turtle, (order Testudines), anyreptile with a body encased in a bonyshell, includingtortoises. Although numerousanimals, frominvertebrates tomammals, have evolved shells, none has an architecture like that of turtles. The turtle shell has a top (carapace) and a bottom (plastron). The carapace and plastron are bony structures that usually join one another along each side of the body, creating a rigid skeletal box. This box, composed ofbone andcartilage, is retained throughout theturtle’s life. Because the shell is anintegral part of the body, the turtle cannot exit it, nor is the shell shed like theskin of some other reptiles.

1 of 2

2 of 2

There are approximately 356 species of turtles living on land in allcontinents exceptAntarctica and in bothsaltwater and fresh water.Tortoises (family Testudinidae) live exclusively on land and have anatomic features distinguishing them from other turtles, but the termtortoise has long been used to refer to other terrestrial testudines as well, such as thebox turtle and thewood turtle. Similarly,terrapin was sometimes used to describe any aquatic turtle but is now largely restricted to theediblediamondback terrapin (Malaclemys terrapin) of the easternUnited States.

Despite turtles’ broad distribution, there are not and never seem to have been a great manyspecies of turtles at any time over the course of their long evolutionary history. The small number of species, however, does not equate to a lack ofdiversity. There are turtles with carapace lengths (the standard way to measure turtles) of less than 10 cm (4 inches), as in the flattenedmusk turtle (Sternotherus depressus), and of more than 1.5 metres (4.9 feet), as in theleatherback sea turtle (Dermochelys coriacea). Some species live in seasonally coldclimates with growing seasons of only about three months; others live in the tropics and grow year-round. Some tortoises rarely see water, while other turtles spend virtually their entire lives in it, be it in a single small pond or traveling the vast openocean.

Both common and rare turtles are kept aspets. In theWestern Hemisphere,pond turtles such as the red-eared slider (Trachemys scripta) and cooters (Pseudemys species) are very often seen inpet stores. The ornate shells that make some species valuable as pets also make themvulnerable toextinction in the wild, since these turtles frequently are found only in small geographic areas or do not breed in captivity.

Before the advent of plastics,tortoiseshell from thehawksbillsea turtle (Eretmochelys imbricata) was used ineyeglass frames and decorative items. Turtles and theireggs have long been eaten in many parts of the world, and they continue to be in great demand commercially. In some areas, local populations and even entire species have been hunted to extinction.

Britannica Quiz

Wild Words from the Animal Kingdom Vocabulary Quiz

Such exploitation is not a recent phenomenon. For example, theNative Americans who settledFlorida quite possibly ate its giant tortoises to extinction as early as 11,500 years ago. The first colonists ofMadagascar eliminated that island’s gianttortoise (Geochelone grandidieri) between 2,300 and 2,100 years ago, and European settlers and sailors eliminated giant tortoises from the island ofMauritius during the 1700s and fromRéunion by the 1840s. Everysea turtle species has long been killed formeat, with its eggs being harvested frombeachnests as soon as they are laid. This practice now endangers many populations of sea turtles. Before 1969, for example, more than 3,000 female leatherback sea turtles emerged from the ocean annually to nest on the beaches ofTerengganu,Malaysia. In the 1990s only 2 to 20 females appeared each year. Their disappearance resulted from years of excessive egg harvesting and the capture andslaughter of juveniles and adults during their migratory search forfood. By the 2010s the species was virtually absent from Terengganu.

Overharvesting is not confined to large species. InChina, turtles large and small are used for both food andmedicine. By the early 1990s, many localpopulations of turtles had disappeared within the country, so turtles began to be imported from around the world. Some species, such as the three-stripedbox turtle, or golden coin turtle (Cuora trifasciata), are so popular for traditional Chinese celebrations and intraditional Chinese medicine that aquaculturists raise them and can sell individual turtles for tens of thousands of dollars (U.S.), an amazing price for areptile less than 20 cm (about 8 inches) long.

Access for the whole family!

Bundle Britannica Premium and Kids for the ultimate resource destination.

Subscribe

Form and function

The turtle’s shell is anadaptation that protects it from predators, whichcompensates for the reptile’s slow crawling speed. Thecarapace andplastron each arose from two types of bone: dermal bones that form in the skin and endochondralbone (bone arising fromcartilage) derived from the skeleton. Evolution has intricately linked these two types of bone to produce the shell of modern turtles. The carapace consists of 10 trunk vertebrae and their ribs, which are overlain by and fused to dermal plates. Another series of dermal plates forms the perimeter of the carapace. The plastron usually contains four pairs of large plates and a single one centred near the front (the anteromedial plate); these plates are large dermal bones, although theanterior ones may contain parts of theshoulder girdle. The shell is variously modified and shaped to meet the needs of defense, feeding, andmovement.

Mosttortoises have high, domed shells, the major exception being thepancake tortoise (Malacochersus tornieri) of southeastern Africa. The pancake tortoise lives among rocky outcroppings, where its flat shell allows it to crawl into crevices to rest. Once in a crevice, the pancake tortoise can inflate itslungs, thus expanding the shell and lodging itself so securely that a predator cannot pull it free. The domed shell of other tortoises and land turtles such asbox turtles (Cuora,Terrapene) seems to be anadaptation that makes the shell difficult for a predator to hold in its mouth and crush. Among aquatic turtles, some groups are swimmers and usually have streamlined shells; streamlining is best-developed in thesea turtles. Other aquatic turtles, such as the matamata (described below) and snapping turtles, are bottom-walkers; their shells are less streamlined and often have ridged carapaces that may assist in camouflage (see alsoconcealing coloration).

The manner in which the neck folds is the maincriteria fordifferentiating the two main groups (suborders) of turtles. All turtles, no matter how long or short their necks, have eight cervicalvertebrae, but those that fold their neck vertically can withdraw the head into the shell. These are the so-called S-necked, or vertical-necked, turtles of the suborderCryptodira (meaning “hidden neck”). Turtles that cannot withdraw the head belong to the suborderPleurodira (meaning “side neck”). (See alsoside-necked turtle;snake-necked turtle.)

In addition to differences in the neck, skulls vary in size and shape between the two groups, though all are made up of the same bony elements. The pleurodiran and cryptodiran turtles differ fundamentally in lowerjawarchitecture and musculature. This difference typically yields a flatter and broaderskull in the pleurodires—an architecture that may have allowed theevolution of the gape-and-suck feeding mechanism seen in many pleurodires and best developed in the South Americanmatamata (Chelus fimbriatus orC. fimbriata). This turtle can quickly enlarge the cavity of itsmouth and throat when striking at passing prey. As the turtle’s head nears its victim, the greatly enlarged cavity acts like avacuum, suckingwater and prey into the mouth. As the throat area is compressed, the mouth is opened to allow water to escape but not the prey. Most carnivorous turtles use a head strike to capture their prey and when scavenging.

No present-day turtles haveteeth; rather, the upper and lower jaws bearkeratinous sheaths that fit onto the skull like a pair offalse teeth. The edges (occasionally with serrations) are sharp and allow turtles to cut pieces of flesh fromcarcasses and quickly kill small prey. The cutting edges are also effective in chopping vegetation into bite-size pieces. Turtles do not chew; those that eatmollusks crush them with a broad, thick sheath inside the mouth.

All the turtle’s senses are well-developed, and they are used in avoiding predators and in finding and capturing food. Theeyes have the typical anatomy of othervertebrates having goodvision. Aquatic turtles have eyes that quickly adjust for aerial or aquatic vision, seeing well in both situations. Tortoises appear to havecolour vision, but colour vision is untested for most turtles. Turtles, particularly aquatic ones, are not strongly olfactory, but all are capable ofsmelling. Some aquatic species have protuberances on the chin in the form of tubercles and papillae. These appear to be mainlytactile, although some are chemosensory (that is, having the ability to sense particular chemical stimuli). The turtleear has an eardrum flush with the surface of the head. A singlebone, the stapes, transmits sound to theinner ear.