Body

Reptiles are vertebrates, or animals with a backbone. Except for snakes, most reptiles have four limbs. Their internal organs are similar to those of birds and mammals.

Reptiles vary greatly in size. Some species of tiny snakes and lizards are less than 2 inches (5 centimeters) long. The largest crocodiles, pythons, and anacondas can grow to more than 30 feet (9 meters) long. The largest lizard is the Komodo dragon, which reaches a length of more than 10 feet (3 meters). Leatherback turtles grow to more than 6¹/₂ feet (2 meters) long and may weigh 1,500 pounds (680 kilograms).

Skin

Reptiles have dry skin covered with scales, which are hardened folds of the upper layer of skin. The scales overlap in many lizards. They are arranged side by side in snakes and alligators and crocodiles. Scales also cover the bony shells of turtles. Reptiles replace their scales throughout their lifetime. For some species, shedding is a continual process. For others it is seasonal. Snakes shed all their scales at once. Lizards, turtles, and alligators and crocodiles shed their scales individually or in patches.

Senses

The development of the sense organs varies among reptiles. Alligators, crocodiles, and most lizards hear fairly well. Snakes and turtles cannot hear very well, but they can sense vibrations in the ground. Snakes and lizards that tunnel underground have very poor eyesight. Reptiles that are active aboveground during the day generally have excellent vision.

Like many other animals, reptiles taste and smell through receptors in the mouth or nose that are sensitive to chemicals. Many reptiles have another chemically sensitive structure called a Jacobson's organ in the mouth. Jacobson's organ is best developed in lizards and snakes. The snake flicks its tongue out and in rapidly. Chemicals present in the air stick to the snake's tongue. Each time the tongue is drawn back into the mouth, the forked tip touches the opening of the organ. The chemicals on the tongue are transferred to the organ for detection. Turtles do not have this sense organ.

Some snakes have heat-sensitive organs in pits or hollows on their heads. They are located on the sides of the head in pit vipers and on the lip scales in some boas and pythons. The bodies of mammals and birds give off heat. Snakes with such heat-sensitive organs can detect and strike their prey even in total darkness.

Feeding

Most reptiles are meat eaters. They feed on a wide variety of insects, mollusks, birds, frogs, mammals, fishes, and even other reptiles. Some reptiles follow their prey, whereas others hide and wait for the prey to approach. For example, the alligator snapper, a turtle found in the southern United States, has a tongue that resembles an earthworm. The turtle uses it as a bait to attract fish into its open mouth. Some snakes inject venom, or poison, into their prey. A few lizards and turtles eat only plants.

Moving

Reptiles use various methods to move from place to place. Most lizards walk on four limbs. Some use only their hind limbs when running. Some snakes and legless lizards move by pushing their belly scales against rough surfaces and pulling their bodies forward. Certain desert snakes use a type of movement called sidewinding. The snake lifts its head and tosses it sideways across the sand in an arc. The body follows.

Flippers or webbed feet help sea turtles swim through the water. Sea snakes use their flattened tails like paddles in the water. Alligators and crocodiles swim by lashing their tails from side to side. No living reptile can fly. However, some varieties of tropical snakes and lizards can glide from tall trees by flattening their bodies.

Regulating body temperature

Reptiles are cold-blooded animals. This means that their body temperature changes when the temperature around them changes. Reptiles warm or cool themselves by moving to warmer or cooler locations. Most reptiles become dormant, or inactive, for a period during winter. This state is called hibernation. Some reptiles go through a phase called estivation, in which they become dormant during the hot, dry part of summer.

Defenses

Reptiles have various ways of protecting themselves from their enemies. Most species will first try to avoid facing their predators by hiding underground or underwater. The coloring of some species allows them to blend in with their surroundings.

Many snakes and lizards will change their body form or posture in order to appear more threatening. Among the most notable warning signals are the rattling of rattlesnakes and the spreading neck displays of cobras. Crocodiles, turtles, and some lizards and snakes hiss loudly when confronted by an enemy. Most reptiles bite when threatened. Alligators and basilisk lizards thrash with their tails. The ball python rolls into a tight ball. Box turtles enclose themselves in their shells. Some snakes and the stinkpot turtle emit foul-smelling odors to discourage predators.