Geckos are lizards belonging to the infraorder Gekkota, found in warm climates throughout the world. They range from 1.6 to 60 cm. Most geckos cannot blink, but they often lick their eyes to keep them clean and moist. They have a fixed lens within each iris that enlarges in darkness to let in more light.
Geckos are unique among lizards in their vocalizations. They use chirping sounds in social interactions with other geckos. 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-Malaygēkoq, which is imitative of the sound the animals make.
All geckos, excluding the Eublepharidae family, lack eyelids and instead have a transparent membrane, which they lick to clean. Nocturnal species have excellent night vision; their eyes are 350 times more sensitive to light than the human eye.
Most gecko species can lose their tails in defense, a process called autotomy. Many species are well known for their specialized toe pads that enable them to climb smooth and vertical surfaces, and even cross indoor ceilings with ease (one hypothesis explains the ability in terms of the van der Waals force). These antics are well-known to people who live in warm regions of the world, where several species of geckos make their home inside human habitations. These species (for example the house gecko) become part of the indoor menagerie and are often welcomed, as they feed on insects, including mosquitoes. Unlike most lizards, geckos are usually nocturnal and are great climbers.
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 long and was discovered in 2001 on a small island off the coast of the Dominican Republic.
Geckos occur in various patterns and colors, and are among the most colorful lizards in the world. Some species can change colour and may be lighter in colour at night. Some species are parthenogenic, which means the female is capable of reproducing without copulating with a male. This improves the gecko's ability to spread to new islands. However, in a situation where a single female gecko populates an entire island, the island will suffer from a lack of genetic variation within the geckos that inhabit it. The gecko's mating call sounds like a shortened bird chirping which attracts males, when they are nearby. Like other reptiles, geckos are ectothermic, producing very little metabolic heat. Essentially a gecko′s body temperature is dependent on its environment. Also, in order to accomplish their main functions — such as locomotion, feeding, reproduction, etc. — geckos must have a relatively elevated temperature.
Shedding or molting
All geckos shed their skin at fairly regular intervals, with species differing in timing and method. Leopard geckos will shed at about two- to four-week intervals. The presence of moisture aids in the shedding. When shedding begins, the gecko will speed the process by detaching the loose skin from its body and eating it.
Close-up of the underside of a gecko's foot as it walks on vertical glass
Gecko toes have special adaptations that allow them to adhere to most surfaces without the use of liquids or surface tension. About 60% of gecko species have adhesive toe pads; 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. The spatula-shaped setae arranged in lamellae on gecko footpads enable attractive van der Waals' forces between the β-keratin lamellae/setae/spatulae structures and the surface. These van der Waals interactions involve no fluids; in theory, a boot made of synthetic setae would adhere as easily to the surface of the International Space Station as it would to a living-room wall, although adhesion varies with humidity. The setae on the feet of geckos are also self-cleaning and will 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.
One study suggested capillary adhesion might play a role, but that hypothesis has been refuted by more recent research. Another study suggests that electrostatic forces may be primarily responsible.
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, this peeling action alters the angle of incidence between millions of individual setae and the surface, reducing the van der Waals force. 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 attraction 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 μm. Human hair varies from 18 to 180 μm, so a human hair could hold between 12 and 1300 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae. Each spatula is 0.2 μm 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.
Recent studies have also revealed that 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.
Development of geckel, a super-sticky adhesive that can attach to both wet and dry surfaces, has been inspired by adhesion in gecko feet.
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 (Peropus 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 is genus of geckos in which there are many varieties.
Hemidactylus frenatus, the Common house gecko, thrives around man 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.
Hemidactylus turcicus, the Mediterranean house gecko, is frequently found in and around buildings, and is an introduced species in the US.
Lepidodactylus lugubris, the mourning gecko, is originally an East Asian and Pacific species; it is equally at home in the wild and residential neighborhoods.
Phelsuma laticauda, the gold dust day gecko, is a diurnal gecko; 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 geckos 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.
^Borsuk-Białynicka, M. (1990). "Gobekko cretacicus gen. et. sp. n., a new gekkonid lizard from the Cretaceous of the Gobi Desert". Acta Palaeontologica Polonica35 (1–2): 67–76.
^Conrad, Jack L.; Norell, Mark A. (1 December 2006). "High-resolution X-ray computed tomography of an Early Cretaceous gekkonomorph (Squamata) from Öösh (Övörkhangai; Mongolia)". Historical Biology18 (4): 405–431. doi:10.1080/08912960600679570.
^Conrad, Jack L. (3 June 2008). "Phylogeny and Systematics of Squamata (Reptilia) Based on Morphology". Bulletin of the American Museum of Natural History310: 1–182. doi:10.1206/310.1.
^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 Zoology265 (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 Technology21 (12–13): 1317–1341. doi:10.1163/156856107782328399. "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) anti-self sticking and (7) non-sticky default state. ... The gecko's adhesive structures are made from ß-keratin (modulus of elasticity [approx.] 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 [approx.] 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."
^ abPuthoff, J.B.; Prowse, M.; Wilkinson, M. and Autumn, K. (2010). "Changes in materials properties explain the effects of humidity on gecko adhesion". Journal of Experimental Biology213 (21): 3699–3704. doi:10.1242/jeb.047654.Cite uses deprecated parameters (help)
^ abProwse, M.S.; Wilkinson, Matt; Puthoff, Jonathan B.; Mayer, George; Autumn, Kellar (2011). "Effects of humidity on the mechanical properties of gecko setae". Acta Biomaterialia7 (2): 733–738. doi:10.1016/j.actbio.2010.09.036. PMID20920615.
^Chen, B.; Gao, H. (2010). "An alternative explanation of the effect of humidity in gecko adhesion: stiffness reduction enhances adhesion on a rough surface". International Journal of Applied Mechanics2: 1–9. doi:10.1142/s1758825110000433.
^Russell, A. P. (1975). "A contribution to the functional analysis of the foot of the Tokay, Gekko gecko (Reptilia: Gekkonidae)". Journal of Zoology, London176: 437–476. doi:10.1111/j.1469-7998.1975.tb03215.x.
^Loskill, P.; Haehl, H.; Grandthyll, S.; Faidt, T.; Mueller, F.; Jacobs, K. (November 2012). "Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions". Adv. Coll. Interf. Sci. 179–182: 107–113. doi:10.1016/j.cis.2012.06.006. PMID22795778.
^Hsu, P. Y.; Ge, L.; Li, X.; Stark, A. Y.; Wesdemiotis, C.; Niewiarowski, P. H.; Dhinojwala, A. (24 August 2011). "Direct evidence of phospholipids in gecko footprints and spatula-substrate contact interface detected using surface-sensitive spectroscopy". Journal of the Royal Society Interface9 (69): 657–664. doi:10.1098/rsif.2011.0370.
^Han, D.; Zhou, K.; Bauer, A.M. (2004). "Phylogenetic relationships among gekkotan lizards inferred from c-mos nuclear DNA sequences and a new classification of the Gekkota". Biological Journal of the Linnean Society83 (3): 353–368. doi:10.1111/j.1095-8312.2004.00393.x.
^Gamble, T.; Bauer, A.M.; Greenbaum, E.; Jackman, T.R. (July 2008). "Out of the blue: A novel, trans-Atlantic clade of geckos (Gekkota, Squamata)". Zoologica Scripta37 (4): 355–366. doi:10.1111/j.1463-6409.2008.00330.x.
^Gamble, Tony; Bauer, Aaron M.; Greenbaum, Eli; Jackman, Todd R. (21 August 2007). "Evidence for Gondwanan vicariance in an ancient clade of gecko lizards". Journal of Biogeography: 070821084123003. doi:10.1111/j.1365-2699.2007.01770.x.