Turtle

For other uses, see Turtle (disambiguation). Chelonian redirects here. For other uses, see Chelonian (disambiguation).

Turtles (order Testudines) are reptiles with a special shell. The shell grows mainly from their ribs. Modern turtles belong to two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked turtles). The groups differ in how the head pulls back into the shell. About 360 kinds of turtles are alive today or were recently alive. They include land-dwelling tortoises and freshwater terrapins. Turtles live on most continents and on some islands. Sea turtles live in much of the ocean. Like other amniotes (reptiles, birds, and mammals), turtles breathe air. They do not lay eggs underwater, though many kinds live in or near water.

A turtle's shell is made mostly of bone. The top part is the rounded carapace. The bottom part is the flatter plastron, or belly-plate. The outside of the shell is covered in scales made of keratin, the same material as hair, horns, and claws. The bones of the carapace grow from ribs that spread out sideways. They become wide, flat plates that join together to cover the body.

Turtles are ectotherms, or "cold-blooded." This means their body temperature changes with their surroundings. Most turtles eat both plants and animals, and they mainly eat plants and slow-moving animals. Many turtles migrate short distances as the seasons change. Sea turtles are the only reptiles that travel long distances to lay their eggs on a favorite beach.

Turtles appear in myths and folktales from all around the world. Some land and freshwater kinds are kept as pets. People have hunted turtles for their meat, for use in traditional medicine, and for their shells. Sea turtles are often caught by accident as bycatch in fishing nets.

Turtle habitats around the world are being destroyed. Because of these problems, many kinds of turtles are gone or are at risk of disappearing.

Naming and etymology

The word turtle comes from the French word tortue (or tortre), which means "turtle" or "tortoise." It is a common name, and people often use it without knowing the scientific differences between animals. In North America, it can refer to the whole group. In Britain, the name is used mostly for sea turtles, while freshwater terrapins and land-dwelling tortoises have their own names. In Australia, non-marine turtles are often called "tortoises," although "turtle" is also widely used for the whole group in science and academic writing.

The scientific name of the group, Testudines, comes from the Latin word testudo, meaning "tortoise." German naturalist August Batsch created this name in 1788. Older names for the group, such as Chelonii and Chelonia, come from an Ancient Greek word meaning "tortoise." Because of a rule about which name was given first, Testudines is the official name. The word chelonian is a formal name for members of this group.

Anatomy and physiology

The largest living turtle is the leatherback turtle. It is also the fourth-largest reptile. It can grow over 2.7 m long and weigh over 500 kg. The largest known turtle was Archelon ischyros, a Late Cretaceous sea turtle that grew up to 4.5 m long. The smallest living turtle is Chersobius signatus from South Africa. It is no more than 10 cm long.

A turtle's shell is unique among vertebrates and protects the animal. It is made of 50–60 bones in two parts. The domed back part is the carapace. The flatter belly part is the plastron. The carapace is joined to the backbone and ribs. The plastron comes from bones of the shoulder girdle, sternum, and gastralia. As the ribs grow, they reach into the dermis to support the carapace. This growth is guided by fibroblast growth factors, including FGF10. The shoulder girdle has a bone called the coracoid, which is similar to a shoulder blade. The shell is covered in epidermal plates called scutes. They are usually arranged like mosaic tiles. Side-necked turtles have extra scutes, and hawksbill sea turtles have scutes that overlap.

Shell shapes depend on how each species lives, and they sometimes with sex differ too. Land turtles have more rounded, dome-shaped shells. Sea turtles have smooth, streamlined shells that reduce drag in the water. Spiky shells can offer protection from predators and camouflage. Some shells are less stiff. Box turtles have bones that are ankylosed, meaning they are fused together. Some shells have hinges. Softshell turtles have soft, rubbery edges. The leatherback has very few shell bones. Instead, it has thick connective tissue.

Turtles have a solid skull. It has no openings for muscles. Instead, the muscles attach to hollows at the back of the skull. This is unusual among amniotes. Some species have large heads and strong bites. The durophagous turtles, which eat animals with hard shells, such as Mesoclemmys nasuta, have the strongest bites. Insectivorous, piscivorous, and omnivorous turtles have weaker bites. Living turtles have no teeth. They have beaks made of keratin sheaths. These beaks may be sharp, serrated, or flat. Flat beaks help break open mollusks. Sea turtles have a secondary palate, which separates the mouth from the nose.

Turtle necks are very flexible. Sea turtles have short necks, while snake-necked turtles have long ones. Even so, all turtles have eight neck vertebrae. Some turtles make sounds using parts of the larynx or glottis. Others use elastin-rich vocal cords.

On land, turtles move slowly. A desert tortoise moves very slowly. Sea turtles can swim quickly. The gopher tortoise digs with its flat front legs. Freshwater turtles have webbing between their toes. Snapping turtles and mud turtles mostly walk along the bottom. Sea turtles and the pig-nosed turtle have flippers. Green sea turtles flap their flippers like bird wings. The Caspian turtle rows with its front legs, which is less efficient. Sea turtles swim about six times as fast as freshwater turtles. Their young swim as well as mackerel. Turtle tails are short. Big-headed turtles use their tails for balance. Male sea turtles have prehensile tails that can grip mates.

Turtles use vision to find food, mates, and their way. The retina has rods for seeing in dim light. It has cones with photopigments for seeing color. Some turtles may detect ultraviolet light. The red-eared slider has seven types of cone cells. Sea turtles focus underwater using the lens, because the cornea does not help with focusing there. Their visual acuity, or sharpness of sight, is best along the horizon. They do not seem to use polarized light. Leatherbacks lack a tapetum. They may be able to spot bioluminescence in deep water.

Turtles have no ear openings. Their eardrum is inside an otic capsule. They hear high sounds in air, up to 500 Hz. They hear lower sounds in water, as the loggerhead sea turtle does. Turtles smell using olfactory and vomeronasal receptors. Green sea turtles have learned to respond to chemicals like triethylamine and cinnamaldehyde.

Because their shells are rigid, turtles breathe in a special way. They use their coelomic cavity. The liver is connected to the lung by the root, and to the stomach by a mesentery. A septum supports the lungs. Muscles such as the transversus abdominis muscle and oblique abdominis muscle move air in and out. All turtles breathe air, and some can respire through the cloaca. Turtles have a circulatory system that carries oxygenated blood around the body. They can survive for a time using anaerobic respiration, with the help of chemical buffering. They can also rest for long periods through aestivation or brumation.

The turtle's heart has two atria and one ventricle. The ventricle connects to the pulmonary artery and the aorta. Freshwater sliders (Trachemys) have a weaker ridge than leatherbacks. When turtles breathe without oxygen, they make lactic acid instead of carbon dioxide. This is different from aerobic respiration, which uses oxygen. Turtles keep water and salts in bladders near the pubis. Tortoise bladders hold solutes and potassium salts. Brackish diamondback terrapins and sea turtles get rid of extra salt through tear glands.

Turtles can regulate their body temperature by basking in the sun. Large species hold heat well, which is called thermal inertia. The Aldabra giant tortoise on Grand Terre Island uses shade to stay cool. Leatherbacks off Nova Scotia use countercurrent heat exchange to stay warm.

Behavior

Most turtles eat both plants and animals. Land turtles eat more plants, and water turtles eat more animals. Many turtles are slow, so they often eat plants or slow animals like snails, worms, and insect larvae. Some turtles, such as the African helmeted turtle and snapping turtles, also eat fish, frogs, reptiles, birds, and mammals. They may ambush their food or find dead animals to eat. The alligator snapping turtle has a worm-like part on its tongue that it uses to lure fish into its mouth. Tortoises eat the most plants, including grass, leaves, and fruit. Many turtles also eat eggshells, bones, hair, and droppings to get extra nutrients. Some turtles have special ways of eating. The yellow-spotted river turtle and the painted turtle may filter feed by swimming with their mouths open and taking in food from the water. What a turtle eats can change with its age, sex, the season, and where it lives. The hawksbill eats sponges, the leatherback eats jellyfish, and the Mekong snail-eating turtle eats snails.

Many people think turtles cannot make sounds, but turtles do make different sounds to talk to each other. One study recorded 53 kinds of turtles, and all of them made sounds. Turtles may travel in groups, and their sounds may help them stay together. The oblong turtle makes many kinds of sounds, from clicks to grunts to drum rolls. Some turtles can learn and remember tasks. The Florida red-bellied cooters remembered tasks for at least 7.5 months. Turtles can also find their way using landmarks and a map-like system. See also: Animal cognition

When they sense danger, turtles may run, freeze, or pull into their shells. Freshwater turtles usually swim away into the water, though the Sonora mud turtle may go on land. A softshell turtle may dive and hide itself in the sea floor. Some turtles bite or give off a bad smell from musk glands. Bell's hinge-back tortoise can play dead. See also: Anti-predator adaptation

Turtles are the only reptiles that migrate over long distances, and some sea turtles travel thousands of kilometers. Some land, freshwater, and estuary turtles, such as the species of Geochelone, Chelydra, and Malaclemys, move shorter distances to lay eggs. Sea turtles nest on a beach and leave the eggs to hatch on their own. The young travel far and then return to the same area to lay eggs. Scientists are still not sure how sea turtles find their way. One idea is imprinting, like salmon, where young turtles learn the smell of their home waters. Another idea is that they use the direction of the Earth's magnetic field at their birth beach. Experiments suggest turtles can sense magnetism and use it for navigation. Studies of genetics in olive ridleys and other sea turtles show that groups return to their own nesting places. See also: Sea turtle migration

Reproduction and life cycle

Turtles have many different ways of mating, but they do not form pair-bonds or social groups. On land, males are often bigger than females. Males fight each other to set up a dominance hierarchy to win mates. Fighting is less common in water-dwelling species, and some males use their size to mate forcibly. In fully aquatic species, males are often smaller and use courtship displays to attract females. Courtship is different in each species. Mounting can be hard, so the male eastern box turtle hooks onto the back of the female's shell. Some females can store sperm from several males, so their egg clutches can have more than one father.

Most turtles lay their eggs on land, and most mothers cover the eggs and leave. Depending on the species, a mother may lay from one egg to over 100. In many species, the temperature of the nest decides whether the young become males or females. Hatchlings break out of their shells using a temporary egg tooth and then dig their way out of the nest. Young turtles face many dangers, but their chances of survival rise once they become adults. Turtles can live a very long time. The oldest living turtle and land animal is said to be a Seychelles giant tortoise named Jonathan, who turned 194 in 2026.

Systematics and evolution

Scientists have tried to explain how turtles evolved, especially how their shells formed. In 1914, Jan Versluys suggested that bony plates in the skin joined with the ribs underneath them. Today, the pareiasaurs that this idea was based on are thought to be unrelated to turtles. Newer fossil finds from the Late Triassic show that the shell grew in stages. Early turtle relatives had partial shells, and a full shell appeared by the end of that time period.

Scientists still debate where turtles fit among reptiles. Some studies placed them closer to lizards and snakes, but many newer studies suggest they are most closely related to birds and crocodilians. Turtles are divided into two living groups, Cryptodira and Pleurodira. Cryptodira pull their necks straight back into their shells, while Pleurodira tuck their necks to the side. Sea turtles have mostly lost the ability to pull their heads inside.

Distribution and habitat

Turtles live all over the world, on continents, on islands, and in the oceans. Some live on land, some live fully in water, and some live partly in water. Sea turtles are mainly found in warm tropical and subtropical areas. Leatherbacks can also be found in the colder parts of the Atlantic and Pacific. All living Pleurodira turtles live in freshwater, and they are found only in the Southern Hemisphere. Cryptodira turtles live on land, in freshwater, and in the sea, and they spread more widely. The areas with the most turtle species, apart from sea turtles, are the Amazon basin, the Gulf of Mexico drainages of the United States, and parts of South and Southeast Asia.

In colder places, there are fewer turtles. Cold makes it harder for them to have young, and long hibernations slow this down. North American species barely reach above the southern border of Canada. Some turtles live high up. For example, Terrapene ornata lives up to 2,000 m (6,600 ft) high in New Mexico. The leatherback sea turtle can dive more than 1,200 m (3,900 ft) deep. Species of the Gopherus genus can survive body temperatures below freezing and above 40 °C (104 °F). They are most active at 26–34 °C (79–93 °F).

Conservation

Turtles are among the animals most at risk of disappearing. Only primates have a higher share of threatened species among vertebrate groups. Of the 360 modern turtle species since 1500 AD, about half are considered threatened, and about 60% are threatened or already extinct. Turtles face many dangers, including habitat destruction, being hunted for food, the pet trade, light pollution, and climate change. Asian turtles are at very high risk, mainly because people have used them for food and medicine for a long time. About 83% of Asia's land and freshwater turtle species are considered threatened. Turtles are disappearing much faster than during the Cretaceous-Tertiary extinction. If this continues, scientists worry that all turtles could be gone within a few centuries.

Hatcheries can help protect turtle eggs from floods, erosion, predators, and poaching. To meet the demand for turtle meat, some Chinese markets have turned to farmed turtles. In 2007, more than a thousand turtle farms were estimated to be operating in China. Even so, many wild turtles are still caught and sent to markets. Conservationists call this "the Asian turtle crisis." In 2000, all Asian box turtles were placed on the CITES list of endangered species. In some American states, it is legal to catch wild turtles. In 2008, the Florida Fish and Wildlife Conservation Commission estimated that about 3,000 pounds of softshell turtles were sent out each week through Tampa International Airport.

Many sea turtles are also accidentally caught as bycatch in longlines, gillnets, and trawling nets. Since the 1980s, the United States has required shrimp trawlers to use turtle excluder devices. These devices help turtles escape from nets. In Australia, Queensland's shark culling program uses shark nets and drum lines. This program has accidentally caught and killed many turtles. Finally, the North American red-eared slider, an invasive ones species, spread after pet turtles were released into the wild. It has been listed among the "world's worst invasive species" because it seems to compete with native turtles.

Human uses

Main article: Cultural depictions of turtles

Further information: World Turtle

Turtles have been part of human cultures since ancient times. They have even traveled into space. In September 1968, the Soviet Union sent two tortoises on the Zond 5 circumlunar flight. They were the first living things from Earth to reach the area around the Moon. Turtles also flew on Zond 6 (1968) and Zond 7 (1969).

In Hindu mythology, the World Turtle, called Kurma or Kacchapa, holds up four elephants that carry the whole world. The turtle is one of the ten avatars or forms of the god Vishnu. The yoga pose Kurmasana is named after it. World Turtles also appear in Algonquian, Iroquois, and Lenape cultures. These peoples tell many versions of the creation story of Turtle Island. In one Iroquois version, a pregnant Sky Woman is caught by birds and lands on Turtle's back. The Earth then grows around her.

A turtle was a symbol of the Ancient Mesopotamian god Enki starting around the 3rd millennium BCE. In an ancient Greek origin myth, only the tortoise did not come to the wedding of Zeus and Hera. As a result, Zeus ordered it to carry its house with it forever. Hermes later made a seven-stringed lyre from a tortoise shell.

In the Shang dynasty, Chinese people practiced plastromancy, a way of telling the future, dating back to 1200 BCE. They wrote questions in the oldest known form of Chinese characters on turtle plastrons, burned them, and read the cracks. Later, the turtle became one of the four sacred animals in Confucianism. In the Han period, steles were placed on stone turtles, a idea later linked with Bixi. Marine turtles are also an important subject in Australian Aboriginal art.

The army of Ancient Rome used a testudo ("tortoise") formation. Soldiers held their shields together to form a shield wall for protection. In Aesop's Fables, "The Tortoise and the Hare" shows how a slower partner may win an unequal race.

Lewis Carroll's 1865 Alice's Adventures in Wonderland includes a Mock Turtle. It is named for a soup meant to imitate an expensive soup made from real turtle meat. In 1896, the French playwright Léon Gandillot wrote a comedy called La Tortue. It came to the Manhattan Theatre in New York in 1898 as The Turtle.

A "cosmic turtle" and the idea of an island appear in Gary Snyder's 1974 novel Turtle Island. A giant turtle also appears in Terry Pratchett's Discworld series, named Great A'Tuin. It first appeared in the 1983 novel The Colour of Magic. A giant fire-breathing turtle called Gamera stars in a series of Japanese monster movies in the kaiju genre. There are twelve films, made from 1965 to 2006. Turtles have also appeared in comics and cartoons, such as the 1984 Teenage Mutant Ninja Turtles.

Some turtles, especially small land and freshwater species, are kept as pets. Demand grew in the 1950s. The US was the main supplier, especially of farm-bred red-eared sliders. The popularity of exotic pets has led to more illegal wildlife trafficking. Poor care can cause tortoises to get sick. Problems include rhinitis (nasal swelling), overgrown beaks, hyperparathyroidism, constipation, and others.

In the early 20th century, people in the United States organized and bet on turtle races. Wild turtle meat is still eaten in some Asian cultures. Turtle soup was once a popular dish in English cuisine.

Turtle shells are widely used in traditional Chinese medicine. Taiwan imported large amounts of hard shells from its neighbors each year for about a decade. A popular medicine made with herbs and turtle shells is guilinggao jelly. The substance tortoiseshell, usually from the hawksbill turtle, has been used for centuries to make jewelry, tools, and decorations. In 1977, the trade in it was banned internationally by CITES.

Some cultures have also used turtle shells to make music. Native American shamans made them into ceremonial rattles. Aztecs, Mayas, and Mixtecs made ayotl drums.