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Cetacea

Cetacea is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel themselves through the water with powerful up-and-down movement of their tail which ends in a paddle-like fluke, using their flipper-shaped forelimbs to maneuver.

While the majority of cetaceans live in marine environments, a small number reside solely in brackish water or fresh water. Having a cosmopolitan distribution, they can be found in some rivers and all of Earth's oceans, and many species inhabit vast ranges where they migrate with the changing of the seasons.

Cetaceans are famous for their high intelligence, complex social behaviour, and the enormous size of some of the group's members. For example, the blue whale reaches a maximum confirmed length of 29.9 m and a weight of 173 tonnes (190 short tons), making it the largest animal ever known to have existed.

There are approximately 89 living species split into two parvorders: Odontoceti or toothed whales (containing porpoises, dolphins, other predatory whales like the beluga and the sperm whale, and the poorly understood beaked whales) and the filter feeding Mysticeti or baleen whales (which includes species like the blue whale, the humpback whale and the bowhead whale). Despite their highly modified bodies and carnivorous lifestyle, genetic and fossil evidence places cetaceans as nested within even-toed ungulates, most closely related to hippopotamus within the clade Whippomorpha.

Cetaceans have been extensively hunted for their meat, blubber and oil by commercial operations. Although the International Whaling Commission has agreed on putting a halt to commercial whaling, whale hunting is still going on, either under IWC quotas to assist the subsistence of Arctic native people or in the name of scientific research, although a large spectrum of non-lethal methods are now available to study marine mammals in the wild. Cetaceans also face severe environmental hazards from underwater noise pollution, entanglement in abandoned ropes and nets, collisions with ships, plastic and heavy metals build-up, to accelerating climate change, but how much they are affected varies widely from species to species, from minimally in the case of the southern bottlenose whale to the baiji (Chinese river dolphin) which is considered to be functionally extinct due to human activity.

Baleen whales and toothed whales
The two parvorders, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have diverged around thirty-four million years ago.

Baleen whales have bristles made of keratin instead of teeth. The bristles filter krill and other small invertebrates from seawater. Grey whales feed on bottom-dwelling mollusks. Rorqual family (balaenopterids) use throat pleats to expand their mouths to take in food and sieve out the water. Balaenids (right whales and bowhead whales) have massive heads that can make up 40% of their body mass. Most mysticetes prefer the food-rich colder waters of the Northern and Southern Hemispheres, migrating to the Equator to give birth. During this process, they are capable of fasting for several months, relying on their fat reserves.

The parvorder of Odontocetes – the toothed whales – include sperm whales, beaked whales, orcas, dolphins and porpoises. Generally their teeth have evolved to catch fish, squid or other marine invertebrates, not for chewing them, so prey is swallowed whole. Teeth are shaped like cones (dolphins and sperm whales), spades (porpoises), pegs (belugas), tusks (narwhals) or variable (beaked whale males). Female beaked whales' teeth are hidden in the gums and are not visible, and most male beaked whales have only two short tusks. Narwhals have vestigial teeth other than their tusk, which is present on males and 15% of females and has millions of nerves to sense water temperature, pressure and salinity. A few toothed whales, such as some orcas, feed on mammals, such as pinnipeds and other whales.

Toothed whales have well-developed senses – their eyesight and hearing are adapted for both air and water, and they have advanced sonar capabilities using their melon. Their hearing is so well-adapted for both air and water that some blind specimens can survive. Some species, such as sperm whales, are well adapted for diving to great depths. Several species of toothed whales show sexual dimorphism, in which the males differ from the females, usually for purposes of sexual display or aggression.

Anatomy

Cetacean bodies are generally similar to those of fish, which can be attributed to their lifestyle and the habitat conditions. Their body is well-adapted to their habitat, although they share essential characteristics with other higher mammals (Eutheria).

They have a streamlined shape, and their forelimbs are flippers. Almost all have a dorsal fin on their backs, but this can take on many forms, depending on the species. A few species, such as the beluga whale, lack them. Both the flipper and the fin are for stabilization and steering in the water.

The male genitals and the mammary glands of females are sunken into the body. The male genitals are attached to a vestigial pelvis.

The body is wrapped in a thick layer of fat, known as blubber. This provides thermal insulation and gives cetaceans their smooth, streamlined body shape. In larger species, it can reach a thickness up to 1/2 m.

Sexual dimorphism evolved in many toothed whales. Sperm whales, narwhals, many members of the beaked whale family, several species of the porpoise family, orcas, pilot whales, eastern spinner dolphins and northern right whale dolphins show this characteristic. Males in these species developed external features absent in females that are advantageous in combat or display. For example, male sperm whales are up to 63% percent larger than females, and many beaked whales possess tusks used in competition among males. Hind legs are not present in cetaceans, nor are any other external body attachments such as a pinna and hair.

Head
Whales have an elongated head, especially baleen whales, due to the wide overhanging jaw. Bowhead whale plates can be 9 m long. Their nostril(s) make up the blowhole, with one in toothed whales and two in baleen whales.

The nostrils are located on top of the head above the eyes so that the rest of the body can remain submerged while surfacing for air. The back of the skull is significantly shortened and deformed. By shifting the nostrils to the top of the head, the nasal passages extend perpendicularly through the skull. The teeth or baleen in the upper jaw sit exclusively on the maxilla. The braincase is concentrated through the nasal passage to the front and is correspondingly higher, with individual cranial bones that overlap.

In toothed whales, connective tissue exists in the melon as a head buckle. This is filled with air sacs and fat that aid in buoyancy and biosonar. The sperm whale has a particularly pronounced melon; this is called the spermaceti organ and contains the eponymous spermaceti, hence the name "sperm whale". Even the long tusk of the narwhal is a vice-formed tooth. In many toothed whales, the depression in their skull is due to the formation of a large melon and multiple, asymmetric air bags.

River dolphins, unlike most other cetaceans, can turn their head 90°. Most other cetaceans have fused neck vertebrae and are unable to turn their head at all.

The baleen of baleen whales consists of long, fibrous strands of keratin. Located in place of the teeth, it has the appearance of a huge fringe and is used to sieve the water for plankton and krill.

Brain
Sperm whales have the largest brain mass of any animal on Earth, averaging 8,000 cm3 and 7.8 kg in mature males. The brain to body mass ratio in some odontocetes, such as belugas and narwhals, is second only to humans. In some whales, however, it is less than half that of humans: 0.9% versus 2.1%.

In cetaceans, evolution in the water has caused changes to the head that have modified brain shape such that the brain folds around the insula and expands more laterally than in terrestrial mammals. As a result, the cetacean prefrontal cortex (compared to that in humans) rather than frontal is laterally positioned.

Brain size was previously considered a major indicator of intelligence. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for cognitive tasks. Allometric analysis of the relationship between mammalian brain mass (weight) and body mass for different species of mammals shows that larger species generally have larger brains. However, this increase is not fully proportional. Typically the brain mass only increases in proportion to somewhere between the two-thirds power (or the square of the cube root) and the three-quarters power (or the cube of the fourth root) of the body mass. mbrain ∝ (mbody)k where k is between two-thirds and three-quarters. Thus if Species B is twice the size of Species A, its brain size will typically be somewhere between 60% and 70% higher. Comparison of a particular animal's brain size with the expected brain size based on such an analysis provides an encephalization quotient that can be used as an indication of animal intelligence.

The neocortex of many cetaceans is home to elongated spindle neurons that, prior to 2019, were known only in hominids. In humans, these cells are thought to be involved in social conduct, emotions, judgment and theory of mind. Cetacean spindle neurons are found in areas of the brain homologous to where they are found in humans, suggesting they perform a similar function.

Skeleton
The cetacean skeleton is largely made up of cortical bone, which stabilizes the animal in the water. For this reason, the usual terrestrial compact bones, which are finely woven cancellous bone, are replaced with lighter and more elastic material. In many places, bone elements are replaced by cartilage and even fat, thereby improving their hydrostatic qualities. The ear and the muzzle contain a bone shape that is exclusive to cetaceans with a high density, resembling porcelain. This conducts sound better than other bones, thus aiding biosonar.

The number of vertebrae that make up the spine varies by species, ranging from forty to ninety-three. The cervical spine, found in all mammals, consists of seven vertebrae which, however, are reduced or fused. This fusion provides stability during swimming at the expense of mobility. The fins are carried by the thoracic vertebrae, ranging from nine to seventeen individual vertebrae. The sternum is cartilaginous. The last two to three pairs of ribs are not connected and hang freely in the body wall. The stable lumbar and tail include the other vertebrae. Below the caudal vertebrae is the chevron bone.

The front limbs are paddle-shaped with shortened arms and elongated finger bones, to support movement. They are connected by cartilage. The second and third fingers display a proliferation of the finger members, a so-called hyperphalangy. The shoulder joint is the only functional joint in all cetaceans except for the Amazon river dolphin. The collarbone is completely absent.

Fluke
Cetaceans have a cartilaginous fluke at the end of their tails that is used for propulsion. The fluke is set horizontally on the body and used with vertical movements, unlike fish and ichthyosaurs, which have vertical tails which move horizontally.

Circulation
Cetaceans have powerful hearts. Blood oxygen is distributed effectively throughout the body. They are warm-blooded, i.e., they hold a nearly constant body temperature.

Respiration
Cetaceans have lungs, meaning they breathe air. An individual can last without a breath from a few minutes to over two hours depending on the species. Cetacea are deliberate breathers who must be awake to inhale and exhale. When stale air, warmed from the lungs, is exhaled, it condenses as it meets colder external air. As with a terrestrial mammal breathing out on a cold day, a small cloud of 'steam' appears. This is called the 'spout' and varies across species in shape, angle and height. Species can be identified at a distance using this characteristic.

The structure of the respiratory and circulatory systems is of particular importance for the life of marine mammals. The oxygen balance is effective. Each breath can replace up to 90% of the total lung volume. For land mammals, in comparison, this value is usually about 15%. During inhalation, about twice as much oxygen is absorbed by the lung tissue as in a land mammal. As with all mammals, the oxygen is stored in the blood and the lungs, but in cetaceans, it is also stored in various tissues, mainly in the muscles. The muscle pigment, myoglobin, provides an effective bond. This additional oxygen storage is vital for deep diving, since beyond a depth around 100 m, the lung tissue is almost completely compressed by the water pressure.

Abdominal organs
The stomach consists of three chambers. The first region is formed by a loose gland and a muscular forestomach (missing in beaked whales); this is followed by the main stomach and the pylorus. Both are equipped with glands to help digestion. A bowel adjoins the stomachs, whose individual sections can only be distinguished histologically. The liver is large and separate from the gall bladder.

The kidneys are long and flattened. The salt concentration in cetacean blood is lower than that in seawater, requiring kidneys to excrete salt. This allows the animals to drink seawater. The urinary bladder is proportionally smaller in cetaceans than in land mammals. The testes are located internally, without an external scrotum. The uterus is bicornuate.

Senses
Cetacean eyes are set on the sides rather than the front of the head. This means only species with pointed 'beaks' (such as dolphins) have good binocular vision forward and downward. Tear glands secrete greasy tears, which protect the eyes from the salt in the water. The lens is almost spherical, which is most efficient at focusing the minimal light that reaches deep water. Odontocetes have little to no ability to taste or smell, while mysticetes are believed to have some ability to smell because of their reduced, but functional olfactory system. Cetaceans are known to possess excellent hearing.

At least one species, the tucuxi or Guiana dolphin, is able to use electroreception to sense prey.

Ears

The external ear has lost the pinna (visible ear), but still retains a narrow ear canal. The three small bones or ossicles that transmit sound within each ear are dense and compact, and differently shaped from those of land mammals. The semicircular canals are much smaller relative to body size than in other mammals.

A bony structure of the middle and inner ear, the auditory bulla, is composed of two compact and dense bones (the periotic and tympanic). It is housed in a cavity in the middle ear; in the Odontoceti (apart from in the physeterids, this cavity is filled with dense foam and completely surrounds the bulla, which is connected to the skull only by ligaments. This may isolate the ear from sounds transmitted through the bones of the skull, something that also happens in bats.

Cetaceans use sound to communicate, using groans, moans, whistles, clicks or the 'singing' of the humpback whale.

Echolocation
Odontoceti are generally capable of echolocation. They can discern the size, shape, surface characteristics, distance and movement of an object. They can search for, chase and catch fast-swimming prey in total darkness. Most Odontoceti can distinguish between prey and nonprey (such as humans or boats); captive Odontoceti can be trained to distinguish between, for example, balls of different sizes or shapes. Echolocation clicks also contain characteristic details unique to each animal, which may suggest that toothed whales can discern between their own click and that of others.

Mysticeti have exceptionally thin, wide basilar membranes in their cochleae without stiffening agents, making their ears adapted for processing low to infrasonic frequencies.

Chromosomes
The initial karyotype includes a set of chromosomes from 2n = 44. They have four pairs of telocentric chromosomes (whose centromeres sit at one of the telomeres), two to four pairs of subtelocentric and one or two large pairs of submetacentric chromosomes. The remaining chromosomes are metacentric—the centromere is approximately in the middle—and are rather small. All cetaceans have chromosomes 2n = 44, except the sperm whales and pygmy sperm whales, which have 2n = 42.

Range and habitat
Cetaceans are found in many aquatic habitats. While many marine species, such as the blue whale, the humpback whale and the orca, have a distribution area that includes nearly the entire ocean, some species occur only locally or in broken populations. These include the vaquita, which inhabits a small part of the Gulf of California and Hector's dolphin, which lives in some coastal waters in New Zealand. River dolphin species live exclusively in fresh water.

Many species inhabit specific latitudes, often in tropical or subtropical waters, such as Bryde's whale or Risso's dolphin. Others are found only in a specific body of water. The southern right whale dolphin and the hourglass dolphin live only in the Southern Ocean. The narwhal and the beluga live only in the Arctic Ocean. Sowerby's beaked whale and the Clymene dolphin exist only in the Atlantic and the Pacific white-sided dolphin and the northern straight dolphin live only in the North Pacific.

Cosmopolitan species may be found in the Pacific, Atlantic and Indian Oceans. However, northern and southern populations become genetically separated over time. In some species, this separation leads eventually to a divergence of the species, such as produced the southern right whale, North Pacific right whale and North Atlantic right whale. Migratory species' reproductive sites often lie in the tropics and their feeding grounds in polar regions.

Thirty-two species are found in European waters, including twenty-five toothed and seven baleen species.

Whale migration
Many species of whales migrate on a latitudinal basis to move between seasonal habitats. For example, the gray whale migrates 10,000 mi round trip. The journey begins at winter birthing grounds in warm lagoons along Baja California, and traverses 5,000 - 7,000 mi of coastline to summer feeding grounds in the Bering, Chuckchi and Beaufort seas off the coast of Alaska.

Sleep
Conscious breathing cetaceans sleep but cannot afford to be unconscious for long, because they may drown. While knowledge of sleep in wild cetaceans is limited, toothed cetaceans in captivity have been recorded to exhibit unihemispheric slow-wave sleep (USWS), which means they sleep with one side of their brain at a time, so that they may swim, breathe consciously and avoid both predators and social contact during their period of rest.

A 2008 study found that sperm whales sleep in vertical postures just under the surface in passive shallow 'drift-dives', generally during the day, during which whales do not respond to passing vessels unless they are in contact, leading to the suggestion that whales possibly sleep during such dives.

Diving
While diving, the animals reduce their oxygen consumption by lowering the heart activity and blood circulation; individual organs receive no oxygen during this time. Some rorquals can dive for up to 40 minutes, sperm whales between 60 and 90 minutes and bottlenose whales for two hours. Diving depths average about 100 m. Species such as sperm whales can dive to 3,000 m, although more commonly 1,200 m.

Social relations
Most cetaceans are social animals, although a few species live in pairs or are solitary. A group, known as a pod, usually consists of ten to fifty animals, but on occasion, such as mass availability of food or during mating season, groups may encompass more than one thousand individuals. Inter-species socialization can occur.

Pods have a fixed hierarchy, with the priority positions determined by biting, pushing or ramming. The behavior in the group is aggressive only in situations of stress such as lack of food, but usually it is peaceful. Contact swimming, mutual fondling and nudging are common. The playful behavior of the animals, which is manifested in air jumps, somersaults, surfing, or fin hitting, occurs more often than not in smaller cetaceans, such as dolphins and porpoises.

Whale song
Males in some baleen species communicate via whale song, sequences of high pitched sounds. These "songs" can be heard for hundreds of kilometers. Each population generally shares a distinct song, which evolves over time. Sometimes, an individual can be identified by its distinctive vocals, such as the 52-hertz whale that sings at a higher frequency than other whales. Some individuals are capable of generating over 600 distinct sounds. In baleen species such as humpbacks, blues and fins, male-specific song is believed to be used to attract and display fitness to females.

Hunting
Pod groups also hunt, often with other species. Many species of dolphins accompany large tunas on hunting expeditions, following large schools of fish. The orca hunts in pods and targets belugas and even larger whales. Humpback whales, among others, form in collaboration bubble carpets to herd krill or plankton into bait balls before lunging at them.

Intelligence

Cetacea are known to teach, learn, cooperate, scheme and grieve.

Smaller cetaceans, such as dolphins and porpoises, engage in complex play behavior, including such things as producing stable underwater toroidal air-core vortex rings or "bubble rings". The two main methods of bubble ring production are rapid puffing of air into the water and allowing it to rise to the surface, forming a ring, or swimming repeatedly in a circle and then stopping to inject air into the helical vortex currents thus formed. They also appear to enjoy biting the vortex rings, so that they burst into many separate bubbles and then rise quickly to the surface. Whales produce bubble nets to aid in herding prey.

Larger whales are also thought to engage in play. The southern right whale elevates its tail fluke above the water, remaining in the same position for a considerable time. This is known as "sailing". It appears to be a form of play and is most commonly seen off the coast of Argentina and South Africa. Humpback whales also display this behaviour.

Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning (thinking about thinking) that humans exploit. Dolphins appear to possess self-awareness. The most widely used test for self-awareness in animals is the mirror test, in which a temporary dye is placed on an animal's body and the animal is then presented with a mirror. Researchers then explore whether the animal shows signs of self-recognition.

Critics claim that the results of these tests are susceptible to the Clever Hans effect. This test is much less definitive than when used for primates. Primates can touch the mark or the mirror, while dolphins cannot, making their alleged self-recognition behavior less certain. Skeptics argue that behaviors said to identify self-awareness resemble existing social behaviors, so researchers could be misinterpreting self-awareness for social responses. Advocates counter that the behaviors are different from normal responses to another individual. Dolphins show less definitive behavior of self-awareness, because they have no pointing ability.

In 1995, Marten and Psarakos used video to test dolphin self-awareness. They showed dolphins real-time footage of themselves, recorded footage and another dolphin. They concluded that their evidence suggested self-awareness rather than social behavior. While this particular study has not been replicated, dolphins later "passed" the mirror test.

Decision-making
Collective decisions are an important part of life as a cetacean for the many species that spend time in groups (whether these be temporary such as the fission-fusion dynamics of many smaller dolphin species or long-term stable associations as are seen in killer whale and sperm whale matrilines). Little is known about how these decisions work, though studies have found evidence messy consensus decisions in groups of sperm whales and leadership in other species like bottlenose dolphins and killer whales.

Reproduction and brooding
Most cetaceans sexually mature at seven to 10 years. An exception to this is the La Plata dolphin, which is sexually mature at two years, but lives only to about 20. The sperm whale reaches sexual maturity within about 20 years and has a lifespan between 50 and 100 years.

For most species, reproduction is seasonal. Ovulation coincides with male fertility. This cycle is usually coupled with seasonal movements that can be observed in many species. Most toothed whales have no fixed bonds. In many species, females choose several partners during a season. Baleen whales are largely monogamous within each reproductive period.

Gestation ranges from 9 to 16 months. Duration is not necessarily a function of size. Porpoises and blue whales gestate for about 11 months. As with all mammals other than marsupials and monotremes, the embryo is fed by the placenta, an organ that draws nutrients from the mother's bloodstream. Mammals without placentas either lay minuscule eggs (monotremes) or bear minuscule offspring (marsupials).

Cetaceans usually bear one calf. In the case of twins, one usually dies, because the mother cannot produce sufficient milk for both. The fetus is positioned for a tail-first delivery, so that the risk of drowning during delivery is minimal. After birth, the mother carries the infant to the surface for its first breath. At birth, they are about one-third of their adult length and tend to be independently active, comparable to terrestrial mammals.

Suckling
Like other placental mammals, cetaceans give birth to well-developed calves and nurse them with milk from their mammary glands. When suckling, the mother actively splashes milk into the mouth of the calf, using the muscles of her mammary glands, as the calf has no lips. This milk usually has a high-fat content, ranging from 16 to 46%, causing the calf to increase rapidly in size and weight.

In many small cetaceans, suckling lasts for about four months. In large species, it lasts for over a year and involves a strong bond between mother and offspring.

The mother is solely responsible for brooding. In some species, so-called "aunts" occasionally suckle the young.

This reproductive strategy provides a few offspring that have a high survival rate.

Lifespan
Among cetaceans, whales are distinguished by an unusual longevity compared to other higher mammals. Some species, such as the bowhead whale (Balaena mysticetus), can reach over 200 years. Based on the annual rings of the bony otic capsule, the age of the oldest known specimen is a male determined to be 211 years at the time of death.

Death
Upon death, whale carcasses fall to the deep ocean and provide a substantial habitat for marine life. Evidence of whale falls in present-day and fossil records shows that deep-sea whale falls support a rich assemblage of creatures, with a global diversity of 407 species, comparable to other neritic biodiversity hotspots, such as cold seeps and hydrothermal vents.

Deterioration of whale carcasses happens through three stages. Initially, organisms such as sharks and hagfish scavenge the soft tissues at a rapid rate over a period of months and as long as two years. This is followed by the colonization of bones and surrounding sediments (which contain organic matter) by enrichment opportunists, such as crustaceans and polychaetes, throughout a period of years. Finally, sulfophilic bacteria reduce the bones releasing hydrogen sulfide enabling the growth of chemoautotrophic organisms, which in turn, support organisms such as mussels, clams, limpets and sea snails. This stage may last for decades and supports a rich assemblage of species, averaging 185 per site.

Disease
Brucellosis affects almost all mammals. It is distributed worldwide, while fishing and pollution have caused porpoise population density pockets, which risks further infection and disease spreading. Brucella ceti, most prevalent in dolphins, has been shown to cause chronic disease, increasing the chance of failed birth and miscarriages, male infertility, neurobrucellosis, cardiopathies, bone and skin lesions, strandings and death. Until 2008, no case had ever been reported in porpoises, but isolated populations have an increased risk and consequentially a high mortality rate.

Origins
The direct ancestors of today's cetaceans are probably found within the Dorudontidae whose most famous member, Dorudon, lived at the same time as Basilosaurus. Both groups had already developed some of the typical anatomical features of today's whales, such as the fixed bulla, which replaces the mammalian eardrum, as well as sound-conducting elements for submerged directional hearing. Their wrists were stiffened and probably contributed to the typical build of flippers. The hind legs existed, however, but were significantly reduced in size and with a vestigial pelvis connection.

Transition from land to sea

The fossil record traces the gradual transition from terrestrial to aquatic life. The regression of the hind limbs allowed greater flexibility of the spine. This made it possible for whales to move around with the vertical tail hitting the water. The front legs transformed into flippers, costing them their mobility on land.

One of the oldest members of ancient cetaceans (Archaeoceti) is Pakicetus from the Middle Eocene of Pakistan. This is an animal the size of a wolf, whose skeleton is known only partially. It had functioning legs and lived near the shore. This suggests the animal could still move on land. The long snout had carnivorous dentition.

The transition from land to sea dates to about 49 million years ago, with the Ambulocetus ("running whale"), also discovered in Pakistan. It was up to 3 m long. The limbs of this archaeocete were leg-like, but it was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly. The snout was elongated with overhead nostrils and eyes. The tail was strong and supported movement through water. Ambulocetus probably lived in mangroves in brackish water and fed in the riparian zone as a predator of fish and other vertebrates.

Dating from about 45 million years ago are species such as Indocetus, Kutchicetus, Rodhocetus and Andrewsiphius, all of which were adapted to life in water. The hind limbs of these species were regressed and their body shapes resemble modern whales. Protocetidae family member Rodhocetus is considered the first to be fully aquatic. The body was streamlined and delicate with extended hand and foot bones. The merged pelvic lumbar spine was present, making it possible to support the floating movement of the tail. It was likely a good swimmer, but could probably move only clumsily on land, much like a modern seal.

Marine animals
Since the late Eocene, about 40 million years ago, cetaceans populated the subtropical oceans and no longer emerged on land. An example is the 18 metre long Basilosaurus, sometimes called Zeuglodon. The transition from land to water was completed in about 10 million years. The Wadi Al-Hitan ("Whale Valley") in Egypt contains numerous skeletons of Basilosaurus, as well as other marine vertebrates.

External phylogeny
Molecular biology, immunology, and fossils show that cetaceans are phylogenetically closely related with the even-toed ungulates (Artiodactyla). Whales' direct lineage began in the early Eocene, around 55.8 million years ago, with early artiodactyls. Most molecular biological evidence suggests that hippos are the closest living relatives. Common anatomical features include similarities in the morphology of the posterior molars, and the bony ring on the temporal bone (bulla) and the involucre, a skull feature that was previously associated only with cetaceans. Since the fossil record suggests that the morphologically distinct hippo lineage dates back only about 15 million years, Cetacea and hippos apparently diverged from a common ancestor that was morphologically distinct from either. The most striking common feature is the talus, a bone in the upper ankle. Early cetaceans, archaeocetes, show double castors, which occur only in even-toed ungulates. Corresponding findings are from Tethys Sea deposits in northern India and Pakistan. The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian plate.

Molecular and morphological evidence suggests that artiodactyls as traditionally defined are paraphyletic with respect to cetaceans. Cetaceans are deeply nested within the artiodactyls; the two groups together form a clade, a natural group with a common ancestor, for which the name Cetartiodactyla is sometimes used. Modern nomenclature divides Artiodactyla (or Cetartiodactyla) into four subordinate taxa: camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus whales (Whippomorpha). The Cetacea's presumed location within Artiodactyla can be represented in the following cladogram:

Internal phylogeny
Within Cetacea, the two parvorders are baleen whales (Mysticeti) which owe their name to their baleen, and toothed whales (Odontoceti), which have teeth shaped like cones, spades, pegs, or tusks, and can perceive their environment through biosonar.

The terms whale and dolphin are informal:

 * Mysticeti:
 * Whales, with four families: Balaenidae (right and bowhead whales), Cetotheriidae (pygmy right whales), Balaenopteridae (rorquals), Eschrichtiidae (grey whales)

 * Odontoceti:
 * Whales: with four families: Monodontidae (belugas and narwhals), Physeteridae (sperm whales), Kogiidae (dwarf and pygmy sperm whales), and Ziphiidae (beaked whales)
 * Dolphins, with five families: Delphinidae (oceanic dolphins), Platanistidae (South Asian river dolphins), Lipotidae (old world river dolphins) Iniidae (new world river dolphins), and Pontoporiidae (La Plata dolphins)
 * Porpoises, with one family: Phocoenidae

The term 'great whales' covers those currently regulated by the International Whaling Commission: the Odontoceti families Physeteridae (sperm whales), Ziphiidae (beaked whales), and Kogiidae (pygmy and dwarf sperm whales); and Mysticeti families Balaenidae (right and bowhead whales), Cetotheriidae (pygmy right whales), Eschrichtiidae (grey whales), as well as part of the family Balaenopteridae (minke, Bryde's, sei, blue and fin; not Eden's and Omura's whales).

Threats
The primary threats to cetaceans come from people, both directly from whaling or drive hunting and indirect threats from fishing and pollution.

Whaling
Whaling is the practice of hunting whales, mainly baleen and sperm whales. This activity has gone on since the Stone Age.

In the Middle Ages, reasons for whaling included their meat, oil usable as fuel and the jawbone, which was used in house construction. At the end of the Middle Ages, early whaling fleets aimed at baleen whales, such as bowheads. In the 16th and 17th centuries, the Dutch fleet had about 300 whaling ships with 18,000 crewmen.

In the 18th and 19th centuries, baleen whales especially were hunted for their baleen, which was used as a replacement for wood, or in products requiring strength and flexibility such as corsets and crinoline skirts. In addition, the spermaceti found in the sperm whale was used as a machine lubricant and the ambergris as a material for pharmaceutical and perfume industries. In the second half of the 19th century, the explosive harpoon was invented, leading to a massive increase in the catch size.

Large ships were used as "mother" ships for the whale handlers. In the first half of the 20th century, whales were of great importance as a supplier of raw materials. Whales were intensively hunted during this time; in the 1930s, 30,000 whales were killed. This increased to over 40,000 animals per year up to the 1960s, when stocks of large baleen whales collapsed.

Most hunted whales are now threatened, with some great whale populations exploited to the brink of extinction. Atlantic and Korean gray whale populations were completely eradicated and the North Atlantic right whale population fell to some 300–600. The blue whale population is estimated to be around 14,000.

The first efforts to protect whales came in 1931. Some particularly endangered species, such as the humpback whale (which then numbered about 100 animals), were placed under international protection and the first protected areas were established. In 1946, the International Whaling Commission (IWC) was established, to monitor and secure whale stocks. Whaling of 14 large species for commercial purposes was prohibited worldwide by this organization from 1985 to 2005, though some countries do not honor the prohibition. The stocks of species such as humpback and blue whales have recovered, though they are still threatened. The United States Congress passed the Marine Mammal Protection Act of 1972 sustain the marine mammal population. It prohibits the taking of marine mammals except for several hundred per year taken in Alaska. Japanese whaling ships are allowed to hunt whales of different species for ostensibly scientific purposes.

Aboriginal whaling is still permitted. About 1,200 pilot whales were taken in the Faroe Islands in 2017, and about 900 narwhals and 800 belugas per year are taken in Alaska, Canada, Greenland, and Siberia. About 150 minke are taken in Greenland per year, 120 gray whales in Siberia and 50 bowheads in Alaska, as aboriginal whaling, besides the 600 minke taken commercially by Norway, 300 minke and 100 sei taken by Japan and up to 100 fin whales taken by Iceland. Iceland and Norway do not recognize the ban and operate commercial whaling. Norway and Japan are committed to ending the ban.

Dolphins and other smaller cetaceans are sometimes hunted in an activity known as dolphin drive hunting. This is accomplished by driving a pod together with boats, usually into a bay or onto a beach. Their escape is prevented by closing off the route to the ocean with other boats or nets. Dolphins are hunted this way in several places around the world, including the Solomon Islands, the Faroe Islands, Peru and Japan (the most well-known practitioner). Dolphins are mostly hunted for their meat, though some end up in dolphinaria. Despite the controversy thousands of dolphins are caught in drive hunts each year.

Fishing
Dolphin pods often reside near large tuna shoals. This is known to fishermen, who look for dolphins to catch tuna. Dolphins are much easier to spot from a distance than tuna, since they regularly breathe. The fishermen pull their nets hundreds of meters wide in a circle around the dolphin groups, in the expectation that they will net a tuna shoal. When the nets are pulled together, the dolphins become entangled under water and drown. Line fisheries in larger rivers are threats to river dolphins.

A greater threat than by-catch for small cetaceans is targeted hunting. In Southeast Asia, they are sold as fish-replacement to locals, since the region's edible fish promise higher revenues from exports. In the Mediterranean, small cetaceans are targeted to ease pressure on edible fish.

Strandings
A stranding is when a cetacean leaves the water to lie on a beach. In some cases, groups of whales strand together. The best known are mass strandings of pilot whales and sperm whales. Stranded cetaceans usually die, because their as much as 90 MT body weight compresses their lungs or breaks their ribs. Smaller whales can die of heatstroke because of their thermal insulation. The causes are not clear. Possible reasons for mass beachings are:
 * toxic contaminants
 * debilitating parasites (in the respiratory tract, brain or middle ear)
 * infections (bacterial or viral)
 * flight from predators (including humans)
 * social bonds within a group, so that the pod follows a stranded animal
 * disturbance of their magnetic senses by natural anomalies in the Earth's magnetic field
 * injuries
 * noise pollution by shipping traffic, seismic surveys and military sonar experiments

Since 2000, whale strandings frequently occurred following military sonar testing. In December 2001, the US Navy admitted partial responsibility for the beaching and the deaths of several marine mammals in March 2000. The coauthor of the interim report stated that animals killed by active sonar of some Navy ships were injured. Generally, underwater noise, which is still on the increase, is increasingly tied to strandings; because it impairs communication and sense of direction.

Climate change influences the major wind systems and ocean currents, which also lead to cetacean strandings. Researchers studying strandings on the Tasmanian coast from 1920 to 2002 found that greater strandings occurred at certain time intervals. Years with increased strandings were associated with severe storms, which initiated cold water flows close to the coast. In nutrient-rich, cold water, cetaceans expect large prey animals, so they follow the cold water currents into shallower waters, where the risk is higher for strandings. Whales and dolphins who live in pods may accompany sick or debilitated pod members into shallow water, stranding them at low tide.

Environmental hazards
Heavy metals, residues of many plant and insect venoms and plastic waste flotsam are not biodegradable. Sometimes, cetaceans consume these hazardous materials, mistaking them for food items. As a result, the animals are more susceptible to disease and have fewer offspring.

Damage to the ozone layer reduces plankton reproduction because of its resulting radiation. This shrinks the food supply for many marine animals, but the filter-feeding baleen whales are most impacted. Even the Nekton is, in addition to intensive exploitation, damaged by the radiation.

Food supplies are also reduced long-term by ocean acidification due to increased absorption of increased atmospheric carbon dioxide. The CO2 reacts with water to form carbonic acid, which reduces the construction of the calcium carbonate skeletons of food supplies for zooplankton that baleen whales depend on.

The military and resource extraction industries operate strong sonar and blasting operations. Marine seismic surveys use loud, low-frequency sound that show what is lying underneath the Earth's surface. Vessel traffic also increases noise in the oceans. Such noise can disrupt cetacean behavior such as their use of biosonar for orientation and communication. Severe instances can panic them, driving them to the surface. This leads to bubbles in blood gases and can cause decompression sickness. Naval exercises with sonar regularly results in fallen cetaceans that wash up with fatal decompression. Sounds can be disruptive at distances of more than 100 km. Damage varies across frequency and species.

Research history
In Aristotle's time, the fourth century BCE, whales were regarded as fish due to their superficial similarity. Aristotle, however, observed many physiological and anatomical similarities with the terrestrial vertebrates, such as blood (circulation), lungs, uterus and fin anatomy. His detailed descriptions were assimilated by the Romans, but mixed with a more accurate knowledge of the dolphins, as mentioned by Pliny the Elder in his Natural history. In the art of this and subsequent periods, dolphins are portrayed with a high-arched head (typical of porpoises) and a long snout. The harbour porpoise was one of the most accessible species for early cetologists; because it could be seen close to land, inhabiting shallow coastal areas of Europe. Much of the findings that apply to all cetaceans were first discovered in porpoises. One of the first anatomical descriptions of the airways of a harbor porpoise dates from 1671 by John Ray. It nevertheless referred to the porpoise as a fish.

"The tube in the head, through which this kind fish takes its breath and spitting water, located in front of the brain and ends outwardly in a simple hole, but inside it is divided by a downward bony septum, as if it were two nostrils; but underneath it opens up again in the mouth in a void."

In the 10th edition of Systema Naturae (1758), Swedish biologist and taxonomist Carl Linnaeus asserted that cetaceans were mammals and not fish. His groundbreaking binomial system formed the basis of modern whale classification.

Culture
Cetaceans have played a role in human culture through history.

Prehistoric
Stone Age petroglyphs, such as those in Roddoy and Reppa (Norway), and the Bangudae Petroglyphs in South Korea, depict them. Whale bones were used for many purposes. In the Neolithic settlement of Skara Brae on Orkney sauce pans were made from whale vertebrae.

Antiquity
The whale was first mentioned in ancient Greece by Homer. There, it is called Ketos, a term that initially included all large marine animals. From this was derived the Roman word for whale, Cetus. Other names were phálaina (Aristotle, Latin form of ballaena) for the female and, with an ironic characteristic style, musculus (Mouse) for the male. North Sea whales were called Physeter, which was meant for the sperm whale Physter macrocephalus. Whales are described in particular by Aristotle, Pliny and Ambrose. All mention both live birth and suckling. Pliny describes the problems associated with the lungs with spray tubes and Ambrose claimed that large whales would take their young into their mouth to protect them.

In the Bible especially, the leviathan plays a role as a sea monster. The essence, which features a giant crocodile or a dragon and a whale, was created according to the Bible by God and should again be destroyed by him. In the Book of Job, the leviathan is described in more detail.

In Jonah there is a more recognizable description of a whale alongside the prophet Jonah, who, on his flight from the city of Nineveh is swallowed by a whale.

Dolphins are mentioned far more often than whales. Aristotle discusses the sacred animals of the Greeks in his Historia Animalium and gives details of their role as aquatic animals. The Greeks admired the dolphin as a "king of the aquatic animals" and referred to them erroneously as fish. Its intelligence was apparent both in its ability to escape from fishnets and in its collaboration with fishermen.

River dolphins are known from the Ganges and – erroneously – the Nile. In the latter case it was equated with sharks and catfish. Supposedly they attacked even crocodiles.

Dolphins appear in Greek mythology. Because of their intelligence, they rescued multiple people from drowning. They were said to love music, probably because of their own song, and in the legends they saved famous musicians, such as Arion of Lesbos from Methymna.

Dolphins belong to the domain of Poseidon and led him to his wife Amphitrite. Dolphins are associated with other gods, such as Apollo, Dionysus and Aphrodite. The Greeks paid tribute to both whales and dolphins with their own constellation. The constellation of the Whale (Ketos, lat. Cetus) is located south of the Dolphin (Delphi, lat. Delphinus) north of the zodiac.

Ancient art often included dolphin representations, including the Cretan Minoans. Later they appeared on reliefs, gems, lamps, coins, mosaics and gravestones. A particularly popular representation is that of Arion or Taras riding on a dolphin. In early Christian art, the dolphin is a popular motif, at times used as a symbol of Christ.

Middle Ages to the 19th century
St. Brendan described in his travel story Navigatio Sancti Brendani an encounter with a whale, between the years 565–573. He described how he and his companions entered a treeless island, which turned out to be a giant whale, which he called Jasconicus. He met this whale seven years later and rested on his back.

Most descriptions of large whales from this time until the whaling era, beginning in the 17th century, were of beached whales, which resembled no other animal. This was particularly true for the sperm whale, the most frequently stranded in larger groups. Raymond Gilmore documented seventeen sperm whales in the estuary of the Elbe from 1723 to 1959 and thirty-one animals on the coast of Great Britain in 1784. In 1827, a blue whale beached itself off the coast of Ostend. Whales were used as attractions in museums and traveling exhibitions. Whalers from the 17th to 19th centuries depicted whales in drawings and recounted tales of their occupation. Although they knew that whales were harmless giants, they described battles with harpooned animals. These included descriptions of sea monsters, including huge whales, sharks, sea snakes, giant squid and octopuses.

Among the first whalers who described their experiences on whaling trips was Captain William Scoresby from Great Britain, who published the book Northern Whale Fishery, describing the hunt for northern baleen whales. This was followed by Thomas Beale, a British surgeon, in his book Some observations on the natural history of the sperm whale in 1835; and Frederick Debell Bennett's The tale of a whale hunt in 1840. Whales were described in narrative literature and paintings, most famously in the novels Moby Dick by Herman Melville and Twenty Thousand Leagues Under the Seas by Jules Verne.

Baleen was used to make vessel components such as the bottom of a bucket in the Scottish National Museum. The Norsemen crafted ornamented plates from baleen, sometimes interpreted as ironing boards.

In the Canadian Arctic (east coast) in Punuk and Thule culture (1000–1600 C.E.), baleen was used to construct houses in place of wood as roof support for winter houses, with half of the building buried under the ground. The actual roof was probably made of animal skins that were covered with soil and moss.

Modern culture
In the 20th century perceptions of cetaceans changed. They transformed from monsters into creatures of wonder, as science revealed them to be intelligent and peaceful animals. Hunting was replaced by whale and dolphin tourism. This change is reflected in films and novels. For example, the protagonist of the series Flipper was a bottle-nose dolphin. The TV series SeaQuest DSV (1993–1996), the movies Free Willy, Star Trek IV: The Voyage Home and the book series The Hitchhiker's Guide to the Galaxy by Douglas Adams are examples.

The study of whale song also produced a popular album, Songs of the Humpback Whale.

Captivity
Whales and dolphins have been kept in captivity for use in education, research and entertainment since the 19th century.

Belugas
Beluga whales were the first whales to be kept in captivity. Other species were too rare, too shy or too big. The first was shown at Barnum's Museum in New York City in 1861. For most of the 20th century, Canada was the predominant source. They were taken from the St. Lawrence River estuary until the late 1960s, after which they were predominantly taken from the Churchill River estuary until capture was banned in 1992. Russia then became the largest provider. Belugas are caught in the Amur Darya delta and their eastern coast and are transported domestically to aquaria or dolphinaria in Moscow, St. Petersburg and Sochi, or exported to countries such as Canada. They have not been domesticated.

As of 2006, 30 belugas lived in Canada and 28 in the United States. 42 deaths in captivity had been reported. A single specimen can reportedly fetch up to US$100,000 (£64,160). The beluga's popularity is due to its unique color and its facial expressions. The latter is possible because while most cetacean "smiles" are fixed, the extra movement afforded by the beluga's unfused cervical vertebrae allows a greater range of apparent expression.

Orcas
The orca's intelligence, trainability, striking appearance, playfulness in captivity and sheer size have made it a popular exhibit at aquaria and aquatic theme parks. From 1976 to 1997, fifty-five whales were taken from the wild in Iceland, nineteen from Japan and three from Argentina. These figures exclude animals that died during capture. Live captures fell dramatically in the 1990s and by 1999, about 40% of the forty-eight animals on display in the world were captive-born.

Organizations such as World Animal Protection and the Whale and Dolphin Conservation campaign against the practice of keeping them in captivity.

In captivity, they often develop pathologies, such as the dorsal fin collapse seen in 60–90% of captive males. Captives have reduced life expectancy, on average only living into their 20s, although some live longer, including several over 30 years old and two, Corky II and Lolita, in their mid-40s. In the wild, females who survive infancy live 46 years on average and up to 70–80 years. Wild males who survive infancy live 31 years on average and can reach 50–60 years.

Captivity usually bears little resemblance to wild habitat and captive whales' social groups are foreign to those found in the wild. Critics claim captive life is stressful due to these factors and the requirement to perform circus tricks that are not part of wild orca behavior. Wild orca may travel up to 160 km in a day and critics say the animals are too big and intelligent to be suitable for captivity. Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of stress. Orcas are well known for their performances in shows, but the number of orcas kept in captivity is small, especially when compared to the number of bottlenose dolphins, with only forty-four captive orcas being held in aquaria as of 2012.

Each country has its own tank requirements; in the US, the minimum enclosure size is set by the Code of Federal Regulations, 9 CFR E § 3.104, under the Specifications for the Humane Handling, Care, Treatment and Transportation of Marine Mammals.

Aggression among captive orcas is common. They attack each other and their trainers as well. In 2013, SeaWorld's treatment of orcas in captivity was the basis of the movie Blackfish, which documents the history of Tilikum, an orca at SeaWorld Orlando, who had been involved in the deaths of three people. The film led to proposals by some lawmakers to ban captivity of cetaceans, and led SeaWorld to announce in 2016 that it would phase out its orca program after various unsuccessful attempts to restore its revenues, reputation, and stock price.

Others
Dolphins and porpoises are kept in captivity. Bottlenose dolphins are the most common, as they are relatively easy to train, have a long lifespan in captivity and have a friendly appearance. Bottlenose dolphins live in captivity across the world, though exact numbers are hard to determine. Other species kept in captivity are spotted dolphins, false killer whales and common dolphins, Commerson's dolphins, as well as rough-toothed dolphins, but all in much lower numbers. There are also fewer than ten pilot whales, Amazon river dolphins, Risso's dolphins, spinner dolphins, or tucuxi in captivity. Two unusual and rare hybrid dolphins, known as wolphins, are kept at Sea Life Park in Hawaii, which is a cross between a bottlenose dolphin and a false killer whale. Also, two common/bottlenose hybrids reside in captivity at Discovery Cove and SeaWorld San Diego.

In repeated attempts in the 1960s and 1970s, narwhals kept in captivity died within months. A breeding pair of pygmy right whales were retained in a netted area. They were eventually released in South Africa. In 1971, SeaWorld captured a California gray whale calf in Mexico at Scammon's Lagoon. The calf, later named Gigi, was separated from her mother using a form of lasso attached to her flukes. Gigi was displayed at SeaWorld San Diego for a year. She was then released with a radio beacon affixed to her back; however, contact was lost after three weeks. Gigi was the first captive baleen whale. JJ, another gray whale calf, was kept at SeaWorld San Diego. JJ was an orphaned calf that beached itself in April 1997 and was transported two miles to SeaWorld. The 680 kg calf was a popular attraction and behaved normally, despite separation from his mother. A year later, the then 8,164.7 kg whale though smaller than average, was too big to keep in captivity, and was released on April 1, 1998. A captive Amazon river dolphin housed at Acuario de Valencia is the only trained river dolphin in captivity.

Here is a list of all the cetaceans that have been taken into captivity for either conservation, research or human entertainment and education purposes currently or in the past, temporarily or permanently.

 * Orca
 * Vaquita
 * False killer whale
 * Pygmy killer whale
 * Long-finned pilot whale
 * Short-finned pilot whale
 * Bottlenose dolphin
 * Beluga whale
 * Pacific white-sided dolphin
 * Risso's dolphin
 * Atlantic white-sided dolphin
 * Narwhal
 * Spotted dolphin
 * Spinner dolphin
 * Common dolphin
 * Harbour porpoise
 * Finless porpoise
 * Commerson's dolphin
 * Rough-toothed dolphin
 * Boto
 * Tucuxi
 * Wholphin
 * Gray whale
 * Pygmy sperm whale
 * Irrawaddy dolphin
 * Indo-Pacific humpback dolphin
 * Baiji
 * Melon-headed whale
 * South Asian river dolphin
 * Minke whale

title: Cetacea


text:
The Canterbury Tales

The Canterbury Tales (Tales of Caunterbury) is a collection of twenty-four stories that runs to over 17,000 lines written in Middle English by Geoffrey Chaucer between 1387 and 1400. It is widely regarded as Chaucer's magnum opus. The tales (mostly written in verse, although some are in prose) are presented as part of a story-telling contest by a group of pilgrims as they travel together from London to Canterbury to visit the shrine of Saint Thomas Becket at Canterbury Cathedral. The prize for this contest is a free meal at the Tabard Inn at Southwark on their return.

It has been suggested that the greatest contribution of The Canterbury Tales to English literature was the popularisation of the English vernacular in mainstream literature, as opposed to French, Italian or Latin. English had, however, been used as a literary language centuries before Chaucer's time, and several of Chaucer's contemporaries—John Gower, William Langland, the Pearl Poet, and Julian of Norwich—also wrote major literary works in English. It is unclear to what extent Chaucer was seminal in this evolution of literary preference.

The Canterbury Tales is generally thought to have been incomplete at the end of Chaucer's life. In the General Prologue, some 30 pilgrims are introduced. According to the Prologue, Chaucer's intention was to write four stories from the perspective of each pilgrim, two each on the way to and from their ultimate destination, St. Thomas Becket's shrine (making for a total of about 120 stories). It is revered as one of the most important works in English literature.

Text
The question of whether The Canterbury Tales is a finished work has not been answered to date. There are 84 manuscripts and four incunabula (printed before 1500) editions of the work, which is more than for any other vernacular English literary text with the exception of Prick of Conscience. This comparison should not be taken as evidence of the Tales' popularity in the century after Chaucer's death, because, according to Derek Pearsall, it is unfair considering that Prick of Conscience had all the benefit of the "preservation of a dogmatic religious subject-matter". Fifty-five of these manuscripts are thought to have been originally complete, while 28 are so fragmentary that it is difficult to ascertain whether they were copied individually or as part of a set. The Tales vary in both minor and major ways from manuscript to manuscript; many of the minor variations are due to copyists' errors, while it is suggested that in other cases Chaucer both added to his work and revised it as it was being copied and possibly as it was being distributed.

There are no manuscripts of the Canterbury Tales surviving in Chaucer's own hand. The two earliest known manuscripts, which both appear to have been copied by the same scribe, are MS Peniarth 392 D (called "Hengwrt"), and the Ellesmere Manuscript, a deluxe, illustrated manuscript. Until the 1940s, scholars tended to prefer the Ellesmere manuscript as closer to Chaucer's intentions; following John M. Manly and Edith Rickert, scholars increasingly favoured Hengwrt. The first version of The Canterbury Tales to be published in print was William Caxton's 1476 edition. It was one of the first books to be printed by Caxton, the first person in England to print books using a printing press. Only 10 copies of this edition are known to exist, including one held by the British Library and one held by the Folger Shakespeare Library.

The copyist of the Hengwrt and Ellesmere manuscripts has been identified as a scrivener named Adam Pinkhurst. Since a poem, apparently by Chaucer, identifies his scribe as a man named "Adam", this has led to the hypothesis that the scribe who copied these two important manuscripts worked with Chaucer and knew him personally. This identification has been the subject of much controversy in the field of Middle English palaeography, though it is widely accepted as plausible.

Order
There is no consensus as to whether a complete version of the Tales exists, and also no consensus regarding Chaucer's intended order of the stories.

Textual and manuscript clues have been adduced to support the two most popular modern methods of ordering the tales. Some scholarly editions divide the Tales into ten "Fragments". The tales that make up a Fragment are closely related and contain internal indications of their order of presentation, usually with one character speaking to and then stepping aside for another character. However, between Fragments, the connection is less obvious. Consequently, there are several possible orders; the one most frequently seen in modern editions follows the numbering of the Fragments (ultimately based on the Ellesmere order). Victorians frequently used the nine "Groups", which was the order used by Walter William Skeat whose edition Chaucer: Complete Works was used by Oxford University Press for most of the twentieth century, but this order is currently seldom followed.

An alternative ordering (seen in the early 15th-century manuscript Harley MS. 7334) places Fragment VIII before VI. Fragments I and II almost always follow each other, just as VI and VII, IX and X do in the oldest manuscripts. Fragments IV and V, by contrast, vary in location from manuscript to manuscript.

Language
Chaucer mainly wrote in a London dialect of late Middle English, which has clear differences from Modern English. From philological research, some facts are known about the pronunciation of English during the time of Chaucer. Chaucer pronounced -e at the end of many words, so that care (except when followed by a vowel sound) was, not as in Modern English. Other nowadays silent letters were also pronounced, so that the word knight was, with both the k and the gh pronounced, not. In some cases, vowel letters in Middle English were pronounced very differently from Modern English, because the Great Vowel Shift had not yet happened. For instance, the long e in wepyng "weeping" was pronounced as, as in modern German or Italian, not as. Below is an IPA transcription of the opening lines of The Merchant's Prologue:

 * 'Wepyng and waylyng, care and oother sorwe
 * I knowe ynogh, on even and a-morwe,'
 * Quod the Marchant, 'and so doon oother mo
 * That wedded been.'

 * 'Weeping and wailing, care and other sorrow
 * I know enough, in the evening and in the morning,'
 * said the Merchant, 'and so do many others
 * who have been married.'

No manuscript exists in Chaucer's own hand; all extant copies were made by scribes. Because the final -e sound was lost soon after Chaucer's time, scribes did not accurately copy it, and this gave scholars the impression that Chaucer himself was inconsistent in using it. It has now been established, however, that -e was an important part of Chaucer's grammar, and helped to distinguish singular adjectives from plural and subjunctive verbs from indicative.

Genre and structure
The Canterbury Tales is a collection of stories built around a frame tale, a common and already long established genre in this period. Chaucer's Tales differs from most other story "collections" in this genre chiefly in its intense variation. Most story collections focused on a theme, usually a religious one. Even in the Decameron, storytellers are encouraged to stick to the theme decided on for the day. The idea of a pilgrimage to get such a diverse collection of people together for literary purposes was also unprecedented, though "the association of pilgrims and storytelling was a familiar one". Introducing a competition among the tales encourages the reader to compare the tales in all their variety, and allows Chaucer to showcase the breadth of his skill in different genres and literary forms.

While the structure of the Tales is largely linear, with one story following another, it is also much more than that. In the General Prologue, Chaucer describes not the tales to be told, but the people who will tell them, making it clear that structure will depend on the characters rather than a general theme or moral. This idea is reinforced when the Miller interrupts to tell his tale after the Knight has finished his. Having the Knight go first gives one the idea that all will tell their stories by class, with the Monk following the Knight. However, the Miller's interruption makes it clear that this structure will be abandoned in favour of a free and open exchange of stories among all classes present. General themes and points of view arise as the characters tell their tales, which are responded to by other characters in their own tales, sometimes after a long lapse in which the theme has not been addressed.

Lastly, Chaucer does not pay much attention to the progress of the trip, to the time passing as the pilgrims travel, or to specific locations along the way to Canterbury. His writing of the story seems focused primarily on the stories being told, and not on the pilgrimage itself.

Style
The variety of Chaucer's tales shows the breadth of his skill and his familiarity with many literary forms, linguistic styles, and rhetorical devices. Medieval schools of rhetoric at the time encouraged such diversity, dividing literature (as Virgil suggests) into high, middle, and low styles as measured by the density of rhetorical forms and vocabulary. Another popular method of division came from St. Augustine, who focused more on audience response and less on subject matter (a Virgilian concern). Augustine divided literature into "majestic persuades", "temperate pleases", and "subdued teaches". Writers were encouraged to write in a way that kept in mind the speaker, subject, audience, purpose, manner, and occasion. Chaucer moves freely between all of these styles, showing favouritism to none. He not only considers the readers of his work as an audience, but the other pilgrims within the story as well, creating a multi-layered rhetoric.

With this, Chaucer avoids targeting any specific audience or social class of readers, focusing instead on the characters of the story and writing their tales with a skill proportional to their social status and learning. However, even the lowest characters, such as the Miller, show surprising rhetorical ability, although their subject matter is more lowbrow. Vocabulary also plays an important part, as those of the higher classes refer to a woman as a "lady", while the lower classes use the word "wenche", with no exceptions. At times the same word will mean entirely different things between classes. The word "pitee", for example, is a noble concept to the upper classes, while in the Merchant's Tale it refers to sexual intercourse. Again, however, tales such as the Nun's Priest's Tale show surprising skill with words among the lower classes of the group, while the Knight's Tale is at times extremely simple.

Chaucer uses the same meter throughout almost all of his tales, with the exception of Sir Thopas and his prose tales. This is a line characterised by five stressed syllables, usually alternating with unstressed syllables to produce lines usually of ten syllables, but often eleven and occasionally nine; occasionally a caesura can be identified around the middle of a line. This metre was probably inspired by French and Italian forms. Chaucer's meter would later develop into the heroic meter of the 15th and 16th centuries sometimes known as riding rhyme, and is an ancestor of iambic pentameter. Chaucer's verse is usually also characterised by couplet rhyme, but he avoided allowing couplets to become too prominent in The Canterbury Tales, and four of the tales (the Man of Law's, Clerk's, Prioress', and Second Nun's) use rhyme royal.

Historical context and themes
In 1386, Chaucer became Controller of Customs and Justice of the Peace and, in 1389, Clerk of the King's Works. It was during these years that Chaucer began working on The Canterbury Tales.

The end of the fourteenth century was a turbulent time in English history. The Catholic Church was in the midst of the Western Schism and, although it was still the only Christian authority in Western Europe, it was the subject of heavy controversy. Lollardy, an early English religious movement led by John Wycliffe, is mentioned in the Tales, which also mention a specific incident involving pardoners (sellers of indulgences, which were believed to relieve the temporal punishment due for sins that were already forgiven in the Sacrament of Confession) who nefariously claimed to be collecting for St. Mary Rouncesval hospital in England. The Canterbury Tales is among the first English literary works to mention paper, a relatively new invention that allowed dissemination of the written word never before seen in England. Political clashes, such as the 1381 Peasants' Revolt and clashes ending in the deposing of King Richard II, further reveal the complex turmoil surrounding Chaucer in the time of the Tales' writing. Many of his close friends were executed and he himself moved to Kent to get away from events in London.

While some readers look to interpret the characters of The Canterbury Tales as historical figures, other readers choose to interpret its significance in less literal terms. After analysis of Chaucer's diction and historical context, his work appears to develop a critique of society during his lifetime. Within a number of his descriptions, his comments can appear complimentary in nature, but through clever language, the statements are ultimately critical of the pilgrim's actions. It is unclear whether Chaucer would intend for the reader to link his characters with actual persons. Instead, it appears that Chaucer creates fictional characters to be general representations of people in such fields of work. With an understanding of medieval society, one can detect subtle satire at work.

Religion
The Tales reflect diverse views of the Church in Chaucer's England. After the Black Death, many Europeans began to question the authority of the established Church. Some turned to Lollardy, while others chose less extreme paths, starting new monastic orders or smaller movements exposing church corruption in the behaviour of the clergy, false church relics or abuse of indulgences. Several characters in the Tales are religious figures, and the very setting of the pilgrimage to Canterbury is religious (although the prologue comments ironically on its merely seasonal attractions), making religion a significant theme of the work.

Two characters, the Pardoner and the Summoner, whose roles apply the Church's secular power, are both portrayed as deeply corrupt, greedy, and abusive. Pardoners in Chaucer's day were those people from whom one bought Church "indulgences" for forgiveness of sins, who were guilty of abusing their office for their own gain. Chaucer's Pardoner openly admits the corruption of his practice while hawking his wares. Summoners were Church officers who brought sinners to the Church court for possible excommunication and other penalties. Corrupt summoners would write false citations and frighten people into bribing them to protect their interests. Chaucer's Summoner is portrayed as guilty of the very kinds of sins for which he is threatening to bring others to court, and is hinted as having a corrupt relationship with the Pardoner. In The Friar's Tale, one of the characters is a summoner who is shown to be working on the side of the devil, not God.

Churchmen of various kinds are represented by the Monk, the Prioress, the Nun's Priest, and the Second Nun. Monastic orders, which originated from a desire to follow an ascetic lifestyle separated from the world, had by Chaucer's time become increasingly entangled in worldly matters. Monasteries frequently controlled huge tracts of land on which they made significant sums of money, while peasants worked in their employ. The Second Nun is an example of what a Nun was expected to be: her tale is about a woman whose chaste example brings people into the church. The Monk and the Prioress, on the other hand, while not as corrupt as the Summoner or Pardoner, fall far short of the ideal for their orders. Both are expensively dressed, show signs of lives of luxury and flirtatiousness and show a lack of spiritual depth. The Prioress's Tale is an account of Jews murdering a deeply pious and innocent Christian boy, a blood libel against Jews that became a part of English literary tradition. The story did not originate in the works of Chaucer and was well known in the 14th century.

Pilgrimage was a very prominent feature of medieval society. The ultimate pilgrimage destination was Jerusalem, but within England Canterbury was a popular destination. Pilgrims would journey to cathedrals that preserved relics of saints, believing that such relics held miraculous powers. Saint Thomas Becket, Archbishop of Canterbury, had been murdered in Canterbury Cathedral by knights of Henry II during a disagreement between Church and Crown. Miracle stories connected to his remains sprang up soon after his death, and the cathedral became a popular pilgrimage destination. The pilgrimage in the work ties all of the stories together and may be considered a representation of Christians' striving for heaven, despite weaknesses, disagreement, and diversity of opinion.

Social class and convention
The upper class or nobility, represented chiefly by the Knight and his Squire, was in Chaucer's time steeped in a culture of chivalry and courtliness. Nobles were expected to be powerful warriors who could be ruthless on the battlefield yet mannerly in the King's Court and Christian in their actions. Knights were expected to form a strong social bond with the men who fought alongside them, but an even stronger bond with a woman whom they idealised to strengthen their fighting ability. Though the aim of chivalry was to noble action, its conflicting values often degenerated into violence. Church leaders frequently tried to place restrictions on jousts and tournaments, which at times ended in the death of the loser. The Knight's Tale shows how the brotherly love of two fellow knights turns into a deadly feud at the sight of a woman whom both idealise. To win her, both are willing to fight to the death. Chivalry was on the decline in Chaucer's day, and it is possible that The Knight's Tale was intended to show its flaws, although this is disputed. Chaucer himself had fought in the Hundred Years' War under Edward III, who heavily emphasised chivalry during his reign. Two tales, Sir Topas and The Tale of Melibee, are told by Chaucer himself, who is travelling with the pilgrims in his own story. Both tales seem to focus on the ill-effects of chivalry—the first making fun of chivalric rules and the second warning against violence.

The Tales constantly reflect the conflict between classes. For example, the division of the three estates: the characters are all divided into three distinct classes, the classes being "those who pray" (the clergy), "those who fight" (the nobility), and "those who work" (the commoners and peasantry). Most of the tales are interlinked by common themes, and some "quit" (reply to or retaliate against) other tales. Convention is followed when the Knight begins the game with a tale, as he represents the highest social class in the group. But when he is followed by the Miller, who represents a lower class, it sets the stage for the Tales to reflect both a respect for and a disregard for upper class rules. Helen Cooper, as well as Mikhail Bakhtin and Derek Brewer, call this opposition "the ordered and the grotesque, Lent and Carnival, officially approved culture and its riotous, and high-spirited underside." Several works of the time contained the same opposition.

Relativism versus realism
Chaucer's characters each express different—sometimes vastly different—views of reality, creating an atmosphere of testing, empathy, and relativism. As Helen Cooper says, "Different genres give different readings of the world: the fabliau scarcely notices the operations of God, the saint's life focuses on those at the expense of physical reality, tracts and sermons insist on prudential or orthodox morality, romances privilege human emotion." The sheer number of varying persons and stories renders the Tales as a set unable to arrive at any definite truth or reality.

Liminality
The concept of liminality figures prominently within The Canterbury Tales. A liminal space, which can be both geographical as well as metaphorical or spiritual, is the transitional or transformational space between a "real" (secure, known, limited) world and an unknown or imaginary space of both risk and possibility. The notion of a pilgrimage is itself a liminal experience, because it centres on travel between destinations and because pilgrims undertake it hoping to become more holy in the process. Thus, the structure of The Canterbury Tales itself is liminal; it not only covers the distance between London and Canterbury, but the majority of the tales refer to places entirely outside the geography of the pilgrimage. Jean Jost summarises the function of liminality in The Canterbury Tales,

"Both appropriately and ironically in this raucous and subversive liminal space, a ragtag assembly gather together and tell their equally unconventional tales. In this unruly place, the rules of tale telling are established, themselves to be both disordered and broken; here the tales of game and earnest, solas and sentence, will be set and interrupted. Here the sacred and profane adventure begins, but does not end. Here, the condition of peril is as prominent as that of protection. The act of pilgrimaging itself consists of moving from one urban space, through liminal rural space, to the next urban space with an ever fluctuating series of events and narratives punctuating those spaces. The goal of pilgrimage may well be a religious or spiritual space at its conclusion, and reflect a psychological progression of the spirit, in yet another kind of emotional space." Liminality is also evident in the individual tales. An obvious instance of this is The Friar's Tale in which the yeoman devil is a liminal figure because of his transitory nature and function; it is his purpose to issue souls from their current existence to hell, an entirely different one. The Franklin's Tale is a Breton Lai tale, which takes the tale into a liminal space by invoking not only the interaction of the supernatural and the mortal, but also the relation between the present and the imagined past.

Reception
While Chaucer clearly states the addressees of many of his poems (the Book of the Duchess is believed to have been written for John of Gaunt on the occasion of his wife's death in 1368), the intended audience of The Canterbury Tales is more difficult to determine. Chaucer was a courtier, leading some to believe that he was mainly a court poet who wrote exclusively for the nobility. He is referred to as a noble translator and poet by Eustache Deschamps and by his contemporary John Gower. It has been suggested that the poem was intended to be read aloud, which is probable as this was a common activity at the time. However, it also seems to have been intended for private reading, since Chaucer frequently refers to himself as the writer, rather than the speaker, of the work. Determining the intended audience directly from the text is even more difficult, since the audience is part of the story. This makes it difficult to tell when Chaucer is writing to the fictional pilgrim audience or the actual reader.

Chaucer's works may have been distributed in some form during his lifetime in part or in whole. Scholars speculate that manuscripts were circulated among his friends, but likely remained unknown to most people until after his death. However, the speed with which copyists strove to write complete versions of his tale in manuscript form shows that Chaucer was a famous and respected poet in his own day. The Hengwrt and Ellesmere manuscripts are examples of the care taken to distribute the work. More manuscript copies of the poem exist than for any other poem of its day except The Prick of Conscience, causing some scholars to give it the medieval equivalent of bestseller status. Even the most elegant of the illustrated manuscripts, however, is not nearly as highly decorated as the work of authors of more respectable works such as John Lydgate's religious and historical literature.

15th century
John Lydgate and Thomas Occleve were among the first critics of Chaucer's Tales, praising the poet as the greatest English poet of all time and the first to show what the language was truly capable of poetically. This sentiment was universally agreed upon by later critics into the mid-15th century. Glosses included in The Canterbury Tales manuscripts of the time praised him highly for his skill with "sentence" and rhetoric, the two pillars by which medieval critics judged poetry. The most respected of the tales was at this time the Knight's, as it was full of both.

Literary additions and supplements
The incompleteness of the Tales led several medieval authors to write additions and supplements to the tales to make them more complete. Some of the oldest existing manuscripts of the tales include new or modified tales, showing that even early on, such additions were being created. These emendations included various expansions of the Cook's Tale, which Chaucer never finished, The Plowman's Tale, The Tale of Gamelyn, the Siege of Thebes, and the Tale of Beryn.

The Tale of Beryn, written by an anonymous author in the 15th century, is preceded by a lengthy prologue in which the pilgrims arrive at Canterbury and their activities there are described. While the rest of the pilgrims disperse throughout the town, the Pardoner seeks the affections of Kate the barmaid, but faces problems dealing with the man in her life and the innkeeper Harry Bailey. As the pilgrims turn back home, the Merchant restarts the storytelling with Tale of Beryn. In this tale, a young man named Beryn travels from Rome to Egypt to seek his fortune only to be cheated by other businessmen there. He is then aided by a local man in getting his revenge. The tale comes from the French tale Bérinus and exists in a single early manuscript of the tales, although it was printed along with the tales in a 1721 edition by John Urry.

John Lydgate wrote The Siege of Thebes in about 1420. Like the Tale of Beryn, it is preceded by a prologue in which the pilgrims arrive in Canterbury. Lydgate places himself among the pilgrims as one of them and describes how he was a part of Chaucer's trip and heard the stories. He characterises himself as a monk and tells a long story about the history of Thebes before the events of the Knight's Tale. John Lydgate's tale was popular early on and exists in old manuscripts both on its own and as part of the Tales. It was first printed as early as 1561 by John Stow, and several editions for centuries after followed suit.

There are actually two versions of The Plowman's Tale, both of which are influenced by the story Piers Plowman, a work written during Chaucer's lifetime. Chaucer describes a Plowman in the General Prologue of his tales, but never gives him his own tale. One tale, written by Thomas Occleve, describes the miracle of the Virgin and the Sleeveless Garment. Another tale features a pelican and a griffin debating church corruption, with the pelican taking a position of protest akin to John Wycliffe's ideas.

The Tale of Gamelyn was included in an early manuscript version of the tales, Harley 7334, which is notorious for being one of the lower-quality early manuscripts in terms of editor error and alteration. It is now widely rejected by scholars as an authentic Chaucerian tale, although some scholars think he may have intended to rewrite the story as a tale for the Yeoman. Dates for its authorship vary from 1340 to 1370.

Books

 * The most well-known work of the 18th century writer Harriet Lee was called The Canterbury Tales, and consists of twelve stories, related by travellers thrown together by untoward accident. In turn, Lee's version had a profound influence on Lord Byron.
 * Henry Dudeney's 1907 book The Canterbury Puzzles contains a part reputedly lost from what modern readers know as Chaucer's tales.
 * Historical-mystery novelist P.C. Doherty wrote a series of novels based on The Canterbury Tales, making use of both the story frame and Chaucer's characters.
 * Science-fiction writer Dan Simmons wrote his Hugo Award winning 1989 novel Hyperion based on an extra-planetary group of pilgrims.
 * Evolutionary biologist Richard Dawkins used The Canterbury Tales as a structure for his 2004 non-fiction book about evolution titled The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution. His animal pilgrims are on their way to find the common ancestor, each telling a tale about evolution.
 * Canadian author Angie Abdou translates The Canterbury Tales to a cross section of people, all snow-sports enthusiasts but from different social backgrounds, converging on a remote back-country ski cabin in British Columbia in the 2011 novel The Canterbury Trail.
 * British poet and performer Patience Agbabi is one of fourteen authors who worked together to tell the stories and experiences of refugees, detainees, and asylum seekers in a book titled Refugee Tales. The collaborative efforts of the writers and displaced people create stories modeled after Chaucer's tale of journey in The Canterbury Tales. This project is rooted in the efforts of the Gatwick Detainees Welfare Group, a non-partisan advocacy group for detained people.

Stage adaptations

 * The Two Noble Kinsmen, by William Shakespeare and John Fletcher, a retelling of "The Knight's Tale", was first performed in 1613 or 1614 and published in 1634.
 * In 1961, Erik Chisholm completed his opera, The Canterbury Tales. The opera is in three acts: The Wyf of Bath's Tale, The Pardoner's Tale and The Nun's Priest's Tale.
 * Nevill Coghill's modern English version formed the basis of a musical version that was first staged in 1964.
 * In 2021, Zadie Smith debuted her first play, The Wife of Willesden, adapting the Wife of Bath's Prologue and Tale as told at a contemporary bar crawl, with the tale set in 17th century Jamaica. The work was originally performed in London and at the American Repertory Theater in Cambridge, Massachusetts in 2023.

Film and television

 * A Canterbury Tale, a 1944 film, jointly written and directed by Michael Powell and Emeric Pressburger, is loosely based on the narrative frame of Chaucer's tales. The movie opens with a group of medieval pilgrims journeying through the Kentish countryside as a narrator speaks the opening lines of the General Prologue. The scene then makes a now-famous transition to the time of World War II. From that point on, the film follows a group of strangers, each with their own story and in need of some kind of redemption, who are making their way to Canterbury together. The film's main story takes place in an imaginary town in Kent and ends with the main characters arriving at Canterbury Cathedral, bells pealing and Chaucer's words again resounding. A Canterbury Tale is recognised as one of the Powell-Pressburger team's most poetic and artful films. It was produced as wartime propaganda, using Chaucer's poetry, referring to the famous pilgrimage, and offering photography of Kent to remind the public of what made Britain worth fighting for. In one scene, a local historian lectures an audience of British soldiers about the pilgrims of Chaucer's time and the vibrant history of England.
 * Pier Paolo Pasolini's 1972 film The Canterbury Tales features several of the tales, some of which cohere to the original tale and others which are embellished. "The Cook's Tale", for instance, which is incomplete in the original version, is expanded into a full story, and "The Friar's Tale" extends the scene in which the Summoner is dragged down to hell. The film includes these two tales as well as "The Miller's Tale", "The Summoner's Tale", "The Wife of Bath's Tale", and "The Merchant's Tale". "The Tale of Sir Topas" was also filmed and dubbed; however, it was later removed by Pasolini, and is now considered lost.
 * Alan Plater retold the stories in a series of plays for BBC2 in 1975: Trinity Tales.
 * On 26 April 1986, American radio personality Garrison Keillor opened "The News from Lake Wobegon" portion of the first live TV broadcast of his A Prairie Home Companion radio show with a reading of the original Middle English text of the General Prologue. He commented, "Although those words were written more than 600 years ago, they still describe spring."
 * Jonathan Myerson directed an animated version of The Canterbury Tales in three parts from 1998 to 2000. The series was nominated for an Oscar (as animated short film) in 1999 and won the BAFTA Award for Best Animated Film in addition to four Primetime Emmys.
 * The 2001 film A Knight's Tale, starring Heath Ledger, takes its title from Chaucer's "The Knight's Tale" and features Chaucer as a character.
 * In 2003, the BBC again featured modern re-tellings of selected tales in their six-episode series Canterbury Tales.

Music

 * British Psychedelic rock band Procol Harum's 1967 hit "A Whiter Shade of Pale" is often assumed to be referencing the Canterbury Tales through the line, "as the miller told his tale." However, lyricist Keith Reid has denied this, saying he had never read Chaucer when he wrote the line.
 * The title of Sting's 1993 album Ten Summoner's Tales alludes to "The Summoner's Tale" and to Sting's birth name, Gordon Sumner.

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title: The Canterbury Tales