Crocodylia /krɒkəˈdɪliə/) is an order of semiaquatic, predatory reptiles known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchian, a subset of archosaurs that appeared about 235 million years ago and were the only survivors of the Triassic–Jurassic extinction event. The order includes the true crocodiles (family Crocodylidae), the alligators and caimans (family Alligatoridae), and the gharial and false gharial (family Gavialidae). Although the term "crocodiles" is sometimes used to refer to all of these, it is less ambiguous to use "crocodilians".

Extant crocodilians have long flat heads with long snouts and tails 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, only show the upper teeth, while crocodiles usually have narrower V-shaped jaws with both rows of teeth visible when closed. Gharials have extremely slender and elongated jaws. 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. They have conical, peg-like teeth and a powerful bite. Like birds, crocodilians possess a four-chambered heart and lungs with unidirectional airflow. Like most other reptiles, they are ectotherms.

Crocodilians are found mainly in the warmer and tropical areas of the Americas, Africa, Asia and Oceania. They live mainly in freshwater habitats, but some can live in saltier environments and even swim out to sea. They have a largely carnivorous diet. Some species like the gharial are specialized feeders, while others like the saltwater crocodile have generalized diets. Crocodilians are generally solitary and territorial, though they sometimes hunt in groups. During the breeding season, dominant males try to monopolize available females. Females lay their eggs in holes or mounds, and similar to many birds, care for their hatched young.

Some species of crocodilians (particularly the Nile crocodile) are known to have attacked humans. Humans are the greatest threat to crocodilian populations through activities that include hunting, poaching, and habitat destruction, but farming of crocodilians has greatly reduced unlawful trading in wild skins. Artistic and literary representations of crocodilians have appeared in human cultures around the world since Ancient Egypt.

"Crocodilia" and "Crocodylia" have been used interchangeably for decades starting with Schmidt's redescription of the group from the formerly defunct term Loricata. Schmidt used the older term "Crocodilia", based on Owen's original name for the group. Wermuth opted for "Crocodylia" as the proper name, basing it on the type genus Crocodylus (Laurenti, 1768). Dundee—in a revision of many reptilian and amphibian names—argued strongly for "Crocodylia". However, it was not until the advent of cladistics and phylogenetic nomenclature that a more solid justification for one spelling over the other was proposed.

Prior to 1988, Crocodilia was a group that encompassed the modern-day animals, as well as their more distant relatives now in the larger groups called Crocodylomorpha and Pseudosuchia. Under its current definition as a crown group (as opposed to a stem-based group), Crocodylia is now restricted to only the last common ancestor of today's crocodilians and all of its descendants (living or extinct).

Crocodilia appears to be a Latinizing of the Greek κροκόδειλος (crocodeilos), which means both lizard and Nile crocodile. Crocodylia, as coined by Wermuth, in regards to the genus Crocodylus appears to be derived from the ancient Greek κρόκη (kroke)—meaning shingle or pebble—and δρîλος or δρεîλος (dr(e)ilos) for "worm". The name may refer to the animal's habit of resting on the pebbled shores of the Nile.

Crocodilians, and birds, are members of the Archosaur clade. Archosaurs are distinguished from other reptiles particularly by two sets of extra openings in the skull: the antorbital fenestra located in front of the animal's eye socket and the mandibular fenestra on the jaw. Archosaurs comprise two main groups: the Pseudosuchia (crocodilians and their relatives), and the Avemetatarsalia (dinosaurs, pterosaurs, and their relatives). The split between these two is assumed to have happened close to the Permian–Triassic extinction event (informally known as the Great Dying).

Crocodylomorpha, the group that later give rise to modern crocodilians, emerged in the Late Triassic. While the most basal crocodylomorphs were large, the ones that gave rise to crocodilians were small, slender, and leggy. This evolutionary grade, the so-called "sphenosuchians" first appeared around Carnian of the Late Triassic. They ate small, fast prey and survived into the Late Jurassic. As the Triassic ended, crocodylomorphs became the only surviving pseudosuchians.

During the early Jurassic period, the dinosaurs became dominant on land, and the crocodylomorphs underwent major adaptive diversifications to fill ecological niches vacated by recently extinguished groups. Mesozoic crocodylomorphs had a much greater diversity of forms than modern crocodilians. Some became small fast-moving insectivores, others specialist fish-eaters, still others marine and terrestrial carnivores, and a few became herbivores. The earliest stage of crocodilian evolution was the protosuchians in the late Triassic and early Jurassic. They were followed by the mesosuchians, which diversified widely during the Jurassic and the Tertiary. The eusuchians first appeared during the Early Cretaceous, and includes modern crocodilians.

Protosuchians were small, mostly terrestrial animals with short snouts and long limbs. They had bony armor in the form of two rows of plates extending from head to tail; this armor would still be found in later species. Their vertebrae were convex on the two main articulating surfaces, and the secondary palate was little developed as it consisted only of a maxilla. The mesosuchians saw a fusion of the palatine bones to the secondary palate and a great extension of the nasal passages behind the palatine and in front of the pterygoid bones. This allowed the animal to breathe through its nostrils while its mouth was open underwater. The eusuchians continued this process with the interior nostrils now opening through an aperture in the pterygoid bones. The vertebrae of eusuchians had one convex and one concave articulating surface. The oldest known eusuchian is Hylaeochampsa vectiana from the Early Cretaceous of the Isle of Wight in the United Kingdom. It was followed by crocodilians such as the Planocraniidae, the so-called 'hoofed crocodiles', in the Palaeogene. Spanning the Cretaceous and Palaeogene periods is the genus Borealosuchus of North America, with six species, though its phylogenetic position is not settled.

The three primary branches of Crocodilia had diverged by the Late Cretaceous. The possible earliest-known members of the group may be Portugalosuchus and Zholsuchus from the Cenomanian-Turonian stages. The classification of Portugalosuchus has been disputed by some researchers who claimed that it may be outside the crown group crocodilians. The morphology-based phylogenetic analyses based on the new neuroanatomical data obtained from its skull using micro-CT scans suggested that this taxon is a crown group crocodilian and a member of the 'thoracosaurs', recovered as a sister taxon of Thoracosaurus within Gavialoidea, though it is uncertain whether 'thoracosaurs' were true gavialoids.

Definitive alligatoroids first appeared during the Santonian-Campanian stages, while definitive longirostres first appeared during the Maastrichtian stage. The earliest known alligatoroids and gavialoids include highly derived forms, which indicates that the time of the actual divergence between the three lineages must have been a pre-Campanian event. Additionally, scientists conclude that environmental factors played a major role in the evolution of crocodilians and their ancestors, with warmer climate being associated with high evolutionary rates and large body sizes.

Crocodylia is cladistically defined as the last common ancestor of Gavialis gangeticus (the gharial), Alligator mississippiensis (American alligator), and Crocodylus rhombifer (the Cuban crocodile) and all of its descendants. The phylogenetic relationships of crocodilians has been the subject of debate and conflicting results. Many studies and their resulting cladograms, or "family trees" of crocodilians, have found the "short-snouted" families of Crocodylidae and Alligatoridae to be close relatives, with the long-snouted Gavialidae as a divergent branch of the tree. The resulting group of short-snouted species, named Brevirostres, was supported mainly by morphological studies which analyzed skeletal features alone.

However, recent molecular studies using DNA sequencing of living crocodilians have rejected this distinct group Brevirostres, with the long-snouted gavialids more closely related to crocodiles than to alligators, with the new grouping of gavialids and crocodiles named Longirostres.

Below is a cladogram from 2021 showing the relationships of the major extant crocodilian groups. This analysis was based off mitochondrial DNA, including that of the recently extinct Voay robustus:

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 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 and relies on cloacal muscles to protrude it out and elastic ligaments and a tendon to pull it back in. The gonads are located near the kidneys.

Crocodilians range in size from the Paleosuchus and Osteolaemus species, which reach 1–1.5 m (3 ft 3 in – 4 ft 11 in), to the saltwater crocodile and Nile crocodile, which reach 6 m (20 ft) and weigh up to 1,000 kg (2,200 lb), though some prehistoric species such as the late Cretaceous Deinosuchus were even larger at up to about 11 m (36 ft) and 3,450 kg (7,610 lb). They tend to be sexually dimorphic, with males much larger than females.

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 steer in a different direction, the limbs are splayed out. Swimming is normally achieved with gentle sinuous movements of the tail, but they can move faster when pursuing or being pursued. Crocodilians are less well-adapted for moving on land, and are unusual among vertebrates in having two different means of terrestrial locomotion: the "high walk" and the "low walk". Their ankle joints flex in a different way from those of other reptiles, a feature they share with some early archosaurs. One of the upper row of ankle bones, the ankle 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 that 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 in other quadrupeds, with the left forelimb moving first, followed by the right hindlimb, then right forelimb, and finally left hindlimb, and repeat. 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 without the body being raised, and is quite different from the sprawling walk of salamanders and lizards. The animal can change from one walk to the other instantaneously, but the high walk is the usual means of locomotion on land. The animal may push its body up and use this form immediately, or may take one or two strides of low walk before raising the body higher. 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 typically slow on land, crocodilians can produce brief bursts of speed, and some can run at 12 to 14 km/h (7.5 to 8.7 mph) for short distances. In some small species such as the freshwater crocodile, a running can progress to galloping. This 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, since the scales are attached to the vertebrae by muscles. 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.

The snout shape of crocodilians varies between species. Alligators and caimans generally have wider, U-shaped snouts while those of crocodiles are typically narrower and V-shaped. The gharial's are extremely elongated. The muscles that close the jaws are larger and more powerful than the ones that open them, and a crocodilian's jaws can be held shut by a person fairly easily. Conversely, the jaws are extremely difficult to pry open. The powerful closing muscles attach at the middle of the lower jaw and 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). In a 2012 study, a saltwater crocodile's bite force was measured even higher, at 3,700 lbf (16 kN). This study also found no correlation between bite force and snout shape. Nevertheless, the gharial's extremely slender jaws are relatively weak and built more 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 pointy. Broad-snouted species have teeth that vary in size, while those of slender-snouted species are more consistent. In general both rows of teeth are visible on crocodiles and gharials when the jaws are closed as 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. Thus they are hidden when the jaws are closed. Crocodilians are homodonts, meaning each of their teeth are all of the same type (they do not possess different tooth types, such as canines and molars) and polyphyodonts are able to replace each of their approximately 80 teeth up to 50 times in their 35- to 75-year lifespan. They are the only non-mammalian vertebrates with tooth sockets. Next to each full-grown tooth there is a small replacement tooth and an odontogenic stem cell in the dental lamina, which can be activated when required. Tooth replacement slows and eventually stops as the animal ages.

The eyes, ears and nostrils of crocodilians are at the top of the head. This 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. This is typical for animals that hunt at night. Crocodilians also possess a tapetum lucidum which enhances vision in low light. When the animal completely submerges, the nictitating membranes cover its eyes. In addition, 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 is fairly good in air, it is significantly weakened underwater. Crocodilians appear to have gone through a "nocturnal bottleneck" early in their history with their eyes losing traits like sclerotic rings, an annular pad of the lens and colored cone oil droplets, giving them dichromatic vision (red-green colorblindness). 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 gets older as they can regrow and replace hair cells. The well-developed trigeminal nerve allows them to detect vibrations in the water (such as those made by potential prey). Crocodilians have just a single olfactory chamber and the vomeronasal organ disappears when they reach adulthood. Behavioural and olfactometer experiments indicate that 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. They appear to have lost their pineal organ, but still show signs of melatonin rhythms.

The skin of crocodilians is clad in non-overlapping scales known as scutes which are covered with in beta-keratin. Many of the scutes are strengthened by bony plates known as osteoderms. They are most numerous on the back and neck of the animal. The belly and underside of the tail possess rows of broad, flat and square-shaped scales. In between crocodilian scales are hinge areas which consist mainly of alpha-keratin. Underneath the surface, the dermis is thick with collagen. Both the head and jaws lack actual 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 that alkaline ions released into the blood from the calcium and magnesium in these 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 that 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, as indirect evidence suggest that 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 them to change color from dark to light and vice versa.

The crocodilian has perhaps the most complex vertebrate circulatory system. It has a four-chambered heart and two ventricles, an unusual trait among extant reptiles, and both a left and right aorta which are connected by a hole called the Foramen of Panizza. Like birds and mammals, crocodilians have separate vessels that direct blood flow to the lungs and the rest of the body respectively. They also have unique cog-teeth-like valves that, when interlocked, direct blood to the left aorta and away from the lungs, and then back around the body. This system may allow the animals to remain submerged for a longer period, but this explanation has been questioned. Other possible reasons for the peculiar circulatory system include assistance with thermoregulatory needs, prevention of pulmonary oedema, or faster recovery from metabolic acidosis. Retaining 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 one or two beats a minute, with little blood flow to the muscle. When it rises and takes a breath, its heart rate almost immediately speeds up, and the muscles receive newly oxygenated blood. Unlike many marine mammals, crocodilians have little myoglobin to store oxygen in their muscles. During diving, an increasing concentration of bicarbonate ions causes haemoglobin in the blood to release oxygen for the muscles.

Crocodilians were traditionally thought to breathe like mammals, with airflow moving in and out tidally, but studies published in 2010 and 2013 conclude that crocodilians breathe more like birds, with airflow moving in a unidirectional loop within the lungs. When a crocodilian inhales, air flows through the trachea and into two primary bronchi, or airways, which split off into narrower secondary passageways. The air continues to move through these, then into even narrower tertiary airways, and then into other secondary airways which were bypassed the first time. The air then flows back into the primary airways and is exhaled.

The lungs of crocodilians are attached to the liver and the pelvis by the diaphragmaticus muscle (analogous of the diaphragm in mammals). 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, and the diaphragmaticus pulls the liver back. When exhaling, the internal intercostal muscles push the ribs inward, 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.

Swimming and diving crocodilians appear to rely on lung volume more for buoyancy than 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 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 fifteen minutes or less at a time, but some can hold their breath for up to two hours under ideal conditions. The maximum diving depth is unknown, but crocodiles can dive to at least 20 m (66 ft).

Vocalizing is produced by vibrating vocal folds in the larynx. The folds of the American alligator have a complex morphology consisting of epithelium, lamina propria and muscle, and according to Riede et al. (2015), "it is reasonable to expect species-specific morphologies in vocal folds/analogues as far back as basal reptiles". Crocodilian vocal folds lack the elasticity of mammalian ones; but the larynx is still capable of complex motor control similar to birds and mammals and can adequately control its fundamental frequency.

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 towards 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 consequently, low energy requirements. They can withstand extended fasting, living on stored fat. Even recently hatched crocodiles are able to survive 58 days without food, losing 23% of their bodyweight during this time.

Crocodilians are ectotherms, relying mostly on their environment to control their body temperature. The sun's heat is the main means of warming for any crocodilian, while immersion in water may either raise its temperature by 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 the 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 but then move into the water for rest of the day. They may also move to land at nightfall, as it is cooler. Gaping with the mouth can provide cooling by evaporation from the mouth lining. By these means, the temperature range of crocodilians is usually maintained between 25 and 35 °C (77 and 95 °F), and mainly stays in the range 30 to 33 °C (86 to 91 °F).

Both the American and Chinese alligator can be found in areas that sometimes experience periods of frost in winter. In cold weather, they remain submerged with their tails in deeper, less cold water and their nostrils just projecting through the surface. If ice forms on the water, they maintain ice-free breathing holes, and there have been occasions when their snouts have become frozen into the ice. Temperature sensing probes implanted in wild American alligators have found that their core body temperatures can descend to around 5 °C (41 °F), but as long as they remain able to breathe they show no ill effects when the weather warms up.

All crocodilians need to maintain a suitable concentration of salt in body fluids. Osmoregulation is related to the quantity of salts and water exchanged with the environment. Intake of water and salts takes place across the lining of the mouth, when water is drunk, incidentally while feeding, and when present in foods. Water is lost during breathing, and both salts and water are lost in the urine and faeces, through the skin, and via salt-excreting glands on the tongue, though these are only present in crocodiles and gharials. The skin is a largely effective barrier to both water and ions, and gaping causes water loss by evaporation. Large animals are better able to maintain homeostasis at times of osmotic stress than smaller ones. 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 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 it is in the surrounding water. The animals are well-hydrated, and the urine in the cloaca is abundant and dilute, nitrogen being excreted as ammonium bicarbonate. Sodium loss is low and mainly takes place through the skin in freshwater conditions. In seawater, the opposite is true. The osmolality in the plasma is lower than the surrounding water, which is dehydrating for the animal. The cloacal urine is much more concentrated, white, and opaque, with the nitrogenous waste being mostly excreted as insoluble uric acid.

Crocodilians are amphibious, living both in water and on land. The last surviving fully terrestrial genus, Mekosuchus, became extinct about 3000 years ago after humans had arrived on its Pacific islands, making the extinction possibly anthropogenic. Typically they are creatures of the tropics; the main exceptions are the American and Chinese alligators, whose ranges consist of the southeastern United States and the Yangtze River, respectively. Florida, in the United States, is the only place that crocodiles and alligators live side by side. Crocodilians live almost exclusively in the lowlands, and do not appear to live above 1,000 metres (3,300 ft). With a range extending from eastern India to New Guinea and northern Australia, the saltwater crocodile is the most widespread species.

Various types of aquatic habitats are used by different crocodilians. Due to their diet, gharials are found in the pools and backwaters of rapid rivers. Caimans prefer warm turbid lakes and ponds and the slower-moving parts of rivers, with the expectation of the dwarf caimans which inhabit cool, relatively clear, fast-flowing waterways, often near waterfalls. The Chinese alligator is found in slow-moving, turbid rivers flowing across China's floodplains whiles the American alligator is highly adaptable and is found in swamps, rivers, or lakes with clear or turbid water. Crocodiles live in marshes, lakes and rivers, and can live in saltier environments including estuaries and mangrove swamps. American and saltwater crocodiles even swim out to sea, however "no species can be considered truly marine". Several extinct species have had marine habitats, including the recently extinct Ikanogavialis papuensis, which occurred in a fully marine habitat in the Solomon Islands coastlines. Climatic factors also affect crocodilians' distribution locally. During the dry season, caimans can be restricted to deep pools in rivers for several months; 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 tâmoûrts but will retreat into rocky shelters or underground during the driest periods and enter aestivation.

Crocodilians also exploit 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. The type of vegetation bordering the rivers and lakes inhabited by crocodilians is mostly tropical rainfiorst. These forests are of great importance to the crocodilians, creating suitable microhabitats where they can flourish. The roots of the trees absorb water when it rains, releasing it back slowly into the environment. This keeps crocodilian habitat moist during the dry season while preventing flooding during the wet season.

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.

Crocodilians are largely carnivorous, and the diets of different species can vary 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 up on fish, insects, and crustaceans, while extremely broad-snouted species with duller teeth, like the Chinese alligator and 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 generalised diets and opportunistically feed on invertebrates, fish, amphibians, other reptiles, birds, and mammals. Though mostly carnivorous, several species of crocodilian have been observed to consume fruit, and this may play a role in seed dispersal.

In general, crocodilians are stalk-and-ambush predators, though hunting strategies vary depending on the individual species and the prey being hunted. Terrestrial prey is stalked from the water's edge and then grabbed and drowned. Gharials and other fish-eating species sweep their jaws from side-to-side to snatch prey, and these animals can leap out of the water to catch birds, bats, and leaping fish. A small animal can be killed by whiplash as the predator shakes its head. When foraging for fish in shallow water, caiman herd them with their tails and bodies. They may also dig for bottom-dwelling invertebrates, and the smooth-fronted caiman will even leave the 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. They 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 spins its body continuously while holding on with its jaws, a maneuver known as the "death roll". During cooperative feeding, some individuals may hold on to the 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 go their separate ways. Food is typically consumed by crocodilians 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 that crocodilians cache kills for later consumption.

Crocodilians are generally polygynous, and individual males try to mate with as many females as they can. Monogamous pairings have been recorded in American alligators, and parthenogenesis has been observed in the American crocodile. 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 the 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 then grasps her with his hindlimbs, placing 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. The number of eggs laid in a single clutch ranges from ten to fifty. 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 (90 °F) produce more males, while those below 31 °C (88 °F) produce more females. However, 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 do occur.

All the hatchlings in a clutch may leave the nest in 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. For spectacled caimans in the Venezuelan Llanos, individual mothers are known to leave their young in the same nurseries, or crèches, and one of the mothers guards them. Hatchlings of some species tend to bask in a group during the day and while during the evening they start to forage separately. 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.

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

Juveniles are highly vocal, both when scattering in the evening and congregating again in the morning. Nearby adults, presumably the parents, may warn them of predators or alert the youngsters 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 the water to send out infrasonic signals that serve to attract females and intimidate rivals. The enlarged boss of the male gharial may serve as a sound resonator.

Another form of acoustic communication is the head slap. This typically starts with an animal in the water elevating its snout and remaining stationary. After some time, the jaws are opened sharply then clamped shut with a biting motion that makes a loud slapping sound, and this 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 the water with their tails. Episodes of head slapping spread through the group. The purpose varies, but it seems have a social function, and is also used in courtship. Dominant individuals intimidate rivals by swimming at the surface, displaying their large body size, and subordinate will submit by holding its head forward above the water with the jaws open and then flee below.

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 breed successfully: nests are submerged, developing embryos are deprived of oxygen, and juveniles get washed away. Despite the maternal care they receive, eggs and hatchlings commonly fall to predation. Numerous predators, both mammalian and reptilian, may raid nests and eat crocodilian eggs. After hatching and making it to the water, young are still under threat.

In addition to terrestrial predators, the hatchlings are also subject to aquatic attacks by fish. Birds take their toll, and in any clutch there may be malformed individuals that are unlikely to survive. In northern Australia, the survival rate for saltwater crocodile hatchlings is only twenty-five percent, but with each succeeding year of life this improves, reaching up to sixty percent by year five. Mortality rates are fairly low among subadults and adults, though they are occasionally preyed on by large cats and snakes. Elephants and hippopotamuses may kill crocodiles defensively. Authorities differ as to whether much cannibalism takes place among crocodilians. Adults do not normally eat their own offspring, but there is some evidence of subadults feeding on juveniles while they themselves may be preyed on by adults. Adults appear more likely to protect juveniles and may chase subadults away 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 and they reach sexual maturity at a certain length, regardless of age. Saltwater crocodiles reach maturity at 2.2–2.5 m (7–8 ft) for females and 3 m (10 ft) for males. Australian freshwater crocodiles take ten years to reach maturity at 1.4 m (4 ft 7 in). The spectacled caiman matures earlier, reaching its mature length of 1.2 m (4 ft) in four to seven years. Crocodilians continue to grow throughout their lives. Males in particular continue to gain in weight as they get older, but this is mostly in the form of extra girth rather than length. Crocodilians can live 35–75 years, and their age can be determined by growth rings in their bones.

Crocodilians are among the most cognitively complex nonavian reptiles. Embryological studies of developing amniotes have shown similar brain structures in the telencephalon between crocodilians, mammals, and birds. Accordingly, several behaviors once thought unique to mammals and birds have been recently discovered in crocodilians. Some crocodilian species have been observed to use sticks and branches to lure nest-building birds, though other authors have argued the purpose, if any, of stick-displaying is ambiguous at best. Several species have been observed to hunt cooperatively, herding and chasing prey. Play, or the free, intrinsically motivated activity by young individuals, has been observed on numerous occasions in crocodilians in both captive and wild settings, with young alligators and crocodiles regularly engaging in object play and social play. It is important to note that not all higher social behaviours are endemic across these clades. A 2023 study of a tinamou bird and American alligator test subjects found that alligators did not appear to engage in visual perspective-taking like the birds. Some researchers have proposed to increase the use of crocodilians as test animals in comparative cognition studies.

Crocodilians are opportunistic predators that are at their most dangerous in water and at the edge of water. Several species are known to attack humans and may do so to defend their territories, nests, or young; by accident, while attacking domestic animals such as dogs; or for food, as larger crocodilians can take prey as big as or bigger than humans. The species on which there is most data are the saltwater crocodile, the Nile crocodile, the mugger crocodile, the American crocodile, the American alligator and the black caiman. Other species which often attacked humans are Morelet's crocodile and the spectacled caiman.

It was estimated that over 1000 attacks by the Nile crocodilies occurred between 2010 and 2020, almost 70% of which were fatal. The species is considered to be the most dangerous large predator in Africa, particularly as it is both widespread and numerous. It can easily sneak up on people or domestic animals that come to the edge of the water. Fisherman, bathers, waders and those washing clothes are particularly vulnerable. Once grabbed and dragged into the water, it is unlikely that the victim will escape. Analysis of attacks show that most take place when crocodiles are guarding nests or newly hatched young.

Saltwater crocodiles have been implicated in over 1300 attacks between 2010 and 2020 with almost half being fatal. Attacks may come from animals of various sizes, but the larger males are generally responsible for fatalities. The bigger the individual, the bigger the prey they need, and humans are the right size. Most of the people attacked have been in water, but they occasionally occur on land. They sometimes attack boats but do not usually appear to be targeting the people in them. Attacks may occur when a human encroaches on the crocodile's territory. American alligators are recorded as making 127 attacks between 2010 and 2020, only six of which were fatal. Alligators are considered to be less aggressive than either the Nile or saltwater crocodile, but the increase in density of the human population in the Everglades has brought people and alligators into proximity and increased the risk of alligator attacks.

Crocodilians have been hunted for their skin, meat, and bones. Their tough skin has been used to make handbags, coats, footwear, wallets and numerous other items. The meat has been compared to that of chicken and may be used as an aphrodisiac. The bones, teeth and pickled heads of those reptiles are used as souvenirs, while other tissues and fluids are ingredients in traditional medicine. Crocodile farms have been established to meet the demand for crocodilian properties. Species bred on these farms are listed under Appendix II by CITES, which allows for regulated trade. A study examining alligator farms in the United States showed that they have generated significant conservation gains, and poaching of wild alligators has greatly diminished.

Several species of crocodilian are traded as exotic pets. They are appealing when young, but crocodilians do not make good pets; they grow large and are both dangerous and expensive to keep. As they grow older, pet crocodilians are often abandoned by their owners, and feral populations of spectacled caimans exist in the United States and Cuba. Most countries have strict regulations for keeping these reptiles.

The blood of alligators and crocodiles contains peptides with antibiotic properties, which may contribute to future antibacterial drugs. Cartilage from crocodiles raised in crocodile farms is also used in research to 3D-print new cartilage for humans by mixing human stem cells with liquified crocodile cartilage after proteins that may trigger the human immune system has been removed.

The IUCN Red List of Threatened Species recognises 26 species of crocodilian and classes 11 of them as threatened including:

• Critically Endangered: Chinese alligator, Philippine Crocodile, Orinoco crocodile, Siamese crocodile, Cuban Crocodile, African Slender-snouted crocodile and gharial.
• Endangered: False gharial
• Vulnerable: American crocodile, mugger crocodile, and dwarf crocodile.

The main threat to crocodilians worldwide is human activity, including hunting and habitat destruction. Early in the 1970s, more than 2 million wild crocodilian skins had been traded, driving down the majority of crocodilian populations, in some cases almost to extinction. Starting in 1973, CITES attempted to prevent trade in body parts of endangered animals, such as the crocodile skins. This proved to be problematic in the 1980s, as crocodiles were abundant and dangerous to humans in some parts of Africa, and it was legal to hunt them. At the Conference of the Parties in Botswana in 1983, it was argued on behalf of aggrieved local people that it was reasonable to sell lawfully hunted skins. In the late 1970s, crocodiles began to be farmed in different countries, started from eggs taken from the wild. By the 1980s, farmed crocodile skins were produced in sufficient numbers to destroy the unlawful trade in wild crocodilians. By 2000, skins from twelve crocodilian species, whether harvested lawfully in the wild or farmed, were traded by thirty countries, and the unlawful trade had almost vanished.

The gharial was historically widespread throughout the major river systems of India, but has undergone a chronic long-term decline since 1943. Major threats have included rampant hunting, accidental catching and water blockage from damns. The gharial population continues to be threatened by environmental hazards such as heavy metals and protozoan parasites. Protection of nests against egg predators has been shown to increase population numbers. The Chinese alligator was historically widespread throughout the eastern Yangtze River system but is currently restricted to some areas in southeastern Anhui thanks to habitat fragmentation and degradation. The wild population is believed to exist only in small fragmented ponds. In 1972, the species was declared a Class I endangered species by the Chinese government and received the maximum amount of legal protection. Since 1979, captive breeding programs were established in China and North America, creating a healthy captive population. In 2008, alligators bred in the Bronx Zoo were successfully reintroduced to Chongming Island. The Philippine crocodile is perhaps the most threatened crocodilian. Hunting and destructive fishing habits have reduced its numbers to around 100 individuals by 2009. In the same year, 50 captive bred crocodiles were released into the wild to help boost the population. Support from local people is crucial for the species' survival.

The American alligator has also suffered serious declines from hunting and habitat loss throughout its range, threatening it with extinction. In 1967 it was listed as an endangered species, but the United States Fish and Wildlife Service and state wildlife agencies in the Southern United States stepped in and worked towards its recovery. Protection allowed the species to recuperate, and in 1987 it was removed from the endangered species list. Similarly in Australia, the saltwater crocodile was heavily hunted to the point where only five percent of its historical numbers in remained in Northern Territory by 1971. Since then, the species was given legal protections and their numbers greatly increased by 2001.

Crocodilians have had prominent roles in the narratives of various cultures around the world and may even have inspired stories of dragons. In Ancient Egyptian religion, Ammit, the devourer of unworthy souls, and Sobek, the god of power, protection, and fertility, are both represented as having crocodile heads. This reflects the Egyptians' view of the crocodile both as a terrifying predator and an important part of the Nile ecosystem. The crocodile was one of several animals that the Egyptians mummified. Crocodiles were also associated with various water deities by peoples of West Africa. During the Benin Empire they symbolised the power of the king or oba and linked him to the living-giving rivers. The Leviathan described in the Book of Job may have been based on a crocodile. In Mesoamerica, the Aztecs had a crocodilian god of fertility named Cipactli who protected crops. In Aztec mythology, the earth deity Tlaltecuhtli is send to bond with a "great caiman". The Maya also worshipped crocodilian gods and even believed that the world was supported on the back of a swimming crocodile.

The gharial is featured in the folk tales of India. In one story, a gharial and a monkey become friends when the monkey gives the gharial fruit but friendship ends after the gharial confess they tried to lure him into this house to eat him. Native American and African American folktales of pair an alligator and with a tricker rabbit; Br'er Rabbit in the African American stories. An Australian Dreamtime story tells of a crocodile ancestor who had fire all to himself, until a "rainbow bird" stole fire-sticks for man; hence the crocodile lives in water.

Ancient historians have described crocodilians from the earliest written records, though often their descriptions contain as much assumption as observation. The Ancient Greek historian Herodotus (c. 440 BC) described the crocodile in detail, though much of his description is fanciful; he claimed that it would lie with its mouth open to permit a "trochilus" bird (possibly an Egyptian plover) to remove leeches. The crocodile was described in the late-13th century Rochester Bestiary, based on classical sources, including Pliny's Historia naturalis (c. 79 AD) and Isidore of Seville's Etymologies. Isidore asserts that the crocodile is named for its saffron colour (Latin croceus, 'saffron') and may be killed by fish with serrated crests sawing into its soft underbelly.

Crocodiles have been reputed to weep for their victims since the 9th century Bibliotheca by Photios I of Constantinople. The story became widely known in 1400 when the English traveller John Mandeville wrote his description of "cockodrills":

In that country [of Prester John] and by all Ind [India] be great plenty of cockodrills, that is a manner of a long serpent, as I have said before. And in the night they dwell in the water, and on the day upon the land, in rocks and in caves. And they eat no meat in all the winter, but they lie as in a dream, as do the serpents. These serpents slay men, and they eat them weeping; and when they eat they move the over jaw, and not the nether jaw, and they have no tongue.

Crocodilians, especially the crocodile, have been recurring characters in stories for children, such as Roald Dahl's The Enormous Crocodile (1978) and Emily Gravett's The Odd Egg (2008). Lewis Carroll's Alice's Adventures in Wonderland (1865) contains the poem How Doth the Little Crocodile. In J. M. Barrie's novel Peter and Wendy (1911), Captain Hook lost his hand to the crocodile. In Rudyard Kipling's Just So Stories (1902), the Elephant's Child acquires his trunk by having his nose pulled very hard by the crocodile.

In movies and shows, crocodilians are often represented as dangerous water obstacles or as monstrous man-eaters like in the horror films Eaten Alive (1977), Alligator (1980), Lake Placid (1999), Crocodile (2000), Primeval (2007) and Black Water (2007). In the film Crocodile Dundee, the title character's nickname comes from the animal that bit off his leg. Some media have attempted to portray these reptiles in more positive or educational light, such as Steve Irwin's wildlife documentary series The Crocodile Hunter.

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