Most reptiles can be classified into three large groups: the turtles (order Chelonia), the snakes and lizards (order Squamata), and the alligators and crocodiles (order Crocodilia). Most reptiles share a number of general morphological features. In general, reptiles are lung-breathing vertebrates with two pairs of limbs and a horny, scaly skin. Reptiles are amniotes , which means that their large, yolky eggs have a protective layer called an amnion which prevents them from drying out on land. Rather than laying eggs, some snakes and lizards bear their young live.
Nonavian reptiles are poikilothermic (cold-blooded) creatures, which means that they derive their body heat from external sources (in contrast to homothermic animals that maintain a constant body temperature through internal mechanisms). Contrary to popular belief, the "cold-bloodedness" of reptiles does not mean that they maintain low body temperatures. Reptiles control their body temperature through a process called thermoregulation, and their internal temperature can fluctuate greatly according to their surroundings. Researchers have found that many reptiles exert precise control over body temperature by moving around to different areas within their surrounding habitat.
Turtles are classified into two sister groups, Pleurodira and Cryptodira. Pleurodires (side-necked turtles) bring their head and neck against their body by bending the neck to the side. Most cryptodires fully retract the head and neck into the shell, although certain groups such as sea turtles and snapping turtles have lost this ability. All pleurodires are aquatic, while cryptodires include terrestrial , aquatic, and marine forms.
Turtles are easily distinguished from other reptiles by the protective shell that encases their body. The turtle shell has two parts, a carapace on top and a plastron under the belly, which incorporate the ribs, vertebrae, and elements of the turtle's pectoral (front limb) girdle. The turtle's head, limbs, and tail can protrude from and retract into the shell to varying degrees, depending on the species. The plastron of the box turtle has a hinge that allows the shell to snap shut and provide further protection for the head.
Turtles can have many different forms of shells. The bony elements of the shell can be covered with scaly, armored plates, (all land turtles and most aquatic and marine turtles) or leathery skin (soft-shelled turtles and the ocean-dwelling leatherback turtle). Terrestrial turtle shells have relatively high domes; the shells of water-dwelling turtles are flatter and more streamlined. Most extant turtles have shells that are less than 60 centimeters (2 feet) long, although sea turtles can grow up to a shell length of 2.7 meters (9 feet).
All turtles lay eggs. They do not guard their eggs or young while they are developing, but instead leave the eggs buried in loose dirt or sand. The survival rate of eggs and newborn turtles is very low, since turtles will produce large numbers of offspring to compensate for high mortality rather than fewer, stronger offspring that would have greater potential to survive. Some turtles lay multiple groups of eggs (called clutches) during each season, and the clutches of sea turtles may contain up to two hundred eggs. Developing embryos get their nutrients from the yolk in the egg and only become carnivorous or omnivorous after they hatch. Most turtles have a varied diet of small insects and worms, crustaceans, nonfibrous plants, or fish. In many turtle species, the sex of the offspring is determined by the temperature at which the eggs are maintained as the young turtles develop.
Crocodilians are large, amphibious, and carnivorous reptiles. Their long bodies, powerful jaws, and muscular tail are covered with heavy armor with bony plates. These aggressive predators have conical teeth and short legs with webbed, clawed toes. The largest crocodilian, the Nile crocodile, reaches a length of about 6 meters (20 feet). Many scientists believe that crocodilians are the closest living relatives to birds, but this relationship is still under debate.
There are three major groups in Crocodilya—alligators, crocodiles, and gavials. These three groups are distinguished by characteristics of the teeth and jaw, although all crocodilians are similar with respect to ecology, morphology, and behavior.
Crocodilians are found in swamps, rivers, and lakes of tropical and sub-tropical habitats in the Northern and Southern Hemispheres. They spend most of their time in the water but are also known to travel long distances over land. They move from place to place in several ways, including sculling, in water (propelling themselves using a side-to-side tale motion), belly sliding (into water), high walking, and galloping. Crocodilians are sit-and-wait predators who float passively in the water while awaiting potential prey. The can knock large mammals into the water with a swing of their powerful tail. With their powerful jaws, some species clamp onto their victim's legs and tear the prey apart by rotating themselves rapidly in the water in an "alligator roll."
Crocodilians lay eggs in nests. After laying her eggs, the mother keeps them warm by covering them with mud and decaying plant material. She guards her eggs until the squeaks of the young signal that the babies are ready to hatch. The mother then knocks the dirt off of the eggs to help the young hatch, but provides no additional care for her young.
Two extant species of tuatara (genus Sphenodon ) belong to this sister group of the squamates. The tuatara is lizardlike in general appearance, but is distinguishable from lizards by a number of characters involving tooth type and arrangement, skull morphology, and genitalia. The tuatara is the only nonavian reptile without a organ used exculsively for sexual intercourse. Like birds, male tuataras transfer sperm to the female while their cloaca (an opening through which feces is expelled, and through which reproduction occurs) are pressed together.
Tuataras are found only in New Zealand, where they live in burrows they construct or those that have been abandoned by other animals. During the daytime they bask in the sun near the entrance to their burrows, but are most active at night. They feed on insects, bird eggs, and young birds. Both species of tuatara are egg layers, and newborn young emerge after a thirteen-month incubation period. Tuataras reach sexual maturation after about twenty years and can live up to the age of fifty. Adult tuataras may reach a total length of up to almost 60 centimeters (two feet).
The human introduction of certain animals to New Zealand has placed many tuatara populations under threat of extinction. Tuataras are slow-moving, and are thus easily captured and eaten by rats, cats, and pigs. An estimated 100,000 tuataras remain on offshore islands near New Zealand. There are no tuataras on the New Zealand mainland. About half of the suriving tuatara live on Stephens Island in Cook Strait, and there have been proposals to relocate some individuals to sanctuaries on other islands so that more of them may be seen in the wild. Tuataras are currently listed as an endangered species by conservation groups, and human interaction with this species is strictly regulated.
The squamates are the most diverse group of non-avian reptiles. Squamata consist of lizards (Sauria), snakes (Serpentes) and amphisbaenians (Amphisbaenia). Lizards are divided into two major groups, Iguania and Scleroglossa. These two groups are generally distinguishable by differences in tongue morphology and function. Iguanians have fleshy tongues that are used while capturing prey. The mucous coating on the tongue helps the lizard to pick up small insects, vertebrates, and plant matter, and aids in swallowing. The tongue of the chameleon is a well-known example of this iguanian trait.
Scleroglossans have thin, forked tongues that are used in chemosensation (the process of gathering information about their environment through detecting chemical cues). These lizards use their tongues for "smelling" the air around them and use their jaws for grabbing prey. Snakes and amphisbaenians are both scleroglossans. Iguanians are territorial sit-and-wait predators; scleroglossans are active foragers that generally do not guard territories.
Most of the three thousand species of saurians (lizards, except for snakes and amphisbaenians) have legs, movable eyelids, external ear openings, and long tails. Some species are limbless. Saurians range in size from the 3-centimeter (1.2-inch) long gecko to the 3-meter (10-foot) long Komodo dragon. All saurians are terrestrial, but some species may live in trees and other foliage, under rocks, or in burrows.
Saurians have different ways of reproducing. Most species are oviparous (egg-laying), while other species are viviparous (bearing live young). Among viviparous lizards, some species rely exclusively on the yolk of the egg to provide nutrients to the developing embryo. In these lizards, the eggs that are retained in the mother's uterus during embryonic development resemble the eggs of oviparous lizards but do not have a hard, calcified shell. Instead, there is a thin shell membrane through which water, gas (oxygen and carbon dioxide), and waste materials can pass. In a smaller number of species, the mother transfers nutrients to her young through a placenta . The placenta is a region of the mother's uterus characterized by a high density of blood vessels. Through these blood vessels, water, gases, and waste products pass between the blood of the mother and her developing young.
Most saurians are diurnal (active only during the daytime) and regulate their body temperature by basking in the sunlight during the early morning hours and hiding in the shade during hotter periods. Geckos have special physiological characteristics that allow them to pursue a nocturnal (active during the nighttime) lifestyle. For example, the muscles of geckos are able to function under cooler temperatures than other lizards are use to. Saurians spend much of their time feeding, mostly on insects. Larger species also feed on other lizards and on small mammals. Several iguanian species feed exclusively on plants.
Serpents (snakes) are secretive, solitary predators that are found in almost all types of habitat worldwide, except near the North and South Poles. They are limbless, lack eyelids, and have only one lung. Their slim, long body form allows them to search for prey in burrows, nests, and crevices. Many snakes are also excellent tree climbers and swimmers, and several species are fully aquatic. Like saurians, snakes as a group exhibit both egg-laying and live-bearing forms of reproduction.
Snakes rely mainly on their senses of sight and smell for information about their surroundings. Individuals use chemical signals to communicate to each other, especially during courtship. Some species of boids (such as boa constrictors) and vipers (such as asps and adders) have also evolved heat-sensing infrared receptors called pit organs. Studies have shown that many snake species that lack pit organs can also sense infrared radiation, although to a lesser degree.
Amphisbaenians are elongate, burrowing squamates. With the exception of the genus Bipes, which has forelimbs, amphisbaenians are limbless. All species lay eggs. They occupy habitats ranging from tropical rain forests to deserts.
see also Phylogenetic Relationships of Major Groups.
Judy P. Sheen
Grasse, Pierre-Paul, ed. Encyclopedia of the Animal World: Reptiles. New York: LaRousse & Co., 1975.
Pough, F. H., Robin M. Andrews, John E. Cadle, Martha L. Crump, Alan H. Savitzky, and Kentwood D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.
Stebbins, Robert C. A Field Guide to Western Reptiles and Amphibians. 2nd ed. Boston: Houghton Mifflin, 1985.
Four reptile groups have species found in aquatic habitats: turtles, lizards, snakes, and crocodilians. Although most must return to land to nest, many have adapted to spend the majority of their lives in water.
Turtles are the oldest living reptiles, having evolved before the dinosaurs, more than 200 million years ago. Of the more than 250 turtle species, about 75 percent utilize aquatic habitats. Found on all continents except Antarctica, these habitats include lakes, bogs, rivers, swamps, estuaries, mangroves, and the open ocean.
Turtle sizes range from the bog turtle, at under 10 centimeters (4 inches) to the leatherback sea turtle, which can reach more than 2 meters (6.5 feet) in length. Some turtle species may live up to 100 years or longer, but most probably live less than half that. All species lay eggs to reproduce and show no parental care.
Turtle shells have two parts, the upper carapace and lower plastron, made up of many separate bones. Hard scale-like scutes usually grow over this bony shell. The soft-shelled turtles have a reduced bony layer in their shell and do not grow scutes, instead covering the shell with leathery skin.
The turtle shell is part of the skeleton with the rib bones fused to the interior portion, so turtles cannot leave their shells. Excluding the sea turtles, most species are able to retract their head and limbs into the shell for protection. Some, like the box turtles, have developed hinges to close off the outside entirely when threatened.
Fresh-water turtles are opportunistic feeders, eating a variety of animals and plants. Fish, snails, frogs, worms, and insects are common prey . Some species actively hunt prey within the water, whereas others rely on surprise attacks. The alligator snapping turtle has a fleshy lure that it moves to attract fish directly into its mouth.
Many fresh-water turtle species throughout the world are endangered , because they are hunted for food, medicine, their shells, and the pet trade. The number of endangered turtles is also increasing because of a loss of wetland habitats owing to human development.
The seven species of sea turtles have fully adapted to an aquatic lifestyle, spending their entire lives in the ocean except for nesting. Their major adaptation to this aquatic existence is strong, paddle-like front flippers that enable the turtle to "fly" through the water. Strong swimmers, sea turtles may migrate great distances between feeding and nesting areas in tropical and subtropical oceans.
Female sea turtles return to beaches in the region of their birth to lay their nests. The turtle digs an egg chamber using her hind flippers, deposits 60 to 160 eggs in the nest, and leaves it to incubate. Females nest several times during one season before returning to their feeding grounds. Most species nest every 2 to 3 years.
Hatchlings dig their way out of the nest and generally emerge at night. To find the water, they orient themselves to the brightest horizon, which traditionally occurred over the open ocean. Unfortunately, coastal development has resulted in artificial lights, leading many hatchlings inland, where they face predation and desiccation (drying out).
All sea turtle species are considered threatened or endangered. For decades, harvesting of eggs and nesting sea turtles significantly reduced populations until protective measures began in the 1970s and 1980s. Continued coastal development along nesting beaches and entanglement in fishing gear are considered the greatest threats to their survival.
Marine iguanas are found along the coast and intertidal waters of the Galapagos Islands. The only marine lizard in the world, it feeds almost exclusively on marine algae. The iguanas can stay submerged while feeding for up to an hour. To raise their body temperatures enough to swim and feed in the cold water, marine iguanas spend much of the day basking on the dark volcanic rock.
The lizards may reach 1.5 meters (5 feet) in length, with almost half that being tail. Salt glands between their eyes excrete excess salt into the nose, where the animal "sneezes" it out of the body. While very few natural predators exist, introduced species such as rats, cats, and dogs are a threat to populations on some of the islands. These animals eat the marine iguana's eggs, which are buried in inland nests.
A few other lizard species are associated with fresh water. The water monitor lizards are found along waterways in forested parts of Asia and Indonesia. They feed on a wide variety of aquatic and terrestrial prey. Similarly, the Chinese crocodile lizard, water dragon lizards, and a few skink species also are known to utilize aquatic habitats to escape predators or find food.
Approximately 70 species of sea snakes exist, primarily in the tropical waters of the Pacific and Indian Oceans. Many are found in shallow waters near coral reefs, mangroves, and estuaries.*
Possessing highly toxic venom, most sea snakes hunt fish along the ocean bottom, exploring crevices in rocks or coral reefs. The venom contains neurotoxins, which disrupt signals from the nerves to the muscles, paralyzing prey. The beaked sea snake, responsible for many human deaths, maintains enough venom to kill more than 50 people. Although almost all sea snakes are potentially lethal to humans, most do not bite nonprey unless provoked.
While similar to the terrestrial snakes from which they evolved, sea snakes have adapted to aquatic habitats. They have a flattened, paddle-like tail to aid in swimming, and valves that seal off the nostrils to keep air in. All snakes have one elongated lung that runs the majority of the body cavity, likely adapted to provide oxygen while snakes swallow large prey. This large lung also allows snakes to stay underwater for long periods of time. Sea snakes further enhance this ability by respiring through their skin, obtaining oxygen directly from the water. These adaptations allow sea snakes to remain submerged for up to 2 hours, depending on activity levels.
Other snake species are found in fresh-water habitats throughout the world. This group includes the largest snake in the world, the giant anaconda. Residing in South America, this snake reaches lengths of 9 meters (30 feet) and can weigh 225 kilograms (500 pounds). This giant predator feeds on turtles, fish, large rodents, deer, tapir, and birds. Although rare, attacks on humans have occurred.
Other than the cottonmouth, the majority of fresh-water snakes in the United States are nonvenomous. Examples include the northern, banded, brown, and green water snakes.
The 23 species of crocodilians include alligators, caimans, crocodiles, and the gharial. They are found in 91 countries around the world, congregated in tropic and subtropic areas. Salt-water crocodiles are the largest living reptiles in the world, with one confirmed at 6.3 meters (20.7 feet) long. Cuvier's dwarf caiman, which reaches only 1.5 meters (5 feet) in length, is the smallest species.
An important aquatic adaptation of crocodilians is a valve that seals off the throat when the animal submerges to prevent water from entering the stomach and lungs. This is utilized in capturing prey underwater or drowning terrestrial (land-based) prey. Internally, the nostrils enter the airway behind this valve, allowing the crocodilians to remain submerged except for their eyes and nostrils.
Another adaptation consists of special sensory cells in the skin, particularly around the head, that are believed to sense pressure changes. These cells likely assist in locating prey in the dark and murky waters in which crocodilians live.
Crocodilians are carnivorous , opportunistic feeders. The majority of species feed primarily on fish, amphibians, turtles, and small birds and animals. Alligator jaws are powerful enough to bite open the shell of turtles or snap limbs of larger mammals.
All the crocodilians reproduce by laying eggs, generally one nest per year. The American alligator builds elaborate mounds of mud and decaying plant materials that incubate the eggs. After nesting, the female remains nearby, protecting the eggs from predators. At the end of the 2-month incubation, the hatchlings call to the mother for assistance in getting out of the nest. The mother picks up the hatchlings and carries them to the water. She provides protection for the hatchlings for up to a year.
see also Endangered Species Act; Life in Water.
Andrew P. Diller
Alderton, David. Turtles and Tortoises of the World. New York: Facts on File, 1988.
Grenard, Steve. Handbook of Alligators and Crocodiles. Malabar, FL: Krieger Publishing, 1991.
Heatwole, Harold. Sea Snakes, 2nd ed. Malabar, FL: Krieger Publishing, 1999.
Lutz, Peter L., and John A. Musick, eds. The Biology of Sea Turtles. Boca Raton, FL:CRC Press, 1997.
Mattison, Christopher. Lizards of the World. New York: Facts on File, 1989.
The Kemp's ridley is the most endangered sea turtle because of overharvesting of eggs at its one nesting beach in Tamaulipas, Mexico. To establish a second nesting location, biologists are relocating eggs to South Padre Island, Texas.
During incubation, the sex of hatchlings is determined by temperature. Cooler temperatures within the nest produce male hatchlings, whereas warmer temperatures produce females.
A majority of female hatchlings are produced at most nesting beaches because of their tropical locations. On emerging from the nest, hatchlings are raised in tanks until they are large enough to avoid most predators, and then are released. After maturing in the wild, these female turtles return to Texas to nest.
* See "Life in Water" for a photograph of a sea snake.
reptile, name for the dry-skinned, usually scaly, cold-blooded vertebrates (see Chordata) of the order Reptilia. Reptiles are found in a variety of habitats throughout the warm and temperate regions (except on some islands), with the greatest variety in the tropics. Reptiles differ from other terrestrial vertebrates (birds and mammals) in that they are cold-blooded, that is, they lack an effective system for regulating their body temperature, which tends to approach that of the environment. For this reason reptiles are not found in the coldest regions of the world, and they hibernate in cool winter areas.
They range in size from 2-in.-long (5-cm) lizards to 30-ft-long (9-m) snakes. They typically have low-slung bodies with long tails, supported by four short legs that project outward from the sides of the body; however, all snakes are limbless. Although reptiles are fundamentally a terrestrial group, some are adapted to living in water. All breathe air by means of lungs and have thick, waterproof skins designed for retaining body moisture. Unlike amphibians, they do not possess gills or breathe water at any stage of their development, and nearly all lay their eggs or bear their young on land.
The reptilian egg has a porous shell and a system of membranes designed to protect the embryo from desiccation. It also has a large quantity of yolk for nourishment. This type of egg is typical of terrestrial vertebrates, and is very different from the simple, unprotected eggs of fishes and amphibians, which are laid in the water. Fertilization is internal in reptiles, and males have copulatory organs. Females of most species lay eggs, but in some the egg is incubated and hatched internally. In a very few there is true live birth, with the young nourished by a primitive placenta instead of an egg yolk.
Types of Reptiles
Living reptiles are classified in four orders. The turtles, order Chelonia, have a protective bony shell, usually covered with horny plates. They are mostly aquatic in habits although some (see tortoise) are adapted to land. They are the oldest living reptiles, having existed nearly unchanged since the Triassic period. Members of the order Crocodilia, which includes alligators, caimans, crocodiles, and gavials, are large, carnivorous reptiles of tropical and subtropical swamps and rivers. They constitute the only remaining order of the great reptilian subclass Archosauria, or ruling reptiles, which includes the extinct dinosaurs. The order Squamata includes the lizards (suborder Sauria) and snakes (suborder Serpentes). Nearly all members of this large and successful modern order are terrestrial. The order Rhynchocephalia has a single living member, the tuatara, a lizardlike reptile of New Zealand.
Reptiles first evolved from amphibians about 250 million years ago in the Carboniferous period and were dominant in the world's fauna during the Mesozoic era, sometimes called the Age of Reptiles. The dinosaurs, the marine Ichthyosaurus and Plesiosaurus, and the flying pterosaurs reached the peak of their development and distribution in the later part of this era (late Cretaceous period). Mammallike reptiles appeared very early in reptilian history and by the Triassic period had given rise to mammals. Bird ancestors arose from precursors of the dinosaurs; the first known birds lived in the Jurassic. The only reptiles that survived into the Cenozoic era belonged to the presently living orders. The approximately 6,000 living reptile species represent a very small fraction of this once vast class.
See R. Conant, Field Guide to Reptiles and Amphibians (1958); A. Bellairs, The Life of Reptiles (2 vol., 1970); K. P. Schmidt and R. F. Inger, Living Reptiles of the World (1957, repr. 1972); H. M. Smith and E. Brodie, Reptiles of North America (1982); H. M. Smith and H. S. Zim, Reptiles and Amphibians (1987).
A reptile is a cold-blooded vertebrate (animal with a backbone) with dry, scaly skin that lays sealed eggs. Dinosaurs once dominated this class, which is now represented by turtles, snakes, lizards, crocodiles, and alligators. Reptiles have endoskeletons (internal skeletons) and are mainly carnivorous (meat-eaters). They do not move especially fast and their name "reptile" means "creeper."
All reptiles have a tough skin made of waterproof scales that resists drying out. In this and many other ways, they are entirely different from amphibians from whom they evolved and with whom they are often confused.
Reptiles do not go through any metamorphosis (a series of distinct changes in which an organism passes as it develops from an egg to an adult) and breathe air through lungs their entire lives. They do, however, lay eggs like amphibians—but even these eggs are substantially different. Reptiles always live on land, although they are ectothermic or cold-blooded like amphibians. This does not mean that they are cold, but rather that their body temperature reflects that of their environment. Unlike warm-blooded vertebrates that use food to keep their internal body temperature constant despite the outside temperature, reptiles have a body temperature that fluctuates with their environment. This can be an advantage since reptiles require less energy and therefore less food, and as a result are able to survive in a habitat where there is little food. As vertebrates, reptiles have an internal skeleton. Turtles are not an exception to this, but they may appear to be. Although they have hard, bony shells above and below their bodies and into which they can withdraw their head and legs for protection, they still have an endoskeleton.
All reptiles reproduce sexually through the joining of male sperm and female eggs. It was the ability of reptiles to produce a sealed egg, or one with a strong membrane, that allowed them to evolve beyond amphibians and remain on land, freeing them from needing water to reproduce or hatch in. The reptilian egg is fertilized internally or within the body of the female, and is called an amniotic egg because it contains the necessary water and food for the embryo to develop and grow before hatching. (This is different than amphibian eggs, which are fertilized than released by the female into the water where they will hatch.) These leathery eggs are usually buried in sand or soil where they hatch, releasing live, miniature versions of their parents.
Although reptiles range in size from a tiny lizard, like the gecko, to a 20-foot (6.1-meter) alligator, they all have skin that is dry to the touch. Reptile skin is made of scales created by a waterproof substance called keratin. The outer layers of this skin are regularly shed by most reptiles as they grow in size.
Reptiles can vary from the slow-moving turtle whose body is encased in a shell and has a jaw like a bird's beak, to the fiercely aggressive crocodile. The tiny gecko lizard can walk on ceilings, where as the basilisk lizard can run on two legs using its tail for balance and can even run across water. A chameleon can change its color to blend in perfectly with its surroundings and catch its prey with its tongue. Snakes can unhinge their jaws and swallow whole a prey that is larger than their own head. They also sniff the air with their tongue, which carries the chemical particles it catches in the air to a specialized detector in the roof of their mouth. Some snakes have hollow teeth to inject venom or poison into the prey they bite, while others coil around their prey and squeeze until it suffocates.
A reptile is an organism in the kingdom Animalia and the class Reptilia. (Kingdoms are the main divisions into which scientists classify all living things on Earth; kingdoms are further subdivided into phylums [or divisions], classes, and orders.) The reptiles include more than 6,000 species grouped into four orders: the turtles (Chelonia), the snakes and lizards (Squamata), the crocodiles and alligators (Crocodilia), and the tuataras (Sphenodonta), large lizardlike animals found only on islands off the coast of New Zealand.
A number of other reptilian orders are now extinct. These include some of the largest animals ever to occupy the planet. Examples include the fishlike ichthyosaurs, the long-necked plesiosaurs, and the huge flying and gliding pterosaurs. The most famous of the extinct reptilian orders were the dinosaurs, that included immense, ferocious predators such as Tyrannosaurus rex and enormously large herbivores (plant-eaters) such as Apatosaurus.
The first reptiles known in the fossil record lived about 340 million years ago. The last representatives of the dinosaurs became extinct about 65 million years ago, after being the dominant large animals of Earth for more than 250 million years.
Reptiles are extremely diverse in their form and function. They characteristically have four legs, although some groups such as the snakes have become legless. They usually have a tail and a body covered with protective scales or plates. These scales are dry, not slimy as some people believe, and have developed from the animal's epidermis (skin).
Reptiles are ectotherms (cold-blooded). This means they warm their bodies by absorbing heat from their environment. Thus, a reptile's body temperature fluctuates with changes in the surrounding temperature. The body temperature of snakes, for example, cools in cold weather and warms up in hot weather. Not surprisingly, external temperature plays a major role in determining the activities of reptiles: they are active when it's warm outside and slow down when its cold.
Reptiles reproduce by internal fertilization. Their eggs (sex cells) have a series of membranes (layers) around the embryo (earliest life-form) that allow the exchange of gases and waste. These eggs, known as amniotic eggs, were an important evolutionary adaptation for conserving moisture and allowed the reptiles to adapt to living on land. Most reptiles are oviparous, meaning they lay eggs in a warm place and the eggs are kept warm until they hatch. Some species are ovoviviparous, meaning the eggs are retained within the female throughout their development so that live young reptiles are born.
Some species of reptiles are dangerous to humans and to animals, including the predatory crocodiles and alligators. Some species of snakes are venomous (poisonous) and may bite people or livestock when threatened. Many species of reptiles are economically important and are hunted as food, for their eggs, or for their skin, which can be manufactured into an attractive leather. Many species are kept as interesting pets or in zoos.
Unfortunately, some people have an unreasonable fear of reptiles that has led to many of them being killed. Additionally many species are endangered because their natural habitats have been taken over for agriculture, forestry, or residential development.
[See also Dinosaurs; Snakes ]
The class Reptilia is composed of about 5,100 species, organized in three very closely interrelated groups: the lizards (order Lacertilia), composed of about 3,165 species; the amphisbaenians (order Amphisbaenia), which consist of about 135 species; and the snakes (order Serpentes), which contain about 1,800 species. According to most experts, lizards appeared in the fossil record in the middle Jurassic, about 165 million years ago (although some authorities place the earliest known fossil lizards in the late Permian, about 250 million years ago). Fossil amphisbaenians have been recorded as early as the late Cretaceous, over 65 million years ago. Snakes are known from the early Cretaceous, about 135 million years ago.
Research has shown that the separation of lizards and snakes into distinct orders is an unnatural artifact of outmoded scientific methodologies and does not reflect the evolutionary history of these animals; significant changes in the classification of the class Reptilia can be expected in the future. Members of the class Reptilia all share numerous characteristics (called synapomorphies) of physiology, behavior, and functional morphology that readily set them apart from amphibians, mammals, turtles, tuataras, and birds. One of the most striking of these is the presence in males of well-developed, paired copulatory organs called hemipenes (all other classes of terrestrial vertebrates have a single penis).
Two of the orders exhibit a combination of characteristics that generally permit ready identification: Lizards generally possess four limbs, ear openings, and eyelids, and snakes lack functional limbs, ear openings, and eyelids. Amphisbaenians differ substantially from lizards and snakes in many ways, most notably by their very short tails, distinctly annulated (ringed) bodies, and the reduction of the right lung (instead of the left lung, as in snakes and limbless lizards).
From small lizards such as geckos, with a snout-vent length sometimes as small as 1.5 centimeters (.59 inches), to the reticulated python of southeastern Asia that reaches 10 meters (32.8 feet) in total length, reptiles display an amazing diversity of size, shape, color, and pattern. Their beauty and comparative ease to maintain as pets has created an entirely new venture, distinct and separate from the science of herpetology, known as herpetoculture. Herpetoculturists worldwide are devoted to the husbandry of many reptiles (mostly snakes), breeding, trading, and selling captive reptiles for fun and profit.
see also Amphibian; Crocodilians; Tuatara; Turtle;
Joseph T. Collins
Gans, Carl, ed. Biology of the Reptilia. Chicago: University of Chicago Press, 1992.
Halliday, Tim R., and Kraig Adler. The Encyclopedia of Reptiles and Amphibians. New York: Facts on File, 1986.
Pough, F. Harvey, R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.
rep·tile / ˈreptəl; ˈrepˌtīl/ • n. 1. a cold-blooded vertebrate animal of a class (Reptilia) that includes snakes, lizards, crocodiles, turtles, and tortoises. They are distinguished by having a dry scaly skin, and typically laying soft-shelled eggs on land. 2. inf. a person regarded with loathing and contempt. • adj. belonging to a reptile or to the class of reptiles. DERIVATIVES: rep·til·i·an / repˈtilēən; -ˈtilyən/ adj. & n. ORIGIN: late Middle English: from late Latin, neuter of reptilis, from Latin rept- ‘crawled,’ from the verb repere.
The class includes the modern crocodiles, lizards and snakes (see Squamata), and tortoises and turtles, as well as many extinct forms, such as the dinosaurs and Pterosauria.