What is a reptile?

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

Books

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.

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Periodicals

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.

David Chiszar, PhD

Hobart M. Smith, PhD