Crocodiles

Among the more distinctive characteristics of the crocodilians are their almost completely four-chambered heart, teeth that are set into sockets in the jaw, a palate that separates the mouth from the nasal chambers, and spongy lungs. These animals also have a protective covering of partially calcified, horny plates on their back. The plates are not connected with each other, and are set into the thick, scaly skin, allowing a great freedom of movement. Crocodilians have a heavy body with squat legs and a large, strong, scale-ridged tail.

Sinusoidal motions of the powerful tail are used to propel the animal while swimming. The tail is also a formidable weapon, used to subdue prey and also for defense. Although they appear to be ungainly and often spend most of their time lying about, crocodilians can actually move quite quickly. Some crocodilians can even lift their body fully above the ground, and run quickly using all four legs; a human cannot outrun a crocodile over a short distance on open land.

Crocodilians have numerous adaptations for living in water. They have webbed feet for swimming slowly and their nostrils, eyes, and ears are set high on the head so they can be exposed even while most of the body and head are below the surface. When a crocodilian is totally submerged, its eye is covered by a semi-transparent nictitating membrane, and flaps of skin seal its nostrils and ears against water inflow. Crocodilians often float motionless in the water, commonly with the body fully submerged and only the nostrils and eyes exposed. To accomplish this behavior, crocodilians regulate their body density by varying the amount of air held in the lungs. Also, the stomach of most adult crocodiles contain stones, to as much as 1% of the animal's total body weight. The stones are thought to be used as buoyancy-regulating ballast.

Crocodilians are poikilothermic, meaning they do not regulate their body temperature by producing and conserving metabolic heat. However, these animals are effective at warming themselves by basking in the sun, and they spend a great deal of time engaged in this activity. Crocodilians commonly bask through much of the day, often with their mouths held open to provide some cooling by the evaporation of water. Only when the day is hottest will these animals re-enter the water to cool down. Most species of crocodilians are nocturnal predators, although they will also hunt during the day if prey is available.

Stories exist of birds entering the open mouths of crocodiles to glean leeches and other parasites. This phenomenon has not been observed by scientists, although it is well known that crocodiles will tolerate certain species of birds picking external parasites from their skin, but not necessarily inside of their mouth.

Male crocodilians are territorial during the breeding season, and chase other males away from places that have good nesting, basking, and feeding habitat. The male animals proclaim their territory by roaring loudly, and sometimes by snapping their jaws together. Intruders are aggressively chased away, but evenly matched animals may engage in vicious fights. Territory-holding males do not actively assemble females into a harem. Rather, they focus on chasing other males away from a territory. Females will enter the defended territory if they consider it to be of high quality.

All crocodilians are predators, and they have large, strong jaws with numerous sharp teeth for gripping their prey. Crocodiles do not have cutting teeth. If they capture prey that is larger than they can eat in a single gulp, it is dismembered by gripping strongly with the teeth and rolling their body to tear the carcass. Some crocodiles will opportunistically cooperate to subdue a large mammal, and then to tear it into bits small enough to be swallowed. However, extremely large animals with tough skin, such as a dead hippopotamus, will be left to rot for some time until the carcass softens and can be torn apart by the crocodiles.

All crocodilians are oviparous, laying hard, white-shelled eggs. The nest may be a pit dug into a beach above high water, or it may be made of heaps of aquatic vegetation, which help to incubate the eggs through heat produced during decomposition. Often, a number of females will nest close to each other, but each builds a separate nest. The nesting grounds are used by the same females, year after year. In many species the female carefully guards and tends her nest. Fairly open, sandy beaches are generally preferred as sites upon which to build the nest or nest mound.

Typically, 20–40 eggs are laid at a time, but this varies with species and the size of the female. Incubation time varies with species and temperature, but ranges, in the case of the Nile crocodile, from 11 to 14 weeks. Predators as diverse as monitor lizards, mongooses, dogs, raccoons, and even ants seek out crocodile nests to eat the eggs and newly hatched young.

An infant crocodilian has a small, so-called "egg tooth" at the end of its snout, which helps it to break out of the shell when ready to hatch. All of the baby crocodilians hatch within a short time of each other, synchronized in part by the faint peeping noises they make during the later stages of incubation. In some crocodilians, the mother assists her babies in hatching, by gently taking eggs into her mouth and cracking them with her teeth. The mother also may guard her offspring for some time after hatching, often allowing them to climb onto her body and head. Female crocodiles are very aggressive against intruders while their eggs are hatching and newborn babies are nearby, and under these circumstances they will even emerge from the water to chase potential predators away. Nevertheless, young crocodilians are vulnerable to being eaten by many predators, and this is a high-risk stage of the life cycle.

Potentially, crocodilians are quite long-lived animals. Individuals in zoos have lived for more than 50 years, and the potential longevity of some species may be as great as a century.