{liz'-urd} Lizards are reptiles closely related to snakes. Well-adapted to life on land, lizards possess a dry, scaly body covering; limbs suited for rapid locomotion; internal fertilization; and hard-shelled eggs for the protection of the embryo. Lizards feed mostly on insects, although a certain number eat plants. Only two species are venomous, the Gila monster (Heloderma suspectum) and the Mexican beaded lizard (H. horridum). LIZARDS AND SNAKES Both snakes and lizards are classified in the same order, Squamata, but lizards are separated into their own suborder, Sauria. Lizards generally can be distinguished from snakes by the presence of two pairs of legs, external ear openings, and movable eyelids, but these convenient external diagnostic features, while absent in snakes, are also absent in some lizards. Lizards can be precisely separated from snakes, however, on the basis of certain internal characteristics. All lizards have at least a vestige of a pectoral girdle (skeletal supports for the front limbs) and sternum (breastbone). The lizard's brain is not totally enclosed in a bony case but has a small region at the front covered only by a membranous septum. The lizard's kidneys are positioned symmetrically and to the rear; in snakes the kidneys are far forward, with the right kidney placed farther front than the left. The lizard's ribs are never forked, as are one or two pairs in the snake. DISTRIBUTION AND HABITAT There are over 3,000 known species of lizards, making them one of the largest and most successful groups of reptiles. They are found on all the continents except Antarctica. The greatest diversity of species occurs in the tropics, but lizards range as far north as the Arctic Circle in Eurasia (Lacerta vivipara) and as far south as southern Chile in South America (Liolaemus, Diplolaemus). In North America lizards barely range into southern Canada. Lizards live at elevations exceeding 4,500 m (15,000 ft) in the Himalayas, in the Andes, and among the volcanoes of Mexico. Although no living species of lizard can truly fly through the air, as did some reptiles of the past, lizards occupy practically every other habitat. Some lizards live in areas of rain and cloud forests soaked by daily rains. Others--such as the crocodile lizard, Shinisaurus, of China, which feeds on fish and tadpoles, the snail-eating caiman lizard, Dracaena, of tidal marshlands in South America, and the brown water lizard, Neusticurus, of South America--have become semiaquatic. Conversely, some lizards have adapted to deserts that receive only 200 mm (about 8 in) or less of rainfall annually. The gecko, Palmatogecko rangei, has extensive webbing on its feet for walking over fine sand. Arboreal forms may exhibit various adaptations, including climbing pads, zygodactylous feet (having the toes arranged into two opposing groups), prehensile tails, and strongly recurved claws. The flying dragons, Draco, of Southeastern Asia have sheets of skin attached to their elongated ribs and are capable of gliding. Burrowing lizards often have reduced eyes and limbs, reduced head plates, smooth scales, and conical heads with rigid skulls to make for easier burrowing. The marine iguana, Amblyrhynchus cristatus, of the Galapagos Islands freely enters the ocean to feed on algae (seaweed) and may forage 15 to 20 m (50 to 65 ft) underwater. This species has salt-secreting glands in the head. GENERAL FEATURES Lizards range in size from 20 to 75 mm (0.75 to 3 in) in exceptionally small forms (Brookesia, Sphaerodactylus) to over 3 m (10 ft) in the Komodo dragon (Varanus komodoensis), but most are closer to 400 mm (16 in) in length. Limbs Limbs are generally well developed, and some species demonstrate bipedalism, standing up and running on their hind legs (Basiliscus, Chlamydosaurus). The race runners, Cnemidophorus, are particularly quick and have been clocked at almost 30 km/h (19 mph). Burrowing forms often have degenerate limbs, and legless lizards move in the same manner as snakes. In some species in the genus, such as the marine iguana, Amblyrhynchus, the tail is vertically flattened, from side to side, an adaptation to swimming. Skin and Teeth The lizard's skin is dry and scaly. The scales are derived from the epidermis and hence are not identical to fish scales, which are derived from another part of the skin, the dermis. Scales may vary from small granules, as in the gecko, to those bearing prominent spines, as in the horned lizard. Many lizards have osteoderms beneath the epidermal skin. These bony nodules are often in a one-to-one ratio with the epidermal scales, providing them with internal support. The lizard's skin is shed periodically, but, unlike that of most snakes, it is shed in patches. A few species in the family Aguinidae shed their skin in its entirety. Skin glands are not numerous in lizards. Most lizards have scent glands at the base of the tail, and males often have glands in front of the anus (preanal) or on the thigh (femoral) that become filled with a hard, waxy, yellow substance. This material may extend from the glands in hairlike columns that rub off on rocks and other surfaces. The significance of these glands is not precisely known, but because they are larger in males, whereas some females lack them completely, and are active during the breeding season, they appear to play a role in courtship. Metachrosis, or the ability to change color under the influence of external stimuli, is a common characteristic of lizards. Many species can darken or lighten their color in response to light, temperature, or emotional state, and a few (chameleons) have developed the ability to alter their color to a remarkable degree. The green anole, Anolis carolinensis, called the American chameleon and found in the southeastern United States, can change from a bright green to brown. A cave lizard, Proctoporus, of Trinidad, is reported to be luminous. Four types of pigment-bearing cells (chromatophores) have been identified in lizards: melanophores, with black or brown pigment; guanophores (leucophores), with whitish pigment; lipophores (xanthophores), with yellow to orange red pigment; and allophores, with red pigment. Lizard coloration may be structural as well as pigmental. Structural colors result from the presence of tiny particles, such as purine crystals, which themselves may be colorless but which selectively reflect the shorter, or bluer, wavelengths of light, making the object appear blue. A combination of structural blue and pigmental yellow can produce green. Lizards bear teeth on the palatines and pterygoids--which are two sets of paired bones in the roof of the mouth--and on the dentaries, or lower jaw. A uniform shape for all the teeth is most common, but in certain species in the families Agamidae and Teiidae the teeth are more or less differentiated into conical and molar types (heterodonty). Lizards' teeth are not set in separate sockets but are fused either to the upper surface of the jaws (acrodont) or to the inside edges (pleurodont). The teeth are generally sharp and conical but may be extremely elongated in some predatory species or serrated or blunt in the larger herbivorous forms or among species with certain feeding specializations. In general, there is little chewing of food among lizards. Many lizards simply seize their prey, bite it and move it about the mouth, and then swallow it. Senses Lizards recognize prey either by visual clues or by chemoreception (scent and taste). The lizard's eye is different from that of a snake in that focusing is accomplished by the contraction of the ciliary muscle of the eye directly squeezing the lens and causing it to change shape. In snakes, focusing is generally accomplished by muscular deformation of the eyeball, which increases its internal pressure and forces the lens forward. Nocturnal lizards usually have a vertical pupil, lack a fovea, and have numerous retinal rods (highly sensitive sensory cells) that distinguish different light intensities. Diurnal species generally have rounded pupils, a fovea, and retinal cones instead of rods; cones distinguish wavelengths of light (color) and hence provide more-detailed daytime vision. Many diurnal lizards have yellow oil in their retinal cones that acts to filter out the shorter wavelengths of light, thereby reducing the amount of unequal refraction (chromatic aberration) characteristic of these wavelengths. In addition to the pair of eyes possessed by most vertebrates, many lizards have a third, or parietal, eye located on the upper rear surface of the head. In some North American species in the family Iguanidae the parietal eye is well developed, having a cornea, a lens, and a retina with sensory elements. The parietal eye has been shown to act as a sort of light meter that helps to regulate the amount of time a lizard exposes itself to the sun. The majority of lizards have eardrums (tympanic membranes), usually located nearly flush with the surface of the skin or just below it. Behind the eardrum is the middle ear cavity, which leads to the middle ear. The ear provides a sense of balance as well as hearing. Some species, such as Holbrookia, appear to lack ears because the ear opening is covered by scaly skin. In burrowing species the external ear opening is small or absent and the middle ear degenerated. The Jacobson's organs are important to the lizard's sense of smell. These organs are paired chambers in the front area of the roof of the mouth that have a connection to the olfactory (scent) lobes of the brain. These chambers detect not only particles present in the mouth cavity but also particles brought to them by the lizard's tongue. By protruding its tongue, a lizard is able to pick up airborne particles, which dissolve in the fluid on the tongue; these particles are then transferred by the tongue to the Jacobson's organs. This process is most efficient in lizards with deeply forked tongues, the tips of which are placed against or thrust into the chambers. In such lizards the sense of smell is relatively keen. The lizard's tongue is not ordinarily used directly in feeding, as an amphibian's is; in chameleons, however, the tongue is long and unforked (and the Jacobson's organs are rudimentary) and is used to catch prey. Internal Features Lizards depend almost entirely on paired lungs to aerate the blood. The heart is three-chambered: the right atrium, which receives deoxygenated blood from the body, is separate from the left atrium, which receives oxygenated blood from the lungs; but the ventricle, which pumps blood to both the body and the lungs, is single. There is, however, an incomplete septum, or wall, across part of the ventricle that essentially separates the two bloodstreams. Female lizards, like other reptiles but in contrast to mammals, have saclike (saccular) ovaries rather than solid ones, and their eggs are polylecithal (large-yolked). Males have two copulatory organs called hemipenes, one on each side of the base of the tail. These are generally short and broad, with pleats or folds, but may also be forked. Either one of the two hemipenes may be used in mating. The majority of lizards lay eggs, but at least half of the lizard families have species that bear live young, with varying degrees of placental formation (mother-embryo connection). BEHAVIOR Parental care is unknown in lizards, but in some forms the female broods, or cares for, the eggs. Growth rates vary greatly, and sexual maturity may be reached in the first breeding season after birth in small species, or not for several years in larger species. Lifespans are poorly documented, but generally the small forms live short lives of 1 to 5 years, whereas larger forms may live for 10 to 20 years or even longer. A notable exception is the small European slow worm, Anguis fragilis, a captive specimen of which reportedly reached 54 years of age. Temperature is one of the most important environmental factors influencing lizard behavior; it directly determines a lizard's metabolic rate. Lizards are ectothermic (cold-blooded) and thus depend primarily on an external heat source to regulate their body temperature. This does not mean, however, that a lizard's body temperature will be the same as that of its immediate environment. Lizards can control their temperature by various behaviors, such as by basking directly in the sun (heliothermy), by positioning themselves (such as under a sunlit rock) in such a way as to gain heat through conduction (thigmothermy), or by seeking shade. A lizard's temperature may be 20 to 30 C degrees (35 to 55 F degrees) higher than the ambient (surrounding) temperature. Most lizards exhibit marked seasonal changes in activity. In order to avoid harsh weather conditions, lizards hibernate in temperate regions during the winter, generally seeking refuge beneath the frost line, or they may estivate during hot, dry climatic conditions, often in deep crevices or fissures in the ground, where they can best retain body moisture. Many lizards are cryptically colored and thereby avoid detection. When disturbed, however, lizards have a variety of defensive actions that include threatening with an open mouth, hissing, inflating the body, positioning the body in such a fashion as to appear as large as possible, biting and scratching, and lashing with the tail. With very few exceptions, only the geckos have a voice and can produce threat calls. Many species are equipped with spinelike scales that make it difficult for a predator to swallow them. Some forms expose brightly colored surfaces in an attempt to frighten enemies. Horned lizards, Phrynosoma, may eject blood from their eyes. Many lizards break their tails (autotomy) when they are confronted by an enemy or roughly handled. This break does not occur between the vertebrae (tail bones) but rather in a zone of weakness in a vertebra itself. These specialized vertebrae can be voluntarily split by muscular contraction; sphincter muscles in the tail stump close off the caudal artery to prevent excessive bleeding. Predators are often attracted to the thrashing, dismembered tail, which allows the lizard to escape. Although tail autotomy may seem like a drastic measure, in actuality it does the lizard little harm, for a new tail is soon regenerated. The new tail, however, is not identical to the original. Its skeletal support consists of a fibrocartilaginous rod instead of vertebrae; its musculature is nonsegmental; its scales are different in size and form; and its color is usually subdued or otherwise altered. EVOLUTION Lizards probably arose from early eosuchian reptiles, particularly such genera as Prolacerta, which lived during the lower Triassic Period, about 220 million years ago. Fossils of lizards have been discovered in Europe in deposits of Triassic age. Several extinct forms (Kuehneosaurus, Icarosaurus) resembled the flying dragon, Draco, which is still found in southeastern Asia. During the Cretaceous Period, about 100 million years ago, certain lizards took to the sea and grew to giant proportions, some species reaching 9 m (30 ft) or more in length. These were the mosasaurs, which developed features similar to those of whales: nostrils on the top of the skull, and paddlelike limbs. CLASSIFICATION The lizards are classified in the order Squamata, suborder Sauria (or Lacertilia). Although authorities may differ on various groupings, the suborder is usually divided into four infraorders. The infraorder Gekkota contains three families: Gekkonidae, Pygopididae, and Xantusiidae (some authorities consider them a single family, Gekkonidae, with four subfamilies). The infraorder Iguania also contains three families: Iguanidae, Agamidae, and Chamaeleonidae. The Anguinomorpha comprises five families: Anguinidae, Xenosauridae, Helodermatidae, Lanthonotidae, and Varanidae. The Scrincomorpha is composed of four families: Cordylidae, Lacertidae, Scrincidae, and Teiidae. Jonathan Campbell Bibliography: Bellairs, Angus, The Life of Reptiles, 2 vols. (1970); Fitch, H. S., The Reproductive Cycles of Lizards and Snakes (1970); Goin, Coleman J., et al., Introduction to Herpetology, 3d ed. (1978); Grzimek, Bernhard, ed., Grzimek's Animal Life Encyclopedia, vol. 6 (1975); Hartman, Jane E., Looking at Lizards (1978); Porter, Kenneth R., Herpetology (1972); Smith, Hobart M., Handbook of Lizards (1946).