Lizards belong to the order Squamata, along
with snakes. There are approximately four
thousand lizard species.
Most lizards reproduce sexually, although some are parthenogenetic. Most lizards are polygynous, with males mating withmore than a single female, although a few, such as Australian sleepy lizards (Trachydosaurus rugosus), are monogamous. Mating occurs after complex social behavior often involving prolonged courtship. Fertilization is internal. Males have paired intromittant organs called hemipenes, one of which is inserted in the female's cloaca during mating. Once eggs are fertilized, the female carries eggs or embryos for various periods of time. The weight of unborn offspring usually reduces the female's ability to run fast, thus affecting her ability to escape predators. Many females change their behavior while gravid to reduce the costs of reproduction. Costs of reproduction are not confined to increased predation risk; energy required for locomotion increases as well due to the added weight that females carry around while gravid. Most lizards produce eggs (oviparity) but many produce live young (viviparity) following extended gestation periods. Females of oviparous species deposit eggs in places that are moist but not wet, such as inside rotted logs or in the ground, often under rocks. Most lizard eggs have pliable, leathery-shelled eggs, but a few, such as geckos, have hard, calcified eggshells. Lizard eggs in the nest are vulnerable to predation by many animals because they cannot move. Eggs contain yolk, which is high in energy and thus a good food source for snakes, mammals, and even some other lizards. Consequently, mortality of lizard eggs in nests is high. Avast majority of oviparous lizards do not provide parental care to eggs, but females of a few, such as five-lined skinks and glass lizards, remain with the eggs, brooding them until they hatch. Hatchlings cut slices in their eggshells with a specialized scale on the front of their jaw, called the egg tooth. Parental care ends once offspring exit the eggs. Hatchlings are fully formed, resembling miniature adults. Females of viviparous species often provide some parental care to neonates (newborns). Most help neonates free themselves from embryonic membranes, often eating the membranes. Afew, such as the large Australian sleepy lizard, engage in extended parental attention, but it does not involve feeding or grooming the young, as in birds and mammals. A number of lizard species in several different families reproduce by parthenogenesis, a process in which females produce daughters that are genetically similar to their mothers without the involvement of males. In such species, potential population growth is extremely high because no energy is wasted on males and every individual produces offspring. Although sex determination in most lizards is chromosomal, as in humans, some species of lizards lack sex chromosomes and have environmental sex determination. Eggs incubated at one set of temperatures produce all males, whereas eggs incubated at another set of temperatures produce all females. The juvenile stage of most lizards is also a high mortality stage. Juvenile lizards are relatively small in body size, and, as a consequence, many predators can easily eat them. Because juveniles do not reproduce, all energy taken in is devoted to growth and maintenance. When lizards reach sexual maturity, growth slows and most energy is directed into reproduction and maintenance. Males use energy in reproductive related behaviors such as territorial defense and courtship, whereas females use energy for production of eggs.
Evolution of Sensing Systems
One of the most fascinating aspects of the natural history of lizards is the variation in relative importance of sensory systems and its consequences. Most lizards in the suborder Iguania are visually oriented, sit-and-wait predators. They capture prey by a process known as tongue prehension; basically, they stick their tongues out and carry insects into the mouth on their tongues. They are colored for camouflage, move very little, and attack moving prey from perches, to which they usually return. Their social systems usually involve territoriality, the defense of a specific area against intruders of the same species. In most instances, territorial behavior is directed toward other males. Lizards in the suborder Scleroglossa use a combination of visual and chemical cues to locate and discriminate prey. They have well-developed vomeronasal systems; they pick up chemicals from the external environment with their tongues and bring them into the mouth, where they are passed over the vomeronasal organ (also called the Jacobson's organ) in the roof of their mouth. This organ transfers information directly to the lizard's brain, allowing it to discriminate prey on the basis of chemicals much like mammals' sense of smell. These lizards capture prey by grasping prey between their jaws. The tongue may be used to help manipulate prey within the mouth but is not used to capture prey. Most of these are active foragers; they travel around, searching for prey, many of which are not moving.
Venomous and Dangerous Lizards
Most lizards will bite in self-defense, and, because all lizards have teeth, some will break the skin. Only two lizard species in the world are venomous, the Gila monster and its close relative, the Mexican beaded lizard. Both have powerful jaws and are difficult to remove should they bite. They do not have fangs like poisonous snakes; rather, they have grooved teeth with venom glands located in the rear of the mouth. Venom moves along the grooved teeth as the lizards grind their jaws while biting. Bites are only rarely fatal. Large monitor lizards are nonvenomous, but their mouths contain high levels of bacteria, which can cause dangerous infections following bites. Some of the larger monitors, such as the Komodo dragon of the Lesser Sunda Islands, have sharp, serrated teeth and can bite completely through the leg muscles of large vertebrates. As a result, their bites are potentially life-threatening even though they rarely attack humans.
Suborders: Iguania (iguanids, chamaeleonids, and agamids); Scleroglossa (all other lizards and snakes); Gekkota (gekkonids, eublepharids, diplodactylids, and pygopodids); Autarchoglossa (teiids, gymnophthalmids, lacertids, xantusiids, dibamids, amphisbaenids, trogonophids, rhineurids, bipedids, scincids, gerrhosaurids, cordylids, anguids, xenosaurids, helodermatids, lanthanotids, varanids, and "snakes")
Geographical location: Every continent except Antarctica
Habitat: A vast majority live on land or in trees in all habitats (desert, savanna, temperate and tropical forest); a few are semiaquatic; one, the GalГЎpagos iguana, dives into the ocean and feeds on algae Gestational period: Highly variable; eggs of Anolis lizards hatch in several weeks, while embryos of some skinks (Mabuya) require a year
Life span: Variable; some small skinks and geckos live one year or less, whereas Gila monsters (Heloderma) and large monitors (Varanus) live thirty years or more
Special anatomy: All lizards have scales (like other reptiles), paired copulatory organs in males (hemipenes), and claws on their feet; most have four legs, an elongate tail, eyelids, and external ear openings; some lizards in several families have lost two or all four legs as adaptations for life underground, some have their ear openings completely covered by scales, some have short tails, and some have their eyes covered by a transparent scale (spectacle); some subterranean lizards, such as the strange worm lizards, have eyes reduced to an eyespot covered by scales
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