(see text for families) Cetacea (/sɪˈteɪʃə/; from Latin cetus 'whale', from Ancient Greek κῆτος (kêtos) 'huge fish, sea monster')[3] is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises.

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.

For example, the blue whale reaches 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.

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.

[9] 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,[10][11] 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.

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.

[15][16] 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.

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.

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.

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

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.

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.

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.

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.

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.

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.

Modern nomenclature divides Artiodactyla (or Cetartiodactyla) into four subordinate taxa: camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus whales (Whippomorpha).

[citation needed] 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.

[106] 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.

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.

[108][109] 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.

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.

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.

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.

[125] 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.

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.

[127] 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.

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.

[132] 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.