Gecko using its dermal flapping to make itself almost invisible against a tree branch. Its head is at the left hand side of the image.
Gecko skin does not generally bear scales, but appears at a macro scale as a papillose surface which is made from hair-like protuberances developed across the entire body. These confer superhydrophobicity, and the unique design of the hair confers a profound antimicrobial action. These protuberances are very small, up to 4 microns in length, and tapering to a point. Gecko skin has been observed to have an anti-bacterial property, killing gram-negative bacteria when it comes in contact with the skin.
The Mossy Leaf-tailed Gecko of Madagascar, U. sikorae, has coloration developed as camouflage, most being grayish brown to black or greenish brown with various markings meant to resemble tree bark; down to the lichens and moss found on the bark. It also has flaps of skin, running the length of its body, head and limbs, known as the dermal flap, which it can lay against the tree during the day, scattering shadows, and making its outline practically invisible.
Geckos are polyphyodonts and able to replace each of their 100 teeth every 3 to 4 months. Next to the full grown tooth there is a small replacement tooth developing from the odontogenicstem cell in the dental lamina. The formation of the teeth is pleurodont; they are fused (ankylosed) by their sides to the inner surface of the jaw bones.
This formation is common in all species in the order Squamata.
Taxonomy and classification
Pores on the skin are often used in classification.
Eublepharis macularius, the leopard gecko, is the most common gecko kept as a pet; it does not have adhesive toe pads and cannot climb the glass of a vivarium.
Gehyra mutilata (Pteropus mutilatus), the stump-toed gecko, is able to vary its color from very light to very dark to camouflage itself; this gecko is at home in the wild, as well as in residential areas.
Gekko gecko, the Tokay gecko, is a large, common, Southeast Asian gecko known for its aggressive temperament, loud mating calls, and bright markings.
Hemidactylus frenatus, the common house gecko, thrives around people and human habitation structures in the tropics and subtropics worldwide.
Hemidactylus garnotii, the Indo-Pacific gecko, is found in houses throughout the tropics, and has become an invasive species of concern in Florida and Georgia in the US.
Hemidactylus mabouia, the tropical house gecko, Afro-American house gecko, or cosmopolitan house gecko, is a species of house gecko native to sub-Saharan Africa and also currently found in North, Central, and South America and the Caribbean.
Phelsuma laticauda, the gold dust day gecko, is diurnal; it lives in northern Madagascar and on the Comoros. It is also an introduced species in Hawaii.
Ptychozoon is a genus of arboreal geckos from Southeast Asia also known as flying or parachute geckos; they have wing-like flaps from the neck to the upper leg to help them conceal themselves on trees and provide lift while jumping.
Most geckos lay a small clutch of eggs, a few are live-bearing and a few can reproduce asexually via parthenogenesis. Geckos also have a large diversity of sex-determining mechanisms including Temperature-dependent sex determination and both XX/XY and ZZ/ZW sex chromosomes with multiple transitions among them over evolutionary time.
This section needs expansion. You can help by adding to it. (February 2020)
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Bauer, Aaron M.; Böhme, Wolfgang; Weitschat, Wolfgang (April 2005). "An Early Eocene gecko from Baltic amber and its implications for the evolution of gecko adhesion". Journal of Zoology. 265 (4): 327-332. doi:10.1017/S0952836904006259.
^Santos, Daniel; Matthew Spenko; Aaron Parness; Kim Sangbae; Mark Cutkosky (2007). "Directional adhesion for climbing: Theoretical and practical considerations". Journal of Adhesion Science and Technology. 21 (12-13): 1317-1341. doi:10.1163/156856107782328399. S2CID53470787. Gecko "feet and toes are a hierarchical system of complex structures consisting of lamellae, setae, and spatulae. The distinguishing characteristics of the gecko adhesion system have been described [as] (1) anisotropic attachment, (2) high pulloff force to preload ratio, (3) low detachment force, (4) material independence, (5) self-cleaning, (6) antiself sticking and (7) nonsticky default state. ... The gecko's adhesive structures are made from ß-keratin (modulus of elasticity [about] 2 GPa). Such a stiff material is not inherently sticky; however, because of the gecko adhesive's hierarchical nature and extremely small distal features (spatulae are [about] 200 nm in size), the gecko's foot is able to intimately conform to the surface and generate significant attraction using van der Waals forces.
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^Russell, A.P.; Baskerville, J.; Gamble, T.; Higham, T. (November 2015). "The evolution of digit form in Gonatodes (Gekkota: Sphaerodactylidae) and its bearing on the transition from frictional to adhesive contact in gekkotans". Journal of Morphology. 276 (11): 1311-1332. doi:10.1002/jmor.20420. PMID26248497. S2CID20296012.