Turtles and Tortoises: Testudines
TURTLES AND TORTOISES: Testudines
Turtles and tortoises, which are in the order Testudines, have bony upper and lower shells that surround much of the body. The upper shell, or carapace (KARE-a-pays), can be tall and rounded, can be flat, or can be some shape in between. The lower shell, or plastron (PLAS-trun), can cover most or just a portion of the bottom of the animal, depending on the species. In most cases, the upper shell connects to the lower shell by way of a bony bridge. In some species, the bridge is made of more flexible tissue called ligament (LIH-guh-ment). The hard shell often is covered with large scales called scutes (SCOOTS). In some species, new scutes grow under the old ones, and the old ones pile up. A person can count the number of scutes in the pile to tell how old the turtle is. Softshell turtles have no scutes. They do have small bony shells, but the bones are covered with leathery or rubbery skin.
Besides shells, another feature of turtles and tortoises is that they have no teeth. Instead they have hard, flat surfaces on their jaws that allow them to grip and tear off bits of plants or animals for feeding. Sometimes these surfaces come to a sharp point in front and look much like the hook on the end of a hawk's or eagle's beak. Turtles with such pointed upper jaws are often said to have horny beaks.
Turtles and tortoises, like birds, dogs, humans, and other animals, are vertebrates (VER-teh-brehts), which means they have a backbone. Turtles and tortoises are unlike all other vertebrates in that their hip and shoulder bones are inside the rib cage instead of outside because the ribs are attached to the upper and lower shells. If the shoulder and hip bones were outside the rib cage, they would have to be outside the shell.
Many turtles and tortoises have long necks, which they can pull back or stretch out. Because some species can pull their necks straight back and others can only pull them sideways, scientists often describe them as being in the hidden-necked group, called Cryptodira, or in the side-necked group, called Pleurodira. A hidden-necked turtle can pull its neck straight back and usually tuck its whole neck and head inside the shell. A side-necked turtle pulls its neck back sideways, often tucking the neck and head along the side of the body and against the bridge between the upper and lower shells.
Turtles come in many sizes. The largest living species is the leatherback sea turtle, which can weigh up to 1,191 pounds (540 kilograms), or more than half a ton. The upper shell can become 8 feet (2.4 meters) long. Some of the smallest of the Testudines are the speckled cape tortoise, flattened musk turtle, and bog turtle. The carapace on each of these three animals barely reaches 4.7 inches (12 centimeters) long.
Turtles and tortoises live on all continents except Antarctica.
Depending on the species, turtles and tortoises can live on land, in fresh water, in the ocean, and along the coast. They live on many of the larger islands of the oceans and on every continent of the world except Antarctica.
Some species of turtles and tortoises are almost completely vegetarian, some eat almost nothing but meat, and still others eat a mix of meat and plants. Many turtles are opportunistic (ahper-too-NIS-tik) feeders, meaning that they eat just about anything they can find, from fruits and leaves to live tadpoles and bits of dead fish. In some species, baby turtles eat mostly insects and other meat but switch to mostly plants as they get older.
BEHAVIOR AND REPRODUCTION
One of the most commonly known behaviors of turtles and tortoises is their ability to pull their legs, tail, neck, and head inside the shell. Many of them hide from attackers this way, but not all of them are able to do it. Side-necked turtles, for example, can pull in the tail and legs but can only tuck their necks along the bridge. Other species, like the big-headed turtle, are hidden-necked but their heads are too large to fit inside the shell. Other turtles have hinges in the lower shell that allow them to draw the lower and upper shells tight against one another once the head, neck, legs, and tail are inside. A few species even have hinges on the upper shell. Attacking animals, or predators (PREH-duh-ters), find it very difficult to get at the turtle's soft body inside such a tightly closed shell, and the turtle usually survives without harm. Besides protecting the turtle from attackers, the shells protect the turtle from drying out too much on hot, dry days. Hinges in the back of the plastron also allow the shell to open wide enough for some female turtles to lay large eggs.
Many water-living turtles are excellent swimmers. Some, such as leatherback turtles, have paddle-like front legs that help them swim hundreds of miles in a year. Others, such as softshell turtles, have webbing between their toes that helps them sweep through the water. Some species of water-living turtles, however, are poor swimmers. American mud and musk turtles, for example, are small to medium-sized turtles that move slowly through the water by walking across the bottom rather than swimming. Land-living turtles and tortoises can get around quite well on the ground, although their shells do not allow enough leg movement for fast running.
Turtles and tortoises are able to protect themselves from predators by hiding inside their shells and in other ways. Many turtles have musk glands, which are small sacs that ooze a substance with a strong odor. This odor may be enough to make a predator stop its attack and leave the area. Some turtles fight back with strong bites. Snapping turtles, for example, are vicious and quickly fling out their long necks to bite at anything or anyone coming too close. Besides having a hard bite, snapping turtles have sharp claws that can badly scratch anyone who picks up the turtle from behind. Other species that are quick to bite include softshell and musk turtles.
During mating season, or courtship, the males of many species of turtles and tortoises try to attract females by methods that can range from head bobbing and gentle rubbing against the female to biting her legs or ramming his shell into hers. Some species, on the other hand, have no such courtship behaviors. Species that live in warmer areas may mate and nest at various times of the year, but those that live in cooler areas usually mate in the fall or spring and nest in the spring or summer. In many species, the female can mate once and lay eggs from that mating for several years.
Most female turtles and tortoises nest by finding a spot on dry land, digging a hole, dropping the eggs inside, and burying them. A few species skip making a hole and simply lay their eggs among leaves on the surface of the ground. Most turtles and tortoises provide no further care for their eggs or young. The Asian giant tortoise is an exception. The female of this species lays her eggs and stays with the nest for a few days to keep away predators. Most of the smaller species of turtles and tortoises lay one to four eggs at a time, but larger species can lay fifty or more. For most turtles and tortoises, the temperature of the nest controls whether the eggs hatch into males or females. A very warm nest usually produces females, and a cooler nest produces males. In some species, an extremely cold nest temperature produces females too. In a few species, the nests have about equal numbers of males and females, no matter what the temperature of the nest. Newly hatched turtles and tortoises, or hatchlings, have a small, hard, tooth-like part on the upper jaw called a caruncle (KAR-un-kul), which helps them break out of the egg. Hatchlings usually head straight for the water or for a hiding spot on land, but a few species that hatch during cold winter months stay underground until spring. Adults of many species that live in colder climates enter a state of deep sleep, or hibernation (high-bur-NAY-shun), during the winter months. Many species that live in hotter areas survive dry weather by entering a state of deep sleep known as estivation (est-ih-VAY-shun).
HOW TURTLES AND TORTOISES USE THEIR SHELLS
A turtle's shell can be important in several ways. It can help the turtle protect itself from attacking animals. The shells of some turtles are so thick and strong that they can even resist the bite of a large crocodile. Other turtles, such as Asian river turtles, often dive very deeply, where the water pressure would be severe enough to crush their lungs if they were not protected by the shell. In turtles that live in very dry places, the shell provides a shield from the sun and helps the turtle keep from drying out too much. Tortoises, which live only on land, use their shells for yet another purpose. They collect rain in the crevices of their upper shells and then tip their bodies forward so the water runs down the sides and into their mouths.
TURTLES, TORTOISES, AND PEOPLE
Many people hunt turtles for food or to use in making traditional medicines. Humans also collect many kinds of turtles and tortoises for the pet trade.
According to the World Conservation Union (IUCN), nearly half of all living species of turtles and tortoises are at risk of becoming extinct. The U.S. Fish and Wildlife Service lists thirteen U.S. species and twenty-four foreign species as Endangered. Many species are at risk because of overhunting and overcollecting or because their habitat is disappearing. Efforts are under way to protect many species.
FOR MORE INFORMATION
Behler, John L., and F. Wayne King. The Audubon Society Field Guide to North American Reptiles and Amphibians. New York: Knopf, 1979.
Burnie, David, and Don E. Wilson, eds. Animal: The Definitive Visual Guide to the World's Wildlife. London: Dorling Kindersley, 2001.
Conant, Roger, and Joseph T. Collins. A Field Guide to Reptiles and Amphibians of Eastern and Central North America. 3rd ed. Boston: Houghton Mifflin, 1998.
Ernst, C. H., and R. W. Barbour. Turtles of the World. Washington, DC: Smithsonian Institution Press, 1989.
Harding, J. H., and J. A. Holman. Michigan Turtles and Lizards. East Lansing: Michigan State University, 1990.
Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.
Stebbins, Robert C. A Field Guide to Western Reptiles and Amphibians. 3rd ed. Boston: Houghton Mifflin, 2003.
Zug, G. R., L. J. Vitt, and J. P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. San Diego, CA: Academic Press, 2001.