Turtle

I

Introduction

Turtle, reptile with a bony or leathery shell. Any shelled reptile can be called a turtle, but in North America people commonly use the word tortoise to designate members of a family of turtles that live entirely on land, reserving the word turtle for species that live in or near water. The word terrapin, of Native American origin, is sometimes used for certain small freshwater turtles, in particular in North America for the diamond-backed terrapin, a turtle that lives in the brackish waters of eastern coastal marshes.

Turtles are ancient life forms that first appeared on Earth during the Triassic Period more than 200 million years ago. Turtles survived the disasters that wiped out the dinosaurs and many other creatures at the end of the Mesozoic Era about 65 million years ago.

Scientists have identified about 300 living species of turtles. Turtles are classified into two suborders: side-necked turtles and hidden-necked turtles. Side-necked turtles protect their heads by folding their necks sideways under the top edge of the shell. These turtles are found only in South America, Africa, and Australia and nearby islands. Hidden-necked turtles pull their heads directly into the shell, using an up-and-down motion of the neck. Hidden-necked turtles are more widespread than side-necked turtles. They include all of the ocean-living and freshwater turtles of North America, Europe, and Asia, as well as the land-living tortoises. Sea turtles, as well as a few other kinds of turtles, are considered members of the hidden-necked group even though they cannot fully withdraw their heads.

Turtles inhabit every continent except Antarctica, in habitats as diverse as ponds, rivers, and oceans; forests and grasslands; and even deserts. Like all living reptiles, turtles are ectotherms—cold-blooded animals whose body temperature is influenced by the outside environment. For this reason, turtles are most abundant in warm tropical and subtropical climates. Eastern North America, however, is home to numerous turtle species, despite its temperate climate.

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Turtles have long fascinated people of many cultures, and they are often used to symbolize wisdom and long life. In many Native American and Asian cultures, turtles are mentioned in myths that explain the origin of the universe. In China and Southeast Asia, turtles are sometimes venerated in religious ceremonies. Despite the reverence turtles have inspired, these animals have a long history of being exploited by humans. People in many parts of the world eat turtle flesh and eggs, and use turtle parts in traditional medicines. Turtle fat is a source of valuable oils. Some turtles, such as the hawksbill turtle, are killed for their decorative shells, which are the source of tortoiseshell used in jewelry. In addition, many thousands of turtles and tortoises are collected and sold as pets. By killing turtles, removing turtles from their natural habitats, and destroying the environments in which turtles thrive, humans have brought many turtle species to the brink of extinction.

II

Physical Description

Turtles vary widely in size. The world's largest turtle—the great leatherback sea turtle—reaches a shell length of 240 cm (96 in) and can weigh over 900 kg (2,000 lb). The giant tortoises of the Galápagos Islands, near South America, and the island of Aldabra, off the East African coast, can reach at least 130 cm (50 in) in length. The largest North American freshwater turtle is the alligator snapping turtle, which can reach a shell length of 80 cm (31.5 in). Among the smallest turtles are the American mud turtles and musk turtles, which have a range that extends from southern Canada through much of South America. Many species in these groups are less than 13 cm (5 in) in shell length. Other small turtles are the rare bog turtle of the eastern United States, at 11 cm (4.5 in); and the tiny speckled cape tortoise of South Africa, whose shell is only 10 cm (4 in) long.

A

Shell

The upper shell of the turtle, under which the head, limbs, and tail can be more or less completely withdrawn, is called the carapace. The lower shell, which encases the belly, is called the plastron. The carapace and plastron are joined together on a turtle’s left and right sides by bony structures called bridges. The inner layer of a turtle’s shell is composed of about 60 bones, including portions of the backbone and the ribs. For this reason, a turtle cannot crawl out of its shell.

In most turtles, the outer layer of the shell is covered with large horny scales called scutes. The scutes are part of the turtle’s outer layer of skin, or epidermis. Scutes are composed of a fibrous protein called keratin, which also makes up the scales of other reptiles. The scutes overlap the seams between the shell bones, adding strength to the shell. Some turtles, like the leatherback sea turtle and the soft-shelled turtles, lack horny scutes, and instead have shells covered with leathery skin.

Some turtles have shells with flexible hinges that allow the turtles to close off the openings in their shells through which they withdraw their heads, tails, and limbs. These hinges protect against predators and help prevent the turtles from drying out when they are on land. Several types of turtles, including Blanding's turtles, which inhabit wetlands in the northeastern United States and Canada, and the American box turtles, have a single hinge across the plastron. American mud turtles have two hinges on their plastrons. African hinge-backed tortoises have a hinge across the back of the carapace.

The shape of a turtle's shell provides clues about its way of life. Most tortoises have high, dome-shaped shells. The shape of these shells makes it difficult for predators to crush these turtles between their jaws. An exception is the African pancake tortoise, which relies on its flat, flexible shell to hide in rock crevices. Most aquatic turtles have flatter, more streamlined shells, which aid in swimming and diving. American musk and snapping turtles have small, cross-shaped plastrons that enable greater leg movement for walking on the bottom of ponds or streams.

Tortoises have rather heavy shells. In contrast, sea turtles and soft-shelled turtles that live continuously in water have lighter shells, which help them avoid sinking in water and increase their swimming speed and agility. These lighter shells have large spaces called fontanelles between the bones. The shells of leatherback turtles are extremely light for their size because they contain many fontanelles and lack horny scutes.

Turtle shells are commonly brown, black, or olive green. In some species, shells may have red, orange, yellow, or gray markings. These markings commonly appear as spots, lines, or irregular blotches. An especially colorful turtle is the eastern painted turtle, which has a yellow plastron and a black or olive shell with red markings around its rim.

B

Head

Most turtles and tortoises have conspicuous eyes placed well forward on the upper sides of their heads. Turtle species that live most of their lives on land usually focus their eyes downward toward objects in front of them. Some water-living turtles, like snapping turtles and soft-shelled turtles, have eyes nearer the top of the head. These turtles can hide from predators and prey in shallow bodies of water, where they lie nearly entirely submerged except for their eyes and nostrils. Sea turtles have glands near their eyes that produce salty tears, ridding the turtles’ bodies of the excess salt that they take in from drinking seawater. When a sea turtle is in the water, its tears are immediately washed away, but when these turtles arrive on land, they look as though they are crying.

Turtles use their jaws to cut and handle food. Instead of teeth, a turtle’s upper and lower jaws are covered by horny ridges, similar to a bird’s beak. Meat-eating turtles commonly have knife-sharp ridges for slicing through their prey. Plant-eating turtles often have ridges with serrated edges that help them cut through tough plants. Turtles use their tongues in swallowing food, but unlike many other reptiles, such as chameleons, they cannot stick out their tongues to capture food.

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