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.
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