lizards

Lizards are scaly-skinned reptiles, closely related to snakes but usually distinguished by the possession of legs, movable eyelids, and external ear openings. Most of the 3,000 living species of lizards live in warm regions of the Earth. Some species of lizards are found nearly at the Arctic Circle in Eurasia and others to the southern tip of South America. The lizards are usually considered part of the reptilian order Squamata.

Lizards are by far the most diverse group of modern reptiles in body shape and size. They range in total length from geckos of 1.2 inches to lizards of 10 feet. The adult weight varies from less than a gram (0.04 ounce) to more than 150 kilograms (330 pounds).

Lizards occupy diverse habitats underground, on elevated vegetation, and on other desert sands. The mosasaurs, an extinct family, were strictly marine. Some were giants, attaining lengths of 10 meters (33 feet), with an elongated head, short neck, and long thin body and tail. No other surviving species are marine, but several are partially aquatic, feeding on freshwater organisms.

Some large species (iguanas) are eaten, and others are used for leather goods. Predators, such as the tegu, can be pests around chicken farms, but the vast majority are vastly insectivorous or consume undesirable rodents. Lizards are valued as subjects for biological research. The varied modes of reproduction and the ability to regulate body temperatures are two of the many areas for study. Lizards are quite popular house pets.

Most lizards reproduce by laying eggs. In some small species, the number of eggs is rather uniform from each egg laying or clutch. For example, all anoles Lay but a single egg at a time; mini geckos lay one or two eggs and some skinks have fixed clutches of two eggs.

Clutch size varies with the size, age, and condition of the mother.  A clutch of four to eight eggs may be considered typical, but large species such as iguanas may lay 50 or more eggs at a time.

Lizard eggs are usually leathery shelled and porous and can expand by absorption of moisture as the embryo grows. An exception occurs in most egg-laying geckos, whose eggs have shells that harden and soon after deposition and then show no further change in size or shape.

Some lizards bear live young. Ovoviviparous forms essentially retain eggs in the oviduct with no definite shell laid down, whereas truly Viviparus forms have a placenta.

Most lizard populations are evenly divided between females and males. Deviations from this pattern are found in parthenogenetic (all female) species, in which the young are produced from unfertilized eggs. Many species dig holes in which the eggs are placed, others bury them under leaf litter or utilize a hole in a tree or cave. Some species, however, notably the five-lined skink of the United States, remain with their eggs throughout incubation time (about six weeks), leaving only infrequently to feed. These skinks turn the eggs regularly and, if the eggs are experimentally scattered, will return them to the nest cavity. As soon as the young disperse, family ties are severed.

Certain lizards, particularly some geckos, are known to be collective egg-layers, many females depositing their eggs at the same site. Whether this is due to social interaction or simply to ideal site conditions has not been determined.

Juvenile lizards are essentially miniature adults; they do not go through any larval phase. They often differ from the adult in color or pattern and in certain body proportions. For example, the heads of hatchling lizards of some species tend to be proportionally larger than those of adults. Certain ornamental structures, such as the throat fan of the male American chameleon (Anolis) or the horns of some true chameleons, develop as the lizards become sexually mature.

Some of the smaller lizards mature very quickly. The young hatch in July and reach sexual maturity that autumn. At this time males undergo spermatogenesis and mating takes place. Large lizards may take several years to reach sexual maturity.

Lizards are cold-blooded and tend to assume the temperature of their surroundings. The preferred body temperature plays a critical physiological role in the life of a lizard. It has been demonstrated in the laboratory that animals kept without the opportunity to achieve the preferred temperature may be unable to attain the normal reproductive condition or may become sterilized There is also evidence that enzymes catalyze reactions most efficiently at or near the preferred body temperature. The diversity of lizard species decreases with increasing latitude or with high altitudes. Water is less of a problem to lizards than is temperature. All reptiles excrete uric acid and hence do not need great amounts of liquid to get rid of nitrogenous wastes. Many lizards have salt glands for active excretion of mineral salts. Deserts, therefore, do not pose severe problems to lizards and serve as a major habitat for the group.

Lizards themselves are food for many birds, mammals, and reptiles, and they have many defensive mechanisms. Some live close to rock piles. When danger threatens, they move into crevices and puff themselves up so that extrication is most difficult. Several spiny-tailed forms also move into crevices and expose only the formidable tail. Perhaps the highest development of this sort of defense is found in the African armadillo lizard (Cordylus), which holds its tail in its mouth with the forefeet and presents a totally spiny appearance to an attacker. To intimidate intruders on its territory. the frilled lizard of Australia (Chlamydosaurus) extends a throat frill almost as wide as the lizard is long. The tails of many lizards break off (autotomize) easily. The broken-off section wriggles rapidly, distracting the predator as the tailless lizard scurries for cover.

Most lizards are quadrupedal, with powerful limb musculature. They are capable of rapid acceleration and possess a great ability to change the direction of motion rapidly. The race runners (Cnemidophorus) can attain speeds of 24 kilometers (15 miles) per hour. A tendency toward elongation of the body is found in some families, often accompanied by reduction of limb length and, not infrequently, complete loss of limbs. Such lizards propel themselves entirely by lateral waves emanating from highly complicated ventral abdominal musculature. Limbless lizards that move quickly on the surface or through sand tend to have elongated tails (Ophisaurus the glass "snake"), whereas the burrowers have extremely reduced tails (amphisbaenians). The burrowers (amphisbaenians in particular) dig by ramming the head into the substrate, then rotating the head around the head joint.

The true chameleons, a predominantly arboreal family, have a different type of highly specialized limb. The digits on each foot are divided into two groups by webs of skin. On the hind limb, three of the toes are on the outside, two on the inside; on the forelimb, the pattern is reversed. Each foot is thus divided into an outer and an inner portion, which can be opposed as the branch is gripped. In addition, some lizards have prehensile tails, which aid in grasping branches.

Some lizards can parachute or glide through the air and make soft landings. The most highly adapted of these are the flying lizards (Draco), which have extensible lateral expansions of the skin, supported by elongate ribs.

Except for openings of nostrils. mouth, eyes, and cloaca, most lizards are completely covered in scales. Scales may be smooth and overlapping or have keels or tubercles. The outer parts of the scales are composed of dead horny tissue made of the protein keratin. The dead layer is shed at intervals and is replaced by proliferating cells in the deep part of the epidermis.

Many lizards can show some colour change. Two groups, the chameleons and the anoles, are particularly noteworthy in this ability. They can change from bright green to deep, chocolate brown, and patterns such as lines and bars may appear and disappear. The pigment cells that permit colour change are melanophores. Within these cells, pigment granules can migrate. In general, the animal is light when the pigment is concentrated and dark when it is dispersed throughout the cells. The actual color seems to be controlled by an interaction of hormones, temperature, and the nervous system.

Lizards are reptiles, a group characterized by the presence of an opening in the temporal bone of the skull. The reptiles include the crocodiles, pterosaurs, dinosaurs, and the more primitive lizards, and snakes.

Ref:

Encyclopedia Britannica

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