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 Latin gekko and English “gecko” stem from the Indonesian–Malay gēkoq, which is imitative of sounds that some species make.
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.
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