Body

Snakes are vertebrates. A vertebrate is an animal with a backbone, or spinal column. A snake's backbone is unusually long. It contains more segments, or vertebrae, than do the backbones of any other major group of animals. The snake's vertebrae are joined in a way that makes the backbone very flexible.

Snakes have long, slender, cylindrical bodies. They do not have arms or legs. A few species have tiny fragments of limbs, but the snakes cannot use them. Many snakes have only one lung. Most of the internal organs, such as the liver and kidneys, are long.

Skin

The bodies of snakes are covered with scales, which are hardened folds of skin. The scales are usually arranged in rows. In some species, the scales on the upper portion of the body are shiny and smooth. In others, a ridge runs lengthwise down the center of each scale.

All reptiles shed their scales throughout their lifetimes. This shedding is called molting. The old layer of scales starts loosening at the lips of the snake. The animal breaks the skin loose by rubbing its head against a rough object, such as a rock. The snake finally crawls out with a new skin, leaving the shed skin intact.

Jaws and teeth

All snakes are meat-eating. Most consume their prey whole, without chewing. Part of the snake's lower jawbone can separate from the upper jaw so that the mouth can open very wide. This lets a snake eat an animal that is larger than the usual size of the snake's mouth.

The snake's prey may still be alive when the snake swallows it. The snake's teeth are curved inward so that live prey cannot easily escape. Most snakes have long, needle-sharp teeth. Vipers and cobras have a pair of hollow fangs in the front of the mouth. They inject venom, or poison, into their prey through these fangs.

Senses

Snakes do not have external ears, and their internal ears are not well developed. They cannot hear most sounds. However, they can detect some vibrations in the ground by means of bones in their lower jaws. Thus, a snake can sense the approach of another animal through ground vibrations.

A transparent layer of skin covers the snake's eyes instead of an eyelid. Many burrowing snakes, which live underground, are blind. Vipers and many other snakes that are active at night have eyes with oval pupils. The pupils enlarge in darkness for better night vision. In bright light, the pupils narrow into slits to protect the snake's sensitive eyes.

Many pit vipers and some pythons and boas can catch their prey in total darkness. They use the sensitive pits or hollows on their heads to detect heat from the prey's body.

Snakes can taste and smell. They also have another similar sense. Snakes flick out their forked tongues to gather information about their surroundings. Chemicals in the air stick to the tongue's moist surface. The tongue is then drawn back in and pressed against a structure in the roof of the mouth. This structure, called Jacobson's organ, allows the snake to distinguish between various chemicals in the air.

Movement

Most snakes move by using the scales on the undersides of their bodies to push against the ground. The snake's body moves in a series of S-shaped loops. Some heavier species, such as some boas and pythons, move in a fairly straight line. To do this, the snake's body inches along like a caterpillar's, with a flow of muscle contractions along the sides.

Some snakes move by folding and unfolding like an accordion. The back end forms a serious of loops or folds to hold that part of the snake in place. The head extends forward until the body is nearly straight again. Then, the front of the snake loops around and the back part is drawn forward. Tree snakes use a similar type of motion to move from branch to branch.

Snakes that live in the desert generally cannot move by any of these methods. The loose sand simply gives way under any kind of push. Instead, many desert snakes use a movement called sidewinding. The front part of the body is thrown upward and sideways in an arch. The front lands several inches to the side of the original location. The rest of the body follows the same path through the air without touching the sand in between. The snake progresses sideways, leaving a track of disconnected marks in the sand.

All snakes can swim. Sea snakes and the Asian water snake have flattened tails that function like paddles in the water. Some sea snakes are helpless on land.

Feeding habits

Vipers, cobras, and their relatives use venom to kill their prey. Several nonpoisonous species are constrictors, including boas, pythons, king snakes, and rat snakes. These snakes coil around their prey and constrict, or squeeze, the animal to death. Most snakes swallow their prey whole, usually head first. A snake may take more than an hour to completely swallow a large meal.

Response to temperature

Snakes can die from excessive heat or cold. Their body temperature is almost totally dependent on the temperature of their surroundings. Snakes slow down in the cold. Many snakes bask in the sun to warm up when they are cool. In cold seasons, some species seek out safe hiding places where they remain completely inactive. This is called hibernation. On the other hand, snakes must avoid prolonged exposure to direct sunlight on hot days.

Defenses

Snakes have many natural enemies, including several kinds of mammals, birds, turtles, and other snakes. To protect themselves, snakes have a variety of defenses. All snakes have teeth, and many, given a chance, will bite an attacker. Some species release venom as they bite. Bites from nonvenomous snakes are usually harmless to humans.

Some snakes use noise to threaten their enemies. The tail vertebrae in rattlesnakes form a “shaker” that makes a rattling noise. Deadly cobras raise the front of the body and spread the neck regions into a hood to scare possible attackers. Some species even pretend to be dead or dying in order to be left alone. Hognose snakes writhe and even bleed from the mouth during their death act.

The coloring of snakes can offer protection from predators in different ways. Many snakes blend in with their environment. For example, many tree snakes are bright green and resemble vines. On the other hand, some venomous species are brightly colored. These snakes use color to advertise their dangerous nature to would-be attackers.