II.

Physical Characteristics

Although all snakes have a long, cylindrical body, many species boast unique modifications suited to particular habitats and lifestyles. Burrowing snakes, for example, have muscular, stout bodies and solidly built heads that they use to push through soil. Sea snakes have flattened, paddlelike tails for swimming, and the long, thin shape of many arboreal, or tree-dwelling, snakes provides agility when navigating between branches. Some snakes, including pythons, retain characteristics that reflect their evolution from lizards or from lizardlike ancestors. These snakes have traces of hind limbs called spurs, which are usually more prominent in males than in females.

The smallest snakes are the blind snakes. One member of this group, the Texas slender blind snake, reaches just 13 cm (slightly more than 5 in) in length when full grown and weighs less than 2 g (less than 0.1 oz). The largest snakes are the anaconda and the reticulated python, both of which grow as long as 10 m (about 33 ft) and can weigh up to 250 kg (about 550 lb). Among most species of snakes, females are larger than males.

A.

Skin

Snake bodies are covered in overlapping scales composed of a horny material called keratin. These transparent scales make up the dry, smooth, outer layer of skin, whose primary function is to prevent water loss. Snakes owe their coloration to pigment cells located in the skin layer below the scales. Most snakes display drab earth tone colors to blend with their natural surroundings. Arboreal species, such as the emerald tree boa, are often vibrant shades of green, a coloration that helps them hide among leafy foliage. Some snakes, such as coral snakes, have brilliant yellow and orange stripes that warn predators of their venomous bite.

Snakes regularly shed the outer layer of their skin as they grow. Even in snakes that are not growing, the scales become drab and worn over time, and must be periodically replaced by a new, healthy layer. Some species of snakes shed their skin about every 20 days, but other species shed it only once a year. In the shedding process, a new layer develops below the surface of the old one, which gradually separates in preparation for being shed, or sloughed. The snake begins the shedding process by rubbing its nose against rocks or other hard objects to separate the old layer from its lips. After the old layer is loosened, the snake crawls out of its old skin, typically shedding it in a single piece.

B.

Skeleton

A snake’s skeleton is lightweight and highly flexible. Like other reptiles, as well as fish, amphibians, birds, and mammals, snakes are vertebrates—that is, they have a backbone made of small, interconnecting bones called vertebrae. Snakes have an especially large number of vertebrae—all snakes have at least 100 vertebrae, and some species have more than 400. By comparison, humans have just 32 vertebrae. Unlike other vertebrates, the ribs of snakes are not anchored in place by a breastbone, or sternum. Instead, each rib is connected to vertebrae and other ribs by elastic muscles. This makes it possible for a snake to greatly expand its ribcage to pass whole, large prey animals through its digestive tract. These sinuous connections also enable snakes to coil up into compact balls.

Snake skull anatomy shows similar modifications. The bones that make up a snake’s jaws are not rigidly attached to its skull. Instead, they are linked to the rest of the skull and to one another by muscles and stretchy tissues called ligaments. Ligaments also join the two halves of the lower jaw, enabling them to stretch far apart. This flexibility is what makes it possible for a snake to eat animals that are much larger than its head. When swallowing prey, a snake advances its jaws stepwise over the animal by anchoring the teeth from one jaw segment into the prey and then slowly moving the other parts of the jaws forward along the prey.

C.

Internal Organs

Snakes share an internal anatomy similar to that of other reptiles, but modified to fit within an extremely narrow space. The snake’s three-chambered heart can move sideways to accommodate large prey animals traveling from the mouth to the stomach. The snake’s respiratory system is also compact: Most snakes rely exclusively on the right lung for respiration. In these animals, the left lung is either very small or nonexistent. Snakes have two kidneys, which are positioned so that the left one lies behind the right one rather than beside it. Similarly, the reproductive organs—a pair of testes for males and a pair of ovaries for females—are situated end-to-end. The snake has an extremely muscular and flexible stomach, a narrow liver, and both large and small intestines. Unlike the small intestines of many other vertebrates, those of snakes are stretched out instead of coiled. Like other reptiles, snakes have a cloaca, an internal chamber that receives wastes from the digestive system and eggs or sperm from the reproductive system before they leave the body. Snakes do not have a urinary bladder; instead, they excrete all their wastes through the rectum.

D.

Senses

A snake obtains information about its environment primarily through the Jacobson’s organ located in the roof of its mouth. The snake continuously flicks out its forked tongue to collect scent particles from the air and the ground. When the tongue draws back into the mouth, the forked tips fit into cell-lined pockets in the Jacobson’s organ, which detects the odors of the particles it receives. This system is keenly sensitive, and snakes rely on it to locate both mates and prey.

Vision and the ability to detect vibrations are also important to the survival of most snakes. Snakes lack eardrums and external ear openings, but they have small bones in their heads that conduct sound. They are able to hear low-frequency sounds and to sense vibrations that travel through the ground or water. The majority of snakes have good eyesight, especially for detecting moving objects, although most burrowing snakes can only distinguish between light and dark.

Pit vipers, boas, and pythons have an unusual adaptation for detecting warm-blooded prey and predators. On the heads of these snakes are small pits lined with cells that are extremely sensitive to heat. These pits enable the snakes to sense the presence of a warm-blooded animal and strike accurately, even in total darkness.

E.

Venom and Fangs

Several groups of snakes inject their prey with poisonous secretions called venom. Most venomous snakes strike out and bite their prey, then wait for their toxic venom to immobilize or even kill the animal before swallowing it whole. Venom is produced by modified salivary glands in the upper jaw and contains a mixture of poisons. Some of these poisons include chemicals that disrupt the nervous system. Others are substances that break down the prey’s tissues, so that the prey animal is partially digested even before it reaches the snake’s stomach. One of the world’s most poisonous snakes is the inland taipan (also known as the fierce snake) of central Australia. Members of one taipan species can deliver enough venom in a single bite to kill nearly 100 people or 250,000 mice.

Snakes typically deliver venom through two hollow teeth called fangs. In a large group of snakes, the vipers, the fangs can be rotated into different positions. When the snake is ready to strike, it points its fangs outward, but under other circumstances it folds them flat against the roof of its mouth. In other poisonous snakes, such as cobras and coral snakes, the fangs are fixed in position. In these snakes, the fangs must be short enough so that the snake can close its mouth without biting itself.

An unusual group of snakes spit or eject venom in a spray, instead of biting. These snakes, which include spitting cobras, use their poison for self-defense rather than for obtaining food. Aiming their spray at the eyes of their enemies, they sometimes reach their target from as far as 2.4 m (8 ft) away.