This article is about the type of reptile. For other uses, see Gecko (disambiguation).
Temporal range: 110 Ma – Recent
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Gold dust day gecko
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 Latin gekko and English “gecko” stem from the Indonesian-Malay gēkoq, 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 an excellent night vision; their color vision 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.
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. Geckos 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 (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 long and was discovered in 2001 on a small island off the coast of the Dominican Republic.
Oligocene-era gecko trapped in amber
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
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Video of Leopard gecko shedding skin
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.
See also technology designed to mimic gecko adhesion and Gecko feet.
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.
Increasing humidity typically fortifies gecko adhesion, even on hydrophobic surfaces, yet is reduced if completely immersed 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. Friction experiments with gecko toes—torn parallel to surfaces—have shown to be influenced also by electrostatic forces.
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.
Uroplatus fimbriatus clinging to glass
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.
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.
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 odontogenic stem 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.
The infraorder Gekkota is divided into seven families, containing numerous genera of gecko species.