Lizard

I

Introduction

Lizard, common name for certain reptiles in the order that also contains snakes, amphisbaenians, and the extinct mosasaurs. Lizards constitute the largest living group of reptiles, with more than 3,000 species in a wide variety of shapes and sizes.

II

Physical Characteristics

Although lizards may resemble salamanders and other amphibians, they differ in that lizards have dry scaly skin, and most have clawed feet and external ear openings. A number of lizards also have very good eyesight compared with most reptiles. Most lizards are small with four legs and a long tail that in many species is fragile and easily broken but will regenerate; the tail of such arboreal (tree-dwelling) species as the chameleon is adapted for grasping branches. The legs of some lizards are greatly shortened, or vestigial, making animals such as the so-called glass snake or slow-worm snake-like in appearance; they are distinguished from true snakes by their movable eyelids and by differences in the structure of the skull bones, especially those of the lower jaw. The bones of the two halves of a lizard's lower jaw are firmly united; those of a snake are separable.

Lizards move in a variety of ways, depending on body structure and habitat. The collared lizard and the basilisk run quickly on well-developed hind legs, and the latter can also run across the surface of water using its greatly developed back legs and whip-like tail. Geckos have special discs on their toes that enable them to cling to smooth surfaces. Some lizards are good swimmers, and a number are adapted for climbing trees.

III

Behaviour

Lizards are cold-blooded and begin the day with a body temperature that is too low for much activity, and they must bask in the sun to increase their body temperature. While basking in the sun they stand sideways to it and flatten and tilt their bodies to maximize heat intake. They can also spread their bodies and change their skin colour from light to dark to increase heat intake. However, in hot desert areas lizards run the risk of overheating and have also developed ways of losing heat.

The diets of lizards vary greatly, depending on species. While most are insectivorous, some, such as the green iguana, are generally herbivorous. The Gila monster, which lives in desert areas, eats reptile and bird eggs as well as small rodents; it also has a fat reserve stored in its tail. Depending on their size, monitors prey on insects, birds and reptiles and their eggs, small mammals, and carrion. One species, the Komodo dragon, is large enough to injure humans.

Like most reptiles, lizards generally do not display parental behaviour. Females of most species lay eggs, which are left to hatch on their own. A few species, however, including the slow-worm, are viviparous (give birth to living young).

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Habitat

Lizards are found in tropical and subtropical regions throughout the world; few species are found in temperate regions and Arctic areas. Some are burrowing or terrestrial animals; some are arboreal, and others are semi-aquatic. Iguanas are found in all three types of habitats.