Crocodiles or true crocodiles are lizard-like semiaquatic reptiles found in tropical brackish, saltwater, and freshwater bodies. They belong to the family Crocodylidae within the order Crocodilia. The term ‘crocodile’ is derived from the Greek word ‘krokódilos,’ which means lizard, indicating their similarity to reptiles.
This group informally refers to all living members of the order Crocodilia, which includes alligators and caimans, gharials and false gharials, and some extinct taxa, although they belong to separate biological families. They are excellent predators because of their unique body structure, with their eyes, ears, and nostrils located right on top of their heads, allowing them to easily scan above the water’s surface while their bodies remain submerged in water.
They vary greatly in size. The largest crocodile species in the world is the saltwater crocodile (Crocodylus porosus), which is typically 17 ft (5.2 m) long but can reach 23 ft (7 m) and weigh over 1,000 kg (2,200 lbs). A saltwater-Siamese hybrid crocodile called Yai is the largest crocodile ever held in captivity, measuring around 6 m (20 ft) in length. Lolong, the largest crocodile ever caught (also a saltwater species), measured more than 20 ft and weighed around 2370 lbs.
In contrast, the African dwarf crocodile (Osteolaemus tetraspis), the world’s smallest crocodile, measures around 1.8 m (5.9 ft). In general, male crocodiles grow larger and more rapidly in size than females.
A crocodile’s external morphology reflects its aquatic and predatory lifestyle. It has a streamlined body, with nostrils, eyes, and ears placed at the highest part of its head. Two pairs of powerful jaws each have a row of conical teeth (sometimes more than 100 in species with long muzzle). These teeth are found in sockets and are replaced continuously throughout their lifespan (polyphyodonty).
They have two pairs of short but sturdy legs, with the forelimbs having five toes. In contrast, the hindlimbs have four toes, which are webbed, either wholly or partially.
A crocodile differs anatomically from alligators and caimans in snout shape and width of jaws. They have a more V-shaped snout than the U-shaped snout of alligators and caimans. Also, their upper and lower jaws are the same width, with the teeth placed along the edge of the upper jaw, visible even when the jaws are closed. The fourth tooth in the lower jaw fits into a socket in the upper jaw.
Crocodiles have diapsid skulls, characterized by two holes or fenestrae behind the eyes on each side. The palate bone connects the nostrils to the glottis and has a rigid tissue called the palatal flap that blocks water entry into the mouth. The tongue is suspended from the palate by a membranous extension, while the teeth arise from the two fenestral sockets.
However, this complex anatomy has evolved over millions of years, starting from the earliest representatives of the crocodilian group, the Protosuchia.
Evolution
They have smooth skin on their sides and bellies, while the dorsal surface is thick, rough, and covered with bony, calcareous scales known as ostracoderm. These scales are highly vascularized (contain many blood capillaries), allowing them to absorb heat through the exoskeletal layer.
Unlike most reptiles with three-chambered hearts, crocodiles, like mammals and birds, have two distinct atria and ventricles comprising a four-chambered heart. The left and the right aorta are connected through an aperture called the Foramen of Panizza.
They possess a pair of eyes with vertical slit-like pupils that protect the cornea from scorching sunlight. A layer of tissue called tapetum lucidum is found on the rear wall of the eye that reflects the incoming light to the retina, enabling effective vision even in low light. Apart from the upper and lower eyelids, crocodiles also have a third called the nictitating membrane, extending over the eyes for protection. Photoreceptor cells called rods and cones provide them with color vision.
Crocodiles also have a highly developed sense of smell aided by a single olfactory chamber in adults, while a sensory vomeronasal organ functions the same in the young. Hearing is facilitated by a thin, cone-shaped tympanic membrane, often covered with flat muscular flaps.
In 1807, Georges Cuvier named Crocodylidae a family under the larger superfamily Crocodyloidea, which also encompasses extinct crocodile relatives. Crocodylidae is presently considered a crown group comprising the most recent common ancestor of the Nile crocodile, the Dwarf crocodile, and their descendants.
Crocodiles are divided into two subfamilies: Crocodylinae and Osteolaeminae. The former contains 13 extant species, while the latter has two extant genera, Osteolaemus and Mecistops, along with three extinct genera.
Recent DNA sequencing studies suggest crocodiles are more closely related to gavialids than alligators. Some morphological studies consider Mecistops to be a basal member of Crocodylinae, which is more closely related to Crocodylus than Osteolaemus.
Crocodiles mainly inhabit the lowlands and waterbodies of humid tropical regions. They are abundant in sub-Saharan Africa, Southeast Asia (including India, Indonesia, and Sri Lanka), the East and West Indies, Mexico, Central America (with around 2,000 crocodiles in Florida), and northern South America. Both freshwater and saltwater crocodiles can be found as far south as Northern Australia.
They are carnivorous animals. While young and smaller crocodiles feed on fish, amphibians, mollusks, crustaceans, and birds, larger species, such as the Nile crocodile and the saltwater crocodile, prey upon large mammals, like wild boar, buffalo, deer, and wildebeest. They also scavenge on carcasses and sometimes cannibalize smaller crocodiles.
Crocodiles usually lurk underwater and scan for their prey for a long time. They suddenly rise out of the water, grasp the prey using their powerful jaw muscles, and bite it with exceptionally high force.
Having gulped the prey, some crocodiles, like the Nile crocodile, use their left aortic arch to direct deoxygenated blood from their muscles to the stomach. The carbon dioxide in the blood increases the stomach acidity and helps dissolve the prey’s flesh easily. Sometimes, they deliberately swallow stones called gastroliths or stomach stones, which act as ballast to balance their bodies or help break the ingested food.
Crocodiles are the most social reptiles. Although they do not particularly form social groups, Mugger crocodiles aggregate during the feeding or basking period. Some species, such as Nile crocodiles, are extremely territorial and always exist in a social hierarchy where the dominant, aggressive male can access the best basking site and resources, while the females are prioritized only during group and surplus feed. Like most social animals with a dominant hierarchy, all male crocodiles are extremely aggressive toward each other while competing for potential females during the mating season.
Among all reptiles, they are the most vocal, producing various sounds depending on their species, age, gender, and size. Most conspecific vocalizations occur during the breeding season for courtship. They make different sounds to assert their territoriality and also during feeding.
While the young produce chirps from within the eggs as a signal of hatching, juveniles emit a high-pitched distress call to alarm the elders of imminent danger. Female crocodiles produce threat calls to alert others of the presence of their eggs in a nest.
Males’ bellow’ by raising their bodies out of the water and slowly moving their tails sideways. They inflate their throats and close their mouths to produce an infrasonic signal underwater that is sharp enough to produce vibration in surrounding objects.
Crocodiles possess remarkable cognitive abilities, such as recognizing prey behavior patterns at different hours of the day or seasons. They also engage in different types of play, such as sliding down slopes (locomotor play) and riding on the backs of other group members (social play).
Crocodiles are long-lived animals, with some individuals living for centuries. Their average lifespan is around 30 to 40 years, but larger species can live for 60 to 70 years.
A male crocodile named Kolya lived in a Russian zoo for 110 to 115 years, while a male freshwater crocodile is estimated to have lived 120 to 140 years in an Australian Zoo.
During courtship, the male and female crocodiles interact through a series of courtship displays, snout-rubbing, and specific vocalizations. Mating usually occurs multiple times underwater, following which the females get ready to build nests or dig holes to lay eggs. While holes are bored in sand, mound nests are built on the ground using vegetation. At night, the females lay between 7 and 95 hard-shelled, translucent eggs (depending on species) for 30 to 40 minutes.
The eggs are incubated for around 80 days, and the incubation temperature determines the offspring’s sex (temperature-dependent sex determination). While eggs incubated at temperatures less than or equal to 30 °C (86 °F) give rise to females, those incubated between 32 and 33°C (90 to 91 °F) predominantly form males. At 31 °C (88 °F), the offspring are of both sexes, while those incubated above 33 °C continue to give rise to males.
When the young are ready to hatch, they squeak from within the eggs. Listening to their calls, the female either abandons the nest or carries the eggs in her mouth and rolls them on the ground to facilitate hatching. The young pierce the eggshell using the egg tooth at the tip of their snouts to set themselves free. The female introduces the group of hatchlings (called a pod or creche) to the water and feeds them for over a year until the next mating season. If the female is absent, the male parent takes care of the pod.
Although crocodiles are formidable predators, humans often hunt them for their use. Their hatchlings are generally preyed upon by crabs, birds, and raccoons.
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