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Cetacea

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Cetacea(/sɪˈteɪʃə/;fromLatincetus'whale', fromAncient Greekκῆτος(kêtos)'huge fish,sea monster')[3]is aninfraorderofaquatic mammalsbelonging to the orderArtiodactylathat includeswhales,dolphinsandporpoises.Key characteristics are their fully aquatic lifestyle,streamlinedbody shape, often large size and exclusivelycarnivorousdiet. 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.[4]

Cetacea
Temporal range:53.5–0MaEarlyEocene– Present
Sperm whale
Humpback whale
Orca
North Atlantic right whale
Amazon river dolphin
Harbor porpoise
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Clade: Cetaceamorpha
Infraorder: Cetacea
Brisson,1762
Subgroups

(see textfor families)

Diversity
Around 94 species

While the majority of cetaceans live in marine environments, a small number reside solely inbrackish waterorfresh water.Having acosmopolitan 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 fortheir high intelligence,complex social behaviour, and the enormous size of some of the group's members. For example, theblue whalereaches a maximum confirmed length of 29.9 meters (98 feet) and a weight of 173 tonnes (190 short tons), making it the largest animal ever known to have existed.[5][6][7]

There are approximately 89[8]living species split into twoparvorders:Odontocetior toothed whales (containingporpoises,dolphins,other predatory whales like thebelugaand thesperm whale,and the poorly understoodbeaked whales) and the filter feedingMysticetiorbaleenwhales (which includes species like theblue whale,thehumpback whaleand thebowhead whale). Despite their highly modified bodies and carnivorous lifestyle, genetic and fossil evidence places cetaceans as nested withineven-toed ungulates,most closely related tohippopotamuswithin the cladeWhippomorpha.

Cetaceans have been extensivelyhuntedfor their meat,blubberandoilby commercial operations. Although theInternational Whaling Commissionhas 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.[9]Cetaceans also face severe environmental hazards from underwaternoise pollution,entanglement in abandoned ropes and nets, collisions with ships, plastic and heavy metals build-up, to acceleratingclimate change,[10][11]but how much they are affected varies widely from species to species, from minimally in the case of thesouthern bottlenose whaleto thebaiji(Chinese river dolphin) which is considered to be functionally extinct due to human activity.[12]

Baleen whales and toothed whales

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Skull of theNorth Atlantic right whale(Mysticeti)
Skull of theorca(Odontoceti)

The two parvorders,baleen whales(Mysticeti) andtoothed whales(Odontoceti), are thought to have diverged around thirty-four million years ago.[13]

Baleen whales have bristles made ofkeratininstead ofteeth.The bristles filterkrilland other smallinvertebratesfrom seawater.Grey whalesfeed on bottom-dwelling mollusks.Rorqualfamily(balaenopterids) use throat pleats to expand their mouths to take in food and sieve out the water.Balaenids(right whalesandbowhead 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 othermarine 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 someorcas,feed on mammals, such aspinnipedsand other whales.

Toothed whales have well-developed senses – their eyesight and hearing are adapted for both air and water, and they have advancedsonar capabilitiesusing theirmelon.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 showsexual dimorphism,in which the males differ from the females, usually for purposes of sexual display or aggression.

Anatomy

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Dolphin 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).[14]

They have a streamlined shape, and their forelimbs areflippers.Almost all have adorsal finon their backs, but this can take on many forms, depending on the species. A few species, such as thebeluga whale,lack them. Both the flipper and the fin are for stabilization and steering in the water.[citation needed]

The male genitals and the mammary glands of females are sunken into the body.[15][16]The male genitals are attached to a vestigialpelvis.[17]

The body is wrapped in a thick layer of fat, known asblubber.This providesthermal insulationand gives cetaceans their smooth, streamlined body shape. In larger species, it can reach a thickness up to one-half meter (1.6 feet).[citation needed]

Sexual dimorphismevolved in many toothed whales. Sperm whales,narwhals,many members of thebeaked whalefamily, several species of theporpoisefamily, orcas,pilot whales,easternspinner dolphinsandnorthern right whale dolphinsshow this characteristic.[18]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.[18][19] Hind legs are not present in cetaceans, nor are any other external body attachments such as apinnaandhair.[20]

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Head profile of abeluga whale,featuring the large "melon" region
Close up of a feedinghumpback whale,showing baleen

Whales have an elongated head, especiallybaleen whales,due to the wide overhanging jaw. Bowhead whale plates can be 9 metres (30 ft) long. Their nostril(s) make up theblowhole,with one in toothed whales and two in baleen whales.[21]

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.[22]The teeth or baleen in the upper jaw sit exclusively on themaxilla.The braincase is concentrated through the nasal passage to the front and is correspondingly higher, with individual cranial bones that overlap.[citation needed]

In toothed whales, connective tissue exists in themelonas a head buckle. This is filled with air sacs and fat that aid in buoyancy andbiosonar.The sperm whale has a particularly pronounced melon; this is called thespermaceti organand contains the eponymousspermaceti,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.[citation needed]

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.[citation needed]

Thebaleenof 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 forplanktonand krill.[citation needed]

Brain

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Brain of thesperm whale,considered the largest brain in the world

Sperm whales have the largest brain mass of any animal on Earth, averaging 8,000 cm3(490 in3) and 7.8 kg (17 lb) in mature males.[23]Thebrain to body mass ratioin some odontocetes, such as belugas and narwhals, is second only to humans.[24]In some whales, however, it is less than half that of humans: 0.9% versus 2.1%.[citation needed]

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.[25]

Brain sizewas previously considered a major indicator ofintelligence.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.Allometricanalysis 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 wherekis 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.[26]Comparison of a particular animal's brain size with the expected brain size based on such an analysis provides anencephalization quotientthat can be used as an indication of animal intelligence.[27]

Theneocortexof many cetaceans is home to elongatedspindle neuronsthat, prior to 2019, were known only inhominids.[28]In humans, these cells are thought to be involved in social conduct, emotions, judgment and theory of mind.[29]Cetaceanspindle neuronsare found in areas of the brain homologous to where they are found in humans, suggesting they perform a similar function.[30]

Skeleton

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Diagram featuring the typical skeletal of a toothed whale (top) and a baleen whale (bottom)

The cetacean skeleton is largely made up ofcortical bone,which stabilizes the animal in the water. For this reason, the usual terrestrial compact bones, which are finely wovencancellous bone,are replaced with lighter and more elastic material. In many places, bone elements are replaced by cartilage and even fat, thereby improving theirhydrostaticqualities. The ear and the muzzle contain a bone shape that is exclusive to cetaceans with a high density, resemblingporcelain.This conducts sound better than other bones, thus aidingbiosonar.

The number ofvertebraethat make up the spine varies by species, ranging from forty to ninety-three. Thecervical 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 thethoracic vertebrae,ranging from nine to seventeen individual vertebrae. Thesternumis 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 thecaudal vertebraeis thechevron 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 theAmazon river dolphin.Thecollarboneis completely absent.

Fluke

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Humpback whale fluke

Cetaceans have acartilaginousfluke at the end of their tails that is used for propulsion. Theflukeis set horizontally on the body and used with vertical movements, unlike fish and ichthyosaurs, which have vertical tails which move horizontally.[31]

Physiology

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Circulation

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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.[32]

Respiration

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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 therespiratoryandcirculatory systemsis of particular importance for the life ofmarine 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 (330 ft), the lung tissue is almost completely compressed by the water pressure.

Abdominal organs

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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 thepylorus.Both are equipped with glands to help digestion. A bowel adjoins the stomachs, whose individual sections can only be distinguishedhistologically.Theliveris large and separate from thegall bladder.[33]

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.[34]Theurinary bladderis proportionally smaller in cetaceans than in land mammals.[35]Thetestesare located internally, without an externalscrotum.[36][37][38][39]The uterus isbicornuate.[37]

Senses

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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 goodbinocular visionforward and downward.Tear glandssecrete 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 functionalolfactory system.[40]Cetaceans are known to possess excellent hearing.[41]

At least one species, thetucuxior Guiana dolphin, is able to useelectroreceptionto sense prey.[42]

Ears

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Biosonar

The external ear has lost thepinna(visible ear), but still retains a narrowear canal.The three small bones orossiclesthat transmit sound within each ear aredense and compact,and differently shaped from those of land mammals. The semicircular canals are much smaller relative to body size than in other mammals.[43]

A bony structure of the middle and inner ear, theauditory 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 thephyseterids,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 inbats.[44]

Cetaceans use sound tocommunicate,using groans, moans, whistles, clicks or the 'singing' of the humpback whale.[42]

Echolocation

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Odontocetiare generally capable ofecholocation.[45]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.[46]

While differences in ear structure associated with echolocating abilities are found amongst Cetacea, cranial asymmetry has also been found to be a factor in the ability to produce sounds used in echolocation. Mysticeti, who don't have the ability to echolocate, possess general symmetry of the skull and facial region, while Odontoceti display a nasofacial asymmetry that is linked to their echolocating abilities.[47]Differences in the level of asymmetry also seem to correlate with differences in the types of sounds produced.[48]

Mysticeti have exceptionally thin, wide basilar membranes in theircochleaewithout stiffening agents, making their ears adapted for processing low toinfrasonicfrequencies.[49]

Chromosomes

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The initialkaryotypeincludes a set ofchromosomesfrom 2n = 44. They have four pairs of telocentric chromosomes (whosecentromeressit at one of thetelomeres), 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 thesperm whalesandpygmy sperm whales,which have 2n = 42.[50]

Ecology

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Range and habitat

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See also:Cetaceans of the Caribbean

Cetaceans are found in many aquatic habitats. While many marine species, such as theblue whale,thehumpback whaleand theorca,have a distribution area that includes nearly the entire ocean, some species occur only locally or in broken populations. These include thevaquita,which inhabits a small part of theGulf of CaliforniaandHector's dolphin,which lives in some coastal waters in New Zealand. Mostriver dolphinspecies live exclusively in fresh water.[51]

Many species inhabit specific latitudes, often in tropical or subtropical waters, such asBryde's whaleorRisso's dolphin.Others are found only in a specific body of water. Thesouthern right whale dolphinand thehourglass dolphinlive only in theSouthern Ocean.Thenarwhaland thebelugalive only in the Arctic Ocean.Sowerby's beaked whaleand theClymene dolphinexist only in the Atlantic and thePacific white-sided dolphinand thenorthern straight dolphinlive only in the North Pacific.[citation needed]

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 thesouthern right whale,North Pacific right whaleandNorth Atlantic right whale.[52]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.[citation needed]

Whale migration

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Many species of whales migrate on a latitudinal basis to move between seasonal habitats. For example, the gray whale migrates 10,000 miles (16,000 km) round trip. The journey begins at winter birthing grounds in warm lagoons along Baja California, and traverses 5,000–7,000 miles (8,000–11,300 km) of coastline to summer feeding grounds in the Bering, Chuckchi and Beaufort seas off the coast of Alaska.[53]

Behaviour

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Sleep

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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 exhibitunihemispheric 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.[54]

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.[55]

Diving

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See also:Physiology of underwater diving § Mammals

While diving, the animals reduce their oxygen consumption by lowering the heart activity and blood circulation; individual organs receive no oxygen during this time. Somerorqualscan dive for up to 40 minutes,sperm whalesbetween 60 and 90 minutes andbottlenose whalesfor two hours. Diving depths average about 100 m (330 ft). Species such as sperm whales can dive to 3,000 m (9,800 ft), although more commonly 1,200 metres (3,900 ft).[56][57]

Social relations

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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.[58]

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.[58]

Whale song

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Males in some baleen species communicate viawhale 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 the52-hertz whalethat sings at a higher frequency than other whales. Some individuals are capable of generating over 600 distinct sounds.[58]In baleen species such as humpbacks, blues and fins, male-specific song is believed to be used to attract and display fitness to females.[59]

Hunting

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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 collaborationbubble carpetsto herd krill or plankton into bait balls before lunging at them.[58]

Intelligence

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Main article:Cetacean intelligence
Bubble net feeding

Cetacea are known to teach, learn, cooperate, scheme and grieve.[60]

Smaller cetaceans, such as dolphins and porpoises, engage in complex play behavior, including such things as producing stable underwatertoroidalair-corevortexrings 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 thehelicalvortex 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.[61]

Killer whaleporpoising

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.[62]Humpback whales also display this behaviour.[citation needed]

Self-awarenessappears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such asmetacognitive reasoning(thinking about thinking) that humans exploit. Dolphins appear to possess self-awareness.[63]The most widely used test for self-awareness in animals is themirror 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.[64]

Critics claim that the results of these tests are susceptible to theClever Hanseffect. This test is much less definitive than when used forprimates.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.[64]

In 1995, Marten and Psarakos used video to test dolphin self-awareness.[65]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.[64]

Decision-making

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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).[66]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.[citation needed]

Life history

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Reproduction and brooding

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Most cetaceans sexually mature at seven to 10 years. An exception to this is theLa 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.[58]

For most species, reproduction is seasonal.Ovulationcoincides with malefertility.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 largelymonogamouswithin each reproductive period.[citation needed]

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 theplacenta,an organ that draws nutrients from the mother's bloodstream. Mammals without placentas either lay minuscule eggs (monotremes) or bear minuscule offspring (marsupials).[citation needed]

Cetaceans usually bear one calf. In the case of twins, one usually dies, because the mother cannot produce sufficient milk for both. In modern cetaceans, the fetus is usually positioned for a tail-first delivery. Contrary to popular belief, this is not to minimize the risk of drowning during delivery. More likely it has to do with the mechanics of birthing and the shape of the fetus.[67]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 terrestrialmammals.[citation needed]

Suckling

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Like other placental mammals, cetaceans give birth to well-developed calves and nurse them with milk from theirmammary 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.[58]

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.[citation needed]

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

This reproductive strategy provides a few offspring that have a high survival rate.[citation needed]

Lifespan

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Among cetaceans, whales are distinguished by an unusual longevity compared to other higher mammals. Some species, such as thebowhead whale(Balaena mysticetus), can reach over 200 years. Based on the annual rings of the bonyotic capsule,the age of the oldest known specimen is a male determined to be 211 years at the time of death.[68]

Death

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Main article:Whale fall

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 otherneriticbiodiversity hotspots, such ascold seepsandhydrothermal vents.[69]

Deterioration of whale carcasses happens through three stages. Initially, organisms such assharksandhagfishscavenge 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 ascrustaceansandpolychaetes,throughout a period of years. Finally, sulfophilic bacteria reduce the bones releasinghydrogen sulfideenabling the growth ofchemoautotrophicorganisms, 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.[69][70]

Disease

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Brucellosisaffects 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 causechronic disease,increasing the chance of failed birth andmiscarriages,male infertility,neurobrucellosis,cardiopathies,bone and skinlesions,strandingsand death. Until 2008, no case had ever been reported in porpoises, but isolated populations have an increased risk and consequentially a high mortality rate.[71]

Evolution

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Main article:Evolution of cetaceans

Fossil history

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Origins

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The direct ancestors of today's cetaceans are probably found within theDorudontidaewhose most famous member,Dorudon,lived at the same time asBasilosaurus.Both groups had already developed some of the typical anatomical features of today's whales, such as the fixed bulla, which replaces the mammalianeardrum,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.[72]

Transition from land to sea

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Fossil of aMaiacetus(red, beige skull) with fetus (blue, red teeth) shortly before the end of gestation[73]

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.[citation needed]

One of the oldest members of ancient cetaceans (Archaeoceti) isPakicetusfrom the Middle Eocene ofPakistan.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 hadcarnivorousdentition.[72]

The transition from land to sea dates to about 49 million years ago, with theAmbulocetus( "running whale" ), also discovered in Pakistan. It was up to 3 m (9.8 ft) 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.[74]The snout was elongated with overhead nostrils and eyes. The tail was strong and supported movement through water.Ambulocetusprobably lived in mangroves inbrackish waterand fed in theriparian zoneas a predator of fish and other vertebrates.[75]

Dating from about 45 million years ago are species such asIndocetus,Kutchicetus,RodhocetusandAndrewsiphius,all of which were adapted to life in water. The hind limbs of these species were regressed and their body shapes resemble modern whales.Protocetidaefamily memberRodhocetusis considered the first to be fully aquatic. The body was streamlined and delicate with extended hand and foot bones. The merged pelviclumbar spinewas 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 modernseal.[72]

Marine animals

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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 longBasilosaurus,sometimes calledZeuglodon.The transition from land to water was completed in about 10 million years. TheWadi Al-Hitan( "Whale Valley" ) in Egypt contains numerous skeletons ofBasilosaurus,as well as other marine vertebrates.[76]

External phylogeny

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Molecular biology,immunology,and fossils show that cetaceans are phylogenetically closely related with theeven-toed ungulates(Artiodactyla). Whales' direct lineage began in the earlyEocene,around 55.8 million years ago, with early artiodactyls.[72]Most molecular biological evidence suggests thathipposare the closest living relatives. Common anatomical features include similarities in themorphologyof the posteriormolars,and the bony ring on the temporal bone (bulla) and the involucre, a skull feature that was previously associated only with cetaceans.[72]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.[77][78][79]The most striking common feature is thetalus,a bone in the upper ankle. Early cetaceans,archaeocetes,show double castors, which occur only in even-toed ungulates. Corresponding findings are fromTethys Seadeposits in northern India and Pakistan. The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian plate.[citation needed]

Cetaceansevolved convergentlyasstreamlinedswimmers with fish andaquatic reptiles.

Molecular and morphological evidence suggests thatartiodactylsas traditionally defined areparaphyleticwith respect to cetaceans. Cetaceans are deeply nested within the artiodactyls; the two groups together form aclade,a natural group with a common ancestor, for which the nameCetartiodactylais 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 withinArtiodactylacan be represented in the followingcladogram:[80][81][82][83][84]

Artiodactyla

Tylopoda(camels)

Artiofabula

Suina(pigs)

Cetruminantia
Ruminantia(ruminants)

Tragulidae(mouse deer)

Pecora(horn bearers)

Cetancodonta/Whippomorpha

Hippopotamidae(hippopotamuses)

Cetacea(whales)

54 mya

Internal phylogeny

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Further information:List of cetaceansandList of extinct cetaceans

Within Cetacea, the twoparvordersarebaleen whales(Mysticeti) which owe their name to their baleen, andtoothed whales(Odontoceti), which have teeth shaped like cones, spades, pegs, or tusks, and can perceive their environment throughbiosonar.[citation needed]

Relationship of extinct and extant cetaceans[85]:
† Extinct taxa

The termswhaleanddolphinare informal:

The term 'great whales' covers those currently regulated by theInternational Whaling Commission:[86]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).[87]

Threats

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The primary threats to cetaceans come from people, both directly from whaling ordrive huntingand indirect threats from fishing and pollution.[88]

Whaling

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Main articles:Whaling,History of Whaling,andDolphin drive hunting
Methods ofwhaling
Japanese research ship whaling mother and calfminke whales.
AnAtlantic white-sided dolphincaught in adrive huntinHvalbaon theFaroe Islandsbeing taken away with a forklift.

Whaling is the practice of hunting whales, mainly baleen and sperm whales. This activity has gone on since theStone Age.[89]

In theMiddle Ages,reasons for whaling included theirmeat,oilusable as fuel and the jawbone, which was used in house construction. At the end of the Middle Ages, early whaling fleets aimed atbaleen whales,such asbowheads.In the 16th and 17th centuries, the Dutch fleet had about 300 whaling ships with 18,000 crewmen.[citation needed]

In the 18th and 19th centuries, baleen whales especially were hunted for theirbaleen,which was used as a replacement for wood, or in products requiring strength and flexibility such ascorsetsandcrinolineskirts. In addition, thespermacetifound in thesperm whalewas used as a machine lubricant and theambergrisas a material for pharmaceutical and perfume industries. In the second half of the 19th century, the explosiveharpoonwas invented, leading to a massive increase in the catch size.[citation needed]

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.[citation needed]

Most hunted whales are now threatened, with some great whale populations exploited to the brink of extinction. Atlantic and Koreangray whalepopulations were completely eradicated and theNorth Atlantic right whalepopulation fell to some 300–600. Theblue whalepopulation is estimated to be around 14,000.[citation needed]

The first efforts to protect whales came in 1931. Some particularly endangered species, such as thehumpback whale(which then numbered about 100 animals),[90]were placed under international protection and the first protected areas were established. In 1946, theInternational 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.[citation needed]

Whales caught 2010–2014, by country

The stocks of species such as humpback and blue whales have recovered, though they are still threatened. The United States Congress passed theMarine Mammal Protection Act of 1972sustain 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.[citation needed]

Aboriginal whalingis still permitted. About 1,200 pilot whales were taken in theFaroe Islandsin 2017,[91]and about 900narwhalsand 800belugasper 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.[92]Iceland and Norway do not recognize the ban and operate commercial whaling. Norway and Japan are committed to ending the ban.[citation needed]

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 theSolomon Islands,theFaroe Islands,Peruand Japan (the most well-known practitioner). Dolphins are mostly hunted for theirmeat,though some end up indolphinaria.Despite the controversy thousands of dolphins are caught in drive hunts each year.[citation needed]

Fishing

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See also:Cetacean bycatch
Dominoesmade of baleen

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 toriver dolphins.[citation needed]

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.[88]

Strandings

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Main article:Cetacean stranding

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 ofpilot whalesand sperm whales.Strandedcetaceans usually die, because their as much as 90 metric tons (99 short tons) body weight compresses their lungs or breaks their ribs. Smaller whales can die of heatstroke because of their thermal insulation.[citation needed]

Beached humpback whale

The causes are not clear. Possible reasons for mass beachings are:[88]

  • 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 pollutionby shipping traffic, seismic surveys and military sonar experiments

Since 2000, whale strandings frequently occurred following militarysonartesting. 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.[93]

Climate changeinfluences 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.[94]

Environmental hazards

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See also:Marine mammals and sonar

Worldwide, use of active sonar has been linked to about 50 marine mammal strandings between 1996 and 2006. In all of these occurrences, there were other contributing factors, such as unusual (steep and complex) underwater geography, limited egress routes, and a specific species of marine mammal—beaked whales—that are suspected to be more sensitive to sound than other marine mammals.

—Rear Admiral Lawrence Rice

Heavy metals, residues of many plant and insect venoms and plastic wasteflotsamare 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.[88]

Damage to theozone layerreduces 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 theNektonis, in addition to intensive exploitation, damaged by the radiation.[88]

Food supplies are also reduced long-term byocean acidificationdue to increased absorption of increased atmospheric carbon dioxide. The CO2reacts with water to formcarbonic acid,which reduces the construction of thecalcium carbonateskeletons of food supplies for zooplankton that baleen whales depend on.[88]

The military and resource extraction industries operate strongsonarand blasting operations. Marine seismic surveys use loud, low-frequency sound that show what is lying underneath the Earth's surface.[95]Vessel traffic also increases noise in the oceans. Such noise can disrupt cetacean behavior such as their use ofbiosonarfor orientation and communication. Severe instances can panic them, driving them to the surface. This leads to bubbles in blood gases and can causedecompression sickness.[96]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 kilometres (62 mi). Damage varies across frequency and species.[citation needed]

Relationship to humans

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Research history

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See also:Cryptid whaleandWhale § In myth, literature and art
A whale as depicted by Conrad Gesner, 1587, inHistoriae animalium

InAristotle'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.[97]His detailed descriptions were assimilated by the Romans, but mixed with a more accurate knowledge of the dolphins, as mentioned byPliny the Elderin hisNatural history.In the art of this and subsequent periods, dolphins are portrayed with a high-arched head (typical of porpoises) and a long snout. Theharbour porpoisewas one of the most accessible species for earlycetologists;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.[98]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.[99][100]

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.

— John Ray, 1671, the earliest description of cetacean airways

In the10th edition of Systema Naturae(1758), Swedish biologist and taxonomistCarl Linnaeusasserted that cetaceans were mammals and not fish. His groundbreaking binomial system formed the basis of modern whale classification.[citation needed]

Culture

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Cetaceans have played a role in human culture through history.[citation needed]

Prehistoric

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Stone Agepetroglyphs,such as those in Roddoy and Reppa (Norway), and theBangudae Petroglyphsin South Korea, depict them.[101][102]Whale bones were used for many purposes. In theNeolithicsettlement of Skara Brae onOrkneysauce pans were made from whale vertebrae.[citation needed]

Antiquity

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Destruction of Leviathan;engraving byGustave Doré,1865
Silver coin withTarasriding a dolphin

The whale was first mentioned inancient GreecebyHomer.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,Latinform of ballaena) for the female and, with an ironic characteristic style, musculus (Mouse) for the male.North Seawhales were called Physeter, which was meant for thesperm whalePhyster macrocephalus.Whales are described in particular by Aristotle,PlinyandAmbrose.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 theBibleespecially, the leviathan plays a role as asea monster.The essence, which features a giant crocodile or a dragon and a whale, was created according to the Bible by God[103]and should again be destroyed by him.[104][105]In theBook of Job,the leviathan is described in more detail.[106][107]

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

Dolphins are mentioned far more often than whales. Aristotle discusses the sacred animals of the Greeks in hisHistoria Animaliumand 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 theGangesand—erroneously—theNile.In the latter case it was equated with sharks and catfish. Supposedly they attacked evencrocodiles.

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 asArionofLesbosfromMethymna.

ConstellationCetus

Dolphins belong to the domain ofPoseidonand led him to his wifeAmphitrite.Dolphins are associated with other gods, such asApollo,DionysusandAphrodite.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 thezodiac.

Ancient art often included dolphin representations, including the CretanMinoans.Later they appeared on reliefs, gems, lamps, coins, mosaics and gravestones. A particularly popular representation is that of Arion orTarasriding on a dolphin. In earlyChristian art,the dolphin is a popular motif, at times used as a symbol ofChrist.

Middle Ages to the 19th century

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See also:History of WhalingandCryptid whales

St. Brendandescribed in his travel storyNavigatio Sancti Brendanian 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.[citation needed]

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 Gilmoredocumented 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.[citation needed]

Depiction of baleen whaling, 1840
Stranded sperm whale engraving, 1598

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.[citation needed]

Among the first whalers who described their experiences on whaling trips was CaptainWilliam Scoresbyfrom Great Britain, who published the bookNorthern Whale Fishery,describing the hunt for northern baleen whales. This was followed byThomas Beale,a British surgeon, in his bookSome observations on the natural history of the sperm whalein 1835; and Frederick Debell Bennett'sThe tale of a whale huntin 1840. Whales were described in narrative literature and paintings, most famously in the novelsMoby DickbyHerman MelvilleandTwenty Thousand Leagues Under the SeasbyJules Verne.[citation needed]

Baleen was used to make vessel components such as the bottom of a bucket in the Scottish National Museum. TheNorsemencrafted ornamented plates from baleen, sometimes interpreted asironingboards.[citation needed]

In the Canadian Arctic (east coast) in Punuk andThuleculture (1000–1600 C.E.),[108]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.[109]

Modern culture

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Further information:Whale § Interactions with humans
Sea World show featuringbottlenose dolphinsandfalse killer whales

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 seriesFlipperwas a bottle-nose dolphin. The TV seriesSeaQuest DSV(1993–1996), the moviesFree WillyandStar Trek IV: The Voyage Home,and the book seriesThe Hitchhiker's Guide to the GalaxybyDouglas Adamsare examples.[110]

The study ofwhale songalso produced a popular album,Songs of the Humpback Whale.[citation needed]

Captivity

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

Belugas

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Main article:Beluga whale § Captivity

Beluga whaleswere the first whales to be kept in captivity. Other species were too rare, too shy or too big. The first was shown atBarnum's MuseuminNew York Cityin 1861.[111]For most of the 20th century, Canada was the predominant source.[112]They were taken from theSt. Lawrence Riverestuary until the late 1960s, after which they were predominantly taken from theChurchill Riverestuary until capture was banned in 1992.[112]Russia then became the largest provider.[112]Belugas are caught in theAmur Daryadelta and their eastern coast and are transported domestically to aquaria ordolphinariainMoscow,St. PetersburgandSochi,or exported to countries such as Canada.[112]They have not been domesticated.[113]

As of 2006, 30 belugas lived in Canada and 28 in the United States. 42 deaths in captivity had been reported.[112]A single specimen can reportedly fetch up to US$100,000 (£64,160). The beluga's popularity is due to its unique color and itsfacial 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.[114]

Orcas

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Main article:Captive killer whales
See also:SeaWorld § Criticism and resulting impact,andIncidents at SeaWorld parks
Ulises the orca, 2009

The orca'sintelligence,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.[115]

Organizations such asWorld Animal Protectionand theWhale and Dolphin Conservationcampaign against the practice of keeping them in captivity.[citation needed]

In captivity, they often develop pathologies, such as thedorsal fincollapse 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.[116]

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 kilometres (100 mi) in a day and critics say the animals are too big and intelligent to be suitable for captivity.[117]Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result ofstress.[118]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-fourcaptive orcasbeing held in aquaria as of 2012.[119]

Each country has its own tank requirements; in the US, the minimum enclosure size is set by theCode of Federal Regulations,9 CFR E § 3.104, under theSpecifications for the Humane Handling, Care, Treatment and Transportation of Marine Mammals.[120]

Dawn Brancheau doing a show four years beforethe incident

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 movieBlackfish,which documents the history ofTilikum,an orca at SeaWorld Orlando, who had been involved in the deaths of three people.[121]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.[122]

Others

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SeaWorldpilot whalewith trainers

Dolphins and porpoises are kept in captivity.Bottlenose dolphinsare 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 arespotted dolphins,false killer whalesandcommon dolphins,Commerson's dolphins,as well asrough-toothed dolphins,but all in much lower numbers. There are also fewer than tenpilot whales,Amazon river dolphins,Risso's dolphins,spinner dolphins,ortucuxiin captivity. Two unusual and rarehybriddolphins, known aswolphins,are kept atSea Life ParkinHawaii,which is a cross between a bottlenose dolphin and afalse killer whale.Also, twocommon/bottlenose hybrids reside in captivity atDiscovery CoveandSeaWorld San Diego.[citation needed]

In repeated attempts in the 1960s and 1970s,narwhalskept in captivity died within months. A breeding pair ofpygmy right whaleswere retained in a netted area. They were eventually released in South Africa. In 1971, SeaWorld captured a California gray whale calf in Mexico atScammon's Lagoon.The calf, later named Gigi, was separated from her mother using a form of lasso attached to her flukes. Gigi was displayed atSeaWorld San Diegofor 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, anothergray whalecalf, was kept atSeaWorld San Diego.JJ was an orphaned calf that beached itself in April 1997 and was transported two miles to SeaWorld. The 680 kilograms (1,500 lb) calf was a popular attraction and behaved normally, despite separation from his mother. A year later, the then 8,164.7 kilograms (18,000 lb) whale though smaller than average, was too big to keep in captivity, and was released on April 1, 1998. A captiveAmazon river dolphinhoused atAcuario de Valenciais the only trainedriver dolphinin captivity.[123][124]

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.[citation needed]

References

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