Turtles arereptiles of theorderTestudines, characterized by a specialshell developed mainly from their ribs. Modern turtles are divided into two major groups, thePleurodira (side necked turtles) andCryptodira (hidden necked turtles), which differ in the way the head retracts. There are 360 living and recently extinctspecies of turtles, including land-dwellingtortoises and freshwaterterrapins. They are found on most continents, some islands and, in the case ofsea turtles, much of the ocean. Like otheramniotes (reptiles,birds, andmammals) they breathe air and do not lay eggs underwater, although many species live in or around water.
Turtle shells are made mostly ofbone; the upper part is the domedcarapace, while the underside is the flatterplastron or belly-plate. Its outer surface is covered inscales made ofkeratin, the material of hair, horns, and claws. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body. Turtles areectotherms or "cold-blooded", meaning that their internal temperature varies with their direct environment. They are generally opportunisticomnivores and mainly feed on plants and animals with limited movements. Many turtlesmigrate short distances seasonally. Sea turtles are the only reptiles that migrate long distances to laytheir eggs on a favored beach.
Turtles haveappeared in myths and folktales around the world. Some terrestrial and freshwater species are widely kept as pets. Turtles have beenhunted for their meat, for use in traditional medicine, and for their shells. Sea turtles are often killed accidentally asbycatch in fishing nets. Turtle habitats around the world are being destroyed. As a result of these pressures, many species are extinct or threatened with extinction.
The wordturtle is borrowed from theFrench wordtortue ortortre 'turtle,tortoise'.[3] It is acommon name and may be used without knowledge of taxonomic distinctions. In North America, it may denote the order as a whole. In Britain, the name is used forsea turtles as opposed to freshwaterterrapins and land-dwelling tortoises. In Australia, which lacks true tortoises (family Testudinidae), non-marine turtles were traditionally called tortoises, but more recently turtle has been used for the entire group.[4]
The name of the order,Testudines (/tɛˈstjuːdɪniːz/ⓘteh-STEW-din-eez), is based on theLatin wordtestudo 'tortoise';[5] and was coined by German naturalistAugust Batsch in 1788.[1] The order has also been historically known asChelonii (Latreille 1800) andChelonia (Ross andMacartney 1802),[2] which are based on theAncient Greek wordχελώνη (chelone) 'tortoise'.[6][7] Testudines is the official order name due to theprinciple of priority.[2] The termchelonian is used as a formal name for members of the group.[1][8]
The largestliving species of turtle (and fourth-largestreptile) is theleatherback turtle, which can reach over 2.7 m (8 ft 10 in) in length and weigh over 500 kg (1,100 lb).[9] The largest known turtle wasArchelon ischyros, aLate Cretaceous sea turtle up to 4.5 m (15 ft) long, 5.25 m (17 ft) wide between the tips of the front flippers, and estimated to have weighed over 2,200 kg (4,900 lb).[10] The smallest living turtle isChersobius signatus of South Africa, measuring no more than 10 cm (3.9 in) in length[11] and weighing 172 g (6.1 oz).[12]
The shell of a turtle is unique amongvertebrates and serves to protect the animal and provide shelter from the elements.[13][14][15] It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back)carapace and the flatter, ventral (belly)plastron. They are connected by lateral (side) extensions of the plastron.[13][16]
Thecarapace is fused with the vertebrae and ribs while the plastron is formed from bones of theshoulder girdle,sternum, andgastralia (abdominal ribs).[13] During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering thedermis (inner skin) of the back to support the carapace. The development is signaled locally by proteins known asfibroblast growth factors that includeFGF10.[17] The shoulder girdle in turtles is made up of two bones, the scapula and thecoracoid.[18] Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.[19]
Development of the shell. The ribs are growing sideways into the carapacial ridge, seen here as a bud, to support the carapace.[17]
The shell is covered inepidermal (outer skin) scales known asscutes that are made ofkeratin, the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total. Carapace scutes are divided into "marginals" around the margin and "vertebrals" over the vertebral column, though the scute that overlays the neck is called the "cervical". "Pleurals" are present between the marginals and vertebrals.[20] Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals.Side-necked turtles additionally have "intergular" scutes between the gulars.[16][21] Turtle scutes are usually structured likemosaic tiles, but some species, like thehawksbill sea turtle, have overlapping scutes on the carapace.[16]
The shapes of turtle shells vary with the adaptations of the individual species, andsometimes with sex. Land-dwelling turtles are more dome-shaped, which appears to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells that allow them to cut through the water. Sea turtles in particular have streamlined shells that reducedrag and increase stability in the open ocean. Some turtle species have pointy or spiked shells that provide extraprotection from predators andcamouflage against the leafy ground. The lumps of a tortoise shell can tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the tip of the plastron is thickened and used for butting and ramming during combat.[22]
Shells vary in flexibility. Some species, such asbox turtles, lack the lateral extensions and instead have the carapace bones fully fused orankylosed together. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract.Softshell turtles have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, but has thickconnective tissue and an outer layer of leathery skin.[23]
The turtle's skull is unique among livingamniotes (which includes reptiles, birds and mammals); it is solid and rigid with no openings for muscle attachment (temporal fenestrae).[24][25] Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the long and narrow skulls of softshells to the broad and flattened skull of themata mata.[25] Some turtle species have developed large and thick heads, allowing for greater muscle mass and stronger bites.[26]
Turtles that are carnivorous ordurophagous (eating hard-shelled animals) have the most powerful bites. For example, the durophagousMesoclemmys nasuta has a bite force of 432 lbf (1,920 N). Species that areinsectivorous,piscivorous (fish-eating), oromnivorous have lower bite forces.[27] Living turtles lack teeth but have beaks made ofkeratin sheaths along the edges of the jaws.[28][13] These sheaths may have sharp edges for cutting meat, serrations for clipping plants, or broad plates for breakingmollusks.[29] Sea turtles, and several extinct forms, have evolved a bonysecondary palate which completely separates the oral and nasal cavities.[30]
The necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such assnake-necked turtles, have long ones. Despite this, all turtle species have eightneck vertebrae, a consistency not found in other reptiles but similar to mammals.[31] Some snake-necked turtles have both long necks and large heads, limiting their ability to lift them when not in water.[26] Some turtles have folded structures in thelarynx orglottis that vibrate to produce sound. Other species haveelastin-richvocal cords.[32][33]
Due to their heavy shells, turtles are slow-moving on land. Adesert tortoise moves at only 0.22–0.48 km/h (0.14–0.30 mph). By contrast, sea turtles can swim at 30 km/h (19 mph).[13] The limbs of turtles are adapted for various means of locomotion and habits and most have five toes. Tortoises are specialized for terrestrial environments and have column-like legs with elephant-like feet and short toes. Thegopher tortoise has flattened front limbs for digging in the substrate. Freshwater turtles have more flexible legs and longer toes withwebbing, giving them thrust in the water. Some of these species, such assnapping turtles andmud turtles, mainly walk along the water bottom, as they would on land. Others, such as terrapins, swim by paddling with all four limbs, switching between the opposing front and hind limbs, which keeps their direction stable.[13][34]
Sea turtles and thepig-nosed turtle are the most specialized for swimming. Their front limbs have evolved into flippers while the shorter hind limbs are shaped more like rudders. The front limbs provide most of the thrust for swimming, while the hind limbs serve as stabilizers.[13][36] Sea turtles such as thegreen sea turtle rotate the front limb flippers like a bird's wings to generate a propulsive force on both the upstroke and on the downstroke. This is in contrast to similar-sized freshwater turtles (measurements having been made on young animals in each case) such as theCaspian turtle, which uses the front limbs like the oars of a rowing boat, creating substantial negative thrust on the recovery stroke in each cycle. In addition, the streamlining of the marine turtles reduces drag. As a result, marine turtles produce a propulsive force twice as large, and swim six times as fast, as freshwater turtles. The swimming efficiency of young marine turtles is similar to that of fast-swimming fish of open water, likemackerel.[35]
Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes.Snapping turtles and thebig-headed turtle have longer tails; the latter uses it for balance while climbing. Thecloaca is found underneath and at the base, and the tail itself houses the reproductive organs. Hence, males have longer tails to contain the penis. In sea turtles, the tail is longer and moreprehensile in males, who use it to grasp mates. Several turtle species have spines on their tails.[37][24]
Thered-eared slider has an exceptional seven types of color-detecting cells in its eyes.[38]
Turtles make use ofvision to find food and mates, avoid predators, and orient themselves. Theretina's light-sensitive cells include bothrods for vision in low light, andcones with three differentphotopigments for bright light, where they have full-color vision. There is possibly a fourth type of cone that detectsultraviolet, as hatchling sea turtles respond experimentally to ultraviolet light, but it is unknown if they can distinguish this from longer wavelengths. A freshwater turtle, thered-eared slider, has an exceptional seven types of cone cell.[38][39][40]
Sea turtles orient themselves on land by night, using visual features detected in dim light. They can use their eyes in clear surface water, muddy coasts, the darkness of the deep ocean, and also above water. Unlike in terrestrial turtles, thecornea (the curved surface that lets light into the eye) does not help to focus light on the retina, so focusing underwater is handled entirely by the lens, behind the cornea. The cone cells contain oil droplets placed to shift perception toward the red part of the spectrum, improving color discrimination. Visual acuity, studied in hatchlings, is highest in a horizontal band with retinal cells packed about twice as densely as elsewhere. This gives the best vision along the visual horizon. Sea turtles do not appear to usepolarized light for orientation as many other animals do. The deep-diving leatherback turtle lacks specific adaptations to low light, such as large eyes, large lenses, or a reflectivetapetum. It may rely on seeing thebioluminescence of prey when hunting in deep water.[38]
Turtles have no ear openings; theeardrum is covered with scales and encircled by a bonyotic capsule, which is absent in other reptiles.[31] Their hearing thresholds are high in comparison to other reptiles, reaching up to 500Hz in air, but underwater they are more attuned to lower frequencies.[41] Theloggerhead sea turtle has been shown experimentally to respond to low sounds, with maximal sensitivity between 100 and 400 Hz.[42]
Turtles haveolfactory (smell) andvomeronasal receptors along the nasal cavity, the latter of which are used to detect chemical signals.[43] Experiments on green sea turtles showed they could learn to respond to a selection of different odorant chemicals such astriethylamine andcinnamaldehyde, which were detected by olfaction in the nose. Such signals could be used in navigation.[44]
The rigid shell of turtles is not capable of expanding and making room for the lungs, as in other amniotes, so they have had to evolve special adaptations for respiration.[45][46][47] The lungs of turtles are attached directly to the carapace above while below, connective tissue attaches them to the organs.[48] They have multiple lateral (side) and medial (middle) chambers (the numbers of which vary between species) and one terminal (end) chamber.[49]
The lungs are ventilated using specific groups of abdominal muscles attached to the organs that pull and push on them.[45] Specifically, it is the turtle's large liver that compresses the lungs. Underneath the lungs, in thecoelomic cavity, the liver is connected to the right lung by theroot, and the stomach is directly attached to the left lung, and to the liver by amesentery. When the liver is pulled down, inhalation begins.[46] Supporting the lungs is a wall orseptum, which is thought to prevent them from collapsing.[50] During exhalation, the contraction of thetransversus abdominis muscle propels the organs into the lungs and expels air. Conversely, during inhalation, the relaxing and flattening of theoblique abdominis muscle pulls the transversus back down, allowing air back into the lungs.[46]
Although many turtles spend large amounts of their lives underwater, all turtles breathe air and must surface at regular intervals to refill their lungs. Depending on the species, immersion periods vary between a minute and an hour.[51] Some species canrespire through the cloaca, which contains large sacs that are lined with many finger-like projections that take up dissolvedoxygen from the water.[52]
Snapping turtle emerging from period ofbrumation, in which it buried itself in mud. Turtles have multiple circulatory and physiological adaptations to enable them to go long periods without breathing.[53]
Turtles share the linkedcirculatory and pulmonary (lung) systems of vertebrates, where the three-chambered heart pumps deoxygenated blood through the lungs and then pumps the returnedoxygenated blood through the body's tissues. The cardiopulmonary system has both structural and physiological adaptations that distinguish it from other vertebrates. Turtles have a large lung volume and can move blood through non-pulmonary blood vessels, including some within the heart, to avoid the lungs while they are not breathing. They can hold their breath for much longer periods than other reptiles and they can tolerate the resulting low oxygen levels. They can moderate the increase in acidity duringanaerobic (non-oxygen-based) respiration bychemical buffering and they can lie dormant for months, inaestivation orbrumation.[53]
The heart has twoatria but only oneventricle. The ventricle is subdivided into three chambers. A muscular ridge enables a complex pattern of blood flow so that the blood can be directed either to the lungs via thepulmonary artery, or to the body via theaorta. The ability to separate the two outflows varies between species. The leatherback has a powerful muscular ridge enabling almost complete separation of the outflows, supporting its actively swimming lifestyle. The ridge is less well developed in freshwater turtles like the sliders (Trachemys).[53]
Turtles are capable of enduring periods of anaerobic respiration longer than many other vertebrates. This process breaks down sugars incompletely tolactic acid, rather than all the way tocarbon dioxide and water as inaerobic (oxygen-based) respiration.[53] They make use of the shell as a source of additional buffering agents for combating increased acidity, and as a sink for lactic acid.[54]
In sea turtles, the bladder is one unit and in most freshwater turtles, it is double-lobed.[55] Sea turtle bladders are connected to two small accessory bladders, located at the sides to the neck of the urinary bladder and above thepubis.[56] Arid-living tortoises have bladders that serve as reserves of water, storing up to 20% of their body weight in fluids. The fluids are normally low insolutes, but higher during droughts when the reptile gainspotassium salts from its plant diet. The bladder stores these salts until the tortoise finds fresh drinking water.[57] To regulate the amount of salt in their bodies, sea turtles and thebrackish-livingdiamondback terrapin secrete excess salt in a thick sticky substance from theirtear glands. Because of this, sea turtles may appear to be "crying" when on land.[58]
Turtles, like other reptiles, have a limited ability toregulate their body temperature. This ability varies between species, and with body size. Small pond turtles regulate their temperature by crawling out of the water and basking in the sun, while small terrestrial turtles move between sunny and shady places to adjust their temperature. Large species, both terrestrial and marine, have sufficient mass to give them substantialthermal inertia, meaning that they heat up or cool down over many hours. TheAldabra giant tortoise weighs up to some 60 kilograms (130 lb) and is able to allow its temperature to rise to some 33 °C (91 °F) on a hot day, and to fall naturally to around 29 °C (84 °F) by night. Some giant tortoises seek out shade to avoid overheating on sunny days. OnGrand Terre Island, food is scarce inland, shade is scarce near the coast, and the tortoises compete for space under the few trees on hot days. Large males may push smaller females out of the shade, and some then overheat and die.[59]
Adult sea turtles, too, have large enough bodies that they can to some extent control their temperature. The largest turtle, the leatherback, can swim in the waters offNova Scotia, which may be as cold as 8 °C (46 °F), while their body temperature has been measured at up to 12 °C (22 °F) warmer than the surrounding water. To help keep their temperature up, they have a system ofcountercurrent heat exchange in the blood vessels between their body core and the skin of their flippers. The vessels supplying the head are insulated by fat around the neck.[59]
Most turtle species are opportunistic omnivores; land-dwelling species are moreherbivorous and aquatic ones morecarnivorous.[26] Generally lacking speed and agility, most turtles feed either on plant material or on animals with limited movements like mollusks, worms, and insect larvae.[13] Some species, such as theAfrican helmeted turtle and snapping turtles, eat fish, amphibians, reptiles (including other turtles), birds, and mammals. They may take them byambush but also scavenge.[60] Thealligator snapping turtle has a worm-like appendage on its tongue that ituses to lure fish into its mouth. Tortoises are the most herbivorous group, consuming grasses, leaves, and fruits.[61] Many turtle species, including tortoises, supplement their diet with eggshells, animal bones, hair, and droppings for extra nutrients.[62]
Turtles generally eat their food in a straightforward way, though some species have special feeding techniques.[13] Theyellow-spotted river turtle and thepainted turtle mayfilter feed by skimming the water surface with their mouth and throat open to collect particles of food. When the mouth closes, the throat constricts and water is pushed out through the nostrils and the gap in between the jaws.[63] Some species employ a "gape-and-suck method" where the turtle opens its jaws and expands its throat widely, sucking the prey in.[13][64][65]
The diet of an individual within a species may change with age, sex, and season, and may also differ between populations. In many species, juveniles are generally carnivorous but become more herbivorous as adults.[13][66] WithBarbour's map turtle, the larger female mainly eats mollusks while the male usually eatsarthropods.[13]Blanding's turtle may feed mainly on snails or crayfish depending on the population. TheEuropean pond turtle has been recorded as being mostly carnivorous much of the year but switching towater lilies during the summer.[67] Some species have developedspecialized diets such as the hawksbill, which eatssponges, the leatherback, which feeds onjellyfish, and theMekong snail-eating turtle.[26][13]
While popularly thought of as mute, turtles make various sounds to communicate.[69][70] One study which recorded 53 species found that all of them vocalized.[71] Tortoises may bellow when courting and mating.[70][31] Various species of both freshwater and sea turtles emit short, low-frequency calls from the time they are in the egg to when they are adults. These vocalizations may serve to create group cohesion whenmigrating.[70] Theoblong turtle has a particularly large vocal range; producing sounds described as clacks, clicks, squawks, hoots, various kinds of chirps, wails,hooos, grunts, growls, blow bursts, howls, and drum rolls.[68]
Play behavior has been documented in some turtle species.[72] In the laboratory,Florida red-bellied cooters can learn novel tasks and have demonstrated a long-term memory of at least 7.5 months.[73] Similarly, giant tortoises can learn and remember tasks, and master lessons much faster when trained in groups.[74] Tortoises appear to be able to retainoperant conditioning nine years after their initial training.[75] Studies have shown that turtles can navigate the environment using landmarks and a map-like system resulting in accurate direct routes towards a goal.[76] Navigation in turtles have been correlated to high cognition function in the medial cortex region of the brain.[76][77]
When sensing danger, a turtle may flee, freeze or withdraw into its shell. Freshwater turtles flee into the water, though theSonora mud turtle may take refuge on land as the shallow temporary ponds they inhabit make them vulnerable.[78] When startled, a softshell turtle may dive underwater and bury itself under the sea floor.[79] If a predator persists, the turtle may bite or discharge from its cloaca. Several species produce foul-smelling chemicals from musk glands. Other tactics include threat displays andBell's hinge-back tortoise canplay dead. When attacked, big-headed turtle hatchlings squeal, possibly startling the predator.[80]
An olive ridley sea turtle nesting on Escobilla Beach,Oaxaca,Mexico. Female sea turtles migrate long distances to nest on favored beaches.
Turtles are the only reptiles that migrate long distances, more specifically the marine species that can travel up to thousands of kilometers. Some non-marine turtles, such as the species ofGeochelone (terrestrial),Chelydra (freshwater), andMalaclemys (estuarine), migrate seasonally over much shorter distances, up to around 27 km (17 mi), to lay eggs. Such short migrations are comparable to those of some lizards, snakes, and crocodilians.[81] Sea turtles nest in a specific area, such as a beach, leaving the eggs to hatch unattended. The young turtles leave that area, migrating long distances in the years or decades in which they grow to maturity, and then return seemingly to the same area every few years to mate and lay eggs, though the precision varies between species and populations. This "natal homing" has appeared remarkable to biologists, though there is now plentiful evidence for it, including from genetics.[82]
How sea turtles navigate to their breeding beaches remains unknown. One possibility isimprinting as insalmon, where the young learn the chemical signature, effectively the scent, of their home waters before leaving, and remember that when the time comes for them to return as adults. Another possible cue is the orientation of theEarth's magnetic field at the natal beach. There is experimental evidence that turtles have an effective magnetic sense, and that they use this innavigation. Proof that homing occurs is derived from genetic analysis of populations of loggerheads, hawksbills, leatherbacks, andolive ridleys by nesting place. For each of these species, the populations in different places have their ownmitochondrial DNA genetic signatures that persist over the years. This shows that the populations are distinct and that homing must be occurring reliably.[82]
Turtles have a wide variety of mating behaviors but do not formpair-bonds or social groups.[83] In terrestrial species, males are often larger than females and fighting between males establishes adominance hierarchy for access to mates. For most semi-aquatic and bottom-walking aquatic species, combat occurs less often. Males of these species instead may use their size advantage tomate forcibly. In fully aquatic species, males are often smaller than females and rely oncourtship displays to gain mating access to females.[84] In green sea turtles, females generally outnumber males.[85]
Courtship varies between species, and with habitat. It is often complex in aquatic species, both marine and freshwater, but simpler in the semi-aquatic mud turtles and snapping turtles. A male tortoise bobs his head, then subdues the female by biting and butting her before mounting.[13] The malescorpion mud turtle approaches the female from the rear, and often resorts to aggressive methods such as biting the female's tail or hind limbs, followed by a mounting.[86]
Female choice is important in some species, and female green sea turtles are not always receptive. As such, they have evolved behaviors to avoid the male's attempts at copulation, such as swimming away, confronting the male followed by biting or taking up a refusal position with her body vertical, her limbs widely outspread, and her plastron facing the male. If the water is too shallow for the refusal position, the females resort to beaching themselves, as the males do not follow them ashore.[85]
All turtles fertilize internally; mounting and copulation can be difficult. In many species, males have a concave plastron that interlocks with the female's carapace. In species like theRussian tortoise, the male has a lighter shell and longer legs. The high, rounded shape of box turtles are particular obstacles for mounting. The maleeastern box turtle leans backward and hooks onto the back of the female's plastron.[87] Aquatic turtles mount in water,[88][89] and female sea turtles support the mounting male while swimming and diving.[90] During copulation, the male turtle aligns his tail with the female's so he can insert his penis into her cloaca.[91] Some female turtles canstore sperm from multiple males and theiregg clutches can have multiple sires.[92][83]
Turtles, including sea turtles, lay their eggs on land, although some lay eggs near water that rises and falls in level, submerging the eggs. While most species build nests and lay eggs where they forage, some travel miles. Thecommon snapping turtle walks 5 km (3 mi) on land, while sea turtles travel even further; the leatherback swims some 12,000 km (7,500 mi) to its nesting beaches.[13][89] Most turtles create a nest for their eggs. Females usually dig a flask-like chamber in the substrate. Other species lay their eggs in vegetation or crevices.[93] Females choose nesting locations based on environmental factors such as temperature and humidity, which are important for developing embryos.[89] Depending on the species, the number of eggs laid varies from one to over 100. Larger females can lay eggs that are greater in number or bigger in size. Compared to freshwater turtles, tortoises deposit fewer but larger eggs. Females can lay multiple clutches throughout a season, particularly in species that experience unpredictablemonsoons.[94]
Most mother turtles do no more in the way of parental care than covering their eggs and immediately leaving, though some species guard their nests for days or weeks.[95] Eggs vary between rounded, oval, elongated, and between hard- and soft-shelled.[96] Most species have theirsex determined by temperature. In some species, higher temperatures produce females and lower ones produce males, while in others, milder temperatures produce males and both hot and cold extremes produce females.[13] There is experimental evidence that the embryos ofMauremys reevesii can move around inside their eggs to select the best temperature for development, thus influencing their sexual destiny.[97] In other species, sex isdetermined genetically. The length of incubation for turtle eggs varies from two to three months for temperate species, and four months to over a year for tropical species.[13] Species that live in warm temperate climates candelay their development.[98]
Hatching young turtles break out of the shell using anegg tooth, a sharp projection that exists temporarily on their upper beak.[13][99] Hatchlings dig themselves out of the nest and find safety in vegetation or water. Some species stay in the nest for longer, be it for overwintering or to wait for the rain to loosen the soil for them to dig out.[13] Young turtles are highly vulnerable to predators, both in the egg and as hatchlings. Mortality is high during this period but significantly decreases when they reach adulthood. Most species grow quickly during their early years and slow down when they mature.[100]
Turtles can live long lives. The oldest living turtle and land animal is said to be aSeychelles giant tortoise namedJonathan, who turned 187 in 2019.[101] AGalápagos tortoise namedHarriet was collected byCharles Darwin in 1835; it died in 2006, having lived for at least 176 years. Most wild turtles do not reach that age. Turtles keep growing new scutes under the previous scutes every year, allowing researchers to estimate how long they have lived.[102] They alsoage slowly.[103] The survival rate for adult turtles can reach 99% per year.[13]
Zoologists have sought to explain the evolutionary origin of the turtles, and in particular of their unique shells. In 1914,Jan Versluys proposed that bony plates in the dermis, calledosteoderms, fused to the ribs beneath them, later called the "Polka Dot Ancestor" by Olivier Rieppel.[19][104] The theory accounted for the evolution of fossilpareiasaurs fromBradysaurus toAnthodon, but not for how the ribs could have become attached to the bony dermal plates.[19]
More recent discoveries have painted a different scenario for the evolution of the turtle's shell. Thestem-turtlesEunotosaurus of the MiddlePermian,Pappochelys of theMiddle Triassic, andEorhynchochelys of theLate Triassic lacked carapaces and plastrons but had shortened torsos, expanded ribs, and lengthened dorsal vertebrae. Also in the Late Triassic,Odontochelys had a partial shell consisting of a complete bony plastron and an incomplete carapace. The development of a shell reached completion with the Late TriassicProganochelys, with its fully developed carapace and plastron.[19][105] Adaptations that led to the evolution of the shell may have originally been for digging and afossorial lifestyle.[105]
The oldest known members of the Pleurodira lineage are thePlatychelyidae, from theLate Jurassic.[106] The oldest known unambiguous cryptodire isSinaspideretes, a close relative of softshell turtles, from the Late Jurassic of China.[107] Turtles became highly diverse during the Cretaceous, as climatic conditions in this period were favourable for their global dispersal.[108] During theLate Cretaceous andCenozoic, members of the pleurodire familiesBothremydidae andPodocnemididae became widely distributed in the Northern Hemisphere due to their coastal habits.[109][110] The oldest known soft-shelled turtles and sea turtles appeared during theEarly Cretaceous.[111][112] Tortoises originated in Asia during theEocene.[113] A late surviving group of stem-turtles, theMeiolaniidae, survived in Australasia into thePleistocene andHolocene.[114]
The turtles' exact ancestry has been disputed. It was believed they were the only surviving branch of the ancientevolutionary gradeAnapsida, which includes groups such asprocolophonids and pareiasaurs. All anapsid skulls lack atemporal opening while all other living amniotes have temporal openings.[115] It was later suggested that the anapsid-like turtle skulls may be due tobackward evolution rather than to anapsid descent.[116] Fossil evidence has shown that early stem-turtles possessed small temporal openings.[105]
Some early morphologicalphylogenetic studies have placed turtles closer toLepidosauria (tuataras,lizards, andsnakes) than toArchosauria (crocodilians and birds).[115] By contrast, severalmolecular studies place turtles either within Archosauria,[117] or, more commonly, as asister group to extant archosaurs,[116][118][119][120] though an analysis conducted by Tyler Lyson and colleagues (2012) recovered turtles as the sister group of lepidosaurs instead.[121] Ylenia Chiari and colleagues (2012) analyzed 248nuclear genes from 16 vertebrates and suggested that turtles share amore recent common ancestor with birds and crocodilians. The date of separation of turtles and birds and crocodilians was estimated to be255 million years ago during the Permian.[122] Throughgenomic-scale phylogenetic study ofultra-conserved elements (UCEs) to clarify the placement of turtles within reptiles, Nicholas Crawford and colleagues (2012) similarly found that turtles are closer to birds and crocodilians.[123]
Using the draft (unfinished) genome sequences of the green sea turtle and theChinese softshell turtle, Zhuo Wang and colleagues (2013) concluded that turtles are likely a sister group of crocodilians and birds.[124] The external phylogeny of the turtles is shown in thecladogram below.[123]
Modern turtles and their extinct relatives with a complete shell are classified within the cladeTestudinata.[125] The most recent common ancestor of living turtles, corresponding to the split between Pleurodira (side-necked species) andCryptodira (hidden necked species), is estimated to have occurred around210 million years ago during the Late Triassic.[126] Robert Thompson and colleagues (2021) comment that living turtles have low diversity, relative to how long they existed. Diversity has been stable, according to their analysis, except for a single rapid increase around theEocene-Oligocene boundary some 30 million years ago, and a large regional extinction at roughly the same time. They suggest that global climate change caused both events, as the cooling and drying caused the land to become arid and turtles to become extinct there, while new continental margins opened up by the climate change provided habitats for other species to evolve.[127]
The cladogram, from Nicholas Crawford and colleagues 2015, shows the internal phylogeny of the Testudines down to the level offamilies.[128][129] The analysis by Thompson and colleagues in 2021 supports the same structure down to the family level.[127]
The different mechanisms of neck retraction in the two suborders of turtles
Turtles are divided into two living suborders: Cryptodira and Pleurodira.[130] The two groups differ in the way the neck is retracted for protection. Pleurodirans retract their neck to the side and in front of the shoulder girdles, whereas cryptodirans retract their neck backward into their shell. These motions are enabled by the morphology and arrangement of neck vertebrae.[131][132] Sea turtles (which belong to Cryptodira) have mostly lost the ability to retract their heads.[133]
The adductor muscles in the lower jaw create a pulley-like system in both subgroups. However, the bones that the muscles articulate with differ. In Pleurodira, the pulley is formed with thepterygoid bones of thepalate, but in Cryptodira the pulley is formed with the otic capsule. Both systems help to vertically redirect theadductor muscles and maintain a powerful bite.[134]
A further difference between the suborders is the attachment of the pelvis. In Cryptodira, the pelvis is free, linked to the shell only by ligaments. In Pleurodira, the pelvis issutured, joined with bony connections, to the carapace and to the plastron, creating a pair of large columns of bone at the back end of the turtle, linking the two parts of the shell.[135]
Turtles are widely distributed across the world's continents, oceans, and islands with terrestrial, fully aquatic, and semi-aquatic species. Sea turtles are mainly tropical and subtropical, but leatherbacks can be found in colder areas of the Atlantic and Pacific.[136] Living Pleurodira all live in freshwater and are found only in the Southern Hemisphere.[137] The Cryptodira include terrestrial, freshwater, and marine species, and these range more widely.[136] The world regions richest in non-marine turtle species are the Amazon basin, theGulf of Mexicodrainages of the United States, and parts of South and Southeast Asia.[138]
For turtles in colder climates, their distribution is limited by constraints on reproduction, which is reduced by long hibernations. North American species barely range above the southern Canadian border.[139] Some turtles are found at high altitudes, for example, the speciesTerrapene ornata occurs up to 2,000 m (6,600 ft) in New Mexico.[140] Conversely, the leatherback sea turtle can dive over 1,200 m (3,900 ft).[141] Species of the genusGopherus can tolerate both below freezing and over 40 °C (104 °F) in body temperature, though they are most active at 26–34 °C (79–93 °F).[142]
Many turtles have been killed accidentally in fishing nets.[143] Some trawlers now use nets fitted withturtle excluders.[144] Seen here, a loggerhead escapes a net so fitted.
Among vertebrate orders, turtles are second only toprimates in the percentage of threatened species. 360 modern species have existed since 1500 AD. Of these, 51–56% are considered threatened and 60% considered threatened or extinct.[145] Turtles face many threats, includinghabitat destruction, harvesting for consumption, the pet trade,[146][147]light pollution,[148] andclimate change.[149] Asian species have a particularly high extinction risk, primarily due to their long-term unsustainable exploitation for food and medicine,[150] and about 83% of Asia's non-marine turtle species are considered threatened.[145] As of 2021, turtle extinction is progressing much faster than during theCretaceous-Tertiary extinction. At this rate, all turtles could be extinct in a few centuries.[151]
Turtlehatcheries can be set up when protection against flooding, erosion, predation, or heavypoaching is required.[152][153][154] Chinese markets have sought to satisfy an increasing demand for turtle meat with farmed turtles. In 2007 it was estimated that over a thousand turtle farms operated in China.[155] All the same, wild turtles continue to be caught and sent to market in large numbers, resulting in what conservationists have called "the Asian turtle crisis".[156][150] In the words of the biologist George Amato, the hunting of turtles "vacuumed up entire species from areas in Southeast Asia", even as biologists still did not know how many species lived in the region.[157] In 2000, all theAsian box turtles were placed on theCITES list of endangered species.[150]
Harvesting wild turtles is legal in some American states,[158] and there has been a growing demand for American turtles in China.[159][160] TheFlorida Fish and Wildlife Conservation Commission estimated in 2008 that around 3,000 pounds of softshell turtles were exported weekly viaTampa International Airport.[160] However, the great majority of turtles exported from the US between 2002 and 2005 were farmed.[159]
Large numbers of sea turtles are accidentally killed inlonglines,gillnets, andtrawling nets asbycatch. A 2010 study suggested that over 8 million had been killed between 1990 and 2008; the Eastern Pacific and the Mediterranean were identified as among the areas worst affected.[143] Since the 1980s, the United States has required allshrimp trawlers to fit their nets withturtle excluder devices that prevent turtles from being entangled in the net and drowning.[144]More locally, other human activities are affecting marine turtles. In Australia,Queensland'sshark culling program, which usesshark nets anddrum lines, has killed over 5,000 turtles as bycatch between 1962 and 2015; including 719 loggerhead turtles and 33 hawksbill sea turtles, which are listed as critically endangered.[161]
Native turtle populations can also be threatened byinvasive ones. The central North American red-eared slider turtle has been listed among the "world's worst invasive species", pet turtle having been released globally. They appear to compete with native turtle species in eastern and western North America, Europe, and Japan.[162][163]
Two tortoises were on the Soviet Union's September 1968Zond 5circumlunar flight, making them the first earthly living things to travel to the vicinity of the Moon. Turtles were also on theZond 6 (1968) and theZond 7 (1969) circumlunar flights.[164][165]
Poster for 1898 production ofThe Turtle at the Manhattan Theatre, Broadway
Terrapin shell leg rattles worn by leadCherokee woman dancer, 20th century
Turtles have featured in human cultures across the world since ancient times. They are generally viewed positively despite not being "cuddly" or flashy; their association with the ancient times and old age have contributed to their endearing image.[166]
InHindu mythology, theWorld Turtle, namedKurma or Kacchapa, supports four elephants on his back; they, in turn, carry the weight of the whole world on their backs.[167][168] The turtle is one of the tenavatars or incarnations of the godVishnu.[167] Theyoga pose Kurmasana is named for the avatar.[169][170] World Turtles are found in Native American cultures including theAlgonquian,Iroquois, andLenape. They tell many versions of thecreation story ofTurtle Island. One version has Muskrat pile up earth on Turtle's back, creating the continent of North America. AnIroquois version has the pregnantSky Woman fall through a hole in the sky between a tree's roots, where she is caught by birds who land her safely on Turtle's back; the Earth grows around her. The turtle here is altruistic, but the world is a heavy burden, and the turtle sometimes shakes itself to relieve the load, causing earthquakes.[167][171][172]
A turtle was the symbol of the Ancient Mesopotamian godEnki from the 3rd millennium BCE onward.[173] An ancient Greekorigin myth told that only the tortoise refused the invitation of the godsZeus andHera to their wedding, as it preferred to stay at home. Zeus then ordered it to carry its house with it, ever after.[174] Another of their gods,Hermes, invented a seven-stringedlyre made with the shell of a tortoise.[175] In theShang dynastyChinese practice ofplastromancy, dating back to 1200 BCE, oracles were obtained by inscribing questions on turtle plastrons using theoldest known form of Chinese characters, burning the plastron, and interpreting the resulting cracks. Later, the turtle was one of thefour sacred animals inConfucianism, while in theHan period,steles were mounted on top of stone turtles, later linked withBixi, the turtle-shelled son of the Dragon King.[176] Marine turtles feature significantly inAustralian Aboriginal art.[168] The army ofAncient Rome used thetestudo("tortoise") formation where soldiers would form ashield wall for protection.[163]
Some turtles, particularly small terrestrial and freshwater species, are kept aspets.[188][189] The demand for pet turtles increased in the 1950s, with the US being the main supplier, particularly of farm-bred red-eared sliders. The popularity for exotic pets has led to an increase in illegalwildlife trafficking. Around 21% of the value of live animal trade is in reptiles, and turtles are among the more popularly traded species.[190] Poor husbandry of tortoises can cause chronicrhinitis (nasal swelling), overgrown beaks,hyperparathyroidism (which softens their skeleton),constipation, various reproductive problems, and injuries from dogs.[188] In the early 20th century, people in the United States have organized and gambled onturtle races.[191]
The flesh of captured wild turtles continues to be eaten in Asian cultures,[192] whileturtle soup was once a popular dish inEnglish cuisine.[193] Gopher tortoise stew has been popular with some groups in Florida.[194] The supposed aphrodisiac or medicinal properties of turtle eggs created a large trade for them in Southeast Asia.[168] Hard-shell turtle plastrons and soft-shell carapaces are widely used intraditional Chinese medicine;Taiwan imported nearly 200 metric tons of hard-shells from its neighbors yearly from 1999 to 2008.[195] A popular medicinal preparation based on herbs and turtle shells isguilinggao jelly.[196] The substancetortoiseshell, usually from the hawksbill turtle, has been used for centuries to make jewelry, tools, and ornaments around the Western Pacific.[168] Hawksbills have accordingly been hunted for their shells.[197] The trading of tortoiseshell was internationally banned in 1977 by CITES.[198] Some cultures have used turtle shells to make music: Native Americanshamans made them into ceremonial rattles, whileAztecs,Mayas, andMixtecs madeayotl drums.[199]
Catching turtles in Australia, 1875
Turtles on sale as food in Canada, 2007
Turtle plastrons for traditional Chinese medicine
A tortoiseshell comb; the material was expensive and decorative, and widely used for small items.[200]
A pet red-eared slider basking on a floating platform under a sun lamp
A notice inBluefields,Nicaragua in 2023 indicating that the capturing and consuming of turtles is prohibited
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