Crocodiles are a living link with the dinosaur-like reptiles of prehistoric times and are the nearest living relatives of the birds (bird). A large variety of crocodilian fossils (fossil) have been discovered that date back 200 million years to the Late Triassic (Triassic Period) Epoch. Fossil evidence also suggests that three major radiations occurred. Only one of the four suborders of crocodiles has survived to modern times. Order Crocodylia includes the “true crocodiles,” alligators (alligator), caimans (caiman), and gavials (gavial).

All crocodiles have a relatively long snout, or muzzle, which varies considerably in shape and proportion. The scales (scale) that cover most of the body generally are arranged in a regular pattern, and thick, bony plates occur on the back. Families and genera are primarily distinguished by differences in skull anatomy. Species are identified principally by the proportions of the snout; by the bony structures on the dorsal, or upper, surface of the snout; and by the number and the arrangement of the scales.

Throughout their range, crocodile populations have declined as human occupation and land use change has reduced their habitat. Many crocodilian species have been greatly depleted by overhunting for their valuable skins—which provide leather for handbags, shoes, belts, and other articles. Local use of crocodiles for meat and medicines is also widespread. People living near crocodiles often dislike them because they become entangled in fishnets, prey on pets and livestock, and occasionally kill people.

Since about 1970, improved national protection, habitat conservation, and international regulation of trade have allowed many populations to recover. About half of the 23 species remain widespread and numerous with little chance of extinction. According to the International Union for the Conservation of Nature and Natural Resources (IUCN), several species (the Chinese alligator 【A. sinensis】, the Orinoco crocodile 【Crocodylus intermedius】, the Philippines crocodile 【C. mindorensis】, the Siamese crocodile 【C. siamensis】, and the Indian gavial 【G. gangeticus】) are critically endangered and face extinction if human pressures on their habitat are not relieved.

Sustainable harvesting, regulated trade, and education have become valuable components of crocodilian conservation. Worldwide, various programs exist to provide incentives and economic benefits to communities that conserve habitat. For example, some communities in the Philippines are paid for each egg made safe from collectors or those intent on destroying nests. Crocodilian conservation has become a model for sustainable resource use. Commercial production by captive breeding, the collection of excess eggs from the wild, and regulated hunting yield 800,000–1,000,000 legal skins each year to international markets.

Crocodiles in captivity are known to have reached ages of 70 or more years, but longevity in the wild is poorly known. life span estimates in the wild are based on growth rates, and limited studies of bone growth rings suggest that the life spans of wild crocodiles and those in captivity may be similar. A Nile crocodile (Crocodylus niloticus) or an estuarine (or saltwater) crocodile (C. porosus) 6 metres (about 20 feet) long may live as long as 80 years. On average, the life span of alligators (alligator) and caimans (caiman) is thought to be between 30 and about 60 years, whereas the true crocodiles are thought to live 50 to 75 years.

Crocodiles are mostly nocturnal animals. They are predators and spend most of their time in the water; although they are also known to make journeys of several kilometres over land. In the first weeks of life, crocodiles eat insects (insect), crustaceans (crustacean), snails (snail), small fishes, frogs (frog), and tadpoles (tadpole). Older crocodiles mainly eat fish and are more apt to prey upon waterfowl and on mammals (mammal). Occasionally, a member of one of the larger species eats a human, though such incidents happen so infrequently that crocodiles cannot be generally regarded as man-eaters.

Crocodiles capture water animals in their jaws with a sideways movement of the muzzle. They have sensitive pressure receptors located in pits in the scales around the mouth that detect motion; these structures assist in the capture of prey in dark or murky water. To catch land animals, a crocodile floats passively or remains motionless at the edge of the water where prey habitually drink. With a sudden lunge, it seizes an unsuspecting animal and drowns it. If the prey is large, a crocodile may grip portions of the victim in its jaws and rotate rapidly in the water to tear the prey apart.

Crocodiles thermoregulate (thermoregulation) by alternately sunning themselves and retiring to shaded areas or cooler water. Larger individuals maintain stable body temperatures in the preferred range of 30–32 °C (86–90 °F) for several hours, even overnight. As a result, these individuals enjoy increased metabolic efficiency. Several studies of wild populations suggest that complex social relations between individuals are expressed as dominance hierarchies (dominance hierarchy) that allow dominant animals better access to preferred sunning and nesting sites. Some crocodiles also dig burrows into the banks of lakes or rivers. Burrows may extend for several metres in length and end in a chamber where individuals seek refuge from drought or cold.

The crocodilian brain is relatively tiny compared with the size of the body; the brain of an alligator that is 4 metres (13 feet) long weighs only 11 grams (0.02 pound). Nevertheless, crocodiles are capable of complex behaviours. They are often curious and show evidence of rapid learning. Captive individuals of some species are known to recognize their keepers and show neither fear nor aggressiveness. These animals beg for food, and some even permit themselves to be petted.

Crocodilian locomotion is the product of a unique body form. The lateral processes of the spine are joined to interlocking bony plates of the dorsal scales by complex ligaments (ligament). This arrangement produces a semirigid “I-beam” structure that encloses the back muscles. This structure is rigid but flexible, and it allows an efficient transfer of energy from the tail when swimming and an erect body posture when walking. When swimming, the crocodile places its legs back against the sides of the body and moves forward by means of lateral wavelike motions of the tail. When walking on land, crocodiles hold themselves high on all four legs. The characteristic sinusoidal (side-to-side) flexure of the body is caused by the movement of a front leg in concert with the opposing hind leg during each step. A cantilevered tail also balances the body. When moving quickly into the water from a bank, crocodiles slide on their bellies and push themselves forward with the feet. Crocodiles are also capable of galloping short distances.

Crocodiles are sexually dimorphic, and adult males are larger than females. Copulation occurs in the water, and it is preceded by a complex courtship in which the animals signal each other using changes in body profile, touch, and vocalization. Chemical signals are probably also part of courtship. The male then mounts the back of the female, and both animals rotate their tails so that the respective cloacae are brought into contact and intromission of the male erectile organ is achieved.

All crocodiles lay hard-shelled eggs, which may weigh 50–160 grams (0.1–0.4 pound) each. A female lays an average of 12–48 eggs per nest, depending upon her age, size, and species. Two general forms of nest building are known. Some species, such as the Nile crocodile (C. niloticus), dig a hole in the ground and refill it with dirt after the eggs are deposited. Others, such as the estuarine (or saltwater) crocodile (C. porosus), build a mound of plant material and soil. The eggs are deposited into the mound, and the sun's heat, the warm environment, and the natural decay of vegetation maintain a warm temperature that aids the development of the embryo. Egg incubation takes from 55 to 100 days. As is the case for many other reptiles, the sex of the developing embryo is determined by the temperature of the incubating egg at a critical time, which is during the first half of its incubation period. Cooler and warmer temperatures produce females, intermediate temperatures produce males, and temperatures near 31 °C (88 °F) produce both sexes. Until hatching occurs, the female usually remains close to the nest to protect the eggs from predators.

After two or three months, the young are fully developed and ready to hatch. While still in the egg, the young may utter squeaks, perhaps signaling that they are ready to emerge. The adult female removes the dirt or other debris from the eggs and assists the hatchlings to the water. In many cases, the female opens the eggs carefully with her tongue and carries the hatchlings in her mouth. The female remains close to her offspring and provides protection from predators for several weeks to months. As the young grow, they disperse, but they may remain near their nest site for over a year. In captivity, males have also been observed assisting in the hatching process and providing hatchling care.

Crocodiles are inhabitants of swamps, lakes, and rivers, although some species make their way to brackish water or to the sea. The estuarine (or saltwater) crocodile (C. porosus) and the American crocodile (C. acutus) are capable of living in marine waters and may swim miles out to sea, although both species normally occupy brackish and freshwater habitats. Glands (gland) in the tongue allow the excretion of excess salt. The smooth-fronted caiman (Paleosuchus trigonatus) of South America prefers rocky, fast-flowing rivers. In West Africa the dwarf crocodile (Osteolaemus tetraspis) is found principally in the rivers of the forest regions. Crocodiles may affect nutrient cycling, ecosystem function, and fisheries (fishery) productivity in the areas they inhabit.

Like many nocturnal animals, crocodiles have eyes (eye, human) with vertical, slit-shaped pupils (pupil); these narrow in bright light and widen in darkness, thus controlling the amount of light that enters. On the back wall of the eye, the tapetum lucidum reflects incoming light, thus utilizing the small amount of light available at night to best advantage. In addition to the protection provided by the upper and lower eyelids, the nictitating membrane (that is, a thin, translucent eyelid) may be drawn over the eye from the inner corner while the lids are open. The delicate eyeball surface is thus protected under the water, while a certain degree of vision is still possible. Unlike the ears (ear, human) of other modern reptiles (reptile), those of the crocodile have a movable, external membranous flap that protects the ears from the water. The sense of smell is well developed and may even operate in the egg prior to hatching.

The entire underside of the crocodile has a regular pattern of scales, which are smaller than those on the upper surface. These scales are rectangular, entirely smooth, and contain little or no bone material. An exception to this condition occurs in caimans of the genera Melanosuchus, Caiman, and Paleosuchus, in which the surface plates on the lower side are also bony. The cloaca—a chamber containing the genital, anal, and urogenital openings—extends longitudinally within the body; it is surrounded by an oval area of small scales on the underside of the body. The cloacal vent is slightly posterior to the attachment of the hind legs at the base of the tail. In contrast to the back and belly, the sides of the body have mostly small knobby scales. As a result, the flanks are distensible, which is necessary for breathing and for the expansion of the body that occurs in gravid females.

The legs of the crocodile are short but powerful. The forefeet (foot) have five toes—the usual number for class Reptilia. The hind legs are more powerfully developed than the front pair, and the hind feet have only four toes, which are wholly or partially webbed. The upper part of the tail carries two crests of high triangular scales that gradually merge near the middle of the tail.

Like all reptiles (reptile), crocodiles are diapsids—i.e., they have two openings on each side of the skull. Crocodiles also show the most important characteristics of the group that includes the dinosaurs (dinosaur) (subclass Archosauria (archosaur)). The skull exhibits distinctly developed upper and lower temporal fenestrae (that is, openings behind the eye sockets); the teeth arise from sockets, and the roof of the skull lacks an opening for the parietal organ—a median, dorsal outgrowth of the brain (see tuatara: Form and function (tuatara)). Within the Archosauria, the crocodiles are a separate order, since they have developed a secondary bony palate, which encloses the nasal passage from the exterior nasal openings to the choanae (internal nostrils). These features occur even in the most primitive representatives of the crocodilian group, namely the Protosuchia of the Late Triassic (Triassic Period) Epoch (about 228–200 million years ago); but their muzzles were very short, and the choanae were relatively far forward on the palate.

As crocodiles continued to evolve, the openings of the choanae tended to be located farther back. In the Mesosuchia of the Jurassic (Jurassic Period) (200–146 million years ago) and Cretaceous (Cretaceous Period) (about 146–65.5 million years ago) periods—to which the long-snouted ocean crocodiles also belong—the choanae were already located at the posterior part of two bones of the skull (palatines). In the true crocodiles (Eusuchia), which appear in the Upper Jurassic, the choanae are entirely enclosed by the pterygoids (that is, the paired bones on the lower part of the cranium). In modern species they are found at the posterior border of the palate. The Sebecosuchia, which existed from the Late Cretaceous (about 100–65.5 million years ago) to the Miocene Epoch (23–5 million years ago), possess skulls with flattened sides, and the choanae lie in a depression in the anterior part of the pterygoids. Modern genetic studies of crocodilian DNA have been used to identify species differences and, in some cases, indicate natural hybridization.

Extinct groups represented only by fossils are indicated by a dagger (†).

Widely different views prevail concerning the classification of the living groups of Eusuchia—the alligators (alligator) and caimans, the true crocodiles, and the gavials (gavial). The three groups have been treated here as distinct families within suborder Eusuchia. Of these, the alligators and the true crocodiles are more closely related to each other, and they are sometimes regarded as constituting two subfamilies of the family Crocodilidae. Some authors regard the gavials as a third subfamily. Tomistoma schlegelii, the Malaysian false gharial, has been placed by some authors in Crocodilidae and by others with the Gavialidae.

In addition, one authority has separated the order Crocodilomorpha into two suborders, Crocodylia and Paracrocodylia. According to this scheme, the Crocodylia include as infraorders those groups given above as suborders. This scheme also contains a suborder, Thalattosuchia.