Reptile

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This article is about the animal class. For other uses, see Reptile (disambiguation).

Reptiles
Temporal range: Late Carboniferous–Present[1]
Tuatara
Saltwater crocodile
Common box turtle
Ladder snake
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Sauropsida
Class: Reptilia
Laurenti, 1768
Extant groups

See text for extinct groups.

Reptiles, as commonly defined, are a group of

amniotic development. Living traditional reptiles comprise four orders: Testudines (turtles), Crocodilia (crocodilians), Squamata (lizards and snakes), and Rhynchocephalia (the tuatara). As of May 2023, about 12,000 living species of reptiles are listed in the Reptile Database.[2] The study of the traditional reptile orders, customarily in combination with the study of modern amphibians, is called herpetology
.

Reptiles have been subject to several conflicting

Dinosauria, are more closely related to living crocodilians than to other reptiles, and are thus nested among reptiles from an evolutionary perspective. Many cladistic systems therefore redefine Reptilia as a clade (monophyletic group) including birds, though the precise definition of this clade varies between authors.[4][3] Others prioritize the clade Sauropsida, which typically refers to all amniotes more closely related to modern reptiles than to mammals.[4]

The earliest known proto-reptiles originated from the

dinosaurs alongside many species of crocodyliforms and squamates (e.g., mosasaurs
). Modern non-bird reptiles inhabit all the continents except Antarctica.

Reptiles are tetrapod

oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades[6]  – the fetus develops within the mother, using a (non-mammalian) placenta rather than contained in an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. 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, which can grow up to 17 mm (0.7 in) to the saltwater crocodile
, Crocodylus porosus, which can reach over 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).

Classification

See also: List of reptiles

Research history

See also: Skull roof
Reptiles, from Nouveau Larousse Illustré, 1897–1904, notice the inclusion of amphibians (below the crocodiles)

In the 13th century, the category of reptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians, and worms", as recorded by Beauvais in his Mirror of Nature.[7] In the 18th century, the reptiles were, from the outset of classification, grouped with the

Systema Naturæ.[8]
The terms reptile and amphibian were largely interchangeable, reptile (from Latin repere, 'to creep') being preferred by the French.[9] J.N. Laurenti was the first to formally use the term Reptilia for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.[10] Today, the two groups are still commonly treated under the single heading herpetology.

"Antediluvian monster", a Mosasaurus discovered in a Maastricht limestone quarry, 1770 (contemporary engraving)

It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, and

amniotic egg
.

The terms Sauropsida ("lizard faces") and

Theropsida ("beast faces") were used again in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on the 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, Protosauria ("first lizards") in which he included some animals today considered reptile-like amphibians, as well as early reptiles.[13]

In 1956,

ichthyosaurs, and sauropterygians.[14]

In the late 19th century, a number of definitions of Reptilia were offered. The biological traits listed by

vertebrae.[15] The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.[16]

The first reptiles had an anapsid type of skull roof, as seen in the Permian genus Captorhinus

The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of

Vertebrate Paleontology.[17][18]
Those four subclasses were:

Phylogenetic classifications group the traditional "mammal-like reptiles", like this Varanodon, with other synapsids, not with extant reptiles

The composition of Euryapsida was uncertain.

Ichthyosaurs were, at times, considered to have arisen independently of the other euryapsids, and given the older name Parapsida. Parapsida was later discarded as a group for the most part (ichthyosaurs being classified as incertae sedis or with Euryapsida). However, four (or three if Euryapsida is merged into Diapsida) subclasses remained more or less universal for non-specialist work throughout the 20th century. It has largely been abandoned by recent researchers: In particular, the anapsid condition has been found to occur so variably among unrelated groups that it is not now considered a useful distinction.[19]

Phylogenetics and modern definition

By the early 21st century, vertebrate paleontologists were beginning to adopt

crocodilians than the latter are to the rest of extant reptiles. Colin Tudge
wrote:

Mammals are a

synapomorphies, as is the proper way. Instead, it is 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 Reptilia are 'non-avian, non-mammalian amniotes'.[16]

Despite the early proposals for replacing the paraphyletic Reptilia with a monophyletic Sauropsida, which includes birds, that term was never adopted widely or, when it was, was not applied consistently.[3]

Bearded dragon (pogona) skeleton on display at the Museum of Osteology

When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988,

Mesosauridae as well as Reptilia sensu stricto.[4][21]

A variety of other definitions were proposed by other scientists in the years following Gauthier's paper. The first such new definition, which attempted to adhere to the standards of the

Homo sapiens. This stem-based definition is equivalent to the more common definition of Sauropsida, which Modesto and Anderson synonymized with Reptilia, since the latter is better known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds, as they are within the clade that includes both lizards and crocodiles.[3]

Taxonomy

See also:
List of snakes

General classification of extinct and living reptiles, focusing on major groups.[22][23]

Phylogeny

The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[24] All genetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha,[24][25][26][27][28][29] though a few have recovered turtles as Lepidosauromorpha instead.[30] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[24]

Amniota

Synapsida (mammals and their extinct relatives)

Sauropsida / Reptilia
†Parareptilia
Eureptilia

†Captorhinidae

Romeriida

†Paleothyris

Diapsida

†Araeoscelidia

Neodiapsida

†Claudiosaurus

†Younginiformes

Crown Reptilia/
Pan-Lepidosauria
/

†Kuehneosauridae

Lepidosauria

Rhynchocephalia (tuatara and their extinct relatives)

Squamata (lizards and snakes)

Lepidosauromorpha
Archelosauria/
Pan-Testudines
/

†

Eosauropterygia

Pan-Archosauria

†Choristodera

s. s.

†

Prolacertiformes

†

Rhynchosauria

†Trilophosaurus

Archosauriformes (crocodiles, birds, dinosaurs and extinct relatives)

Pantestudines
s. l.
Sauria
(total group)

The position of turtles

The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.

sister clade to the archosaurs, the group that includes crocodiles, non-avian dinosaurs, and birds.[33] However, in their comparative analysis of the timing of organogenesis, Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade within Sauropsida, outside the saurian clade altogether.[34]

Evolutionary history

Main article: Evolution of reptiles

Origin of the reptiles

An early reptile Hylonomus
Archaeopteryx lithographica
perched on the foreground tree stump

The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs.[21][failed verification]

The oldest known animal that may have been an

Ma show typical reptilian toes and imprints of scales.[38] These tracks are attributed to Hylonomus, the oldest unquestionable reptile known.[39]
It was a small, lizard-like animal, about 20 to 30 centimetres (7.9 to 11.8 in) long, with numerous sharp teeth indicating an insectivorous diet.[40] Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote)[41] and Paleothyris, both of similar build and presumably similar habit.

However,

microsaurs have been at times considered true reptiles, so an earlier origin is possible.[42]

Rise of the reptiles

The earliest amniotes, including stem-reptiles (those amniotes closer to modern reptiles than to mammals), were largely overshadowed by larger stem-tetrapods, such as

Carboniferous Rainforest Collapse.[43] This sudden collapse affected several large groups. Primitive tetrapods were particularly devastated, while stem-reptiles fared better, being ecologically adapted to the drier conditions that followed. Primitive tetrapods, like modern amphibians, need to return to water to lay eggs; in contrast, amniotes, like modern reptiles – whose eggs possess a shell that allows them to be laid on land – were better adapted to the new conditions. Amniotes acquired new niches at a faster rate than before the collapse and at a much faster rate than primitive tetrapods. They acquired new feeding strategies including herbivory and carnivory, previously only having been insectivores and piscivores.[43] From this point forward, reptiles dominated communities and had a greater diversity than primitive tetrapods, setting the stage for the Mesozoic (known as the Age of Reptiles).[44] One of the best known early stem-reptiles is Mesosaurus, a genus from the Early Permian
that had returned to water, feeding on fish.

A 2021 examination of reptile diversity in the Carboniferous and the Permian suggests a much higher degree of diversity than previously thought, comparable or even exceeding that of synapsids. Thus, the "First Age of Reptiles" was proposed.[42]

Anapsids, synapsids, diapsids, and sauropsids

A = Anapsid,
B = Synapsid,
C = Diapsid

It was traditionally assumed that the first reptiles retained an

Diapsida ("two arches").[45] The function of the holes in these groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.[31]

Turtles have been traditionally believed to be surviving parareptiles, on the basis of their anapsid skull structure, which was assumed to be primitive trait.[50] The rationale for this classification has been disputed, with some arguing that turtles are diapsids that evolved anapsid skulls, improving their armor.[21] Later morphological phylogenetic studies with this in mind placed turtles firmly within Diapsida.[51] All molecular studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extant archosaurs.[26][27][28][29]

Permian reptiles

With the close of the

therapsids.[52]

The parareptiles, whose massive

pareiasaurian parareptiles reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).[52]

Early in the period, the modern reptiles, or crown-group reptiles, evolved and split into two main lineages: the Archosauromorpha (forebears of turtles, crocodiles, and dinosaurs) and the Lepidosauromorpha (predecessors of modern lizards and tuataras). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.

Mesozoic reptiles

The close of the Permian saw the greatest mass extinction known (see the

birds.[54]

The

ichthyosaurs) and the sauropterygians, which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphs[4] or to archosauromorphs,[55][56][57] and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.[58]

Cenozoic reptiles

Varanus priscus was a giant carnivorous goanna lizard, perhaps as long as 7 metres and weighing up to 1,940 kilograms[59]
choristodere, the latest surviving order of extinct reptiles. The last known choristoderes are known from the Miocene
, around 11.3 million years ago

The close of the

birds survived. This dramatic extinction pattern at the end of the Mesozoic led into the Cenozoic. Mammals and birds filled the empty niches left behind by the reptilian megafauna and, while reptile diversification slowed, bird and mammal diversification took an exponential turn.[44] However, reptiles were still important components of the megafauna, particularly in the form of large and giant tortoises.[61][62]

After the extinction of most archosaur and marine reptile lines by the end of the Cretaceous, reptile diversification continued throughout the Cenozoic. Squamates took a massive hit during the K–Pg event, only recovering ten million years after it,[63] but they underwent a great radiation event once they recovered, and today squamates make up the majority of living reptiles (> 95%).[64][65] Approximately 10,000 extant species of traditional reptiles are known, with birds adding about 10,000 more, almost twice the number of mammals, represented by about 5,700 living species (excluding domesticated species).[66]

Species diversity of living reptiles (2013)[67]
Reptile group Described species Percent of reptile species
Squamates 9193 96.3%
- Lizards 5634 59%
- Snakes 3378 35%
- Amphisbaenians 181 2%
Turtles 327 3.4%
Crocodilians 25 0.3%
Rhynchocephalians 1 0.01%
Total 9546 100%

Morphology and physiology

Circulation

Thermographic image of monitor lizards

All

systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chambered heart varies 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.[68]

Juvenile iguana heart bisected through the ventricle, bisecting the left and right atrium

For example, iguana hearts, like the majority of the squamate hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.[69] The main structures of the heart are the sinus venosus, the pacemaker, the left atrium, the right atrium, the atrioventricular valve, the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge, pulmonary veins, and paired aortic arches.[70]

Some squamate species (e.g., pythons and monitor lizards) have three-chambered hearts that become functionally four-chambered hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle 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.[71]

Crocodilians have an anatomically four-chambered heart, similar to birds, but also have two systemic aortas and are therefore capable of bypassing their pulmonary circulation.[72] In turtles, the ventricle is not perfectly divided, so a mix of aerated and nonaerated blood can occur.[73]

Metabolism

Sustained energy output (joules) of a typical reptile versus a similar size mammal as a function of core body temperature. The mammal has a much higher peak output, but can only function over a very narrow range of body temperature.

Modern non-avian reptiles exhibit some form of cold-bloodedness (i.e. some mix of poikilothermy, ectothermy, and bradymetabolism) so that they have limited physiological means of keeping the body temperature constant and often rely on external sources of heat. Due to a less stable core temperature than birds and mammals, reptilian biochemistry requires enzymes capable of maintaining efficiency over a greater range of temperatures than in the case for warm-blooded animals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals; for many lizards, it falls in the 24–35 °C (75–95 °F) range,[74] while extreme heat-adapted species, like the American desert iguana Dipsosaurus dorsalis, can have optimal physiological temperatures in the mammalian range, between 35 and 40 °C (95 and 104 °F).[75] While the optimum temperature is often encountered when the animal is active, the low basal metabolism makes body temperature drop rapidly when the animal is inactive.

As in all animals, reptilian muscle action produces heat. In large reptiles, like

leatherback turtles, the low surface-to-volume ratio allows this metabolically produced heat to keep the animals warmer than their environment even though they do not have a warm-blooded metabolism.[76] This form of homeothermy is called gigantothermy; it has been suggested as having been common in large dinosaurs and other extinct large-bodied reptiles.[77][78]

The benefit of a low resting metabolism is that it requires far less fuel to sustain bodily functions. By using temperature variations in their surroundings, or by remaining cold when they do not need to move, reptiles can save considerable amounts of energy compared to endothermic animals of the same size.[79] A crocodile needs from a tenth to a fifth of the food necessary for a lion of the same weight and can live half a year without eating.[80] Lower food requirements and adaptive metabolisms allow reptiles to dominate the animal life in regions where net calorie availability is too low to sustain large-bodied mammals and birds.

It is generally assumed that reptiles are unable to produce the sustained high energy output necessary for long distance chases or flying.[81] Higher energetic capacity might have been responsible for the evolution of warm-bloodedness in birds and mammals.[82] However, investigation of correlations between active capacity and thermophysiology show a weak relationship.[83] Most extant reptiles are carnivores with a sit-and-wait feeding strategy; whether reptiles are cold blooded due to their ecology is not clear. Energetic studies on some reptiles have shown active capacities equal to or greater than similar sized warm-blooded animals.[84]

Respiratory system

X-ray fluoroscopy videos of a female American alligator showing contraction of the lungs while breathing

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.[85] 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.[86]

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". The airways form a number of double tubular chambers within each lung. On inhalation and exhalation air moves through the airways in the same direction, thus creating a unidirectional airflow through the lungs. A similar system is found in birds,[87] monitor lizards[88] and iguanas.[89]

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 against damage 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.[90]

Turtles and tortoises

Red-eared slider taking a gulp of air

How turtles 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 those turtles breathe. The varied results indicate that turtles have found a variety of solutions to this problem.

The difficulty 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).[91]

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.[92] This is because they use their abdominal muscles to breathe during locomotion. The last species to have been studied is the red-eared slider, which also breathes during locomotion, but takes 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.[92]

Sound production

Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited to

vocal cords, which have elastin-rich connective tissue.[93][94]

Hearing in snakes

Hearing in humans relies on 3 parts of the ear; the outer ear that directs sound waves into the ear canal, the middle ear that transmits incoming sound waves to the inner ear, and the inner ear that helps in hearing and keeping their balance. Unlike humans and other mammals, snakes do not possess an outer ear, a middle ear, and a tympanum but have an inner ear structure with cochleas directly connected to their jawbone.[95] They are able to feel the vibrations generated from the sound waves in their jaw as they move on the ground. This is done by the use of mechanoreceptors, sensory nerves that run along the body of snakes directing the vibrations along the spinal nerves to the brain. Snakes have a sensitive auditory perception and can tell which direction sound being made is coming from so that they can sense the presence of prey or predator but it is still unclear how sensitive snakes are to sound waves traveling through the air.[96]

Skin

Skin of a sand lizard, showing squamate reptiles iconic scales

Reptilian skin is covered in a horny epidermis, making it watertight and enabling reptiles to live on dry land, in contrast to amphibians. Compared to mammalian skin, that of reptiles is rather thin and lacks the thick dermal layer that produces leather in mammals.[97] Exposed parts of reptiles are protected by

lepidosaurs, such as lizards and snakes, the whole skin is covered in overlapping epidermal scales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurs.[citation needed] The scales found in turtles and crocodiles are of dermal, rather than epidermal, origin and are properly termed scutes.[citation needed
] In turtles, the body is hidden inside a hard shell composed of fused scutes.

Lacking a thick dermis, reptilian leather is not as strong as mammalian leather. It is used in leather-wares for decorative purposes for shoes, belts and handbags, particularly crocodile skin.

Shedding

Reptiles shed their skin through a process called ecdysis which occurs continuously throughout their lifetime. In particular, younger reptiles tend to shed once every five to six weeks while adults shed three to four times a year.[98] Younger reptiles shed more because of their rapid growth rate. Once full size, the frequency of shedding drastically decreases. The process of ecdysis involves forming a new layer of skin under the old one. Proteolytic enzymes and lymphatic fluid is secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.[99] Snakes will shed from the head to the tail while lizards shed in a "patchy pattern".[99] Dysecdysis, a common skin disease in snakes and lizards, will occur when ecdysis, or shedding, fails.[100] There are numerous reasons why shedding fails and can be related to inadequate humidity and temperature, nutritional deficiencies, dehydration and traumatic injuries.[99] Nutritional deficiencies decrease proteolytic enzymes while dehydration reduces lymphatic fluids to separate the skin layers. Traumatic injuries on the other hand, form scars that will not allow new scales to form and disrupt the process of ecdysis.[100]

Excretion

bladder. Excess salts are also excreted by nasal and lingual salt glands
in some reptiles.

In all reptiles, the urinogenital ducts and the rectum both empty into an organ called a cloaca. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards, but is lacking in the monitor lizard, the legless lizards. It is absent in the snakes, alligators, and crocodiles.[101]

Many turtles and lizards have proportionally very large bladders.

Galapagos tortoise had a bladder which could store up to 20% of its body weight.[102] Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.[103]: 143  Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid in osmoregulation.[104]

Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity.[105] Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to have calculi.[106]

Digestion

A colubrid snake, Dolichophis jugularis, eating a legless lizard, Pseudopus apodus. Most reptiles are carnivorous, and many primarily eat other reptiles and small mammals.
Gastroliths from a plesiosaur

Most reptiles are insectivorous or carnivorous and have simple and comparatively short digestive tracts due to meat being fairly simple to break down and digest.

masticate their food.[107] Their poikilotherm metabolism has very low energy requirements, allowing large reptiles like crocodiles and large constrictors to live from a single large meal for months, digesting it slowly.[80]

While modern reptiles are predominantly carnivorous, during the early history of reptiles several groups produced some herbivorous

pareiasaurs; and in the Mesozoic several lines of dinosaurs.[44] Today, turtles are the only predominantly herbivorous reptile group, but several lines of agamas and iguanas have evolved to live wholly or partly on plants.[108]

Herbivorous reptiles face the same problems of mastication as herbivorous mammals but, lacking the complex teeth of mammals, many species swallow rocks and pebbles (so called

ballast, stabilizing them in the water or helping them to dive.[111] A dual function as both stabilizing ballast and digestion aid has been suggested for gastroliths found in plesiosaurs.[112]

Nerves

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.[113] Due to their short cochlea, reptiles use electrical tuning to expand their range of audible frequencies.

Vision

Most reptiles are diurnal animals. The vision is typically adapted to daylight conditions, with color vision and more advanced visual depth perception than in amphibians and most mammals.

Reptiles usually have excellent vision, allowing them to detect shapes and motions at long distances. They often have poor vision in low-light conditions. Birds, crocodiles and turtles have three types of

blind snakes
, vision is reduced.

Many

pineal eye or pineal gland. This "eye" does not work the same way as a normal eye does as it has only a rudimentary retina and lens and thus, cannot form images. It is, however, sensitive to changes in light and dark and can detect movement.[114]

Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of

pit vipers, but are also found in boas and pythons. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.[b]

Most reptiles, as well as birds, possess a nictitating membrane, a translucent third eyelid which is drawn over the eye from the inner corner. In crocodilians, it protects its eyeball surface while allowing a degree of vision underwater.[117] However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called the brille, spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.[118]

Reproduction

Crocodilian egg diagram
(1) eggshell, (2) yolk sac, (3) yolk (nutrients), (4) vessels, (5) amnion, (6) chorion, (7) air space, (8) allantois, (9) albumin (egg white), (10) amniotic sac, (11) crocodile embryo, (12) amniotic fluid
Common house geckos mating, ventral view with hemipenis inserted in the cloaca
Most reptiles reproduce sexually, for example this Trachylepis maculilabris skink
Reptiles have amniotic eggs with hard or leathery shells, requiring internal fertilization when mating.

Reptiles generally

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, only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm.[120]

Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An

Plesiosauria.[6]

Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother. This form of 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 suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

Some reptiles exhibit temperature-dependent sex determination (TDSD), in which the incubation temperature determines whether a particular egg hatches as male or female. TDSD is most common in turtles and crocodiles, but also occurs in lizards and tuatara.[122] To date, there has been no confirmation of whether TDSD occurs in snakes.[123]

Longevity

Giant tortoises are among the longest-lived vertebrate animals (over 100 years by some estimates) and have been used as a model for studying longevity.[124] DNA analysis of the genomes of Lonesome George, the iconic last member of Chelonoidis abingdonii, and the Aldabra giant tortoise Aldabrachelys gigantea led to the detection of lineage-specific variants affecting DNA repair genes that might contribute to our understanding of increased lifespan.[124]

Cognition

See also: Animal cognition

Reptiles are generally considered less intelligent than mammals and birds.

white rats at learning to navigate mazes.[130] Another study found that giant tortoises are capable of learning through operant conditioning, visual discrimination and retained learned behaviors with long-term memory.[131] Sea turtles have been regarded as having simple brains, but their flippers are used for a variety of foraging tasks (holding, bracing, corralling) in common with marine mammals.[132]

There is evidence that reptiles are

sentient and able to feel emotions including anxiety and pleasure.[133]

Defense mechanisms

Many small reptiles, such as snakes and lizards, that live on the ground or in the water are vulnerable to being preyed on by all kinds of carnivorous animals. Thus, avoidance is the most common form of defense in reptiles.[134] At the first sign of danger, most snakes and lizards crawl away into the undergrowth, and turtles and crocodiles will plunge into water and sink out of sight.

Camouflage and warning

Phelsuma deubia
 on a palm frond

Reptiles tend to avoid confrontation through camouflage. Two major groups of reptile predators are birds and other reptiles, both of which have well-developed color vision. Thus the skins of many reptiles have cryptic coloration of plain or mottled gray, green, and brown to allow them to blend into the background of their natural environment.[135] Aided by the reptiles' capacity for remaining motionless for long periods, the camouflage of many snakes is so effective that people or domestic animals are most typically bitten because they accidentally step on them.[136]

When camouflage fails to protect them,

Rattlesnakes
rapidly vibrate the tip of the tail, which is composed of a series of nested, hollow beads to ward off approaching danger.

In contrast to the normal drab coloration of most reptiles, the lizards of the genus Heloderma (the

beaded lizard) and many of the coral snakes have high-contrast warning coloration, warning potential predators they are venomous.[138] A number of non-venomous North American snake species have colorful markings similar to those of the coral snake, an oft cited example of Batesian mimicry.[139][140]

Alternative defense in snakes

Further information: Venom and Evolution of snake venom

Camouflage does not always fool a predator. When caught out, snake species adopt different defensive tactics and use a complicated set of behaviors when attacked. Some species, like cobras or hognose snakes, first elevate their head and spread out the skin of their neck in an effort to look large and threatening. Failure of this strategy may lead to other measures practiced particularly by cobras, vipers, and closely related species, which use

play dead when in danger; some, including the grass snake, exude a foul-smelling liquid to deter attackers.[143][144]

Defense in crocodilians

When a

crocodilian
is concerned about its safety, it will gape to expose the teeth and tongue. If this does not work, the crocodilian gets a little more agitated and typically begins to make hissing sounds. After this, the crocodilian will start to change its posture dramatically to make itself look more intimidating. The body is inflated to increase apparent size. If absolutely necessary, it may decide to attack an enemy.

A White-headed dwarf gecko with shed tail

Some species try to bite immediately. Some will use their heads as sledgehammers and literally smash an opponent, some will rush or swim toward the threat from a distance, even chasing the opponent onto land or galloping after it.[145] The main weapon in all crocodiles is the bite, which can generate very high bite force. Many species also possess canine-like teeth. These are used primarily for seizing prey, but are also used in fighting and display.[146]

Shedding and regenerating tails

Main article: Autotomy

skinks, and some other lizards that are captured by the tail will shed part of the tail structure through a process called autotomy and thus be able to flee. The detached tail will continue to thrash, creating a deceptive sense of continued struggle and distracting the predator's attention from the fleeing prey animal. The detached tails of leopard geckos can wiggle for up to 20 minutes. The tail grows back in most species, but some, like crested geckos, lose their tails for the rest of their lives.[147] In many species the tails are of a separate and dramatically more intense color than the rest of the body so as to encourage potential predators to strike for the tail first. In the shingleback skink and some species of geckos, the tail is short and broad and resembles the head, so that the predators may attack it rather than the more vulnerable front part.[148]

Reptiles that are capable of shedding their tails can partially regenerate them over a period of weeks. The new section will however contain cartilage rather than bone, and will never grow to the same length as the original tail. It is often also distinctly discolored compared to the rest of the body and may lack some of the external sculpting features seen in the original tail.[149]

Relations with humans

Further information: Human uses of reptiles

In cultures and religions

Main article:
Reptiles in culture
Painting of fighting "Laelaps" (now Dryptosaurus) by Charles R. Knight (1897)

Dinosaurs have been widely depicted in culture since the English palaeontologist Richard Owen coined the name dinosaur in 1842. As soon as 1854, the Crystal Palace Dinosaurs were on display to the public in south London.[150][151] One dinosaur appeared in literature even earlier, as Charles Dickens placed a Megalosaurus in the first chapter of his novel Bleak House in 1852.[c] The dinosaurs featured in books, films, television programs, artwork, and other media have been used for both education and entertainment. The depictions range from the realistic, as in the television documentaries of the 1990s and first decade of the 21st century, to the fantastic, as in the monster movies of the 1950s and 1960s.[151][153][154]

The snake or serpent has played a powerful

Tree of Knowledge of Good and Evil.[158]

The turtle has a prominent position as a symbol of steadfastness and tranquility in religion, mythology, and folklore from around the world.

cosmological myths of several cultures a World Turtle carries the world upon its back or supports the heavens.[161]

Medicine

See also: Epidemiology of snakebites
The Rod of Asclepius symbolizes medicine

Deaths from

cytotoxic effect of snake venom is being researched as a potential treatment for cancers.[164]

Gila monsters produce compounds that reduce plasma glucose; one of these substances is now used in the anti-

Alzheimer's drug.[167]

Geckos have also been used as folk medicine, especially in China, without any evidence that they have any active compounds.[168] Turtles have been used in Chinese traditional medicine for thousands of years, with every part of the turtle believed to have medical benefits (again, without scientific evidence). Growing demand for turtle meat has placed pressure on vulnerable wild populations of turtles.[169]

Commercial farming

See also: Crocodile farm, Snake farm, and Turtle farming

Crocodiles are protected in many parts of the world, and are

Crocodile leather is made into wallets, briefcases, purses, handbags, belts, hats, and shoes. Crocodile oil has been used for various purposes.[171]

Snakes are also farmed, primarily in East and Southeast Asia, and their production has become more intensive in the last decade. Snake farming has been troubling for conservation in the past as it can lead to overexploitation of wild snakes and their natural prey to supply the farms. However, farming snakes can limit the hunting of wild snakes, while reducing the slaughter of higher-order vertebrates like cows. The energy efficiency of snakes is higher than expected for carnivores, due to their ectothermy and low metabolism. Waste protein from the poultry and pig industries is used as feed in snake farms.[172] Snake farms produce meat, snake skin, and antivenom.

Turtle farming is another known but controversial practice. Turtles have been farmed for a variety of reasons, ranging from food to traditional medicine, the pet trade, and scientific conservation. Demand for turtle meat and medicinal products is one of the main threats to turtle conservation in Asia. Though commercial breeding would seem to insulate wild populations, it can stoke the demand for them and increase wild captures.[173][169] Even the potentially appealing concept of raising turtles at a farm to release into the wild is questioned by some veterinarians who have had some experience with farm operations. They caution that this may introduce into the wild populations infectious diseases that occur on the farm, but have not (yet) been occurring in the wild.[174][175]

Reptiles in captivity

Further information: Herpetoculture

A herpetarium is a zoological exhibition space for reptiles and amphibians.

In the Western world, some snakes (especially relatively docile species such as the

anoles,[178] and geckos (such as the popular leopard gecko and the crested gecko).[177]

Turtles and tortoises are increasingly popular pets, but keeping them can be challenging due to their particular requirements, such as temperature control, the need for UV light sources, and a varied diet. The long lifespans of turtles and especially tortoises mean they can potentially outlive their owners. Good hygiene and significant maintenance is necessary when keeping reptiles, due to the risks of Salmonella and other pathogens.[179] Regular hand-washing after handling is an important measure to prevent infection.

See also

Main article: Outline of reptiles

Notes

  1. ^ This taxonomy does not reflect modern molecular evidence, which places turtles within
    Diapsida
    .
  2. ^ "The copperhead is a pit viper and, like others pit vipers, it has heat-sensitive pit organs on each side of its head between the eye and the nostril. These pits detect objects that are warmer than the environment and enable copperheads to locate nocturnal, mammalian prey."[116]
  3. ^ "Michaelmas term lately over, and the Lord Chancellor sitting in Lincoln's Inn Hall. Implacable November weather. As much mud in the streets, as if the waters had but newly retired from the face of the earth, and it would not be wonderful to meet a Megalosaurus, forty feet long or so, waddling like an elephantine lizard up Holborne Hill."[152]

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  177. ^ a b Virata, John B. "5 Great Beginner Pet Lizards". Reptiles Magazine. Retrieved 28 May 2017.
  178. ^ McLeod, Lianne. "An Introduction to Green Anoles as Pets". The Spruce. Retrieved 28 May 2017.
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Further reading
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