Geckos are mostly relatively small, insectivorous, and night-active, and many species are noteworthy for their ability to vocalize and to climb. They are represented by more than 1,000 species distributed almost worldwide from southern South America to southern Siberia, although they are most numerous and diverse in the tropics. They are especially widespread on oceanic islands, which they have colonized very successfully. Although many geckos are arboreal, they are also numerous in arid areas where rock-living, terrestrial, and burrowing species predominate.
The most striking feature of many geckos is their feet. Almost every one of the 50 genera has a unique design of the toes that is related to its particular mode of locomotion. In some terrestrial forms the toes are narrow and unmodified underneath. In the burrowing Web-footed gecko of the Namib Desert, the toes are connected by skin supported by small bones to form scoops for excavating sand. The Barking gecko of southern Africa is one of several geckos to use toe fringes to assist in digging and walking on sand. In climbing geckos, the toes are expanded into pads. The undersides of these pads are divided into broad, overlapping scales called scansors. Each scansor in turn bears tens, or even hundreds of thousands, of tiny, hairlike projections (setae), each about 10-150 micrometers long. In the Tokay gecko, each seta may have more than 100 branches, each ending in a flat, spatulalike tip about 0.2 micrometers across. These tips interact with the substrate on which the animal climbs, forming weak, temporary molecular bonds. Although individually minute, these forces, summed across all of the toes of the gecko, provide a tremendous adhesive ability - sufficient to allow the lizard to climb even on glass.
In some geckos, the tail tips may also bear setae and be used as a “fifth limb” in climbing. The tails of other geckos may be broad and flat, as in the leaf-tailed geckos of Madagascar (genus Uroplatus). All true geckos have fracture planes in the tails, which often break in encounters with predators or rivals.
Geckos have depressed, soft-skinned bodies and large heads. They often have loose flaps of skin on their flanks, which in the parachute geckos (genus Ptychozoon) have been modified into “wings” for gliding. Gecko eyes are covered by a transparent covering (the spectacle), and eyelids are lacking; when dust or debris adheres to the eye, geckos use their mobile tongues to wipe it away. In nocturnal species, the eyes are usually large. Geckos can regulate the amount of light entering the eye by changing the shape of the pupil from a series of pinholes or a narrow vertical slit in sunlight to a nearly round opening in total darkness. Nocturnal geckos are usually rather drab in color. In contrast, diurnal geckos such as the day geckos of Madagascar may be bright green with blue, red, or yellow markings; in these species the eyes are smaller and the pupil may be round, even in direct light.
Geckos locate food through a combination of visual and chemical cues. The vast majority feed on insects, arachnids, and other small invertebrates. The large Tokay gecko, however, is capable of eating other lizards, small snakes, birds, and mammals as well. Day geckos supplement insect prey with fruit and pollen or nectar from flowers.
Many geckos have a well-developed larynx and vocal cords and can produce a diversity of modulated chirps, clicks, growls, and barks. Barking geckos even form large choruses, with each male declaring its territory and attempting to attract mates. Visual signals are also used, especially in diurnal species, and chemical communication may also play some role in courtship.
Most geckos lay two eggs with hard, calcareous shells, but some smaller species produce only one egg per clutch. Eggs may be laid in shallow pits, under bark, or on plant or rock surfaces. Some species, such as the Fan-footed gecko, lay eggs communally, and many individuals may place their clutches in a few favorable laying sites. Many of the gecko species that have successfully colonized small, distant oceanic islands are all-female forms that reproduce clonally.
Some geckos, especially island species, are threatened by habitat loss. At least one giant day gecko, Phelsuma edwardnewtoni, has become extinct on Rodrigues Island in the Indian Ocean.