Crocodilians are predatory, semi-aquatic reptiles of the order Crocodilia or Crocodylia. This group includes true crocodiles (family Crocodylidae), alligators and caimans (family Alligatoridae), and gharials (family Gavialidae). The name of the order originates from the type genus of this group, Crocodylus.
These large, lizard-like reptiles are believed to have originated in the Late Cretaceous Period (about 94 million years ago) and are phylogenetically the closest relatives of dinosaurs and birds. They are lizard-like in appearance and possess elongated and dorsoventrally flattened snouts that vary among the different members. As carnivorous animals, crocodilians mostly feed on fish, crustaceans, mollusks, birds, and mammals.
The largest crocodilian is the saltwater crocodile (Crocodylus porosus), which is typically 7 feet (5.2 meters) long but can reach up to 23 feet (7 meters). In contrast, the smallest living, Cuvier’s dwarf caiman, measures up to 1.6 meters (5.2 feet) in males and up to 1.2 meters (3.9 feet) in females.
They are sturdy, lizard-like in appearance, with long, flattened snouts and laterally compressed tails. The snout shape varies among the different species, with crocodiles having either broad or slender snouts, while alligators and caimans possess broad ones. In contrast, gharials have distinctively long snouts that make them easily recognizable.
They have two pairs of highly reduced limbs, of which the front pair has five digits with little or no webbing, whereas the hind pair has four webbed digits, followed by a rudimentary fifth one.
Crocodilians possess specialized cartilaginous extensions of bone from the ribs that allow the thorax to collapse during diving, while their broad pelvis accommodates large quantities of food. Both males and females have a single outlet at the base of the tail called the cloaca, where the intestinal, urinary, and genital tracts drain. They are sexually dimorphic, with the males usually larger than the females.
All crocodilians have extremely powerful jaw muscles that also add to the bite force of these reptiles. The closing muscles of the jaws are attached to the median portion of the lower jaw, while the hinge attaches to a joint called the atlanto-occipital joint that allows the animal to open its mouth wide. The closing muscles of the jaw are more massive than the ones that open them.
They are typically homodonts (with all their teeth of the same type) with conical, peg-like teeth that often vary depending on the species. For example, while slender-snouted species have uniform teeth, broad-snouted species have teeth of varying sizes. Crocodilians are the only non-mammalian vertebrates that have tooth sockets.
Although reptiles generally have three-chambered hearts with two atria and a single ventricle, crocodilians are an exception. They have characteristic four-chambered hearts (like birds and mammals) with two atria and two ventricles. The two main arteries for circulation, the left and right aorta, are connected by a hole called the Foramen of Panizza. All four heart chambers are partitioned by valves that prevent the blood from backflowing.
The respiratory organ in crocodilians is the lungs, which are connected to the liver and the pelvis by the diaphragmaticus muscle. When a crocodilian inhales air, the external intercostal muscles expand the ribs, while the ischiopubic muscle pushes the hip downward and the belly outward, allowing more space to accommodate the air. While exhaling, the internal intercostal muscles push the ribs inward, followed by the rectus abdominis muscle, which pulls the hips and liver forward and the belly inward. This respiratory mechanism is called the ‘hepatic piston’ as the lungs take over the space of the liver during expansion.
They have a relatively short digestive tract with a highly acidic stomach that divides into two chambers: a muscular gizzard for grinding meat and a digestive chamber that allows enzymes to break food chemically. The stomach also contains swallowed stones called gastroliths or gizzard stones that assist in food grinding. It also receives deoxygenated blood from the lungs, which helps form gastric acid secretions in the stomach.
Since these reptiles submerge themselves underwater, their eyes specialize in low-light vision. Behind the retina, a reflective layer called tapetum lucidum enhances vision in darkness. The fovea, a pit composed of photoreceptive cone cells, forms a horizontal bar in the retina’s middle. When completely submerged, a thin, transparent, or translucent membrane, known as the nictitating membrane, covers their eyes. The glands found on this membrane produce a lubricant that keeps the eyes clean.
The crown group, Archosauria, constitutes the most recent common ancestor of crocodilians, birds, and all their descendants. It comprises the false crocodiles (clade Pseudosuchia) and Avemetatarsalia, comprising all living crocodilians and archosaurs more closely related to crocodilians than birds.
The phylogenetic relationships between the order Crocodilia have remained debatable. Many cladistic studies had initially placed the two short-snouted crocodilian families, Alligatoridae and Crocodylidae, close to each other. The family of long-snouted crocodilians, Gavialidae, was kept as a divergent branch of the phylogenetic tree. The resulting short-snouted crocodilians were grouped under the clade Brevirostres based on morphological evidence.
However, recent DNA sequencing of extant crocodiles suggests the long-snouted gavialids are more closely related to crocodiles than to alligators, thus placing both groups under a new clade called Longirostres.
These reptiles live partly on land and in water. Although they are mostly found in the tropics, the American and Chinese alligators extend as far as the southeastern United States and the Yangtze River of the Tibetian Plateau in China. In the United States, Florida is the only state where alligators and crocodiles live side by side. The saltwater crocodile is the most widely distributed of all crocodilians, with a range extending from eastern India to New Guinea and Australia.
While some crocodilians prefer swamps, ponds, and lake edges with plentiful sunlight, others spend most of their time in lower stretches of rivers. They rarely venture into the sea and sometimes inhabit brackish-water mangrove swamps and estuaries.
Asian gharials are found in river pools and backwaters, whereas South American dwarf caimans often inhabit areas near waterfalls. Similarly, the American alligator, being highly adaptable, lives in almost every aquatic environment, such as swamps, rivers, or lakes with clear or turbid water. On the other hand, the Chinese alligator is primarily found in slow-moving, turbid rivers across floodplains. In Mauritania, desert crocodiles live in caves and burrows during summer, while they are found in cool pockets of water called gueltas in monsoons.
They are carnivores that usually feed on insects, crustaceans, birds, fish, and small mammals. Chinese alligators and broad-snouted caimans specialize in consuming hard-shelled mollusks.
Crocodilians communicate using a range of sounds, such as roars, growls, barks, bellows, whines, hisses, and grunts. While inside their eggs, the young start to squeal and announce their hatching, whereas, once outside the egg, they produce yelps and grunts, usually in response to external stimuli. Similarly, the parents vocalize to alert the juveniles against imminent danger or hint at the availability of resources.
While alligators are the most noisy crocodilians, some crocodiles are almost mute. The American alligator is exceptionally noisy, producing about seven bellows at a ten-second interval, while its males emit infrasonic signals underwater to attract their mates. Adult females of New Guinea and Siamese crocodiles are found roaring when confronting other adults, whereas Nile crocodiles grunt or bellow when threatened.
Sometimes, they also communicate through a ‘head slap,’ in which they open their jaws and shut them forcefully, producing a loud, slapping sound. They then submerge their heads underwater, creating plenty of bubbles on the water’s surface. This type of acoustic communication is used to court females and maintain sociality in groups.
All adult crocodilians are extremely territorial and usually prefer leading solitary lives, while some are occasionally gregarious and aggregate during adverse environmental conditions. Male saltwater crocodiles form territories they maintain throughout the year, often including female nesting sites within them. They guard these regions vehemently and defend them from their conspecifics.
While crocodilians usually live between 35 and 75 years, the oldest known species, an Australian crocodile (that has lived in captivity), is estimated to be around 120 years old.
Crocodilians are polygynous, with the males having multiple female partners. However, monogamy has been observed in American alligators.
Usually, the dominant male defends his territory containing the female nesting colonies and tries to attract a potential mate through courtship. American alligator males rub themselves against the females, circle around them, and display different patterns during swimming. The male grasps the female with his hindlimbs and places his tail under hers to align their cloacas so that males can insert their penis. Copulation usually occurs in water, persisting for up to 15 minutes, during which the copulating pair constantly submerges and rises to the surface.
Depending on the species, the females prepare nets, mounds, or holes for laying eggs within a month of mating. These nests are made by assembling vegetation, litter, sand, or soil, following which around ten to fifty eggs are laid in a single clutch. These eggs, covered by hard calcareous shells, are incubated for two to three months. The temperature at which the eggs are incubated determines the sex of the offspring (Temperature-dependent Sex Determination). Nest temperatures above 32°C (90°F) result in a higher proportion of male offspring, while temperatures below 31°C (88°F) lead to the production of more females. Although hatchlings of both sexes are found in most nests, some single-sex clutches are also found in the wild.
The mother assists the hatchlings in getting to the water by carrying them in her mouth. She stays close to them until they mature and can join the adults. In American alligators, the young associate with adults in one to two years, whereas saltwater and Nile crocodile juveniles take a few months to do so.
Though crocodilians themselves are predators, crabs, birds, and mammals, such as raccoons, often target their eggs and juveniles for food. Humans are their greatest predators as they hunt them for their skin and also destroy their natural habitats.
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