Lizards are perhaps the most familiar of the reptiles. They differ dramatically from dinosaurs and from others in the nature of their skeletons. Most lizards are dwellers on the land and active by day, however, there are also many that live much of their lives underground, and thereby avoid predators.
Underground species are found in such diverse families as the skinks, snake lizards and glass lizards. Those that are active on the surface from time to time escape by rapidly diving, burrowing or sinking into soft soil. Many are most active at night when their potential predators are not about.
One of the most distinctive features of lizards is their horny skin folded in scales. Scales vary greatly in appearance from small to granular to large and plate-like. They touch one another (juxtaposed) or overlap (imbricate), and are smooth or possess one or more ridges (keels).
This covering of scales reduces water loss and may facilitate the uptake of solar energy. Certain scales have been modified into sharp spines which can dissuade attackers. Some are fortified with internal horny plates called osteoderms. The horned lizards of North America and the moloch of Australia are especially spiny, offering attackers an unpalatable mouthful of sharp scales.
A curious feature of lizards is their pineal body in whom this part of the brain is connected to a sort of “third eye.” A cranial bone is perforated at this point to allow an extension of nervous tissue from the brain to a light-sensitive transparent disk on top of the head. The pineal body may be involved in regulating the animal’s “biological clock,” influenced by the recurring pattern of day and light.
All lizards possess a well-developed tongue which can be extended. Many species constantly “sense” the world around them with this organ, sampling molecules from the environment and retrieving them into the mouth as chemical information on food, mates, territories and predators.
The bones of the skull are usually rather loosely attached to one another, and the cranial vault of most lizards is open toward the snout. The tongue is well developed in most lizards. The external ear opening is usually visible, and there is movable eyelid in most species.
The site of chemical sensation is called the vomeronasal, or Jacobson’s organ. It occurs in a variety of vertebrates including amphibians, snakes, bison and members of the cat family. This type of chemosensation is different from smell and taste, of which lizards are also capable.
The tongue has other uses such as cleaning the transparent spectacle of the lidless eyes. In the Great Plains skink the mother regularly licks her eggs. The extraordinarily long, sticky and highly extensible tongues of chameleons are used to capture distant insects.
The familiar scurrying of lizards is not possible for some, for in many burrowers and some surface dwellers, the limbs are reduced or absent. Most species, however, have four legs with five toes each.
Lizards are well known for their agility and swiftness. Typically, when under attack, most species attempt to escape, but the slow and clumsy blue-tongued skinks from Australia almost invariably threaten the predator with a gaping mouth, strong jaws, bright blue tongue and bouts of hisses.
The beaded lizards, venomous and usually slow-moving, also gape and hiss. Many species escape predators by climbing trees, rocks or man-made structures.
The special feet modifications of geckos and anoles permit them to climb almost any surface. Several species of geckos move with ease, upside down, across ceilings.
In contrast to venomous snakes, the venom of the venomous lizards in the genus Heloderma is produced by multi-compartmentalized glands on the lower jaw from which ducts lead onto grooved teeth along the length of the mandible.
When confronted by a snake or other predator, a lizard will sometimes voluntarily shed (autotomize) its tail in one or more pieces. Before shedding the tail, some lizards slowly undulate the vertically erect tail from side-to-side.
This movement is believed to cause the predator to shift the focus of its attack from the more vulnerable head and trunk, improving the tail-less lizard’s chance of escaping unharmed. Tail regeneration itself may require substantial energy.
Although voluntary tail-loss may work, camouflage or crypsis is by far the most effective way to avoid predators. Many lizards exhibit patterns and coloration that blend with their background. Chisel-teeth lizards and chameleons can change in a few seconds from display to camouflage. Crypsis is further enhanced if the lizard remains motionless, but confronted directly by predators, such as birds, mammals and other reptiles, several other behavioral and physical strategies are employed.
Another ploy in avoiding predation is to “play dead” or, alternatively, to become very rigid (“tonic immobility”). Some predators stop their attack when the prey becomes limp or rigid and appears lifeless. They rely on behavioral cues in performing their attack, and a “dead” lizard does not supply these.