reptile

1. Any of various cold-blooded, usually egg-laying vertebrates of the class Reptilia, such as a snake, lizard, crocodile, turtle, or dinosaur, having an external covering of scales or horny plates and breathing by means of lungs.
2. A person regarded as despicable or treacherous.

[Middle English reptil, from Old French reptile, from Late Latin rēptile, from neuter of Latin rēptilis, creeping, from rēptus, past participle of rēpere, to creep.]

The reptiles

The difference between amphibians and reptiles is that reptiles exhibit a suite of characteristics understandable as adaptations to life on land at increasing distance from water. Although many species of amphibians live on land in adulthood, most have an aquatic larval stage, and few can exist for long without moisture even during their terrestrial stages of life. Amphibians are tied to water—most species are not found more than a few meters from water or from moist soil, humus, or vegetation. Reptiles of many species are relatively liberated from water and can inhabit both mesic (moist) and xeric (dry) environments. Reptiles need water for various physiological processes, as do all living things, but some reptiles can obtain the water they need from the foods they eat and through conservative metabolic processes without drinking or by drinking only infrequently. Understanding the nature of reptiles requires focus on their techniques for maintaining favorable water balance in habitats where water may not be readily available and where moist microniches may be uncommon.

Characteristics

Most reptiles have horny skin, almost always cornified as scales or larger structures called scutes or plates. Such integuments resist osmotic movement of water from body compartments or tissues into the surrounding air or soil, thus minimizing desiccation. There are times in the lives of snakes and lizards when their skin becomes permeable to water, as when the animals are preparing to shed their old skin. During such times they seek out favorable hiding places that protect them not only from predators but also from water loss. The combination of integumentary impermeability (most of the time) and innate preferences for favorable microclimates during vulnerable periods allows reptiles to retain body water rather than to lose it to arid surroundings. Some reptiles are known to drink water that condenses on their scales when they reside in cool burrows.

Added to the mechanisms for retaining body water is an excretory system that is considerably advanced over those in fishes and many amphibians. The kidneys are integral components of the circulatory system. They allow constant, efficient filtration of blood. Most aquatic organisms excrete nitrogenous waste as ammonia. Ammonia readily diffuses across skin or gills, provided plenty of water is present, but is not efficiently excreted by the kidneys. Ammonia is highly toxic, and animals cannot survive if this substance accumulates in their bodies. Terrestrial organisms excrete nitrogenous waste in the form of urea or uric acid, which are less toxic and which require less water than does excretion of ammonia. Urea is the main nitrogenous waste in terrestrial amphibians, whereas uric acid (which requires very little water) is the main nitrogenous effluent in reptiles. Finally, some desert-dwelling reptiles have a remarkable ability to tolerate high plasma urea concentrations during drought. This characteristic allows the animals to minimize water loss that would be coincident with excretion. Rather than being excreted, nitrogenous waste is simply retained as urea, and water is conserved. When a rainfall finally occurs, reptiles (e.g., the desert tortoise Gopherus agassizii) drink copiously, eliminate wastes stored in the bladder, and begin filtering urea from the plasma. Within days their systems return to normal, and the tortoises store a large volume of freshwater in their bladders to deal with the next drought.

Feeding

Feeding in a water medium among vertebrates can take several forms ranging from detritus feeding (ingestion of decaying organic matter on the substrate) to neuston feeding (ingestion of tiny organisms residing in the surface film). Probably the most common mechanism of obtaining food is suction feeding, whereby the predator creates a current by sucking water into the expanded buccal cavity and out through gills, causing prey to be captured in the mouth. Most fish rely on suction feeding, and this mechanism contributes to the effectiveness of detritivores, neustonivores, and aquatic predators. As a consequence, most fish have relatively weak mouths and low bite strength. There are exceptions, such as sharks, but the general rule is that fish depend on suction more than on biting, a circumstance that works effectively because of the liquid nature of the water medium and the associated friction arising between the medium and objects suspended in it. Aquatic amphibians also use suction feeding, although some species have lingual and jaw prehension, particularly during terrestrial stages. The transition to land dwelling among most reptiles has necessitated a revolution in oral structures and kinematics to cope with the less dense medium of air. Because suction feeding does not work effectively in air, jaw prehension with consequent increases in bite strength has been emphasized in the evolution of most reptiles. Jaw prehension involves increased number and volume of the jaw-suspending muscles and increased surface area of muscle origins. Associated with this development was the appearance of temporal openings in the dermal bone surrounding the brain, because these openings allowed some of the jaw-suspending muscles to escape from the constraints of the dermal-chondral fossae and to attach at origin sites on the lateral and dorsal surfaces of the skull.

Skulls

The number and position of temporal openings have been used to classify reptiles into taxonomic groups, and the highlights of this classification system are reviewed here. Reptile skulls lacking temporal vacuities are said to be anapsid (without openings). This group includes the fossil order Cotylosauria, also called stem reptiles because of their ancestral position to all higher reptiles and hence to birds and mammals. The turtles, order Testudines, also are anapsid. Synapsid skulls have a single temporal opening on each side. The opening is positioned relatively low along the lateral surface of the skull, within the squamosal and postorbital bones. All synapsid reptiles (orders Pelycosauria, Therapsida, and Mesosauria) are extinct, but they are of great interest because of their ancestral position relative to the mammals. The parapsid condition also has a single vacuity on each side, but it is located rather high on the dorsolateral surface of the skull, within the supratemporal and postfrontal bones. Extinct, fishlike members of the order Ichthyosauria constitute the single order of parapsid reptiles, but these animals were probably closely related to euryapsid reptiles that had a single vacuity in much the same position except that it also invaded the dorsal aspects of the squamosal and postorbital bones. Orders of euryapsids were Placodontia and Sauropterygia, both marine and extinct, in the Triassic and Cretaceous periods, respectively. The diapsid condition is characterized by two temporal vacuities on each side of the skull. Major orders include Thecodontia (small crocodilian-like reptiles ancestral to birds and to all of the archosaurs), Crocodylia, Saurischia (dinosaurs with ordinary reptile-type hips), Ornithischia (dinosaurs with bird-type hips), Pterosauria (flying reptiles), Squamata (lizards, snakes, and several extinct groups), Eosuchia (extinct transitional forms that led to squamates), and Rhynchocephalia (mostly extinct, lizard-like diapsids with one surviving lineage, the tuatara [Sphenodon punctatus] on islands associated with New Zealand; S. punctatus may be a superspecies containing two or more separable species).

The order Testudines, which contains all living and extinct turtles, has traditionally been grouped with the primitive cotylosaurs because of common possession of the anapsid condition. Most herpetologists and paleontologists have agreed on this matter for many years. Molecular geneticists, however, have found evidence that turtles may actually be closely related to diapsid reptiles. This finding suggests that the anapsid condition of turtles may be secondary. That is, turtles may have evolved from ancestors that possessed two temporal vacuities on each side of their skulls, but in the course of evolution, turtles lost these openings. Essentially the same idea was proposed early in the twentieth century, not on the basis of genetic evidence but on the basis of a paleontological scenario involving a series of extinct but turtle-like diapsid fossils. Few at that time could accept the possibility that temporal vacuities once evolved would ever be abandoned, so this notion was dismissed and has resided in scientific limbo ever since. It has been revived on the strength of genetic data, and this much derided "preposterous idea" may become accepted.

It appears as if there is a contradiction associated with the anapsid status of turtles. Whereas some species are suction feeders with relatively weak mouths, others, such as snapping turtles, have profound bite strength. How is this strength produced, given the absence of temporal openings that would allow large jaw-suspending muscles to anchor (originate) on the dorsal surface of the skull? It turns out that many species of turtles have an analogous adaptation in which sections of dermal bone on the side and back of the skull have become emarginated or notched. Temporal openings are holes surrounded by bone. Emarginations are missing sections of the edges of the flat bones that form the ventral or pleural borders of the skull. With substantial sections of these bones missing, jawsuspending muscles have the same opportunity to escape from the dermal-chondral fossae as is made possible by vacuities. Although turtles are, strictly speaking, anapsid, some have taken an alternative pathway that leads to the bite strength necessary for effective jaw prehension of substantial prey or for tearing vegetation. If the anapsid condition is secondary, turtles have substituted an analogous trait that accomplished much the same biophysical effect as did the former temporal vacuities.

Reproduction

The earliest reptile fossils known are from the Upper Carboniferous period, approximately 270 million years ago, but by this time several of the reptilian orders were already in evidence, including both anapsid cotylosaurs and synapsid pelycosaurs. This finding implies that reptile evolution began much earlier. Another implication is that temporal vacuities (empty spaces) and emarginations (notches), although widely distributed in reptiles, are not defining characteristics of this class of vertebrates, because several groups do not have them. The earliest defining characteristics may never be known unless some very early fossils in good condition are found. It is likely that a desiccation-resistant integument was present. Another area on which to focus is the egg and the reproductive process. The egg is macrolecithal (contains much yolk) and is surrounded by a hard shell in turtles, crocodilians, and geckos and a soft or parchment-like shell in the other squamates. In either case, a shelled egg requires that fertilization occur before shell formation. This means that fertilization must take place within the female's body (i.e., in her oviducts) rather than externally as is typical of fishes and amphibians. Consequently, most male reptiles possess copulatory organs that deposit sperm into the cloaca of the female. From the cloaca the sperm cells migrate up the oviduct guided by chemical stimuli. Male turtles and crocodilians have a single penis homologous to the penis of mammals. This organ develops during embryogenesis from the medial aspect of the embryonic cloaca. Male lizards and snakes have paired hemipenes, which develop during embryogenesis from the right and left lateral aspects of the embryonic cloaca. Some male snakes have bifurcated hemipenes, so the males appear to have four copulatory organs. Thus internal fertilization is the rule among extant reptiles. Even tuatara, the males of which lack copulatory organs, transfer sperm in the manner of most birds with a so-called cloacal kiss involving apposition of male and female cloacae and then forceful expulsion of seminal fluid directly into the female's cloaca. Internal fertilization is necessary because of shell formation around eggs. Many reptiles live far from standing or running water, thus external fertilization in the manner of most fishes or amphibians would be associated with risk of desiccating both sperm and eggs.

The oviducts of some female reptiles are capable of storing sperm in viable condition for months or even years. In some turtles and snakes, fertilization can occur three years after insemination. Theoretically, a female need not mate each year, but she might nevertheless produce young each year using sperm stored from an earlier copulation. Although this interesting possibility has been known from observation of captive reptiles for approximately five decades, we still do not know whether or how often female reptiles use it under natural conditions. Another curiosity of reptile reproduction is that the females of some species of lizards and snakes are capable of reproducing parthenogenetically, even though reproduction in these species normally occurs sexually. (These species should not be confused with others that only reproduce parthenogenetically. This is not a widespread mode of reproduction in reptiles, but it is known to occur in several species of lizards and at least one snake.) Facultative parthenogenesis has only recently been discovered among captive reptiles, and there is as yet no information on whether it occurs in nature.

Macrolecithal eggs allow embryos to complete development within the egg or within the mother in the case of viviparity, such that the neonate is essentially a miniature version of its parents rather than a larva that must complete development during an initial period of posthatching life, as is common among amphibians. The reptilian embryo lies at the top of the large supply of yolk, and cell division does not involve the yolk, which becomes an extra embryonic source of nourishment for the growing embryo. A disk called the vitelline plexus surrounds the embryo and is the source of the three membranes (chorion, amnion, and allantois) that form a soft "shell" within the outer shell of the reptilian egg. Together these structures defend the water balance of the developing embryo and store waste products. Although reptile eggs absorb water from the substrate in which they are deposited, these eggs do not have to be immersed in water as is required for the eggs of most amphibians. Immersion of most reptile eggs results in suffocation of the embryos. Female reptiles deposit their eggs in carefully selected terrestrial sites that provide adequate soil moisture and protect the eggs from extremes of temperature.

Some species have another strategy for protecting embryos from abiotic and biotic exigencies. These reptiles retain the embryos and incubate them within the maternal body. The mother's thermoregulatory and osmoregulatory behaviors contribute to the embryos' welfare and to the mother's welfare. The mother's predator-avoidance behaviors can enhance the fitness of embryos exposed to greater predation elsewhere. In view of these potential advantages, which in some habitats might be considerable, it is not surprising that live-bearing has evolved many times in reptiles, although it is quite rare in amphibians. All crocodilians, turtles, and tuatara are egg layers. At least 19% of lizard species and 20% of snakes are live-bearers. Cladistic studies have shown that viviparity has evolved independently many times within squamates, in at least 45 lineages of lizards and 35 lineages of snakes. It also appears that viviparity is an irreversible trait and that once viviparity evolves, oviparous descendants rarely occur. The term embryo retention is used for species in which females retain embryos until very near the completion of embryogenesis when shells are added. The eggs are deposited and then hatch within 72–96 hours. Examples include the North American smooth green snake Liochlorophis vernalis, and the European sand lizard Lacerta agilis. Most important to understand is that the embryos are lecithinotrophic (nourishment of the embryos comes entirely from the yolk) with no additional postovulatory contribution from the mother. The mother, however, may play a role in gas exchange of the embryos. This process can involve proliferation of maternal capillaries in the vicinity of the embryos, a form of rudimentary placentation. Some species that give birth to live young also have lecithinotrophic embryos that undergo rudimentary placentation. Some embryo-retaining species eventually add a shell to their eggs and oviposit them within a few days of hatching. Others never add a shell, and the young are simply born alive, although they need to extricate themselves from the extraembryonic membranes that surround them. Many herpetologists prefer to abandon the term ovoviviparous because this word connotes that shelled eggs hatch in the maternal oviduct. No species is known in which this occurs. Accordingly, the term viviparous is used for all live-bearers, and herpetologists recognize that considerable variation exists in the degree to which viviparous embryos are matrotrophic (supported by maternal resources through a placenta).

Although females of oviparous species deposit their eggs in sheltered positions, the vagaries of climate can result in relative cooling or heating of oviposition sites with associated changes in moisture. This realization has led to considerable research on the effects of these abiotic factors on embryonic development. It is now known that within the range of 68–90°F (20–32°C), incubation time can vary as much as fivefold, and that neonatal viability is inversely related to incubation time. Hatchlings from rapidly developing embryos at high temperatures perform poorly on tests of speed and endurance relative to hatchlings from slower-developing embryos at lower temperatures. The slower-developing embryos typically give rise to larger hatchlings than do their rapidly developing counterparts. In the context of this work, it was found that the sex ratio of hatchling turtles varied depending on incubation temperature. In several species of tortoise (Gopherus and Testudo), for example, almost all embryos became males at low incubation temperatures (77–86°F [25–30°C]), and most became females between 88°F and 93°F (31–34°C). Temperature-dependent sex determination (TSD) is known to be widespread, occurring in 12 families of turtles, all crocodilians, the tuatara, and in at least three families of lizards. However, the effect of temperature differs in the various groups. Most turtles exhibit the pattern described, whereas most crocodilians and lizards exhibit the opposite pattern, females being produced at low incubation temperatures and males at higher ones. In a few crocodilians, turtles, and lizards females are produced at high and low incubation temperatures and males at intermediate temperatures. It is possible that some viviparous species experience TSD, in which case the thermoregulatory behavior of the mother would determine the sex of the embryos, but this phenomenon has not been observed.

The effect of the discovery of TSD has been enormous. Almost all developmental biologists previously believed that sex in higher vertebrates was genetically determined. This phenomenon has important implications for the management of threatened or endangered populations, especially if the program contains a captive propagation component. Unless care is taken to incubate eggs at a variety of temperatures, the program could end up with a strongly biased sex ratio. Reflection on the effects of global warming on reptiles exhibiting TSD generates the worry that extinction could be brought about from widely skewed sex ratios.

Diversity of reptiles

Reptiles range in body form from crocodilians to squamates, tuatara, and turtles. This diversity borders on trivial, however, in comparison with the range of forms and lifestyles that existed during the Jurassic and Cretaceous periods. This point can be further appreciated by considering locomotion among lizards with well-developed legs. Although some species are capable of quick movement, the gait of all lizards is basically the same as that of salamanders. The legs extend from the sides and must support the body through right angles, greatly limiting body mass and speed. Within the context of these constraints, lizards do quite well, but their locomotion remains relatively primitive. Truly advanced locomotion, with the legs directly under the body, occurs among mammals, but this pattern of limb suspension evolved in dinosaurs and was clearly a part of their long period of success. All extant reptiles are ectotherms, deriving their body heat from radiation, conduction, or convection, whereas mammals and birds are endotherms, producing body heat by energy-consuming metabolic activity. Thus we see the primitive condition in the reptiles and the advanced condition in the birds and mammals. There is now good reason to believe that at least some dinosaurs were endotherms. Accordingly, it is important to keep in mind that the diversity of extant reptiles is but a fraction of the diversity exhibited by this class of vertebrates during earlier phases of its natural history.

Locomotion

The basic pattern of the tetrapod limbs of amphibians is preserved in reptiles: a single proximal bone is followed distally by paired bones. In the fore limb is the humerus followed by the radius and ulna. In the hind limb is the femur followed by the tibia and fibula. The wrist and hand are formed from the carpal and metacarpal bones, and the ankle and foot are formed from the tarsals and metatarsals, five or fewer digits bearing horny claws distal to both wrist and ankle. Reptile orders show enormous variation in the precise form and arrangement of these basic elements and in their behavioral deployment. In squamates these elements are abandoned in favor of serpentine locomotion, which requires an elongate body and therefore an increased number of vertebrae, more than 400 in some snakes. Serpentine locomotion depends on friction between the animal and the substrate, which in some animals is accomplished by pressing the posterior edges of the belly scales against stationary objects so that Newton's third law (for every action there is an equal and opposite reaction) can operate. Some lizards have lost their limbs and use serpentine movement. Others with perfectly fine legs will, in bunch grass habitats, fold the limbs against the body and exhibit facultative serpentine movement, presumably because this type of movement produces faster escape behavior than does ordinary running in tangled vegetation. The twisting and bending of the trunk required in serpentine movement enhance the danger of vertebral dislocation. This selective pressure has been answered by the development of an extra pair of contact points between adjacent vertebrae in snakes, bringing the total number of articular points to five per vertebra. The result is that each vertebra is essentially locked to the next and resists dislocating forces arising from roll, pitch, and yaw.

Brain

The brain and spinal cord exhibit several advanced characteristics in reptiles relative to amphibians, including larger size and greater definition of structural divisions and greater development of the cerebral cortex. Neural connections between the olfactory bulbs, the corpus striatum, and several other subcortical structures have become clearly established in reptiles, and these connections have been conserved in subsequent evolution such that they are present in mammals, including humans. This set of connections is sometimes referred to as the "reptilian brain" or "R-complex" and is thought to represent a neural circuit necessary for the mediation of basic functions such as predation and mating as well as the affective concomitants associated with social behaviors ranging from cooperation to aggression. In the study of mammals, we speak of the regulation of emotion by components of the reptilian brain. Herpetologists are generally reluctant to speak of emotion in their animals, but they have no difficulty recognizing the existence of the neural circuit in question and in understanding that it contributes to social and reproductive activities. Whether this contribution is limited to the organization of motor patterns or whether emotion also is involved remains an open question.

Eyes

Sensory structures of reptiles exhibit variations in size and complexity that are roughly correlated with ecological variation and phylogeny. For example, lizards considered to be primitive, such as those of the family Chamaeleonidae, are primarily visually guided in the context of predation as well as in the contexts of social and reproductive behavior. This reliance on vision is reflected in the wonderful mobility of the eyes, the size of the optic lobes, and in the brilliant color patterns in the family. The phenomenon of "excited coloration" (color changes reflecting emotional or motivational states) involves socially important signals that can only be appreciated with vision. More advanced lizards, such as those in the family Varanidae, place greater emphasis on their nasal and vomeronasal chemosensory systems. Associated with this characteristic is a shift in the morphology and deployment of the tongue, which in varanids is used mainly to pick up nonvolatile molecules and to convey them to the vomeronasal organs. There is an associated shift from insectivory to carnivory. In snakes, which may be derived from a varanid-like ancestor, these shifts have been carried to an even greater extreme.

Ears

Audition presents an interesting problem in reptiles. Snakes and some lizards have no external ear, although the middle and inner ears are present. In species with a distinct external auditory meatus, there is little doubt about the existence of a sense of hearing, although it is generally thought that only sounds of low frequency are detected. In species lacking an external ear, seismic sounds are probably conducted by the appendicular and cranial skeletons to the inner ear. It has been suggested, however, that the lungs might respond to airborne sounds and transmit them to the inner ear via the pharynx and eustachian tube. Although no reptiles are known for having beautiful voices, many generate sounds. For example, male alligators bellow, and this sound undoubtedly serves social functions. Many snakes hiss, some growl, and a fair number issue sounds with their tails either with a rattle or by lashing the tail against the substrate. Such sounds are generally aimed at predators or other heterospecific intruders, and herpetologists have believed that the issuing organism was deaf to its own sound, unless the sound had a seismic component. Perhaps this view can be altered if the concept of pneumatic reception of airborne sound is corroborated.

Other senses

Cutaneous sense organs are common, including those sensitive to pain, temperature, pressure, and stretching of the skin. Although pain and temperature receptors are best known on the heads of reptiles, these receptors are not confined there. The mechanoreceptors that detect touch, pressure, and stretch are present over the body, especially within the hinges of scales. Receptors that detect infrared radiation (heat) are also of dermal and epidermal origin. In boas and pythons, these receptors are associated with the lips. In pitvipers such as rattlesnakes, a membrane containing heat receptors is stretched across the inside of each pit approximately 0.04–0.08 in (1–2 mm) below the external meatus. The geometry of the bilateral pits is such that their receptive fields overlap, allowing stereoscopic infrared detection. The nerves of the pits project to the same brain areas as do the eyes, giving rise to images containing elements from the visible part of the spectrum as well as the infrared part. When a pitviper is in the process of striking a mouse, the snake's mouth is wide open with fangs erect, so that the pits and eyes are oriented up rather than straight ahead toward the prey. It turns out that in the roof of the mouth near the fangs are additional infrared sensitive receptors that appear to take over guidance of the strike during these final moments.

Reptiles also possess proprioceptors associated with muscles, tendons, ligaments, and joints. Proprioceptors report the positions of body components to the brain, allowing the brain to orchestrate posture and movement. Another class of internal receptor contains taste buds, which are located in the lining of the mouth and on the tongue. In reptiles with slender, forked tongues specialized for conveying nonvolatile chemicals to the vomeronasal organs, lingual taste buds are generally absent, but taste buds may be present elsewhere in the mouth.

Teeth

With a few notable exceptions, the teeth of extant reptiles are unspecialized; that is, most teeth look alike, and the dentition is called homodont (Latin for "alike teeth"). The teeth may vary considerably in size along the length of the tooth-bearing bones, especially in snakes, because the teeth are deciduous and are replaced regularly. This type of dentition is called polyodont. Teeth are present on the bones of the upper and lower jaw and on other bones forming the roof of the mouth (palatine and pterygoid). If teeth are ankylosed (cemented by calcification) to the inside of jawbones, the dentition is pleurodont. This is the condition of all snakes and most lizards. If the teeth are ankylosed to a bony ridge along the jawbones, as in some lizards, the dentition is acrodont. Crocodilian teeth are situated in sockets, as are the teeth of mammals, and this dentition is called thecodont. The most spectacular type of tooth specialization in extant reptiles involves the fangs of venomous snakes. These fangs are hollow, elongated teeth on each side of the front of the upper jaw, although some species have solid, grooved fangs on each side of the rear of the upper jaw. In front-fanged snakes, venom is forcefully injected through the fangs and exits into the prey through slitlike openings on the lower anterior face of each fang. In rear-fanged snakes, venom runs under little pressure along the grooves and enters prey as the rear fangs successively embed themselves into prey during swallowing. Among the front-fanged species are those with folding fangs that are normally held parallel to the roof of the mouth and rotated down into position as needed. Other front-fanged snakes have less mobility associated with their fangs, which are therefore always in the biting position. The fangs typically are much longer in species with folding fangs than in species with fixed front fangs. With the exception of fangs, most teeth in extant reptiles are used to grip prey, although some lizards have specialized, blunt teeth that crush snail shells. Some extinct reptiles had far more specialized tooth patterns than do the surviving groups.

Venom

All reptiles possess salivary glands that lubricate food and begin the process of digestion. Saliva also cleans the teeth by digesting pieces of organic matter that might adhere to the teeth or be stuck between adjacent teeth. The venom that has evolved in snakes undoubtedly arose from salivary glands, and it has retained its original digestive function. Venom contains elements that immobilize and kill prey, and it facilitates digestion. It has been conclusively demonstrated in force-feeding experiments in which rattlesnakes fed envenomated mice completed the digestion process significantly quicker than did conspecifics fed identical euthanized mice that had not been envenomated. Similar studies have been completed with comparable results for a variety of species, including rear-fanged snakes. In some rear-fanged snakes, venom is apparently used only for digestion and not for subduing prey or for defense. In the Mexican beaded lizard (Heloderma horridum) and the Gila monster (H. suspectum), the only venomous lizards, venom is apparently used strictly for defense and not for acquisition or digestion of prey.

Energy

In some snakes and lizards, very long periods of time can occur between successive meals, and the reptiles exhibit an interesting form of physiological economy by down-regulating their digestive machinery. This process saves energy, because maintaining functional digestive tissue in the absence of food would require considerable caloric costs. Reptiles retain this down-regulated condition until the next meal has been secured, at which time the gut is up-regulated.

Exercise

Gas exchange occurs through lungs. Most snakes have only one lung (on the left). The heart has three chambers, two atria and one ventricle, except in crocodilians, in which a second ventricle is present, producing a four-chambered heart much like that of mammals. Even in reptiles with a three-chambered heart, a septum exists within the ventricle and minimizes mixing of oxygenated and nonoxygenated blood. Researchers have studied the physiological mechanisms associated with exhaustive locomotion and have found interesting parallels between reptiles and mammals in the rapidity of recovery from exhaustion. A major difference, however, is that mammals exhibit a so-called exercise effect (exercise-induced ability to mobilize greater levels of oxygen and, hence, to work harder than was possible before exercise), whereas no reptile has yet been shown to do this.

Conservation

New species of reptiles continue to be discovered. This is especially true of lizards. Hence the numbers that follow are approximations subject to change. We currently recognize 285 species of turtles, 23 crocodilians, two tuatara, 4,450 lizards, and 2,900 snakes. One of the authors of this chapter (H. M. S.) has named approximately 300 species in his career and is working on projects that will almost certainly add species to the list. In countries such as the United States, where numerous herpetologists have studied the fauna thoroughly, it is relatively unlikely that new species will be discovered. Nevertheless, herpetologists sometimes find reasons to justify the splitting of previously recognized species into two or more species. Third World countries present an entirely different situation because they possess few indigenous herpetologists, and some of these countries have only rarely been visited by herpetologists. Consequently, new species are quite likely to be found in these lands, especially those in the tropics and subtropics. It has been estimated that in most such countries, approximately 30% of the reptile fauna remains to be discovered. Thus much basic work remains to be done. At the same time, we must be mindful of the rate at which species are currently being lost to deforestation, habitat fragmentation, pollution, overharvesting, invasion of harmful exotic species, and other anthropogenic causes. We are now facing a situation in which we are losing species to extinction before they have been given proper scientific names. During the past decade, amphibian biologists have justifiably called attention to the worldwide decline of many salamanders and anurans. Without doubt this is a serious problem, but it has overshadowed the fact that reptiles have been suffering the same fate.

Many of the same factors responsible for amphibian declines have been insidiously working their decimating effects on reptiles. At the heart of the problem is the human population, now much more than six billion, and a drastically uneven distribution of resources. Many people living in areas of high reptile diversity are unable to eke out a living and are therefore tempted to exploit their native fauna, legally or illegally, and to engage in other economic activities that eventually have negative repercussions on the fauna. Hunting of reptiles occurs for local consumption, sale of hides or shells, sale of live animals to the pet trade, and sale of meat or other body parts as exotic food or medicines. China has almost extinguished its turtle fauna, for example, and has put catastrophic pressure on the turtle population in the rest of Southeast Asia. Chinese dealers also purchase several species of turtles during their active seasons in North America, particularly snapping turtles and softshells, for shipment to Asia. A team of biologists conducting a survey of tortoises in Madagascar found hundreds of dead animals, all with their livers removed. Local rumor revealed that these organs are made into an exotic pâté that is shipped to Asia. Although the mathematics of sustainable harvesting have been well worked out and can provide the basis for enlightened commercial practices and population management, the rate at which turtles have been harvested in China, Southeast Asia, Madagascar, and elsewhere is greatly exceeding the rate required for sustainable yields.

A similar situation developed in connection with hides of various reptiles, including crocodilians and several large lizards and snakes. In the case of crocodilians, management programs aimed at providing sustainable yields were developed in several countries, and these measures proved successful, so much so that the species involved recovered from endangered status. This experience indicates that the conservation strategy of management for sustainable yield can work if it is carefully implemented on the basis of good ecological and demographic data and if the harvest is carefully monitored. Enthusiastic participation of local people is an important element of the success of such programs as they have been carried out in Africa, Asia, and South America. It may not be too late to put these ideas into practice to save the turtle fauna of Asia. In the case of the crocodilians, declining populations quickly allowed several secondary events, such as explosive growth in populations of fish that were prey of crocodilians and reductions in populations of fish that depended on the deep holes made by crocodilians. An added benefit of sustainable yield programs was that these perturbations were reversed as the crocodilian populations were restored. It is probable that secondary effects of Asian turtle harvesting will make themselves known in the near future because turtle burrows are homes for a variety of other creatures. Eliminating turtles makes the ecosystem inhospitable for animals that depend on turtles. In short, enlightened management may be a tool for creating sustainable yield and for habitat restoration.

Resources

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

Bennett, A. F. "The Energetics of Reptilian Activity." In Biology of the Reptilia. Vol. 13, Physiology, edited by C. Gans and F. H. Pough. New York: Academic Press, 1982.

Carroll, R. L. "The Origin of Reptiles." In Origins of the Higher Groups of Tetrapods: Controversy and Consensus, edited by H. P. Schultze and L. Trueb. Ithaca, NY: Comstock, 1991.

Fitch, H. S. Reproductive Cycles in Lizards and Snakes. Lawrence, KS: University of Kansas Natural History Museum, 1970.

Garland, T., Jr. "Phylogenetic Analyses of Lizard Endurance Capacity in Relation to Body Size and Body Temperature." In Lizard Ecology: Historical and Experimental Perspectives, edited by L. T. Vitt and E. R. Pianka. Princeton, NJ: Princeton University Press, 1994.

Greenberg, N., and MacLean, P. D., eds. Behavior and Neurology of Lizards. Rockville, MD: National Institute of Mental Health, 1978.

Pieau, C. "Temperature and Sex Differentiation in Embryos of Two Chelonians, Emys orbicularis L. and Testudo graeca L." In Intersexuality in the Animal Kingdom, edited by R. Reinboth. New York: Springer-Verlag, 1975.

Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.

Zug, G. R., L. J. Vitt, and J. P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. New York: Academic Press, 2001.

de Cock Buning, T. "Thermal Sensitivity as a Specialization for Prey Capture and Feeding in Snakes." American Zoologist 23 (1983): 363–75.

Gans, C., and P. F. A. Maderson. "Sound-Producing Mechanisms in Recent Reptiles: A Review and Comment." American Zoologist 13 (1973): 1195–203.

Guillette, L. J., Jr., R. E. Jones, K. T. Fitzgerald, and H. M. Smith. "Evolution of Viviparity in the Lizard Genus Sceloporus." Herpetologica 36 (1980): 201–15.

Hedges, S. B., and L. L. Poling. "A Molecular Phylogeny of Reptiles." Science 283 (1999): 998–1001.

Packard, G. C., and M. J. Packard. "Evolution of the Cleidoic Egg among Reptilian Antecedents of Birds." American Zoologist 20 (1980): 351–62.

Schuett, G. W., P. J. Fernandez, W. F. Gergits, N. J. Casna, D. Chiszar, H. M. Smith, J. G. Mitton, S. P. Mackessy, R. A. Odum, and M. J. Demlong. "Production of Offspring in the Absence of Males: Evidence for Facultative Parthenogenesis in Bisexual Snakes." Herpetological Natural History 5 (1997): 1–10.

Schwenk, K. "The Evolution of Chemoreception in Squamate Reptiles: A Phylogenetic Approach." Brain Behavior and Evolution 41 (1993): 124–37.

Secor, S. M., and J. Diamond. "Adaptive Responses to Feeding in Burmese Pythons: Pay before Pumping." Journal of Experimental Biology 198 (1995): 1313–25.

[Article by: David Chiszar, PhD; Hobart M. Smith, PhD]

For more information on reptile, visit Britannica.com.

A class of scaly vertebrates that usually reproduce by laying eggs. Lizards, snakes, turtles, and alligators are reptiles. Reptiles are cold-blooded animals.

See Dinosaur, Lizard, Serpent, Snake, or Turtle.

"Reptilia" redirects here. For other uses, see Reptile (disambiguation).

Reptiles Fossil range: 320–0 Ma PreЄ Є O S D C P T J K Pg N Carboniferous – Recent
From left to right: Spectacled Caiman (Caiman crocodilus), Green Sea Turtle (Chelonia mydas,), Eastern Diamondback Rattlesnake (Crotalus adamanteus) and Tuatara (Sphenodon punctatus)
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata (unranked) Amniota Class: Reptilia Laurenti, 1768
Included groups
Crocodilia Sphenodontia Squamata Testudines For extinct orders, see text
Excluded groups
Aves Mammalia

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded amniotes that have skin covered in scales or scutes as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and lay amniote eggs, whose embryos are surrounded by the amnion membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized:

• Crocodilia (crocodiles, gavials, caimans, and alligators): 23 species
• Sphenodontia (tuatara from New Zealand): 2 species
• Squamata (lizards, snakes, and amphisbaenids ("worm-lizards"): approximately 7,900 species
• Testudines (turtles, tortoises, and terrapins): approximately 300 species

The majority of reptile species are oviparous (egg-laying) although certain species of squamates are capable of giving live birth. This is achieved, either through ovoviviparity (egg retention), or viviparity (offspring born without use of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, that grows to only 1.6 cm (0.6 in), to the saltwater crocodile that may reach 6 m in length and weigh over 1,000 kg. The science dealing with reptiles is called herpetology.

Contents 1 Classification 1.1 History of classification 1.2 Taxonomy 1.3 Phylogeny 2 Evolutionary history 2.1 Rise of the reptiles 2.2 Anapsids, synapsids and sauropsids 2.3 Permian reptiles 2.4 Mesozoic, the "Age of Reptiles" 2.5 Demise of the dinosaurs 3 Systems 3.1 Circulatory 3.2 Respiratory 3.2.1 Reptilian lungs 3.2.2 Turtles and tortoises 3.2.3 Palate 3.3 Skin 3.4 Excretory 3.5 Digestive systems 3.6 Nervous system 3.7 Vision 3.8 Reproductive 4 References 5 Further reading 6 See also 7 External links

Classification

History of classification

Reptiles are a paraphyletic group. If birds were included would it be monophyletic.

The reptiles were from the outset of classification grouped with the amphibians. Linnaeus working from species poor Sweden where the common adder and grass Snake are often found hunting in water, included all reptiles and amphibians in class "III - Amphibia" in his Systema Naturae.^[1] The terms "reptile" and "amphibian" were largely interchangeable, "reptiles" being preferred by the French.^[2] Josephus Nicolaus Laurenti were the first to formally use the term "Reptilia" for an expanded, though basically similar selection of reptiles and amphibians to that of Linnaeus.^[3] Not until the turn of the century did it become clear that reptiles and amphibians are in fact quite different animals, and Pierre André Latreille erected the class Batracia for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds and mammals.^[4]

The British anatomist Thomas Henry Huxley made Latreille's definition popular, and together with Richard Owen expanded Reptilia to include the various fossil “Antediluvian monster”, including the mammal-like Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into Mammals, Sauroids, and Ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of Sauropsida and Ichthyopsida for the two.^[5]

Around the end of the 19th century, the class reptilia had come to included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term. However, in recent years, many taxonomists^[who?] have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:

Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synapomorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptila are 'non-avian, non-mammalian amniotes'.^[6]

The terms "Sauropsida" ("Lizard Faces") and "Theropsida" ("Beast Faces") were taken up again in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on one hand (Sauropsida) and mammals and their extinct relatives (Theropsida) on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, the Protosauria ("First Lizards") which included some Paleozoic amphibians as well as early reptiles.^[7]

In 1956 D.M.S. Watson observed that the first two groups diverged very early in reptilian history, and so he divided Goodrich's Protosauria among them. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals respectively. Thus his Sauropsida included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata (lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropyterygians.^[8]

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology^[9]) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Those divisions were:

• Anapsida – no fenestrae
• Synapsida – one low fenestra (no longer considered true reptiles)
• Euryapsida – one high fenestra (now included within Diapsida)
• Diapsida – two fenestrae

All of the above but Synapsida fall under Sauropsida.

Taxonomy

Classification to order level, after Benton, 2004.^[10]

• Series Amniota
  • Class Synapsida
    • Order Pelycosauria*
    • Order Therapsida
      • Class Mammalia
  • Class Sauropsida
    • Subclass Anapsida
      • Order Testudines (turtles)
    • Subclass Diapsida
      • Order Araeoscelidia
      • Order Younginiformes
      • Infraclass Ichthyosauria
      • Infraclass Lepidosauromorpha
        • Superorder Sauropterygia
          • Order Placodontia
          • Order Nothosauroidea
          • Order Plesiosauria
        • Superorder Lepidosauria
          • Order Sphenodontia (tuatara)
          • Order Squamata (lizards & snakes)
      • Infraclass Archosauromorpha
        • Order Prolacertiformes
        • Division Archosauria
          • Subdivision Crurotarsi
            • Superorder Crocodylomorpha
              • Order Crocodilia
              • Order Phytosauria
              • Order Rauisuchia
              • Order Rynchosauria
          • Subdivision Avemetatarsalia
            • Infradivision Ornithodira
              • Order Pterosauria
              • Superorder Dinosauria
                • Order Saurischia
                  • Class Aves
                • Order Ornithischia

Phylogeny

The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by Laurin and Gauthier (1996), presented as part of the Tree of Life Web Project.^[11]

Amniota

Synapsida Reptilia unnamed Anapsida Mesosauridae unnamed Millerettidae unnamed Lanthanosuchidae unnamed Nyctiphruretia unnamed Pareiasauria Procolophonoidea ?Testudines (turtles, tortoises, and terrapins) Romeriida Captorhinidae unnamed Protorothyrididae* Diapsida Araeoscelidia unnamed Younginiformes Sauria ?Ichthyosauria ?Sauropterygia Lepidosauromorpha (lizards, snakes, tuatara, and their extinct relatives) Archosauromorpha (crocodiles, birds, and their extinct relatives)

Evolutionary history

Rise of the reptiles

The early reptile Hylonomus

Mesozoic scene showing typical reptilian megafauna, the dinosaurs Europasaurus holgeri and Iguanodon, the early bird Archaeopteryx perched on the forground treestump.

Megalania was a giant, carnivorous goanna that might have grown to as long as 7 metres, and weighed up to 1,940 kilograms (Molnar, 2004).

The origin of the reptiles lays about 320-310 million years back, in the steaming swamps of the late Carboniferous, when the first reptiles evolved from advanced reptilomorph labyrinthodonts.^[12] The oldest traces of reptiles is a series of footprints from the fossil strata of Nova Scotia, dated to 315 million years old.^[13] The tracks are attributed to Hylonomus, the oldest known reptile in the biological sense of the word.^[14] It was a small, lizard-like animal, about 20 to 30 cm (8-12 inche) long, with numerous sharp teeth indicating an insectivorous diet.^[15] Other examples include Westlothiana (for the moment considered to be more related to amphibians than amniotes)^{[citation needed]} and Paleothyris, both of similar build and presumably habit. One of the best known early reptiles is Mesosaurus, a genus of early reptiles from the early Permian that had returned to water, living off fish. The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.

Anapsids, synapsids and sauropsids

A = Anapsid, B = Synapsid, C = Diapsid

The first reptiles are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc.^[16] Turtles are believed by some to be surviving Anapsids, as they also share this skull structure, but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor (see Parareptilia).^[17] Both sides have strong evidence, and the conflict has yet to be resolved.^{[18][19] [20]}

Very early after the first reptiles appeared, two branches split off.^[21] One lead to the Synapsida (the "mammal-like reptiles" or "stem mammals"), having two openings in the skull roof behind the eyes high , the other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The function of the holes in bout groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.^[22] The diapsids and later anapsids are classed as the "true reptiles", the Sauropsida.^[7].

Permian reptiles

With the close of the Carboniferous, reptiles became the dominant tetrapod fauna. While the terrestrial reptilomorph labyrinthodonts still existed, the mammal-like reptiles evolved the first terrestrial megafauna in the form of pelycosaurs like Edaphosaurus and the carnivorous Dimetrodon. In the mid-Permian the climate turned dryer, resulting in a faunal turnover. The primitive pelycosaurs where replaced by the more advanced therapsids.^[23]

The anapsid reptiles, with their massive skulls without postorbital holes, continued and flourished throughout the Permian. The pareiasaurs reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).^[24]

Early in the period, the diapside reptiles split into two lineages, the lepidosaurs (forefathers of modern snakes, lizards, and tuataras). The group remained lizard-like and relatively small and inconspicuous during the whole periode.

Mesozoic, the "Age of Reptiles"

The close of the Permian saw the greatest mass extinction known (see the Permian–Triassic extinction event). Most of the earlier anapsid/synapsid megafauna disappeared, making room for the archosauromorph diapsids. The archosaurs was characterized by elongated hind-legs and an erect pose, the early forms looking somewhat like long legged crocodiles. The archosaurs became the dominant group during the Triassic, developing into the well known dinosaurs and pterosaurs, as well as crocodiles and phytosaurs. Some of the dinosaurs developed into the largest land animals ever to have lived, making the Mesozoic popularly known as the "Age of Reptiles". The dinosaurs also deveoped smaller forms, including the feather-bearing smaller theropds. In the mid Jurassic, these gave rise to the first birds.^[25]

The lepidosauromorph diapsids may have been ancestral to the sea reptiles.^[26] Developing into the ichthyosaurs and sauropterygians, they came to dominate the Mesozoic seas.

The Therpasids came under increasing pressure from the archosaurs the early Mesozoic and developed into increasingly smaller and more nocturnal forms, the first mammals being the only survivors of the line by late Jurassic.

Demise of the dinosaurs

The close of the Cretacious saw the demise of the Mesozoic reptilian megafauna (see the Cretaceous–Tertiary extinction event). Of the large marine reptiles, only the sea turtles are left, and of the dinosaurs, only the small feathered theropods survived in the form of birds. The major surviving reptilian line is the lepidosaurs, of which the snakes are currently the most numerous and widespread representatives. The end of the “Age of Reptiles”, opened up for the “Age of Mammals”. Despite this, reptiles are still a major fauna component, particularly in tropical climates. There are about 8200 extant species of reptiles (whereof almost half are snakes), compared to 5400 species of mammals (of which ⅔ are rodents and bats). The most numerous modern group with reptilian roots are the birds, with over 9000 species.

Systems

Circulatory

Thermographic image of a monitor lizard.

Most reptiles have a three-chamber heart consisting of two atria, one variably-partitioned ventricle, and two aorta that go the systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chamber heart is variable depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be a fitness advantage.^[27]

There are some interesting exceptions to the general physiology. For instance, crocodilians have an anatomically four-chambered heart, but also have two systemic aorta and are therefore capable only of bypassing their pulmonary circulation.^[28] Also, some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.^[29]

Respiratory

Reptilian lungs

All reptiles breathe using lungs. Aquatic turtles have developed more permeable skin, and some species have modified their cloaca to increase the area for gas exchange (Orenstein, 2001). Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In squamates, the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al., 2003). Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepatic piston."

Turtles and tortoises

Red-eared slider taking a gulp of air.

How turtles and tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles & tortoises have found a variety of solutions to this problem. The problem is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al., 2003). They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is red-eared sliders, which also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells (ibid).

Palate

Most reptiles lack a secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains from getting kicked in by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.

Skin

The hind leg of an iguana, showing iguanas' iconic scales.

Reptilian skin is covered in a horny epidermis, making it watertight and enable reptiles to live on dry land, in contrast to the amphibians. Compared to mammals, reptilian skin is rather thin, and lack the thick dermal layer that produces leather in mammals.^[30] Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base, forming armour. In turtles, the body is hidden inside a hard shell composed on fused scutes. In the lepidosaurians like lizards and snakes, the whole skin is covered in epidermal scales. Such scales where once thought to be typical of the class Reptilia as a whole, but are actually found only in lepidosaurians. The scales found in turtles and crocodiles are of dermal origin rather than epidermal, and are properly termed scutes.

Excretory

Excretion is performed mainly by two small kidneys. In diapsids, uric acid is the main nitrogenous waste product; turtles, like mammals, mainly excrete urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure present in the nephrons of birds and mammals, called a Loop of Henle. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt glands in some reptiles.

Digestive systems

Watersnake Malpolon monspessulanus eating a lizard. Most reptiles are carnivorous, and quite a few primarily eat other reptiles

Most reptiles are carnivorous and have rather simple and not overly long guts, meat being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting about the fact that they can not divide and masticate their food like mammals do, and their lower metabolism. Being cold blooded their energy requirement is about a 5th to a 10th of that of a mammal of the same size. Large reptiles like crocodiles and the large constrictors can basically live from a single large meal for months, digesting it slowly.

While modern reptiles are predominately carnivorous, this has not always been so. During the early history of reptiles, several groups produced big-bodied herbivorous megafauna, in the Paleozoic the Pareiasaurs and the synapsid Dicynodonts, and in the Mesozoic several lines of Dinosaurs. Today the turtles are the only predominantly herbivorous reptile group, but several lines of agams and iguanas have developed to live wholly or partly from plants.

Herbivorous reptiles face the same problems of mastication as herbivorous mammals, but lacking the complex mammal teeth, quite a few species swallow rocks and pebbles to aid in digestion, so called gastrolithes. The rocks are washed around in the stomach helping to grind up plant matter. Fossil gastrolithes has also been found associated with sauropods. Sea turtles, crocodiles and marine iguanas also use the gastrolithes as ballast, helping them to dive.

Nervous system

The reptilian nervous system contains the same basic part of the amphibian brain, but the reptile cerebrum and cerebellum are slightly larger. Most typical sense organs are well developed with certain exceptions most notably the snake's lack of external ears (middle and inner ears are present). There are twelve pairs of cranial nerves.[2]

Reptiles are not generally considered particularly intelligent when compared to mammals and birds.^[31] Their brains fall well below those of mammals in size relative to the body, the encephalisation quotient being about one tenth of that of mammals.^[32] The crocodiles have brains in the higher size range and show a fairly complex social structure. Larger lizards like the monitors are known to exhibit complex behaviour, including cooporation.^[33] The Komodo dragon is known to engage in play.^[34]

Vision

Most reptiles are diurnal animals. The vision is typically adapted to daylight condition, with colour vision and advanced visual depth perception compared to amphibians and most mammals. In some species vision is reduced, such as blindsnakes.^[35] Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of pits sensitive to infrared radiation (heat). Such heat sensitive pits are particularly well developed in the pit vipers, but also found in boas and pythons. These allows the snakes to sense the body heat from birds and mammals, making pitvipers able to hunt rodents in the dark.

Reproductive

Reptiles have amniote eggs with hard or leathery shells, requiring internal fertilization.

Most reptiles reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Tuataras lack copulatory organs, so the male and female simply press their cloacas together as the male excretes sperm.^[36] Most reptiles, however, have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis, while squamates including snakes and lizards possess a pair of hemipenes.

Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic life. There are no larval stages of development. Viviparity and ovoviviparity have only evolved in Squamates, and a substantial fraction of the species utilize this mode of reprduction, including all boas and most vipers. The degree of viviparity varies: some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a placenta.

Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females are able to produce a unisexual diploid clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). In captivity, Komodo dragons (varanidae) have reproduced by parthenogenesis.

Parthenogenetic species are also suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

References

1. ^ Linnaeus, Carolus (1758) (in Latin). Systema naturae per regna tria naturae :secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. (10th edition ed.). Holmiae (Laurentii Salvii). http://www.biodiversitylibrary.org/bibliography/542. Retrieved on 2008-09-22. 
2. ^ Encyclopaedia Britannica, 9th ed. (1878). original text
3. ^ Laurenti, J.N. (1768): Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena. Facsimile, showing the mixed composition of his Reptilia
4. ^ Latreielle, P.A. (1804): Nouveau Dictionnaire à Histoire Naturelle, xxiv., cited in Latreille's Familles naturelles du règne animal, exposés succinctement et dans un ordre analytique, 1825
5. ^ Huxley, T.H. (1863): The Structure and Classification of the Mammalia. Hunterian lectures, presented in Medical Times and Gazette, 1863. original text
6. ^ Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262. 
7. ^ ^{a b} Goodrich, E.S. (1916). "On the classification of the Reptilia". Proceedings of the Royal Society of London 89B: 261–276. doi:10.1098/rspb.1916.0012. 
8. ^ Watson, D.M.S. (1957). "On Millerosaurus and the early history of the sauropsid reptiles". Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences 240 (673): 325–400. doi:10.1098/rstb.1957.0003. 
9. ^ Romer, A.S. (1933). Vertebrate Paleontology. University of Chicago Press. , 3rd ed., 1966.
10. ^ Benton, Michael J. (2004). Vertebrate Paleontology (3rd ed.). Oxford: Blackwell Science Ltd.. ISBN 0632056371. 
11. ^ Laurin, M. and Gauthier, J.A. (1996). "Amniota. Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives." Version 01 January 1996. http://tolweb.org/Amniota/14990/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/
12. ^ Laurin, M.; Reisz, R. R. (1995). "A reevaluation of early amniote phylogeny". Zoological Journal of the Linnean Society 113: 165–223.  (abstract)
13. ^ Falcon-Lang, H.J., Benton, M.J. & Stimson, M. (2007): Ecology of early reptiles inferred from Lower Pennsylvanian trackways. Journal of the Geological Society, London, 164; no. 6; pp 1113-1118. article
14. ^ Earliest Evidence For Reptiles
15. ^ Palmer, D., ed (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 62. ISBN 1-84028-152-9. 
16. ^ Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
17. ^ Laurin, M.; Reisz, R. R. (1995). "A reevaluation of early amniote phylogeny". Zoological Journal of the Linnean Society 113: 165–223.  (abstract)
18. ^ Benton, M. J. (2000). Vertebrate Paleontology (2nd ed. ed.). London: Blackwell Science Ltd. ISBN 0632056142. , 3rd ed. 2004 ISBN 0632056371
19. ^ Zardoya, R.; Meyer, A. (1998). "Complete mitochondrial genome suggests diapsid affinities of turtles". Proc Natl Acad Sci U S A 95 (24): 14226–14231. ISSN 0027-8424. http://www.pubmedcentral.gov/articlerender.fcgi?artid=24355. 
20. ^ Rieppel, O.; deBraga, M. (1996). "Turtles as diapsid reptiles". Nature 384: 453–455. 
21. ^ van Tuninen, M. & Hadly, E.A. (2004): Error in Estimation of Rate and Time Inferred from the Early Amniote Fossil Record and Avian Molecular Clocks. Journal of Mulecular Biology, no 59: pp 267-276 PDF
22. ^ Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
23. ^ Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York - ISBN 9780471384618.
24. ^ Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York - ISBN 9780471384618.
25. ^ Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York - ISBN 9780471384618.
26. ^ Gauthier J. A. (1994): The diversification of the amniotes. In: D. R. Prothero and R. M. Schoch (ed.) Major Features of Vertebrate Evolution: 129-159. Knoxville, Tennessee: The Paleontological Society.
27. ^ Hicks, James (2002). "The Physiological and Evolutionary Significance of Cardiovascular Shunting Patterns in Reptiles". News in Physiological Sciences 17: 241–245. 
28. ^ Axelsson, Michael; Craig E. Franklin (1997). "From anatomy to angioscopy: 164 years of crocodilian cardiovascular research, recent advances, and speculations.". Comparative Biochemistry and Physiology A 188 (1): 51–62. doi:10.1016/S0300-9629(96)00255-1. 
29. ^ Wang, Tobias; Altimiras, Jordi; Klein, Wilfried; Axelsson, Michael (2003). "Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures". The Journal of Experimental Biology 206: 4242–4245. doi:10.1242/jeb.00681. PMID 14581594. 
30. ^ Hildebran, M. & Goslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & sons inc, New York. 635 pages ISBN 0-471-29505-1
31. ^ Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
32. ^ Figure of relative brain size in vertebrates
33. ^ King, Dennis & Green, Brian. 1999. Goannas: The Biology of Varanid Lizards. University of New South Wales Press. ISBN 0-86840-456-X, p. 43.
34. ^ Tim Halliday (Editor), Kraig Adler (Editor). Firefly Encyclopedia of Reptiles and Amphibians. Hove: Firefly Books Ltd. pp. 112, 113, 144, 147, 168, 169. ISBN 1-55297-613-0. 
35. ^ [1]
36. ^ Lutz, Dick (2005), Tuatara: A Living Fossil, Salem, Oregon: DIMI PRESS, ISBN 0-931625-43-2

Further reading

• Colbert, Edwin H. (1969). Evolution of the Vertebrates (2nd ed.). New York: John Wiley and Sons Inc.. ISBN 0471164666. 
• Klein, Wilfied; Abe, Augusto; Andrade, Denis; Perry, Steven (2003). "Structure of the posthepatic septum and its influence on visceral topology in the tegu lizard, Tupinambis merianae (Teidae: Reptilia)". Journal of Morphology 258 (2): 151–157. doi:10.1002/jmor.10136. 
• Landberg, Tobias; Mailhot, Jeffrey; Brainerd, Elizabeth (2003). "Lung ventilation during treadmill locomotion in a terrestrial turtle, Terrapene carolina". Journal of Experimental Biology 206 (19): 3391–3404. doi:10.1242/jeb.00553. PMID 12939371. 
• Laurin, Michel and Gauthier, Jacques A.: Diapsida. Lizards, Sphenodon, crocodylians, birds, and their extinct relatives, Version 22 June 2000; part of The Tree of Life Web Project
• Orenstein, Ronald (2001). Turtles, Tortoises & Terrapins: Survivors in Armor. Firefly Books. ISBN 1-55209-605-X. 
• Pianka, Eric; Vitt, Laurie (2003). Lizards Windows to the Evolution of Diversity. University of California Press. pp. 116–118. ISBN 0-520-23401-4. 
• Pough, Harvey; Janis, Christine; Heiser, John (2005). Vertebrate Life. Pearson Prentice Hall. ISBN 0-13-145310-6. 

See also

• List of reptiles
• Lists of reptiles by region
• Amniote
• Anapsida
• Synapsida
• Diapsida

External links

Find more about Reptile on Wikipedia's sister projects: Definitions from Wiktionary

Textbooks from Wikibooks
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Learning resources from Wikiversity

Wikispecies has information related to: Reptilia

The Wikibook Dichotomous Key has a page on the topic of Reptilia

• The Reptile Database
• Reptile Phylogeny
• World Reptile Amphibian Information Center
• Reptile images

v • d • e Extant Chordata classes by subphylum Kingdom Animalia · Subkingdom Eumetazoa · (unranked) Bilateria · Superphylum Deuterostomia Urochordata (Tunicates) Ascidiacea (Ascidians) · Thaliacea · Appendicularia (Larvaceans) · Sorberacea Cephalochordata (Lancelets) Leptocardii Craniata Myxini (Hagfish) · Hyperoartia (Lampreys) · Chondrichthyes (Cartilaginous fish) · Actinopterygii (Ray-finned fish) · Sarcopterygii (Lobe-finned fish) · Amphibia (Amphibians) · Sauropsida (Reptiles) · Aves (Birds) · Mammalia (Mammals)

v • d • e Extant sauropsida orders by subclass Kingdom: Animalia · Phylum: Chordata · Subphylum: Vertebrata Anapsida Chelonia Testudines Diapsida Lepidosauromorpha Sphenodontia · Squamata Archosauromorpha Crocodilia

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Dansk (Danish)
n. - [zool.] krybdyr, reptil
adj. - krybende

Nederlands (Dutch)
reptiel

Français (French)
n. - (Zool, fig) reptile
adj. - de reptile, reptilien

Deutsch (German)
n. - Reptil, Kriechtier
adj. - kriechend, abscheulich

Ελληνική (Greek)
n. - (ζωολ., μτφ.) ερπετό
adj. - έρπων, δουλικός

Italiano (Italian)
rettile

Português (Portuguese)
n. - réptil (m)
adj. - réptil

Русский (Russian)
рептилия, подлец

Español (Spanish)
n. - reptil, persona rastrera, vil
adj. - reptante, rastrero, bajo, despreciable

Svenska (Swedish)
n. - reptil, kräldjur
adj. - krälande, lömsk, giftig

中文（简体）(Chinese (Simplified))
爬虫动物, 卑鄙的人, 爬虫, 爬行的, 卑鄙的, 爬虫类的

中文（繁體）(Chinese (Traditional))
n. - 爬蟲動物, 卑鄙的人, 爬蟲
adj. - 爬行的, 卑鄙的, 爬蟲類的

한국어 (Korean)
n. - 파충류 동물, 비열한 인간, 악랄한 인간
adj. - 파행하는, 기어 다니는, 비열한

日本語 (Japanese)
n. - 爬行動物, 卑劣な人間
adj. - 爬行する, はい回る, 卑劣な

العربيه (Arabic)
‏(الاسم) الزاحف (صفه) شخص حقير, زاحف‏

עברית (Hebrew)
n. - ‮חיה זוחלת, זוחל‬
adj. - ‮זוחל‬

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