The Body of a Turtle

Shell. Most species of turtles can pull their head, legs, and tail into their shell for protection. A few kinds of turtles, particularly sea turtles, cannot withdraw into their shell.

A turtle's shell consists of two layers. The inner layer is made up of bony plates and is actually part of the skeleton. Among most species, the outer layer consists of hard, horny structures called scutes, which are formed from skin tissue. Soft-shelled turtles and the leatherback turtle have an outer layer of tough skin rather than scutes. The part of the shell that covers the turtle's back is called the carapace, and the part that covers the belly is called the plastron. The carapace and the plastron are joined along each side of the body by a bony structure called the bridge.

Most turtles that live on land have a high, domed shell. Those that live in water have a flatter, more streamlined shell. Some species of turtles, including Blanding's turtle, box turtles, and mud turtles, have a hinged plastron. These turtles can close the plastron tightly against the carapace after withdrawing into their shell.

The shells of some kinds of turtles are plain black, brown, or dark green. But others have bright green, orange, red, or yellow markings.

Head. The head of most species of turtles is covered by hard scales. Turtles have no teeth, but they have a beak with a hard, sharp edge that they use to cut food. Many turtles have powerful jaws, with which they tear food and capture prey.

Legs and feet. A turtle's legs and feet vary according to the habitat of the species. Land turtles, particularly tortoises, have heavy, short, clublike legs and feet. Most freshwater turtles have longer legs and webbed feet. Sea turtles have legs shaped like long paddles, with flippers instead of feet.

The hip bones and shoulder bones of the turtles, unlike those of any other animal, are inside the ribcage. This unusual feature enables most kinds of turtles to pull their legs inside their shell. The species that are unable to withdraw their legs cannot do so because the shell is too small.

Senses. Turtles have a well-developed sense of sight and of touch. Scientific experiments indicate that they also have a good sense of smell, at least for nearby objects. Turtles have a middle ear and inner ear, and a tympanic membrane (eardrum) forms their outer ear. A turtle can hear low-pitched sounds about as well as a human being can.

The Life of A Turtle

Young. Turtles hatch from eggs, which are fertilized within the female's body. One mating can result in the fertilization of all the eggs of a female for several years. Most species lay their eggs between late spring and late autumn, and some lay eggs more than once during this period. A green turtle may lay as many as seven clutches (groups) of eggs during one breeding season.

All turtles, including sea and freshwater species, lay their eggs on land. Among most species, the female digs a hole in the ground with her back feet when ready to lay her eggs. She lays the eggs in the hole and covers them with soil, sand, or rotting plant matter. The number of eggs laid varies. An African pancake tortoise lays only one egg per clutch, but a sea turtle may lay 200 eggs at a time.

The female turtle walks away after covering her eggs and does not return. The warmth of the sun hatches the eggs. The temperature at which the eggs are incubated also determines the sex of the hatchlings. Newly hatched turtles must dig their way to the surface of the ground, obtain food, and protect themselves--all on their own.

Many animals prey on turtle eggs and newborn turtles. Various birds and mammals flock to beaches and eat baby sea turtles as they crawl toward the water. Fish attack many others as they enter the sea. Skunks, raccoons, and snakes dig up the nests of freshwater turtles and devour the eggs.

Scientists believe turtles live longer than any other backboned animal. Some box turtles and tortoises have lived more than 100 years. Most of a turtle's growth occurs during the animal's first 5 to 10 years. The turtle continues to grow after reaching this age, but at a much slower rate.

Food. Most kinds of turtles eat both animals and plants. The organisms eaten by a turtle vary among the species. A few kinds of turtles, including green turtles and tortoises, feed almost entirely on plants. Certain freshwater species, such as map turtles and soft-shelled turtles, eat chiefly animals.

Hibernation. Turtles, like other cold-blooded animals, cannot remain active in cold weather. Species that live in regions with harsh winters must hibernate. Most freshwater turtles hibernate by burrowing into the warm, muddy bottom of a pond, stream, or other body of water. Land turtles bury themselves in soil or under rotting vegetation.

Some species of turtles survive hot, dry periods by going into a state of limited activity called estivation. Estivation somewhat resembles hibernation.

Kinds of Turtles

 There are seven main groups of turtles: (1) mud and musk turtles, (2) pond and marsh turtles, (3) sea turtles, (4) side-necked turtles, (5) snapping turtles, (6) soft-shelled turtles, and (7) tortoises.

Mud and musk turtles make up a family of 22 fresh-water species. They live in the Western Hemisphere, particularly in Central America. Mud and musk turtles of the United States include the common mud turtle, common musk turtle, razor-backed musk turtle, and yellow mud turtle.

Few mud and musk turtles grow more than 6 inches (15 centimeters) long. But these turtles have large heads and strong jaws, and they may bite. When disturbed, these turtles give off a foul-smelling substance called musk from glands on the bridge in front of their hind legs. The common musk turtle, whose musk has a particularly strong, unpleasant scent, is often called the "stinkpot."

Pond and marsh turtles form the largest family of turtles--about 90 species. Members of this family live in Asia, Europe, North and South America, and northern Africa. Pond and marsh turtles of North America include the box turtle, chicken turtle, diamondback terrapin, map turtle, painted turtle, red-eared turtle, spotted turtle, and wood turtle. Many of these species are brightly colored, with green, red, or yellow markings on their head, legs, and shell. Most pond and marsh turtles found in the United States are small, but some kinds may grow more than a foot (30 centimeters) long. The majority of pond and marsh turtles live in lakes, ponds, rivers, streams, and tidewater areas. A few species, including box turtles and wood turtles, dwell mainly on land.

Snapping turtles make up a family of large, fresh-water turtles that live only in North America, Central America, and northern South America. There are two species of snappers. The common snapper, which may be found from Canada to Ecuador, grows as long as 19 inches (47 centimeters). The other species, the alligator snapper, lives in the Central and Southeastern United States. Alligator snappers are the largest turtles of North America, except for sea turtles. An alligator snapper may measure more than 24 inches (60 centimeters) long and weigh over 200 pounds (91 kilograms).

Snapping turtles eat small water animals, such as fish, frogs, insects, snails, and young waterfowl. They also feed on plants and algae (simple aquatic organisms). Snappers have a large head and strong jaws. They may bite fiercely if disturbed. A snapper's small shell does not give much protection, and so it depends on its strong, sharp-edged jaws for defense.

Soft-shelled turtles make up a family of 21 species. These freshwater turtles have a shell covered by smooth skin. They live in Africa, Asia, and North America. Three species of soft-shelled turtles--the smooth softshell, the spiny softshell, and the Florida softshell--live in the continental United States. Two species of Chinese softshells, both Asian species, are found in Hawaii.

Unlike other turtles, softshells have fleshy lips that cover their beak. Most kinds of softshells also have a long, tube-shaped nose that they push above the surface of the water in order to breathe. Most softshells do not grow much longer than a foot (30 centimeters), but some species measure up to 3 feet (91 centimeters) long. A soft-shelled turtle may bite when disturbed, and it can strike with lightning speed.

Tortoises form a family of about 50 species. These land turtles live in Africa, Asia, Europe, and North and South America, and on certain ocean islands. The tortoises of the Aldabra Islands and the Galapagos Islands are the world's largest land turtles. These huge reptiles may measure up to 4 feet (1.2 meters) long and may weigh up to 600 pounds (270 kilograms).

Three species of tortoises live in the United States. The desert tortoise makes its home in the dry areas of the Southwest. The gopher tortoise lives in sandy-soiled areas of the Southeastern United States. The Texas tortoise is found in scrub forests of southern Texas.

Tortoises live only on land. Most species are slow-moving creatures with a high, domed shell. But the African pancake tortoise has a flat, flexible shell. When in danger, this tortoise runs quickly into a crack in a nearby rock. It then takes a deep breath and inflates its body, wedging itself tightly in the crack.

Turtles and human beings

The activities of human beings are a serious threat to the survival of many turtles, and turtle conservation must improve to prevent certain species from becoming extinct. Wildlife experts classify more than 40 kinds of turtles as endangered. These rare turtles include many types of tortoises and most sea turtles.

People have long used turtle meat and eggs for food and turtle shells as ornaments. The most threatened species of turtles include the most economically valuable ones. For example, the green turtle is a popular food in many parts of the world. The use of its meat and eggs by humans has seriously endangered its survival. The hawksbill turtle also has almost been killed off because tortoise shell, a substance that is used in making ornamental objects, comes from its carapace. People further endanger turtles by poisoning their homes with pollution. People also continually replace forests, swamps, and other natural areas with cities and farms. This activity almost ensures the extinction of certain kinds of turtles.

Some governments forbid the capture of rare species of turtles. Turtle preserves have been established in certain areas, and scientists are experimenting with raising valuable species on turtle farms. But zoologists must know more about how turtles live in the wild to save many of the endangered ones.

Scientific classification. Turtles make up the order Testudines (sometimes called Chelonia) in the class Reptilia and the phylum Chordata.