Lizard is the common name used for all squamate reptiles that are not snakes (and to a lesser extent amphisbaenians or "worm lizards"), encompassing over 7,000 species. They have a cosmopolitan distribution, ranging across all continents except Antarctica, as well as most oceanic island chains. Extant lizard species vary in size from the 13.5 mm (0.53 in) nano-chameleon and the 14–18 mm (0.55–0.71 in) Jaragua dwarf gecko to the 3-meter-long Komodo dragon. Most lizards are quadrupedal with a side-to-side swaying gait, although some lizards are capable of running on hindlimbs. Some lineages (known as "legless lizards") have secondarily lost their limbs and developed a snake-like elongated body due to convergent evolution. Some lizards, such as the arboreal Draco, are capable of gliding flight using fanned-out gular membranes and costal patagia as wings. They are often territorial, the males fighting off other males and signalling, often with bright colours, to attract mates and to intimidate rivals. Lizards are mainly carnivorous, often being ambush predators; smaller lizard species eat insects and other invertebrates, while the larger monitor lizards can prey upon tetrapods as big as water buffalo. Lizards are able to thrive in a wide range of habitats, from wetlands to deserts, from alpine meadows to coastal caves, from arctic tundra to tropical islands like the Galápagos. Some small lizards like the common house gecko and pale-flecked garden sunskink are able to adapt to niches within human settlements and become a part of the urban wildlife. As most lizards are relatively small, they are preyed upon by larger animals such as mammals, birds, other reptiles, and even fish and large frogs, as well as predatory invertebrates such as spiders, praying mantises and giant centipedes, and thus have a variety of antipredator adaptations including venom, camouflage, reflex bleeding and the ability to self-amputate their tail as a decoy that distracts the predator. Snakes descended from lizards, so cladistic classification systems often regard "lizard" as a paraphyletic group, and some lizards are more closely related to snakes than they are to other lizards. To regard lizards as a monophyletic group, one would have to classify snakes as a clade/subgroup of specialized lizards, and this would also make "lizards" and "squamates" synonyms.
Anatomy
Largest and smallest The adult length of species within the suborder ranges from a few centimeters for chameleons such as Brookesia micra and geckos such as Sphaerodactylus ariasae to nearly 3 m (10 ft) in the case of the largest living varanid lizard, the Komodo dragon. Most lizards are fairly small animals.
Distinguishing features
Lizards typically have rounded torsos, elevated heads on short necks, four limbs and long tails, although some are legless. Lizards and snakes share a movable quadrate bone, distinguishing them from the rhynchocephalians, which have more rigid diapsid skulls. Some lizards such as chameleons have prehensile tails, assisting them in climbing among vegetation. As in other reptiles, the skin of lizards is covered in overlapping scales made of keratin. This provides protection from the environment and reduces water loss through evaporation. This adaptation enables lizards to thrive in some of the driest deserts on earth. The skin is tough and leathery, and is shed (sloughed) as the animal grows. Unlike snakes which shed the skin in a single piece, lizards slough their skin in several pieces. The scales may be modified into spines for display or protection, and some species have bone osteoderms underneath the scales.
The dentitions of lizards reflect their wide range of diets, including carnivorous, insectivorous, omnivorous, herbivorous, nectivorous, and molluscivorous. Species typically have uniform teeth suited to their diet, but several species have variable teeth, such as cutting teeth in the front of the jaws and crushing teeth in the rear. Most species are pleurodont, though agamids and chameleons are acrodont. Komodo dragons have iron-reinforced teeth, with high concentrations of iron strengthening the tips and serrated edges. This natural, orange-pigmented iron coating acts as a protective, wear-resistant layer that keeps their teeth sharp to tear through. The tongue can be extended outside the mouth, and is often long. In the beaded lizards, whiptails and monitor lizards, the tongue is forked and used mainly or exclusively to sense the environment, continually flicking out to sample the environment, and back to transfer molecules to the vomeronasal organ responsible for chemosensation, analogous to but different from smell or taste. In geckos, the tongue is used to lick the eyes clean: they have no eyelids. Chameleons have very long sticky tongues which can be extended rapidly to catch their insect prey. Three lineages, the geckos, anoles, and to a lesser extent chameleons, have modified the scales under their toes to form adhesive pads. The pads are composed of millions of tiny setae (hair-like structures) which fit closely to the substrate to adhere using van der Waals forces; no liquid adhesive is needed. In addition, the toes of chameleons are divided into two opposed groups on each foot (zygodactyly), enabling them to perch on branches as birds do.
Physiology
Locomotion
Aside from legless lizards, most lizards are quadrupedal and move using gaits with alternating movement of the right and left limbs with substantial body bending. This body bending prevents significant respiration during movement, limiting their endurance, in a mechanism called Carrier's constraint. Several species can run bipedally, and a few can prop themselves up on their hindlimbs and tail while stationary. Several small species such as those in the genus Draco can glide: some can attain a distance of 60 metres (200 feet), losing 10 metres (33 feet) in height. Some species, like geckos and chameleons, adhere to vertical surfaces including glass and ceilings. Some species, like the common basilisk, can run across water.
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