Lizard

Article Outline

Introduction; Physical Characteristics; Behavior of Lizards; Habitat; Evolution of Lizards; Lizards and People

Special Anatomy and Defenses

Some lizards have specialized anatomic features. While many types of lizards can use their tongues to catch small prey, chameleons have evolved an extremely long muscular tongue that can be rapidly shot from the mouth to grab insects up to a body-length away. Geckos have microscopically modified scales on toe pads that allow them to cling to and climb on virtually any surface, including smooth glass. The frilled lizard has skin on its neck that can be expanded as a threat, making the animal appear larger to predators. When cornered, the horned lizard can spray an intruder with blood from the corners of its eyes. The chuckwalla may retreat to a crevice and inflate its lungs with air, increasing its size so much that it cannot be pulled out of the narrow space. The alligator lizard has protective, armor-like scales, while the agile swift darts behind the rocks or among the fallen trees of its habitat.

Recent studies show that a number of types of lizards, including monitors and iguanas, have venom-secreting glands in their mouths but lack specialized teeth for delivering a potent bite. Low doses of venom in their saliva may help subdue small prey. Only two lizards can give a true venomous bite: the beaded lizard and the Gila monster. Both lizards have grooves in their teeth to help channel poison from large glands, but they use their venom mainly for defense. The bite of a Komodo dragon contains extremely septic bacteria as well as doses of venom, and is designed to bring down large prey. A large animal bitten by a Komodo dragon usually becomes severely weakened or dies from infection after a few days. The giant lizard then finishes off its victim.

Activity and Metabolism

Like other living reptiles, lizards are cold-blooded, or ectothermic, meaning they do not generate heat internally and use their environment to control their body temperature. Lizards bask in the sun to raise their body temperature to become active or seek shade or water to avoid overheating. Because lizards do not need to eat extra food to maintain their body heat and metabolism, they can survive on fewer meals than warm-blooded mammals and birds need, and can tolerate long periods without eating.

Most lizards are only capable of short bursts of fast running or other high levels of activity. Their sprawling side-to-side limb movements restrict expansion of their lungs to breathe and their three-chambered hearts do not effectively separate oxygen-rich blood from oxygen-depleted blood. Monitor lizards, however, have evolved special features that allow them to sustain vigorous high-oxygen activity for long periods, similar to mammals. Monitors have a special muscular pouch on their throats to pump extra air into their lungs. They also have a special valve in their hearts that can close off to circulate oxygen-rich blood during high-activity, similar to the four-chambered heart found in crocodiles, birds, and mammals. At other times, the three-chambered heart in monitors allows both types of blood to mix, the condition typical of other lizards.

Behavior of Lizards

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Diets

The diets of lizards vary greatly, depending on species. While most eat insects, some, such as the green iguana, generally eat plants. 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 be a threat to humans.

Reproduction

A number of lizards stage elaborate displays to defend territory and attract mates, sometimes using brightly colored body parts. In some species, males may wrestle rivals to mate with females. Like snakes, male lizards have a reproductive organ with two penises or hemipenes, which are usually contained within small sacs at the base of the tail. During mating, males insert one of their hemipenes into the cloaca of the female.

Most lizards lay soft-shelled eggs in moist soil or decaying plants. In most cases the sex of offspring is determined by the temperature at which eggs are incubated. A number of species, including the slowworm, some chameleons, and the shingle back lizard, give birth to live young by retaining their eggs internally until they hatch. A few lizards can lay fertile eggs without mating, a form of reproduction called parthenogenesis.

Typical of most reptiles, lizards generally do not display parental behavior. Most females leave their eggs to hatch on their own. The females of some monitor lizards such as the Komodo dragon may guard their nests from predators until the eggs hatch but do not protect the young after birth, unlike crocodilians.

Habitat