Lizards

Lizard Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Vertebrata

Subphylum: Tetrapoda

Class: Reptilia

Subclass: Diapsida

Superorder: Lepidosauria

Order: Squamata

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: Most live on land or in trees in all habitats (desert, savanna, temperate and tropical forest). A few are semiaquatic. One, the Galápagos 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 elongated tail, eyelids, and external ear openings. Some lizards, belonging to several families, lost two or all four legs as they adapted for life underground. Other traits belonging to some, but not all, lizards include ear openings completely covered by scales, short tails, and eyes covered by a transparent scale (spectacle). Subterranean lizards, such as the strange worm lizards, have eyes reduced to an eyespot covered by scales.

Lizards belong to the order Squamata, along with snakes. Approximately four thousand lizard species exist on Earth.

Lizard Reproduction

Most lizards reproduce sexually, although some are parthenogenetic. Most lizards are polygynous, with males mating with more than a single female, although a few, such as the Australian sleepy lizard (Trachydosaurus rugosus), are monogamous. Mating occurs after complex social behavior, often involving prolonged courtship. Fertilization is internal. Males have paired intromittent organs, called hemipenes, one of which is inserted in the female’s cloaca during mating. Once the eggs are fertilized, the female carries the 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, and the energy required for locomotion also increases due to the added weight that expectant females carry.

Most lizards produce eggs (oviparity), but many produce live young (viviparity) following extended gestation periods. Females of the oviparous species deposit eggs in places that are moist, 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 predators because they cannot move. Eggs contain yolk, which is high in energy and a good food source for snakes, mammals, and even other lizards. Consequently, mortality of lizard eggs is high. Most 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 the offspring exits the egg. 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. A few, such as the large Australian sleepy lizard, engage in extended parental care, but this does not involve feeding or grooming the young, as with birds and mammals.

Several lizard species in various 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 all individuals produce 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 range of temperatures will produce all males, whereas eggs incubated at another range of temperatures will produce all females.

The juvenile stage of most lizards is also a high mortality stage. Juvenile lizards are relatively small, and, consequently, many predators can easily eat them. Because juveniles do not reproduce, all energy 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, while females use energy for egg production.

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—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 based on chemicals much like a mammal's sense of smell. These lizards capture prey by grasping it between their jaws. The tongue may be used to help manipulate prey in the mouth but is not used to capture prey. Most of these are active foragers—they move 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 can 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. Instead, 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 rarely fatal. Large monitor lizards are nonvenomous, but their mouths contain high levels of bacteria, which can cause dangerous infections following bites. Some 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, though they rarely attack humans.

Principal Terms

Active or Wide Foraging: Moving about in search of prey

Autotomy: Loss of the tail by controlled muscle contractions resulting in breakage at intervertebral sutures

Behavioral Thermoregulation: Maintaining relatively constant body temperature by shuttling between warm and cool microhabitats

Ecdysis: Sloughing off of old skin, which is replaced by new skin from underneath

Lecithotrophy: Nutrition of developing offspring from yolk reserves within the egg

Matrotrophy: Nutrition of developing offspring directly from the mother; in the case of lizards, via a placenta

Oviparity: Production of shelled eggs by females

Parthenogenesis: Reproduction in which unfertilized eggs develop into females genetically identical to their mothers

Sit-and-Wait Foraging: Sitting in one place, waiting, and attacking prey as they move

Spectacle: Transparent scale covering the eye as a replacement for the eyelid; occurs in some lizards and all snakes

Viviparity: Production of live young by females

Bibliography

Auffenberg, Walter. The Behavioral Ecology of the Komodo Monitor. University Presses of Florida, 1981.

Cogger, Harold G. Reptiles and Amphibians of Australia. 7th ed. Ralph Curtis Books, 2018.

Cogger, Harold G., and Richard G. Zweifel, eds. Encyclopedia of Reptiles and Amphibians. 2nd ed. Academic Press, 1998.

Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. 4th ed. Prentice Hall, 2016.

Pruett, Hailey. "Types of Lizards: The 15 Lizard Species You Should Know!" A-Z Animals, 9 Mar. 2023, a-z-animals.com/blog/types-of-lizards-the-15-lizard-species-you-should-know. Accessed 25 June 2023.

Vitt, Laurie J., and Eric R. Pianka, eds. Lizard Ecology: Historical and Experimental Perspectives. Princeton University Press, 1994.

Vitt, Laurie J., Janalee P. Caldwell, and George R. Zug. Herpetology: An Introductory Biology of Amphibians and Reptiles. 4th ed. Academic Press, 2014.