Animals

Crocodilia (sometimes spelled Crocodylia) is the order of reptiles that includes alligators, caimans, crocodiles, gharials (or gavials) and false gharials (or false gavials).

There are three families of crocodilian - Alligatoridae (alligators and caimans), Crocodylidae (crocodiles) and Gavialidae (gharials and  false gharials).

Most of the crocodilians have evolved to be powerful carnivores that can kill prey much larger than themselves.

Saltwater crocodileThe earliest crocodilian fossils that we know of come from the Triassic Period, which occurred from about 250 to about 200 million years ago.

There are 23 species of crocodilian living on Earth today.

Most live in tropical and semitropical regions of Africa, Asia, North America and South America.

The saltwater crocodile (Crocodylus porosus), also known as the estuarine crocodile,  is the largest living crocodilian on Earth, as well as the largest living reptile in the world. Adult male saltwater crocodiles may be as much as 17 feet long. There have been reports of saltwater crocodiles over 20 feet long.

Saltwater crocodiles live in Australia, India and Southeast Asia

The smallest crocodilian on Earth is Cuvier's dwarf caiman, which lives in South America. It grows to about 4 or 5 feet long.

Many crocodilian species are now threatened with extinction because of habitat loss and overhunting.

Cuvier's dwarf caimanCrocodilians are often hunted for their thick hides, which are used to make shoes and luggage.

Some people consider crocodile meat to be a delicacy.

Physical Characteristics

Alligators, crocodiles and other crocodilians have heavy bodies with squat legs and strong heavy tails, which they use as weapons.

They also use their tails to propel themselves forward in water.

Crocodilians are very fast swimmers. They can swim at speeds of up to 20 miles an hour.

American alligator

They have webbed feet.

Despite its squat appearance, a crocodilian can move very quickly on land

Over a short distance, a crocodilian can run up to 11 miles an hour and can outrun a human.

A crocodilian's skin acts like armor. It is covered in scales that do not overlap. These scales have small, bony plates underneath them.

Crocodilians have strong jaws, with teeth that are set in bony sockets.

When an alligator or a crocodile loses a tooth, it grows back. A crocodilian's teeth are continually replaced until the crocodilian becomes very old.

A crocodilian's eyes, ears and nostrils are high on its head, so they can remain above water while most of the crocodile's body is hidden below the surface.

A crocodilian's ears and nostrils are covered with movable skin flaps that close when the crocodile dives.

GharialWhen a crocodilian is under water, a valve at the back of the throat, which is known as a palatal valve, closes off the lungs. This allows the crocodilian to open its mouth underwater without drowning.

Crocodilians are the only reptiles with four-chambered hearts. Mammals and birds also have four-chambered hearts. Most reptiles have three-chambered hearts.

Eyes

Crocodilians have excellent vision in daylight and in the dark.

A crocodilian has vertical pupils - like a cat's pupils - that can open wider than a human's eyes to let in more light.

A layer of tissue, known as a tapetum lucidum, lies behind a crocodile's retina and reflects light back through the retina. This makes it easier for the crocodile to see at night.

The tapetum lucidum makes the crocodile's eyes glow in the dark.

Many vertebrates have tapeta lucida.

A transparent membrane, which is known as a third eyelid, or a nictitating membrane, covers a crocodilian's eyes. The nictitating membrane protects the eyes when the crocodilian dives underwater.

Digestion

A crocodilian's stomach is divided into two sections.

The first section resembles a bird's gizzard and contains gastroliths - small stones that are swallowed by the crocodile.

These gastroliths help to grind food.

They also act as ballast, allowing the crocodilian to float in the water with only its eyes and nostrils exposed, so that it can hide while waiting for prey to come close.

The second part of a crocodilian's stomach secretes very strong acid. A crocodilian's stomach is more acidic than the stomach of any other animal.

This extremely high level of stomach acid allows the crocodilian to digest every part of its prey - even bones and horns.

Dermal Pressure Receptors

Crocodilians have pits on their on their skin that allow them to detect changes in pressure in the water.

These sensory organs, which look like black spots or stubble, are known as dermal pressure receptors. They have a role that is similar to the role of the lateral line in fish.

Dermal pressure receptors may also detect chemicals or the level of salt in the water

Behavior

Crocodiles spend most of their time in the water.

They are usually more active at night than during the day, when they spend a large amount of time basking in the sun in order to regulate their body temperatures.

A crocodilian will often hold its mouth open when it basks. This allows the water in the crocodilian's mouth to evaporate, cooling off the crocodile.

If the crocodilian  feels threatened, it will retreat underwater.

Alligators, crocodiles, and caimans usually eat fish, but they may also attack large animals, such as turtles, pigs, monkeys or buffalo.

They may attack humans, but they do not do so very often.

Gharials eat fish almost exclusively.

Young crocodilians eat small fish and invertebrates.

A crocodilian hunts by waiting motionless in the water with only the top of its head exposed.

It adjusts the level at which it floats in the water by eating and disgorging gastroliths, as well as by changing the amount of air in its lungs.

When its prey is nearby, the crocodilian snaps its jaws around it and then drags it into the water to drown it.

Crocodilians do not chew their food.

If its prey is relatively small, a crocodilian will swallow it whole.

When a crocodilian attacks a larger animal, it will grip it tightly with its jaws, roll to throw the prey off balance, then drags it into deeper water so that it drowns.

The crocodilian will then tear large chunks off its victim by rolling around with it - shaking its head and thrashing.

This behavior is known as a death roll.

It will swallow the chunks whole.

Reproduction

Crocodilians lay eggs in nests on land.

Depending on the species, a nest may be a hole in a sandy beach or a mound of soil and plant material.

A mother crocodilian will stay by her nest to defend her eggs from predators.

When her babies are about to hatch, they will cry from inside their eggs.

When the mother hears their cry, the mother will dig the eggs out of the nest if they are buried, and she may help the babies bite their way out of the eggs.

She will usually protect her offspring until they are about a year old.

A mother crocodilian may allow her offspring to ride on her back.

In some species, the young will swim into the mother's mouth when they are threatened.

The sex of a crocodilian is determined by the temperature of the egg in which it develops.

Difference between the Alligators and Crocodiles

The head of an alligator has a different shape than the head of a crocodile.

An alligator's head is shorter and wider than a crocodile's head.

The snout of an alligator looks like the letter "U", while a crocodile's snout resembles the letter "V".

An alligator's upper jaw is wider than its lower jaw. When it closes its mouth, the teeth in its lower jaw fit inside sockets in its upper jaw, and are not visible. Only the teeth of the upper jaw can be seen.

A crocodile, however, has upper and lower jaws that are the same width. When a crocodile's mouth is closed, both its upper and lower teeth are visible.

Both crocodiles and alligators have a very large fourth tooth in the bottom jaw. When an alligator's mouth is closed, this tooth is hidden. It is always exposed in a crocodile.

The dermal pressure receptors of alligators can be found only on their jaws. Crocodiles have dermal pressure receptors all over their bodies.

Alligators usually live in freshwater while crocodiles usually live in saltwater.

Both alligators and crocodiles have glands in their tongues that allow them to get rid of excess salt. However, these glands function better in crocodiles than in alligators.

Alligators are usually less aggressive and smaller than crocodiles.

Classification

Crocodilians are members of the clade Archosauria, which also includes birds and dinosaurs.

A clade is a group of organisms with a common ancestor. All of the descendants of that ancestor belong to the clade.

Archosaurs, along with snakes, lizards and tuataras, are diapsids.

A diapsid is an amniote that has two temporal fenestrae (holes) on each side of its skull or is descended from an amniote with two temporal fenestrae on each side of its skull.