The Uropeltidae, also commonly known as shield-tail snakes, shield-tailed snakes or earth snakes, are a family of primitive, nonvenomous, burrowing snakes native to Peninsular India and Sri Lanka. Seven or eight genera are recognized, depending on whether Teretrurus rhodogaster is treated in its own genus or as part of Brachyophidium. The family comprises over 50 species. These snakes are not well known in terms of their diversity, biology, and natural history.

Description

Snakes in the family Uropeltidae are small snakes, with adults growing to a total length (including tail) of 20–75 cm. They are adapted to a fossorial way of life, which is apparent in their anatomy. The skull is primitive and inflexible, with a short, vertical quadrate bone and rigid jaws; the coronoid bone is still present in the lower jaw. The orbital bones are absent, the supratemporal is vestigial, and the eyes are small and degenerate, not covered by a brille, but by large polygonal shields. However, the pelvis and hind limbs, the presence of which is also considered a primitive trait, have disappeared in this family.

The tail is characteristic, ending in either an enlarged rigid scale with two points, or more often an upper surface with a subcircular area covered with thickened spiny scales, or a much enlarged spiny plate. The ventral scales are much reduced in size. The body is cylindrical and covered with smooth scales.

Behaviour and natural history

Many species of shield-tail snakes are rather poorly known in terms of natural history. Field studies indicate that most species are obligate burrowers and may often come out on to soil surface during rainy nights. Even roadkills of these snakes have been recorded by field biologists during peak monsoon rains. They seem to prefer the humus-rich topsoil layers and rarely burrow deeper inside (like during very hot or dry weather).

When approached by predators, these snakes do not bite like most snakes, but coil their bodies into a ball and hide their heads tucked underneath. Some may poke with their harmless tail tip, like a worm snake. Many have a drab and dull-coloured back, but a very bright, contrastingly coloured underside (such as bright yellow, red, etc.) to startle predators by turning upside down and twitching. This aposematic colouration wards off would-be predators.

Feeding

The diets of shield-tail snakes consist mostly of invertebrates, particularly earthworms, and many species have actually been observed in the wild by researchers to eat earthworms. Frank Wall, who dissected many species for analysing the gut contents to study the diet, remarks about the presence of worms and mud.

Reproduction

All members of the family Uropeltidae retain eggs that hatch within the body of the mother (ovoviviparity).

The Typhlopidae are a family of blind snakes. They are found mostly in the tropical regions of Africa, Asia, the Americas, and all mainland Australia and various islands. The rostral scale overhangs the mouth to form a shovel-like burrowing structure. They live underground in burrows, and since they have no use for vision, their eyes are mostly vestigial. They have light-detecting black eye spots, and teeth occur in the upper jaw. Typhlopids do not have dislocatable lower jaw articulations restricting them to prey smaller than their oral aperture. All species in the family Typhlopidae are fossorial and feed on social fossorial invertebrates such as termites and ants. The tracheal lung is present and chambered in all species. One species, the Brahminy’s blind snake, is the only unisexual snake, with the entire population being female and reproducing via parthenogenesis. The tail ends with a horn-like scale. Most of these species are oviparous. Currently, 18 genera are recognized containing over 200 species.

Description

The total length of most species ranges from 7 to 28 cm, with maximum recorded sizes reaching 97 cm. The snout is distinctly protruding, and the head shields are specialized for a burrowing lifestyle. The skull is dense and compact, with reduced kinesis compared to more evolutionarily advanced snakes. The mouth is positioned on the ventral surface of the head. In some species, the eye region is externally indistinguishable.

The body shows clear adaptations to a fossorial life. The body covered with smooth, rounded scales. Dorsal and ventral scales are of equal size. The tail is very short and thick, ending in a sharp spine. The strongly reduced eyes are visible only as dark spots beneath the thick horny shields.

The premaxillary and maxillary bones do not make contact. The premaxillary, palatine, and mandibular bones lack teeth, while the maxillary bones bear a few small teeth at their posteroinferior ends. The nasal bones are broadly connected with the frontal and prefrontal bones. Supratemporal bones are absent.

All genera retain vestiges of the pelvis, and in some species rudimentary hindlimbs are also present.

Crocodilia is an order of semiaquatic, predatory reptiles that are known as crocodilians. The order includes the true crocodiles (family Crocodylidae), the alligators and caimans (family Alligatoridae), and the gharial and false gharial (family Gavialidae).

Extant crocodilians have flat heads with long snouts and tails that are compressed on the sides, with their eyes, ears, and nostrils at the top of the head. Alligators and caimans tend to have broader U-shaped jaws that, when closed, show only the upper teeth, whereas crocodiles usually have narrower V-shaped jaws with both rows of teeth visible when closed. Gharials have extremely slender, elongated jaws. The teeth are conical and peg-like, and the bite is powerful. All crocodilians are good swimmers and can move on land in a “high walk” position, traveling with their legs erect rather than sprawling. Crocodilians have thick skin covered in non-overlapping scales and, like birds, crocodilians have a four-chambered heart and lungs with unidirectional airflow.

Like most other reptiles, crocodilians are ectotherms or ‘cold-blooded’. They are found mainly in the warm and tropical areas of the Americas, Africa, Asia, and Oceania, usually occupying freshwater habitats, though some can live in saline environments and even swim out to sea. Crocodilians have a largely carnivorous diet; some species like the gharial are specialized feeders while others, like the saltwater crocodile, have generalized diets. They are generally solitary and territorial, though they sometimes hunt in groups. During the breeding season, dominant males try to monopolize available females, which lay their eggs in holes or mounds and, like many birds, they care for their hatched young.

Though there is diversity in snout and tooth shape, all crocodilian species have essentially the same body morphology. They have solidly built, lizard-like bodies with wide, cylindrical torsos, flat heads, long snouts, short necks, and tails that are compressed from side to side. Their limbs are reduced in size; the front feet have five mostly non-webbed digits, and the hind feet have four webbed digits and an extra fifth. The pelvis and ribs of crocodilians are modified; the cartilaginous processes of the ribs allow the thorax to collapse when submerging and the structure of the pelvis can accommodate large amounts of food, or more air in the lungs. Both sexes have a cloaca, a single chamber and outlet near the tail into which the intestinal, urinary and genital tracts open. It houses the penis in males and the clitoris in females. The crocodilian penis is permanently erect; it relies on cloacal muscles to protrude it, and elastic ligaments and a tendon to retract it. The gonads are located near the kidneys.

Crocodilians range in size from the dwarf caimans and African dwarf crocodiles, which reach 1–1.5 m, to the saltwater crocodile and Nile crocodile, which reach 6 m and weigh up to 1,000 kg. Some prehistoric species such as the late-Cretaceous Deinosuchus were even larger, at up to about 11 m and 3,450 kg. Crocodilians tend to be sexually dimorphic; males are much larger than females.

Locomotion

Crocodilians are excellent swimmers. During aquatic locomotion, the muscular tail undulates from side to side to drive the animal through the water while the limbs are held close to the body to reduce drag. When the animal needs to stop or change direction, the limbs are splayed out. Swimming is normally achieved with gentle sinuous movements of the tail, but the animals can move more quickly when pursuing or being pursued. Crocodilians are less well-adapted for moving on land, and are unusual among vertebrates in having two means of terrestrial locomotion: the “high walk” and the “low walk”. The ankle joints flex in a different way from those of other reptiles, a feature crocodilians share with some early archosaurs. One of the upper row of ankle bones, the talus bone, moves with the tibia and fibula, while the heel bone moves with the foot and is where the ankle joint is located. The result is the legs can be held almost vertically beneath the body when on land, and the foot swings during locomotion as the ankle rotates.

The limbs move much the same as those of other quadrupeds; the left forelimb moves first, followed by the right hindlimb, then right forelimb, and finally left hindlimb. The high walk of crocodilians, with the belly and most of the tail held off the ground and the limbs held directly under the bodies, resembles that of mammals and birds. The low walk is similar to the high walk, but the body is not raised, and is quite different from the sprawling walk of salamanders and lizards. Crocodilians can instantly change from one walk to the other; the high walk is the usual means of locomotion on land. The animal may immediately push up its body up use this form, or it may take one or two strides of low walk before raising the body. Unlike most other land vertebrates, when crocodilians increase their pace of travel, they increase the speed at which the lower half of each limb (rather than the whole leg) swings forward, so stride length increases while stride duration decreases.

Though they are typically slow on land, crocodilians can produce brief bursts of speed; some can run at 12 to 14 km/h for short distances. In some small species, such as the freshwater crocodile, running can progress to galloping, which involves the hind limbs launching the body forward and the fore limbs subsequently taking the weight. Next, the hind limbs swing forward as the spine flexes dorso-ventrally, and this sequence of movements is repeated. During terrestrial locomotion, a crocodilian can keep its back and tail straight because muscles attach the scales to the vertebrae. Whether on land or in water, crocodilians can jump or leap by pressing their tails and hind limbs against the substrate and launching themselves into the air. A fast entry into water from a muddy bank can be effected by plunging to the ground, twisting the body from side to side and splaying out the limbs.

Jaws and teeth

The snout shape of crocodilians varies between species. Alligators and caimans generally have wide, U-shaped snouts while those of crocodiles are typically narrower and V-shaped. The snouts of the gharials are extremely elongated. The muscles that close the jaws are larger and more powerful than the ones that open them, and a human can quite easily hold shut a crocodilian’s jaws, but prying open the jaws is extremely difficult. The powerful closing muscles attach at the middle of the lower jaw. The jaw hinge attaches behind the atlanto-occipital joint, giving the animal a wide gape. A folded membrane holds the tongue stationary.

Crocodilians have some of the strongest bite forces in the animal kingdom. In a study published in 2003, an American alligator’s bite force was measured at up to 2,125 lbf (9.45 kN) and in a 2012 study, a saltwater crocodile’s bite force was measured at 3,700 lbf (16 kN). This study found no correlation between bite force and snout shape, though the gharial’s extremely slender jaws are relatively weak and are built for quick jaw closure. The bite force of Deinosuchus may have measured 23,000 lbf (100 kN), even greater than that of theropod dinosaurs like Tyrannosaurus.

Crocodilian teeth vary from dull and rounded to sharp and pointed. Broad-snouted species have teeth that vary in size, while those of slender-snouted species are more consistent. In general, in crocodiles and gharials, both rows of teeth are visible when the jaws are closed because their teeth fit into grooves along the outside lining of the upper jaw. By contrast, the lower teeth of alligators and caimans normally fit into holes along the inside lining of the upper jaw, so they are hidden when the jaws are closed. Crocodilians are homodonts, meaning each of their teeth are of the same type; they do not have different tooth types, such as canines and molars. Crocodilians are polyphyodonts; they are able to replace each of their approximately 80 teeth up to 50 times in their 35-to-75-year lifespan. Crocodilians are the only non-mammalian vertebrates with tooth sockets. Next to each full-grown tooth is a small replacement tooth and an odontogenic stem cell in the dental lamina that can be activated when required. Tooth replacement slows and eventually stops as the animal ages.

Sense organs

The eyes, ears and nostrils of crocodilians are at the top of the head; this placement allows them to stalk their prey with most of their bodies underwater. When in bright light, the pupils of a crocodilian contract into narrow slits, whereas in darkness they become large circles, as is typical for animals that hunt at night. Crocodilians’ eyes have a tapetum lucidum that enhances vision in low light. When the animal completely submerges, the nictitating membranes cover its eyes. Glands on the nictitating membrane secrete a salty lubricant that keeps the eye clean. When a crocodilian leaves the water and dries off, this substance is visible as “tears”. While eyesight in air is fairly good, it is significantly weakened underwater. Crocodilians appear to have undergone a “nocturnal bottleneck” early in their history, during which their eyes lost traits like sclerotic rings, an annular pad of the lens and coloured cone oil droplets, giving them dichromatic vision (red-green colourblindness). Since then, some crocodilians appear to have re-evolved full-colour vision.

The ears are adapted for hearing both in air and underwater, and the eardrums are protected by flaps that can be opened or closed by muscles. Crocodilians have a wide hearing range, with sensitivity comparable to most birds and many mammals. Hearing in crocodilians does not degrade as the animal ages because they can regrow and replace hair cells. The well-developed trigeminal nerve allows them to detect vibrations in water, such as those made by potential prey. Crocodilians have a single olfactory chamber and the vomeronasal organ disappears when they reach adulthood. Behavioural and olfactometer experiments indicate crocodiles detect both air-borne and water-soluble chemicals, and use their olfactory system for hunting. When above water, crocodiles enhance their ability to detect volatile odorants by gular pumping, a rhythmic movement of the floor of the pharynx. Crocodiles appear to have lost their pineal organ but still show signs of melatonin rhythms.

Skin and scales

The skin of crocodilians is clad in non-overlapping scales known as scutes that are covered by beta-keratin. Many of the scutes are strengthened by bony plates known as osteoderms. Scutes are most numerous on the back and neck of the animal. The belly and underside of the tail have rows of broad, flat, square-shaped scales. Between crocodilian scales are hinge areas that consist mainly of alpha-keratin. Underneath the surface, the dermis is thick with collagen. Both the head and jaws lack scales and are instead covered in tight, keratinised skin that is fused directly to the bones of the skull and which, over time, develop a pattern of cracks as the skull develops. The skin on the neck and sides is loose. The scutes contain blood vessels and may act to absorb or release heat during thermoregulation. Research also suggests alkaline ions released into the blood from the calcium and magnesium in the dermal bones act as a buffer during prolonged submersion when increasing levels of carbon dioxide would otherwise cause acidosis.

Some scutes contain a single pore known as an integumentary sense organ. Crocodiles and gharials have these on large parts of their bodies, while alligators and caimans only have them on the head. Their exact function is not fully understood, but it has been suggested they may be mechanosensory organs. There are prominent, paired integumentary glands in skin folds on the throat, and others in the side walls of the cloaca. Various functions for these have been suggested; they may play a part in communication—indirect evidence suggests they secrete pheromones used in courtship or nesting. The skin of crocodilians is tough and can withstand damage from conspecifics, and the immune system is effective enough to heal wounds within a few days. In the genus Crocodylus, the skin contains chromatophores, allowing animals to change colour from dark to light and vice versa.

Circulation

Crocodilians may have the most-complex vertebrate circulatory system with a four-chambered heart and two ventricles, an unusual trait among extant reptiles. Both have left and right aorta are connected by a hole called the Foramen of Panizza. Like birds and mammals, crocodilians have vessels that separately direct blood flow to the lungs and the rest of the body. They also have unique, cog-teeth-like valves that when interlocked direct blood to the left aorta and away from the lungs, and then around the body. This system may allow the animals to remain submerged for a lengthy period, but this explanation has been questioned. Other possible reasons for the peculiar circulatory system include assistance with thermoregulatory needs, prevention of pulmonary oedema, and quick recovery from metabolic acidosis. Retention of carbon dioxide within the body permits an increase in the rate of gastric acid secretion and thus the efficiency of digestion, and other gastrointestinal organs such as the pancreas, spleen, small intestine, and liver also function more efficiently.

When submerged, a crocodilian’s heart may beat at only once or twice a minute, with little blood flow to the muscle. When it rises and takes a breath, its heart rate almost immediately increases and the muscles receive newly oxygenated blood. Unlike many marine mammals, crocodilians have little myoglobin to store oxygen in their muscles. While diving, an increasing concentration of bicarbonate ions causes haemoglobin in the blood to release oxygen for the muscles.

Respiration

Crocodilians were traditionally thought to breathe like mammals, with airflow tidally moving in and out, but studies published in 2010 and 2013 conclude respiration in crocodilians is more bird-like, with airflow moving in a unidirectional loop within the lungs. During inhalation, air flows through the trachea and into two primary bronchi (airways) that divide into narrower secondary passageways. The air continues to move through these, then into even narrower tertiary airways, and then into other secondary airways that were bypassed the first time. The air then flows back into the primary airways and is exhaled.

In crocodilians, the diaphragmaticus muscle, which is analogous to the diaphragm in mammals, attaches the lungs to the liver and pelvis. During inhalation, the external intercostal muscles expand the ribs, allowing the animal to take in more air, while the ischiopubis muscle causes the hips to swing downwards and push the belly outward, while the diaphragmaticus pulls the liver back. When exhaling, the internal intercostal muscles push the ribs inwards while the rectus abdominis pulls the hips and liver forwards and the belly inward. Crocodilians can also use these muscles to adjust the position of their lungs, controlling their buoyancy in the water. An animal sinks when the lungs are pulled towards the tail and floats when they move back towards the head. This allows them to move through the water without creating disturbances that could alert potential prey. They can also spin and twist by moving their lungs laterally.

When swimming and diving, crocodilians appear to rely on lung volume for buoyancy more than for oxygen storage. Just before diving, the animal exhales to reduce its lung volume and reach negative buoyancy. When diving, the nostrils of a crocodilian shut tight. All species have a palatal valve, a membranous flap of skin at the back of the oral cavity (mouth) that protects the oesophagus and trachea when the animal is underwater. This enables them to open their mouths underwater without drowning. Crocodilians typically remain underwater for up to fifteen minutes, but under ideal conditions, some can hold their breath for up to two hours. The depth to which crocodilians can dive is unknown, but crocodiles can dive to at least 20 m.

Crocodilians vocalize by vibrating vocal folds in the larynx. The folds of the American alligator have a complex morphology consisting of epithelium, lamina propria and muscle. Although crocodilian vocal folds lack the elasticity of mammalian ones, the larynx is still capable of complex motor control similar to that in birds and mammals, and can adequately control its fundamental frequency.

Digestion

Crocodilian teeth can only hold onto prey, and food is swallowed unchewed. The stomach consists of a grinding gizzard and a digestive chamber. Indigestible items are regurgitated as pellets. The stomach is more acidic than that of any other vertebrate and contains ridges for gastroliths, which play a role in the crushing of food. Digestion takes place more quickly at higher temperatures. When digesting a meal, CO2-rich blood near the lungs is redirected to the stomach, supplying more acid for the oxyntic glands. Compared to crocodiles, alligators digest more carbohydrates relative to protein. Crocodilians have a very low metabolic rate and thus low energy requirements. They can withstand extended fasting by living on stored fat. Even recently hatched crocodiles are able to survive 58 days without food, losing 23% of their bodyweight during this time.

Thermoregulation

Crocodilians are ectotherms (‘cold-blooded’), relying mostly on their environment to control their body temperature. The main means of warming is sun’s heat, while immersion in water may either raise its temperature via thermal conduction or cool the animal in hot weather. The main method for regulating its temperature is behavioural; temperate-living alligators may start the day by basking in the sun on land and move into water for the afternoon, with parts of the back breaking the surface so it can still be warmed by the sun. At night, it remains submerged and its temperature slowly falls. The basking period is longer in winter. Tropical crocodiles bask briefly in the morning and move into water for rest of the day. They may return to land at nightfall when the air cools. Animals also cool themselves by gaping the mouth, which cools by evaporation from the mouth lining. By these means, the temperature range of crocodilians is usually maintained between 25 and 35 °C, and mainly stays in the range 30 to 33 °C.

Osmoregulation

All crocodilians need to maintain a suitable concentration of salt in body fluids. Osmoregulation is related to the quantity of salts and water that are exchanged with the environment. Intake of water and salts occurs across the lining of the mouth, when water is drunk, incidentally while feeding, and when present in foods. Water is lost during breathing, and salts and water are lost in the urine and faeces, through the skin, and in crocodiles and gharials via salt-excreting glands on the tongue. The skin is a largely effective barrier for water and ions. Gaping causes water loss by evaporation. Large animals are better able than small ones to maintain homeostasis at times of osmotic stress. Newly hatched crocodilians are much less tolerant of exposure to salt water than are older juveniles, presumably because they have a higher surface-area-to-volume ratio.

The kidneys and excretory system are much the same as those in other reptiles, but crocodilians do not have a bladder. In fresh water, the osmolality (the concentration of solutes that contribute to a solution’s osmotic pressure) in the plasma is much higher than that of the surrounding water. The animals are well-hydrated, the urine in the cloaca is abundant and dilute, and nitrogen is excreted as ammonium bicarbonate. Sodium loss is low and mainly occurs through the skin in freshwater conditions. In seawater, the opposite is true; the osmolality in the plasma is lower than that of the surrounding water, causing the animal to dehydrate. The cloacal urine is much more concentrated, white, and opaque, and nitrogenous waste is mostly excreted as insoluble uric acid.

Distribution and habitat

Crocodilians are amphibious, living both in water and on land. The last-surviving, fully terrestrial genus Mekosuchus became extinct about 3,000 years ago after humans had arrived on the Pacific islands it inhabited, making the extinction possibly anthropogenic. Crocodilians are typically creatures of the tropics; the main exceptions are the American and Chinese alligators, whose ranges are the southeastern United States and the Yangtze River, respectively. Florida, United States, is the only place where the ranges of crocodiles and alligators coincide. Crocodilians live almost exclusively in lowland habitiat, and do not appear to live above 1,000 m. With a range extending from eastern India to New Guinea and northern Australia, the saltwater crocodile is the widest-spread species.

Crocodilians use various types of aquatic habitats. Due to their diet, gharials are found in pools and backwaters of rapidly flowing rivers. Caimans prefer warm, turbid lakes and ponds, and slow-moving parts of rivers, although the dwarf caiman inhabits cool, relatively clear, fast-flowing waterways, often near waterfalls. The Chinese alligator is found in slow-moving, turbid rivers that flow across China’s floodplains. The highly adaptable American alligator is found in swamps, rivers and lakes with clear or turbid water. Crocodiles live in marshes, lakes and rivers, and can live in saline environments including estuaries and mangrove swamps. American and saltwater crocodiles swim out to sea. Several extinct species, including the recently extinct Ikanogavialis papuensis, which occurred in coastlines of the Solomon Islands, had marine habitats. Climatic factors locally affect crocodilians’ distribution. During the dry season, caimans can be restricted for several months to deep pools in rivers; in the rainy season, much of the savanna in the Orinoco Llanos is flooded, and they disperse widely across the plain. West African crocodiles in the deserts of Mauritania mainly live in gueltas and floodplains but they retreat underground and to rocky shelters, and enter aestivation during the driest periods.

Crocodilians also use terrestrial habitats such as forests, savannas, grasslands and deserts. Dry land is used for basking, nesting and escaping from temperature extremes. Several species make use of shallow burrows on land to keep cool or warm, depending on the environment. Four species of crocodilians climb trees to bask in areas lacking a shoreline. Tropical rainforests bordering rivers and lakes inhabited by crocodilians are of great importance to them, creating microhabitats where they can flourish. The roots of the trees absorb rainwater and slowly release it back into the environment. This keeps crocodilian habitat moist during the dry season while preventing flooding during the wet season.

Behaviour

Adult crocodilians are typically territorial and solitary. Individuals may guard basking spots, nesting sites, feeding areas, nurseries, and overwintering sites. Male saltwater crocodiles defend areas with several female nesting sites year-round. Some species are occasionally gregarious, particularly during droughts, when several individuals gather at remaining water sites. Individuals of some species may share basking sites at certain times of the day.

Feeding

Crocodilians are largely carnivorous. The diets of species varies with snout shape and tooth sharpness. Species with sharp teeth and long, slender snouts, like the Indian gharial and Australian freshwater crocodile, are specialized for snapping fish, insects, and crustaceans. Extremely broad-snouted species with blunt teeth, like the Chinese alligator and the broad-snouted caiman, are equipped for crushing hard-shelled molluscs. Species whose snouts and teeth are intermediate between these two forms, such as the saltwater crocodile and American alligator, have generalized diets and opportunistically feed on invertebrates, fish, amphibians, reptiles, birds and mammals. Though mostly carnivorous, several species of crocodilian have been observed consuming fruit, and this may play a role in seed dispersal.

In general, crocodilians are stalk-and-ambush predators, though hunting strategies vary between species an their prey. Terrestrial prey is stalked from the water’s edge, and grabbed and drowned. Gharials and other fish-eating species sweep their jaws from side-to-side to snatch prey; these animals can leap out of water to catch birds, bats and leaping fish. A small prey animal can be killed by whiplash as the predator shakes its head. When foraging for fish in shallow water, caiman use their tails and bodies to herd fish and may dig for bottom-dwelling invertebrates. The smooth-fronted caiman will leave water to hunt terrestrial prey.

Crocodilians are unable to chew and need to swallow food whole, so prey that is too large to swallow is torn into pieces. Crocodilians may be unable to deal with a large animal with a thick hide, and may wait until it becomes putrid and comes apart more easily. To tear a chunk of tissue from a large carcass, a crocodilian continuously spins its body while holding prey with its jaws, a manoeuvre that is known as the death roll. During cooperative feeding, some individuals may hold onto prey while others perform the roll. The animals do not fight, and each retires with a piece of flesh and awaits its next feeding turn. After feeding together, individuals may depart alone. Crocodilians typically consume prey with their heads above water. The food is held with the tips of the jaws, tossed towards the back of the mouth by an upward jerk of the head and then gulped down. There is no hard evidence crocodilians cache kills for later consumption.

Reproduction and parenting

Crocodilians are generally polygynous, and individual males try to mate with as many females as they can. Monogamous pairings of American alligators have been recorded. Dominant male crocodilians patrol and defend territories, which contain several females. Males of some species, like the American alligator, try to attract females with elaborate courtship displays. During courtship, crocodilian males and females may rub against each other, circle around, and perform swimming displays. Copulation typically occurs in water. When a female is ready to mate, she arches her back while her head and tail dip underwater. The male rubs across the female’s neck and grasps her with his hindlimbs, and places his tail underneath hers so their cloacas align and his penis can be inserted. Intermission can last up to 15 minutes, during which time the pair continuously submerge and surface. While dominant males usually monopolise females, single American alligator clutches can be sired by three different males.

Depending on the species, female crocodilians may construct either holes or mounds as nests, the latter made from vegetation, litter, sand or soil. Nests are typically found near dens or caves. Those made by different females are sometimes close to each other, particularly in hole-nesting species. Clutches may contain between ten and fifty eggs. Crocodilian eggs are protected by hard shells made of calcium carbonate. The incubation period is two to three months. The sex of the developing, incubating young is temperature dependant; constant nest temperatures above 32 °C produce more males, while those below 31 °C produce more females. Sex in crocodilians may be established in a short period of time, and nests are subject to changes in temperature. Most natural nests produce hatchlings of both sexes, though single-sex clutches occur.

All of the hatchlings in a clutch may leave the nest on the same night. Crocodilians are unusual among reptiles in the amount of parental care provided after the young hatch. The mother helps excavate hatchlings from the nest and carries them to water in her mouth. Newly hatched crocodilians gather together and follow their mother. Both male and female adult crocodilians will respond to vocalizations by hatchlings. Female spectacled caimans in the Venezuelan Llanos are known to leave their young in nurseries or crèches, and one female guards them. Hatchlings of some species tend to bask in a group during the day and start to forage separately in the evening. The time it takes young crocodilians to reach independence can vary. For American alligators, groups of young associate with adults for one-to-two years while juvenile saltwater and Nile crocodiles become independent in a few months.

Communication

Crocodilians are the most vocal of the non-avian reptiles. They can communicate with sounds, including barks, bellows, chirps, coughs, growls, grunts, hisses, moos, roars, toots and whines. Young start communicating with each other before they are hatched. It has been shown the young will repeat, one after another, a light tapping noise near the nest. This early communication may help young to hatch simultaneously. After breaking out of the egg, a juvenile produces yelps and grunts, either spontaneously or as a result of external stimuli. Even unrelated adults respond quickly to juvenile distress calls.

Juveniles are highly vocal, both when scattering in the evening and congregating in the morning. Nearby adults, presumably the parents, may warn young of predators or alert them to the presence of food. The range and quantity of vocalisations vary between species. Alligators and caimans are the noisiest while some crocodile species are almost completely silent. In some crocodile species, individuals “roar” at others when they get too close. The American alligator is exceptionally noisy; it emits a series of up to seven throaty bellows, each a couple of seconds long, at ten-second intervals. It also makes various grunts, growls and hisses. Males create vibrations in water to send out infrasonic signals that attract females and intimidate rivals. The enlarged boss of the male gharial may serve as a sound resonator.

The head slap is another form of acoustic communication. This typically starts when an animal in water elevates its snout and remaining stationary. After some time, the jaws are sharply opened then clamped shut with a biting motion that makes a loud, slapping sound that is immediately followed by a loud splash, after which the head may immerse below the surface and blow bubbles from the throat or nostrils. Some species then roar while others slap water with their tails. Episodes of head slapping spread through the group. The purpose varies and it seems have a social function, and is also used in courtship. Dominant individuals intimidate rivals by swimming at the surface and displaying their large body size, and subordinates submit by holding their head forward above water with the jaws open and then flee below.

Growth and mortality

Eggs and hatchlings have a high death rate, and nests face threats from floods, drying, overheating, and predators. Flooding is a major cause of failure of crocodilians to successfully breed; nests are submerged, developing embryos are deprived of oxygen and juveniles are swept away. Despite the maternal care they receive, eggs and hatchlings are commonly lost to predation. Predators, both mammalian and reptilian, may raid nests and eat crocodilian eggs. After hatching and reaching water, young are still under threat.

In addition to terrestrial predators, young are subject to aquatic attacks by fish. Birds take their toll, and malformed individuals are unlikely to survive. In northern Australia, the survival rate for saltwater crocodile hatchlings is 25 percent but this improves with each year of life, reaching up to 60 percent by year five. Mortality rates among subadults and adults are low, though they are occasionally preyed upon by large cats and snakes. Elephants and hippopotamuses may defensively kill crocodiles. Authorities are uncertain how much cannibalism occurs among crocodilians. Adults do not normally eat their own offspring but there is some evidence of subadults feeding on juveniles, while subadults may be preyed on by adults. Adults appear more likely to protect juveniles and may chase away subadults from nurseries. Rival male Nile crocodiles sometimes kill each other during the breeding season.

Growth in hatchlings and young crocodilians depends on the food supply. Animals reach sexual maturity at a certain length, regardless of age. Saltwater crocodiles reach maturity at 2.2–2.5 m for females and 3 m for males. Australian freshwater crocodiles take ten years to reach maturity at 1.4 m. The spectacled caiman matures earlier, reaching its mature length of 1.2 m in four to seven years. Crocodilians continue to grow throughout their lives; males in particular continue to gain weight as they age, but this is mostly in the form of extra girth rather than length. Crocodilians can live for 35–75 years; their age can be determined by growth rings in their bones.

Turtles are reptiles of the order Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked turtles), which differ in the way the head retracts. There are 360 living and recently extinct species of turtles, including land-dwelling tortoises and freshwater terrapins. They are found on most continents, some islands and, in the case of sea turtles, much of the ocean. Like other amniotes (reptiles, birds, and mammals) they breathe air and do not lay eggs underwater, although many species live in or around water.

Turtle shells are made mostly of bone; the upper part is the domed carapace, while the underside is the flatter plastron or belly-plate. Its outer surface is covered in scales made of keratin, the material of hair, horns, and claws. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body. Turtles are ectotherms or “cold-blooded”, meaning that their internal temperature varies with their direct environment. They are generally opportunistic omnivores and mainly feed on plants and animals with limited movements. Many turtles migrate short distances seasonally. Sea turtles are the only reptiles that migrate long distances to lay their eggs on a favored beach.

Size

The largest living species of turtle (and fourth-largest reptile) is the leatherback turtle, which can reach over 2.7 m in length and weigh over 500 kg. The largest known turtle was Archelon ischyros, a Late Cretaceous sea turtle up to 4.5 m long, 5.25 m wide between the tips of the front flippers, and estimated to have weighed over 2,200 kg. The smallest living turtle is Chersobius signatus of South Africa, measuring no more than 10 cm in length and weighing 172 g.

Shell

The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements. It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) carapace and the flatter, ventral (belly) plastron. They are connected by lateral (side) extensions of the plastron.

The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralia (abdominal ribs). During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the dermis (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as fibroblast growth factors that include FGF10. The shoulder girdle in turtles is made up of two bones, the scapula and the coracoid. Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.

The shapes of turtle shells vary with the adaptations of the individual species, and sometimes with sex. Land-dwelling turtles are more dome-shaped, which appears to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells that allow them to cut through the water. Sea turtles in particular have streamlined shells that reduce drag and increase stability in the open ocean. Some turtle species have pointy or spiked shells that provide extra protection from predators and camouflage against the leafy ground. The lumps of a tortoise shell can tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the tip of the plastron is thickened and used for butting and ramming during combat.

Shells vary in flexibility. Some species, such as box turtles, lack the lateral extensions and instead have the carapace bones fully fused or ankylosed together. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. Softshell turtles have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, but has thick connective tissue and an outer layer of leathery skin.

Head and neck

The turtle’s skull is unique among living amniotes (which includes reptiles, birds and mammals); it is solid and rigid with no openings for muscle attachment (temporal fenestrae). Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the long and narrow skulls of softshells to the broad and flattened skull of the mata mata. Some turtle species have developed large and thick heads, allowing for greater muscle mass and stronger bites. In several species, some individuals can develop larger heads and thicker muscles than others, a phenomena known as ‘megacephaly’. This is likely due to differences in diet.

Turtles that are carnivorous or durophagous (eating hard-shelled animals) have the most powerful bites. For example, the durophagous Mesoclemmys nasuta has a bite force of 432 lbf (1,920 N). Species that are insectivorous, piscivorous (fish-eating), or omnivorous have lower bite forces. Living turtles lack teeth but have beaks made of keratin sheaths along the edges of the jaws. These sheaths may have sharp edges for cutting meat, serrations for clipping plants, or broad plates for breaking mollusks. Sea turtles, and several extinct forms, have evolved a bony secondary palate which completely separates the oral and nasal cavities.

The necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such as snake-necked turtles, have long ones. Despite this, all turtle species have eight neck vertebrae, a consistency not found in other reptiles but similar to mammals. Some snake-necked turtles have both long necks and large heads, limiting their ability to lift them when not in water. Some turtles have folded structures in the larynx or glottis that vibrate to produce sound. Other species have elastin-rich vocal cords.

Limbs and locomotion

Due to their heavy shells, turtles are slow-moving on land. A desert tortoise moves at only 0.22–0.48 km/h. By contrast, sea turtles can swim at 30 km/h. The limbs of turtles are adapted for various means of locomotion and habits and most have five toes. Tortoises are specialized for terrestrial environments and have column-like legs with elephant-like feet and short toes. The gopher tortoise has flattened front limbs for digging in the substrate. Freshwater turtles have more flexible legs and longer toes with webbing, giving them thrust in the water. Some of these species, such as snapping turtles and mud turtles, mainly walk along the water bottom, as they would on land. Others, such as terrapins, swim by paddling with all four limbs, switching between the opposing front and hind limbs, which keeps their direction stable.

Sea turtles and the pig-nosed turtle are the most specialized for swimming. Their front limbs have evolved into flippers while the shorter hind limbs are shaped more like rudders. The front limbs provide most of the thrust for swimming, while the hind limbs serve as stabilizers. Sea turtles such as the green sea turtle rotate the front limb flippers like a bird’s wings to generate a propulsive force on both the upstroke and on the downstroke. This is in contrast to similar-sized freshwater turtles (measurements having been made on young animals in each case) such as the Caspian turtle, which uses the front limbs like the oars of a rowing boat, creating substantial negative thrust on the recovery stroke in each cycle. In addition, the streamlining of the marine turtles reduces drag. As a result, marine turtles produce a propulsive force twice as large, and swim six times as fast, as freshwater turtles. The swimming efficiency of young marine turtles is similar to that of fast-swimming fish of open water, like mackerel.

Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes. Snapping turtles and the big-headed turtle have longer tails; the latter uses it for balance while climbing. The cloaca is found underneath and at the base, and the tail itself houses the reproductive organs. Hence, males have longer tails to contain the penis. In sea turtles, the tail is longer and more prehensile in males, who use it to grasp mates. Several turtle species have spines on their tails.

Senses

Turtles make use of vision to find food and mates, avoid predators, and orient themselves. The retina’s light-sensitive cells include both rods for vision in low light, and cones with three different photopigments for bright light, where they have full-color vision. There is possibly a fourth type of cone that detects ultraviolet, as hatchling sea turtles respond experimentally to ultraviolet light, but it is unknown if they can distinguish this from longer wavelengths. A freshwater turtle, the red-eared slider, has an exceptional seven types of cone cell.

Sea turtles orient themselves on land by night, using visual features detected in dim light. They can use their eyes in clear surface water, muddy coasts, the darkness of the deep ocean, and also above water. Unlike in terrestrial turtles, the cornea (the curved surface that lets light into the eye) does not help to focus light on the retina, so focusing underwater is handled entirely by the lens, behind the cornea. The cone cells contain oil droplets placed to shift perception toward the red part of the spectrum, improving color discrimination. Visual acuity, studied in hatchlings, is highest in a horizontal band with retinal cells packed about twice as densely as elsewhere. This gives the best vision along the visual horizon. Sea turtles do not appear to use polarized light for orientation as many other animals do. The deep-diving leatherback turtle lacks specific adaptations to low light, such as large eyes, large lenses, or a reflective tapetum. It may rely on seeing the bioluminescence of prey when hunting in deep water.

Turtles have no ear openings; the eardrum is covered with scales and encircled by a bony otic capsule, which is absent in other reptiles. Their hearing thresholds are high in comparison to other reptiles, reaching up to 500 Hz in air, but underwater they are more attuned to lower frequencies. The loggerhead sea turtle has been shown experimentally to respond to low sounds, with maximal sensitivity between 100 and 400 Hz.

Turtles have olfactory (smell) and vomeronasal receptors along the nasal cavity, the latter of which are used to detect chemical signals. Experiments on green sea turtles showed they could learn to respond to a selection of different odorant chemicals such as triethylamine and cinnamaldehyde, which were detected by olfaction in the nose. Such signals could be used in navigation.

Breathing

The rigid shell of turtles is not capable of expanding and making room for the lungs, as in other amniotes, so they have had to evolve special adaptations for respiration. The lungs of turtles are attached directly to the carapace above while below, connective tissue attaches them to the organs. They have multiple lateral (side) and medial (middle) chambers (the numbers of which vary between species) and one terminal (end) chamber.

The lungs are ventilated using specific groups of abdominal muscles attached to the organs that pull and push on them. Specifically, it is the turtle’s large liver that compresses the lungs. Underneath the lungs, in the coelomic cavity, the liver is connected to the right lung by the root, and the stomach is directly attached to the left lung, and to the liver by a mesentery. When the liver is pulled down, inhalation begins. Supporting the lungs is a wall or septum, which is thought to prevent them from collapsing. During exhalation, the contraction of the transversus abdominis muscle propels the organs into the lungs and expels air. Conversely, during inhalation, the relaxing and flattening of the oblique abdominis muscle pulls the transversus back down, allowing air back into the lungs.

Although many turtles spend large amounts of their lives underwater, all turtles breathe air and must surface at regular intervals to refill their lungs. Depending on the species, immersion periods vary between a minute and an hour. Some species can respire through the cloaca, which contains large sacs that are lined with many finger-like projections that take up dissolved oxygen from the water.

Circulation

Turtles share the linked circulatory and pulmonary (lung) systems of vertebrates, where the three-chambered heart pumps deoxygenated blood through the lungs and then pumps the returned oxygenated blood through the body’s tissues. The cardiopulmonary system has both structural and physiological adaptations that distinguish it from other vertebrates. Turtles have a large lung volume and can move blood through non-pulmonary blood vessels, including some within the heart, to avoid the lungs while they are not breathing. They can hold their breath for much longer periods than other reptiles and they can tolerate the resulting low oxygen levels. They can moderate the increase in acidity during anaerobic (non-oxygen-based) respiration by chemical buffering and they can lie dormant for months, in aestivation or brumation.

The heart has two atria but only one ventricle. The ventricle is subdivided into three chambers. A muscular ridge enables a complex pattern of blood flow so that the blood can be directed either to the lungs via the pulmonary artery, or to the body via the aorta. The ability to separate the two outflows varies between species. The leatherback has a powerful muscular ridge enabling almost complete separation of the outflows, supporting its actively swimming lifestyle. The ridge is less well developed in freshwater turtles like the sliders (Trachemys).

Turtles are capable of enduring periods of anaerobic respiration longer than many other vertebrates. This process breaks down sugars incompletely to lactic acid, rather than all the way to carbon dioxide and water as in aerobic (oxygen-based) respiration. They make use of the shell as a source of additional buffering agents for combating increased acidity, and as a sink for lactic acid.

Osmoregulation

In sea turtles, the bladder is one unit and in most freshwater turtles, it is double-lobed. Sea turtle bladders are connected to two small accessory bladders, located at the sides to the neck of the urinary bladder and above the pubis. Arid-living tortoises have bladders that serve as reserves of water, storing up to 20% of their body weight in fluids. The fluids are normally low in solutes, but higher during droughts when the reptile gains potassium salts from its plant diet. The bladder stores these salts until the tortoise finds fresh drinking water. To regulate the amount of salt in their bodies, sea turtles and the brackish-living diamondback terrapin secrete excess salt in a thick sticky substance from their tear glands. Because of this, sea turtles may appear to be “crying” when on land.

Thermoregulation

Turtles, like other reptiles, have a limited ability to regulate their body temperature. This ability varies between species, and with body size. Small pond turtles regulate their temperature by crawling out of the water and basking in the sun, while small terrestrial turtles move between sunny and shady places to adjust their temperature. Large species, both terrestrial and marine, have sufficient mass to give them substantial thermal inertia, meaning that they heat up or cool down over many hours. The Aldabra giant tortoise weighs up to some 60 kg and is able to allow its temperature to rise to some 33 °C on a hot day, and to fall naturally to around 29 °C by night. Some giant tortoises seek out shade to avoid overheating on sunny days. On Grand Terre Island, food is scarce inland, shade is scarce near the coast, and the tortoises compete for space under the few trees on hot days. Large males may push smaller females out of the shade, and some then overheat and die.

Adult sea turtles, too, have large enough bodies that they can to some extent control their temperature. The largest turtle, the leatherback, can swim in the waters off Nova Scotia, which may be as cold as 8 °C, while their body temperature has been measured at up to 12 °C warmer than the surrounding water. To help keep their temperature up, they have a system of countercurrent heat exchange in the blood vessels between their body core and the skin of their flippers. The vessels supplying the head are insulated by fat around the neck.

Diet and feeding

Most turtle species are opportunistic omnivores; land-dwelling species are more herbivorous and aquatic ones more carnivorous. Generally lacking speed and agility, most turtles feed either on plant material or on animals with limited movements like mollusks, worms, and insect larvae. Some species, such as the African helmeted turtle and snapping turtles, eat fish, amphibians, reptiles (including other turtles), birds, and mammals. They may take them by ambush but also scavenge. The alligator snapping turtle has a worm-like appendage on its tongue that it uses to lure fish into its mouth. Tortoises are the most herbivorous group, consuming grasses, leaves, and fruits. Many turtle species, including tortoises, supplement their diet with eggshells, animal bones, hair, and droppings for extra nutrients.

Turtles generally eat their food in a straightforward way, though some species have special feeding techniques. The yellow-spotted river turtle and the painted turtle may filter feed by skimming the water surface with their mouth and throat open to collect particles of food. When the mouth closes, the throat constricts and water is pushed out through the nostrils and the gap in between the jaws. Some species employ a “gape-and-suck method” where the turtle opens its jaws and expands its throat widely, sucking the prey in.

The diet of an individual within a species may change with age, sex, and season, and may also differ between populations. In many species, juveniles are generally carnivorous but become more herbivorous as adults. With Barbour’s map turtle, the larger female mainly eats mollusks while the male usually eats arthropods. Blanding’s turtle may feed mainly on snails or crayfish depending on the population. The European pond turtle has been recorded as being mostly carnivorous much of the year but switching to water lilies during the summer. Some species have developed specialized diets such as the hawksbill, which eats sponges, the leatherback, which feeds on jellyfish, and the Mekong snail-eating turtle.

Communication and intelligence

While popularly thought of as mute, turtles make various sounds to communicate. One study which recorded 53 species found that all of them vocalized. Tortoises may bellow when courting and mating. Various species of both freshwater and sea turtles emit short, low-frequency calls from the time they are in the egg to when they are adults. These vocalizations may serve to create group cohesion when migrating. The oblong turtle has a particularly large vocal range; producing sounds described as clacks, clicks, squawks, hoots, various kinds of chirps, wails, hooos, grunts, growls, blow bursts, howls, and drum rolls.

Play behavior has been documented in some turtle species. In the laboratory, Florida red-bellied cooters can learn novel tasks and have demonstrated a long-term memory of at least 7.5 months. Similarly, giant tortoises can learn and remember tasks, and master lessons much faster when trained in groups. Tortoises appear to be able to retain operant conditioning nine years after their initial training. Studies have shown that turtles can navigate the environment using landmarks and a map-like system resulting in accurate direct routes towards a goal. Navigation in turtles have been correlated to high cognition function in the medial cortex region of the brain.

Defense

When sensing danger, a turtle may flee, freeze or withdraw into its shell. Freshwater turtles flee into the water, though the Sonora mud turtle may take refuge on land as the shallow temporary ponds they inhabit make them vulnerable. When startled, a softshell turtle may dive underwater and bury itself under the sea floor. If a predator persists, the turtle may bite or discharge from its cloaca. Several species produce foul-smelling chemicals from musk glands. Other tactics include threat displays and Bell’s hinge-back tortoise can play dead. When attacked, big-headed turtle hatchlings squeal, possibly startling the predator.

Migration

Turtles are the only reptiles that migrate long distances, more specifically the marine species that can travel up to thousands of kilometers. Some non-marine turtles, such as the species of Geochelone (terrestrial), Chelydra (freshwater), and Malaclemys (estuarine), migrate seasonally over much shorter distances, up to around 27 km, to lay eggs. Such short migrations are comparable to those of some lizards, snakes, and crocodilians. Sea turtles nest in a specific area, such as a beach, leaving the eggs to hatch unattended. The young turtles leave that area, migrating long distances in the years or decades in which they grow to maturity, and then return seemingly to the same area every few years to mate and lay eggs, though the precision varies between species and populations. This “natal homing” has appeared remarkable to biologists, though there is now plentiful evidence for it, including from genetics.

How sea turtles navigate to their breeding beaches remains unknown. One possibility is imprinting as in salmon, where the young learn the chemical signature, effectively the scent, of their home waters before leaving, and remember that when the time comes for them to return as adults. Another possible cue is the orientation of the Earth’s magnetic field at the natal beach. There is experimental evidence that turtles have an effective magnetic sense, and that they use this in navigation. Proof that homing occurs is derived from genetic analysis of populations of loggerheads, hawksbills, leatherbacks, and olive ridleys by nesting place. For each of these species, the populations in different places have their own mitochondrial DNA genetic signatures that persist over the years. This shows that the populations are distinct and that homing must be occurring reliably.

Reproduction and life cycle

Turtles have a wide variety of mating behaviors but do not form pair-bonds or social groups. In terrestrial species, males are often larger than females and fighting between males establishes a dominance hierarchy for access to mates. For most semi-aquatic and bottom-walking aquatic species, combat occurs less often. Males of these species instead may use their size advantage to mate forcibly. In fully aquatic species, males are often smaller than females and rely on courtship displays to gain mating access to females. In green sea turtles, females generally outnumber males.

Courtship varies between species, and with habitat. It is often complex in aquatic species, both marine and freshwater, but simpler in the semi-aquatic mud turtles and snapping turtles. A male tortoise bobs his head, then subdues the female by biting and butting her before mounting. The male scorpion mud turtle approaches the female from the rear, and often resorts to aggressive methods such as biting the female’s tail or hind limbs, followed by a mounting.

Female choice is important in some species, and female green sea turtles are not always receptive. As such, they have evolved behaviors to avoid the male’s attempts at copulation, such as swimming away, confronting the male followed by biting or taking up a refusal position with her body vertical, her limbs widely outspread, and her plastron facing the male. If the water is too shallow for the refusal position, the females resort to beaching themselves, as the males do not follow them ashore.

All turtles fertilize internally; mounting and copulation can be difficult. In many species, males have a concave plastron that interlocks with the female’s carapace. In species like the Russian tortoise, the male has a lighter shell and longer legs. The high, rounded shape of box turtles are particular obstacles for mounting. The male eastern box turtle leans backward and hooks onto the back of the female’s plastron. Aquatic turtles mount in water, and female sea turtles support the mounting male while swimming and diving. During copulation, the male turtle aligns his tail with the female’s so he can insert his penis into her cloaca. Some female turtles can store sperm from multiple males and their egg clutches can have multiple sires.

Turtles, including sea turtles, lay their eggs on land, although some lay eggs near water that rises and falls in level, submerging the eggs. While most species build nests and lay eggs where they forage, some travel miles. The common snapping turtle walks 5 km (3 mi) on land, while sea turtles travel even further; the leatherback swims some 12,000 km to its nesting beaches. Most turtles create a nest for their eggs. Females usually dig a flask-like chamber in the substrate. Other species lay their eggs in vegetation or crevices. Females choose nesting locations based on environmental factors such as temperature and humidity, which are important for developing embryos. Depending on the species, the number of eggs laid varies from one to over 100. Larger females can lay eggs that are greater in number or bigger in size. Compared to freshwater turtles, tortoises deposit fewer but larger eggs. Females can lay multiple clutches throughout a season, particularly in species that experience unpredictable monsoons.

Most mother turtles do no more in the way of parental care than covering their eggs and immediately leaving, though some species guard their nests for days or weeks. Eggs vary between rounded, oval, elongated, and between hard- and soft-shelled. Most species have their sex determined by temperature. In some species, higher temperatures produce females and lower ones produce males, while in others, milder temperatures produce males and both hot and cold extremes produce females. There is experimental evidence that the embryos of Mauremys reevesii can move around inside their eggs to select the best temperature for development, thus influencing their sexual destiny. In other species, sex is determined genetically. The length of incubation for turtle eggs varies from two to three months for temperate species, and four months to over a year for tropical species. Species that live in warm temperate climates can delay their development.

Hatching young turtles break out of the shell using an egg tooth, a sharp projection that exists temporarily on their upper beak. Hatchlings dig themselves out of the nest and find safety in vegetation or water. Some species stay in the nest for longer, be it for overwintering or to wait for the rain to loosen the soil for them to dig out. Young turtles are highly vulnerable to predators, both in the egg and as hatchlings. Mortality is high during this period but significantly decreases when they reach adulthood. Most species grow quickly during their early years and slow down when they mature.

Turtles can live long lives. The oldest living turtle and land animal is said to be a Seychelles giant tortoise named Jonathan, who turned 187 in 2019. A Galápagos tortoise named Harriet was collected by Charles Darwin in 1835; it died in 2006, having lived for at least 176 years. Most wild turtles do not reach that age. Turtles keep growing new scutes under the previous scutes every year, allowing researchers to estimate how long they have lived. They also age slowly. The survival rate for adult turtles can reach 99% per year.

Squamata is the largest order of reptiles; most members of which are commonly known as lizards, with the group also including snakes. With over 11,991 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Squamates are distinguished by their skins, which bear horny scales or shields, and must periodically engage in molting. They also possess movable quadrate bones, making possible movement of the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very widely to accommodate comparatively large prey. Squamates are the most variably sized living reptiles, ranging from the 16 mm dwarf gecko (Sphaerodactylus ariasae) to the 6.5 m reticulated python (Malayopython reticulatus). The now-extinct mosasaurs reached lengths over 14 m.

Among other reptiles, squamates are most closely related to the tuatara, the last surviving member of the once diverse Rhynchocephalia, with both groups being placed in the superorder Lepidosauria.

Description

The trunk is covered with horny scales, scutes, or granules. The quadrate bone typically articulates with the cranium in a mobile manner. Of the temporal arches, only the upper is retained, or both may be absent. The pterygoid bones do not articulate with the vomer, and the transverse bone is usually present.

The teeth are attached to the upper or inner surface of the jaws, being of either the acrodont or pleurodont type. The vertebrae are amphicoelous or procoelous, with two or three sacral vertebrae when present. The ribs bear a single head, while abdominal ribs are absent or rudimentary.

The pineal foramen may be present or absent. Jacobson’s organ is generally well developed. The copulatory organ is paired, and the cloacal opening is transverse.

Reproduction

The male members of the group Squamata have hemipenes, which are usually held inverted within their bodies, and are everted for reproduction via erectile tissue like that in the mammalian penis. Only one is used at a time, and some evidence indicates that males alternate use between copulations. The hemipenis has a variety of shapes, depending on the species. Often it bears spines or hooks, to anchor the male within the female. Some species even have forked hemipenes (each hemipenis has two tips). Due to being everted and inverted, hemipenes do not have a completely enclosed channel for the conduction of sperm, but rather a seminal groove that seals as the erectile tissue expands. This is also the only reptile group in which both viviparous and ovoviviparous species are found, as well as the usual oviparous reptiles. The eggs in oviparous species have a parchment-like shell. The only exception is found in blind lizards and three families of geckos (Gekkonidae, Phyllodactylidae and Sphaerodactylidae), where many lay rigid and calcified eggs. Some species, such as the Komodo dragon, can reproduce asexually through parthenogenesis.

Studies have been conducted on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates. Ritual combat between males for the females with which they want to mate includes topping, a behavior exhibited by most viperids, in which one male twists around the vertically elevated fore body of his opponent and forcing it downward. Neck biting commonly occurs while the snakes are entwined.

Parthenogenesis is a natural form of reproduction in which the growth and development of embryos occur without fertilization. Agkistrodon contortrix (copperhead snake) and Agkistrodon piscivorus (cottonmouth snake) can reproduce by facultative parthenogenesis; they are capable of switching from a sexual mode of reproduction to an asexual mode.

Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex-determining chromosomes, and females a ZW pair. However, the Colombian rainbow boa, Epicrates maurus, can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny. The WW females are likely produced by terminal automixis.

When female sand lizards mate with two or more males, sperm competition within the female’s reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness. On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives. This preference may enhance the fitness of progeny by reducing inbreeding depression.