A turtle is an animal in armor. Much of its body lies within a protective shell, which has openings for the turtle's four chunky legs, short tail, and head. When danger threatens, many turtles pull legs, tail, and head into the shell. But unlike some animals that live in shells, such as hermit crabs and snails, a turtle cannot crawl out of its shell. The shell is part of the turtle's body.
All turtles belong to the class of backboned animals known as reptiles. This class also includes snakes, lizards, and crocodiles. Turtles are the oldest group. The first turtles crawled about on earth more than 250,000,000 years ago. Turtles have changed very little since that time.
Turtles are found in almost all temperature and tropical regions of the world. Many turtles spend all or most of their lives in fresh water. They may live in swamps, ponds, running streams, or even roadside ditches. They come up on dry land to sun themselves or to lay eggs. Other turtles live completely on land. Still others live in warm seas, sometimes following warm currents far northward.
The name "turtle" is often used to identify those animals that live in water. The name "tortoise" frequently refers to a turtle that lives on land. The American Indian name "terrapin" usually refers to small freshwater turtles, especially those used for food. But these groupings are not strictly scientific. In this article, all of these animals will be referred to generally as turtles, though the proper name for a specific animal, such as Galápagos tortoise, will be used.
There are some 250 kinds of turtles. They range in size from tiny bog turtles about 3 inches (7 centimeters) long to leatherbacks almost 9 feet (2.5 meters) long that weigh 1,500 pounds (680 kilograms).
A Turtle's Shell
A turtle's shell is made of two parts that are joined at the sides by bony bridges. The upper part of the shell is called the carapace, from a Spanish word for "shield." The lower part is called the plastron, from an Italian word for "breastplate."
The carapace and the plastron are made up of many small, flat, bony shields, or plates. These plates are fused with (joined to) the turtle's backbone and ribs and parts of its shoulders and hips. The bony plates are covered with broad, thin scales called laminae. The laminae are made of material something like your fingernails. (The hawksbill sea turtle was once hunted and killed for this material, which was called tortoiseshell. Before plastics were developed, tortoiseshell was widely used for combs, the frames of eyeglasses, and other objects.) The laminae give a turtle its color. Depending on the kind of turtle, this color may be brown, black, or olive green. (The plastron is usually lighter than the carapace.) These dull colors blend perfectly with a turtle's usual surroundings.
Some turtles, however, have brightly colored markings. Spotted turtles are covered with polka dots. More spots develop as the turtles get older. Painted turtles have bright red, yellow, or orange borders on their smooth olive or dark-brown shells. The colors are so bright that they look as though they were painted on. The bright colors may help turtles find one another. Scientists have discovered that turtles see colors and are especially sensitive to red.
As a rule, turtles that spend all or part of their time on land rely on their hard shell to protect them from the teeth and claws of hungry animals. Otters, raccoons, crocodiles, snakes, bears, and large birds are some of the animals that prey on turtles. A typical land turtle draws its head, tail, and legs into its shell for protection. The stout, scaly legs form a sort of armored door. Other turtles, such as the freshwater box and mud turtles, can shut up their shell completely. The plastron of these turtles is hinged and, when closed, fits firmly against the carapace.
Mud and musk turtles defend themselves by hissing and withdrawing into their shells. They have a further means of defense. They can give off a bad-smelling yellow fluid called musk. Some scientists think this odor may also serve to attract a mate. Other turtles squirt out water or body wastes when bothered.
A few turtles drive off their enemies by biting and clawing. The snapping turtles, for example, are quick to attack with a vicious bite. A snapper has a small plastron. Its legs, tail, and neck do not fit into the shell. However, the snapper has other defenses. Its tail is armed with a ridge of spines, and its snapping jaws take care of the front end. The soft-shelled turtles also rely on their sharp claws and crushing jaws for defense.
How Turtles Breathe
The turtle's rigid shell prevents the animal from breathing in the usual way, that is, moving the ribs to fill the lungs with air. The turtle's ribs are firmly fixed to the inside of its shell, so in order to breathe the turtle uses two special sets of muscles. One set pulls the other body organs away from the lungs, filling the lungs with air. Then a second set of muscles pulls the organs against the lungs, forcing the air out.
One deep breath may last a turtle several hours. Some freshwater turtles can remain underwater for several days (except during hibernation, when they can remain underwater for several months). They do this by lying still on the bottom, thereby using up very little oxygen. Some kinds can even take a little oxygen from the water, as a fish does, by using specialized body tissues.
In climates where the ground and water freeze in winter, turtles survive the cold by hibernating. They may burrow in the muddy bottoms of ponds or streams or crawl under decaying vegetation. When a turtle hibernates, it uses up very little oxygen. Even if it spends months underwater, a hibernating turtle does not have to come up for air. Its body does not have the same oxygen needs that it has when the turtle is active during the warmer months.
How Turtles Move
Turtles are famous for the slowness of their movements. And most turtles do move slowly on land, their legs sprawled awkwardly out to the sides. A box turtle, for example, heaves its body along, clawing at the ground, with its toes spread wide apart. If it falls on its back, it has a hard time righting itself.
The turtles that live entirely on land are the slowest and most awkward of all turtles. They have high, domed shells and stumpy legs that end in small feet. Most of these turtles walk on the tips of their toes. This is true of the largest land turtles as well as the smallest. A giant tortoise of the Galápagos Islands in the Pacific may have a weight of 500 pounds (227 kilograms) or more to move.
In water, however, turtles move more easily. Most freshwater turtles are good swimmers. They have webbed feet and smooth, arched shells. Strangely enough, freshwater turtles can move more quickly on land than most land turtles can. The members of one family can even run. These are the soft-shelled turtles, which have a soft, leathery skin (instead of horny scales) covering the bony plates.
Ocean turtles move awkwardly on land. But in water they swim steadily along with strong, winglike strokes of their huge flippers. A sea turtle, such as a hawksbill, a loggerhead, or especially the green turtle, is somewhat streamlined. Its body is heart-shaped, tapering toward the tail. Its front flippers are longer and stronger than the hind ones.
Most turtles will eat anything they can catch. This may be earthworms, snails, slugs, insects, mussels, shrimps, fish, frogs, or even small birds and mice. The ocean-dwelling green turtle feeds mainly on seaweeds. Land turtles, too, often eat plants. Map turtles, mud and musk turtles, snapping turtles, and most other water-dwelling turtles eat dead animals from time to time.
A turtle has no teeth. However, its beak has sharp edges and its jaws are strong. A turtle seizes its prey and holds the wriggling creature in its beak. If the prey is too large to be swallowed whole, the turtle uses its claws to tear off pieces until the food is small enough.
Land turtles feed as they go, shearing off bits of leaves, flowers, or fruits when they feel hungry. The green sea turtle grazes on underwater plants. Other sea turtles actively swim after jellyfish, crabs, and other slow-moving creatures. Freshwater turtles for the most part hunt in a different way. They lie on the bottom of a pond or river and wait for something to come along. This is what the snapping turtles do, for example. Because they are poor swimmers, these animals hide in the mud. But when a snapper sees something moving in the water, such as a fish or the legs of a bird, its long neck darts out and its sharp beak grabs the prey.
Reproduction and Life Cycle
Turtles may mate on land or in the water, depending on the species. After a single mating, female turtles may store sperm in their bodies and lay fertilized eggs for several years.
All turtles lay eggs on land. They lay their eggs in warm, sunny places, usually not far from water. The eggs of most turtles are oval, but some are round. Some eggs have soft, rubbery shells; others have hard, brittle shells. They may be as large as tennis balls or as small as kidney beans. The eggs are usually white.
When a female turtle finds a good spot to lay her eggs, she starts digging, first using one hind leg and then the other. When the hole is as deep as her legs can reach, she starts laying her eggs. Most turtles lay from two to 25 eggs at a time. Sea turtles may lay 150 or more.
When all the eggs are laid, the female turtle fills in the nest with sand or dirt. Once she has completed her task of concealing the nest, the female turtle goes back into the water or moves away through the underbrush. She has no further interest in her eggs.
The beginning of a turtle's life is a very dangerous time. Bears, skunks, raccoons, and other animals may dig up and eat the eggs. And even if the eggs hatch, birds, lizards, and other hungry animals may gobble up the tiny young turtles. People also use turtles for food. Many kinds of turtles and turtle eggs are eaten throughout the world.
If the young turtles survive until adulthood, they may live a long time. Turtles live longer than any other animal except people. Some turtles are known to have lived more than 100 years. But a very large turtle is not necessarily an old one. Turtles reach their full size in less than ten years. After that, turtles continue to grow, but very slowly.
Instincts and Migration
All turtles have good eyesight and hearing. They also have a good sense of smell and touch. In addition, some — and perhaps all — turtles have a sense that seems unrelated to the usual senses. This sense is more like an instinct. It is responsible for controlling turtle behavior during various stages of the animals' lives. For example, when water turtles of any kind hatch, the hatchlings promptly make their way to water. They do this even when they cannot see the water. Scientists think that a turtle can somehow distinguish the difference between light over water and light over land.
Some green turtles seem to have a "compass sense." They may cross more than 1,000 miles (1,600 kilometers) of water to their nesting grounds on Ascension Island, an island halfway between Africa and South America. The turtles arrive at Ascension in December. They remain there until June, laying eggs every twelve days. Scientists tagged some females during one nesting season. Later a few of the tagged turtles were found feeding on seaweeds off the coast of Brazil. Three years later some of the females returned to the island beach where they were tagged. They laid their eggs in the same stretch of beach.
Such long-distance migrations still puzzle scientists. They know that strong ocean currents carry the young turtles to the coastal waters of Brazil. But why do the turtles return to Ascension Island? And how do they find their way there — against the current — during the thousand-mile journey? Perhaps some day we will better understand the behavior of these fascinating reptiles.