Lizards are a large and widespread group of reptiles of the order Squamata, with nearly 5,000 species ranging across all continents except Antarctica. Most lizards have four limbs, external ears, and a tail. Many can shed their tails in order to escape from predators, although this trait is not universal. Vision, including color vision, is particularly well developed in lizards, and most communicate with body language or bright colors on their bodies as well as via pheromones. The adult length of species within the order range from a few centimeters for some Caribbean geckos to nearly three meters (9 feet, 6 inches) in Komodo Dragons.
[edit] Description
Any generic description of lizards is often complicated by the fact that many typical lizard traits are either retentions from their evolutionary ancestors (such as the basic, 4-limbed, tetrapod body form) or are either lost or changed in some species (loss of limbs, loss of external ears, loss of the tail, etc.)
Lizards are reptiles, and universally possess scaly skin and a skull with many fused or reduced bones. Most lizards retain the typical tetrapod body plan of a short neck, four limbs of roughly equal size ending in five toes each, a moderately long body, and a long tail. Most lizards possess external ears and have movable eyelids. Encompassing forty families, there is tremendous variety in color, appearance and size. Most lizards are oviparous, though a few species are viviparous. Many are also capable of regeneration of lost limbs or tails. Almost all lizards are carnivorous, though most are so small that insects are their primary prey. About 120 species (3%) are known to be herbivorous.[1] A few species are omnivorous, and others have reached sizes where they can prey on other vertebrates. Many lizards are good climbers or fast sprinters. Some can run bipedally, such as the collared lizard, and some can even run across the surface of water to escape, namely the basilisk. Many lizards can change colour in response to their environments or in times of stress. The most familiar example is the chameleon, but more subtle colour changes occur in other lizard species as well, such as the anole, also known as the "American chameleon", "house chameleon" or "chamele".
Some lizard species, including the glass lizard and flap-footed lizards, have lost their legs or reduced them to the point they are non-functional. However, some vestigial structures remain. Snakes, which evolved from the ancestors of monitor lizards, are characterized by lack of eyelids, lack of an external ear, a forked tongue, and having a highly elongate body (as opposed to a normal body but extremely long tail). While any given legless lizard species (of which there are many) may match on one or two of these characteristics, they invariably differ from snakes in others. For example, flap-footed lizards lack eyelids as do true snakes, but can be distinguished by their external ears.
Lizards are part of the reptile family, meaning that they have no inner means of achieving homeostasis. As a result, they must keep careful watch of their body temperature. This need requires lizards to live in areas with consistently high temperatures. Lizards are rarely seen in the upper half of the United States and most European countries.
[edit] Senses and communication
Lizards employ many diverse methods of communication. Like many other animals, they have an acute sense of smell, detecting scents of their prey or pheromones from other lizards. The primary organ of scent in lizards is a vomeronasal organ in the roof of the mouth, and lizards gather scents by flicking out their tongues, then retracting them and delivering the captured odor molecules to this organ. Some large carnivorous lizards, such as tegus and monitor lizards, have forked tongues like snakes, to take advantage of this organ better. As a result, many male lizards possess enlarged pores on the underside of their thighs, which they rub against objects to mark their territory.
While most lizards can hear well, few are capable of vocalizations or otherwise making noise. The exception to this rule is the geckos, which communicate through a wide variety of barks, chirps and whistles, with each species having specific patterns and sounds.
Sight is quite important for most lizards, both for locating prey and for communication, and as such, many lizards have highly acute color vision. Most lizards rely heavily on body language, using specific postures, gestures and movements to define territory, resolve disputes, and entice mates. Some species of lizard also utilize bright colors, such as the iridescent patches on the belly of Sceloporus. These colors would be highly visible to predators, so are often hidden on the underside or between scales and only revealed when necessary.
A particular innovation in this respect is the dewlap, a brightly colored patch of skin on the throat, usually hidden between scales. When a display is needed, the lizards erect the hyoid bone of their throat, resulting in a large vertical flap of brightly colored skin beneath the head which can be then used for communication. Anoles are particularly famous for this display, with each species having specific colors, including patterns only visible under ultraviolet light, as lizards can often see UV.
[edit] Evolution and relationships
The retention of the basic tetrapod body form by lizards makes it tempting to assume any similar animal, alive or extinct, is also a lizard. However, this is not the case, and lizards are part of a well-defined group.
