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{{Short description|Infraorder of mammals}}
{{Automatic taxobox
| fossil_range      = {{geological range|53.5|0|earliest=|latest=|PS=}}Early [[Eocene]] – Present
| image = {{Multiple image
| perrow            = 2/2/2
| total_width       = 275
| caption_align = center
| image1            =  Mother and baby sperm whale.jpg
| caption1          = [[Sperm whale]]
| image2            = Humpback stellwagen edit.jpg
| caption2          = [[Humpback whale]]
| image3            = Killerwhales jumping.jpg
| caption3          = [[Orca]]
| image4            = Eubalaena glacialis with calf.jpg
| caption4          = [[North Atlantic right whale]]
| image5            = Amazonas-Flussdelfin Orinoko3.jpg
| caption5          = [[Amazon river dolphin]] 
| image6            = Marsvin (Phocoena phocoena) light.jpg
| caption6          = [[Harbor porpoise]]
| border            = infobox
}}
| taxon             = Cetacea
| authority         = [[Mathurin Jacques Brisson|Brisson]], 1762
| subdivision_ranks = Subgroups
| subdivision       =
*†[[Pakicetidae]]
*†[[Ambulocetidae]]
*†[[Remingtonocetidae]]
*†[[Protocetidae]] ([[paraphyletic]])
*'''Pelagiceti''' <small>Uhen, 2008</small><ref>{{cite journal |author=Uhen, M.D. |year=2008 |title=New protocetid whales from Alabama and Mississippi, and a new Cetacean clade, Pelagiceti |journal=[[Journal of Vertebrate Paleontology]] |volume=28 |issue=3 |pages=589–593 |doi=10.1671/0272-4634(2008)28[589:NPWFAA]2.0.CO;2 |jstor=20490986 |s2cid=86326007}}</ref>
**†[[Basilosauridae]] (paraphyletic)
**†[[Kekenodontidae]]
**'''Neoceti''' <small>Fordyce & de Muizon, 2001</small><ref>{{cite conference |author1=Fordyce, E. |author2=de Muizon, C. |year=2001 |title=Evolutionary history of the cetaceans: a review |editor1=Mazin, J.-M. |editor2=de Buffrénil, V. |book-title=Secondary Adaptations of Tetrapods to Life in the Water: Proceedings of the international meeting, Poitiers, 1996 |publisher=[[:de:Verlag Dr. Friedrich Pfeil|Verlag Dr. Friedrich Pfeil]] |place=München, Germany |pages=169–233 |isbn=3-931516-88-1 |lccn=2002550356 |oclc=52121251 |ol=20591860M}}</ref>
***[[Mysticeti]]
***[[Odontoceti]]
''([[#Classification|see text]] for families)''
| diversity        = Around 94 species
| diversity_link   = List of cetaceans
}}

'''Cetacea''' ({{IPAc-en|s|ɪ|ˈ|t|eɪ|ʃ|ə}}; {{etymology|la|cetus|[[whale]]}}, {{etymology|grc|''κῆτος'' ({{grc-transl|[[Cetus (mythology)|κῆτος]]}})|[[list of largest fish|huge fish]], [[sea monster]]}})<ref>{{cite journal |url=http://aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/2001/AquaticMammals_27-02/27-02_Ranneft.PDF |archive-url=https://web.archive.org/web/20160327235028/http://www.aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/2001/AquaticMammals_27-02/27-02_Ranneft.PDF |archive-date=2016-03-27 |url-status=live |first1=D.|last1=M. Raneft |first2=H.|last2=Eaker |first3=R.|last3=W. Davis |year=2001 |title=A guide to the pronunciation and meaning of cetacean taxonomic names |journal=Aquatic Mammals |volume=27 |issue=2 |page=185}}</ref> is an [[infraorder]] of [[aquatic mammal]]s belonging to the order [[Artiodactyla]] that includes [[whale]]s, [[dolphin]]s 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.<ref>{{cite journal|last=E. Fish|first=Frank|year=2002|title=Balancing Requirements for Stability and Maneuverability in Cetaceans|journal=Integrative and Comparative Biology|volume=42|issue=1|pages=85–93|doi=10.1093/icb/42.1.85|pmid=21708697|s2cid=25036870|doi-access=free}}</ref>

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 [[cetacean intelligence|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 {{convert|29.9|m|ft|abbr=off|sp=us}} and a weight of 173 tonnes (190 short tons), making it the largest animal ever known to have existed.<ref name="Wood">Wood, Gerald ''The Guinness Book of Animal Facts and Feats'' (1983) {{ISBN|978-0-85112-235-9}}</ref><ref>{{cite web|last1=Davies|first1=Ella|date=2016-04-20|title=The longest animal alive may be one you never thought of|url=http://www.bbc.com/earth/story/20160420-the-longest-animal-alive-may-not-be-the-blue-whale|access-date=2018-02-14|website=BBC Earth|language=en}}</ref><ref>{{cite web|title=Largest mammal|url=http://www.guinnessworldrecords.com/world-records/largest-mammal|website=Guinness World Records}}</ref>

There are approximately 89<ref>{{Cite web|last=Perrin|first=W.F.|date=2020|title=World Cetacea Database|url=http://www.marinespecies.org/cetacea|access-date=2020-12-12|website=marinespecies.org}}</ref> living species split into two [[parvorder]]s: [[Odontoceti]] or toothed whales (containing [[porpoise]]s, [[dolphin]]s, other predatory whales like the  [[beluga whale|beluga]] and the [[sperm whale]], and the poorly understood [[beaked whale]]s) 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 ungulate]]s, most closely related to [[Hippopotamidae|hippopotamus]] within the clade [[Whippomorpha]].

Cetaceans have been extensively [[whaling|hunted]] for their meat, [[blubber]] and [[Whale oil|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.<ref>{{cite journal |last1=Notarbartolo di Sciara |first1=G. |last2=Briand |first2=F. |date=2004 |title=Investigating the Roles of Cetaceans in Marine Ecosystems - An overview |journal=CIESM Workshop Monographs |volume=25 |pages=1–15}}[https://www.researchgate.net/publication/289532221]</ref> Cetaceans also face severe environmental hazards from underwater [[Noise pollution#Wildlife|noise pollution]], entanglement in abandoned ropes and nets, collisions with ships, plastic and heavy metals build-up, to accelerating [[climate change]],<ref name="wdcs2">{{cite book|author=Cara E. Miller|url=http://uk.whales.org/sites/default/files/whales-and-dolphins-in-the-pacific-islands.pdf|title=Current State of Knowledge of Cetacean Threats, Diversity, and Habitats in the Pacific Islands Region|publisher=Whale and Dolphin Conservation Society|year=2007|isbn=978-0-646-47224-9|access-date=5 September 2015|archive-url=https://web.archive.org/web/20150908003502/http://uk.whales.org/sites/default/files/whales-and-dolphins-in-the-pacific-islands.pdf|archive-date=8 September 2015|url-status=dead}}</ref><ref name="Nowacek2">{{cite journal|last1=Nowacek|first1=Douglas|last2=Donovan|first2=Greg|last3=Gailey|first3=Glenn|last4=Racca|first4=Roberto|last5=Reeves|first5=Randall|last6=Vedenev|first6=Alexander|last7=Weller|first7=David|last8=Southall|first8=Brandon|date=2013|title=Responsible Practices for Minimizing and Monitoring Environmental Impacts of Marine Seismic Surveys with an Emphasis on Marine Mammal|journal=Aquatic Mammals|volume=39|issue=4|pages=356–377|doi=10.1578/am.39.4.2013.356}}<!--|access-date=April 2, 2016--></ref> 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.<ref name="baiji">{{cite news|last=Lovgren|first=Stefan|date=December 14, 2006|title=China's Rare River Dolphin Now Extinct, Experts Announce|work=National Geographic News|publisher=[[National Geographic Society]]|location=Washington, D.C.|url=http://news.nationalgeographic.com/news/2006/12/061214-dolphin-extinct.html|archive-url=https://web.archive.org/web/20061218185108/http://news.nationalgeographic.com/news/2006/12/061214-dolphin-extinct.html|url-status=dead|archive-date=December 18, 2006|access-date=2015-10-18}}</ref>
== 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]]).<ref name="autogenerated2">{{cite news |last1=Groves |last2=Colin |first3=Peter |last3=Grubb |title=Ungulate taxonomy |newspaper=JHU Press |year=2011}} {{page needed|date=April 2018}}</ref>

The male genitals and the mammary glands of females are sunken into the body.<ref name="Thewissen2013">{{cite book |author=[[Hans Thewissen|Thewissen]], J.G.M.  |title=The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea |url=https://books.google.com/books?id=8QPyBwAAQBAJ&q=(penis+OR+genital)&pg=PA383 |date=11 November 2013 |publisher=Springer  |isbn=978-1-4899-0159-0 |pages=383–}}</ref><ref name="Miller2007">{{cite book |author=Miller, Debra Lee |title=Reproductive Biology and Phylogeny of Cetacea: Whales, Porpoises and Dolphins |url=https://books.google.com/books?id=sD3NBQAAQBAJ |date=2007 |publisher=CRC Press |isbn=978-1-4398-4257-7}}</ref> The male genitals are attached to a vestigial [[pelvis]].<ref>{{Cite book |last=Tinker |first=Spencer Wilkie |url=https://books.google.com/books?id=ASIVAAAAIAAJ&pg=PA93 |title=Whales of the World |date=1988-01-01 |publisher=Brill Archive |isbn=978-0-935848-47-2 |language=en}}</ref>

[[Sexual dimorphism]] evolved in many toothed whales. Sperm whales, [[narwhal]]s, many members of the [[beaked whale]] family, several species of the [[porpoise]] family, orcas, [[pilot whale]]s, eastern [[spinner dolphin]]s and [[northern right whale dolphin]]s show this characteristic.<ref name="ReferenceA">{{cite journal |last1=Dines |first1=James |last2=Mesnick |first2=Sarah |last3=Ralls |first3=Katherine |last4=May-Collado |first4=Laura |last5=Agnarsson |first5=Ingi |last6=Dean |first6=Matthew |title=A trade-off between precopulatory and postcopulatory trait investment in male cetaceans |journal=Evolution |date=2015 |volume=69 |issue=6 |pages=1560–1572 |doi=10.1111/evo.12676 |pmid=25929734|s2cid=18292677 }}</ref> 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.<ref name="ReferenceA" /><ref>{{cite journal |last1=Dalebout |first1=Merel |last2=Steel |first2=Debbie |last3=Baker |first3=Scott |title=Phylogeny of the Beaked Whale Genus ''Mesoplodon'' (Ziphiidae: Cetacea) Revealed by Nuclear Introns: Implications for the Evolution of Male Tusks|journal=Systematic Biology |date=2008 |volume=57 |issue=6 |pages=857–875 |doi=10.1080/10635150802559257 |pmid=19085329|s2cid=205729032 |doi-access=free }}</ref>
Hind legs are not present in cetaceans, nor are any other external body attachments such as a [[pinna (anatomy)|pinna]] and [[hair]].<ref>{{Cite web|url=http://www.eurekalert.org/pub_releases/2006-05/uof-haw052206.php|title=How ancient whales lost their legs, got sleek and conquered the oceans|date=2006-05-22|website=EurekAlert|publisher=University of Florida|access-date=2016-03-20}}</ref>

=== Head ===
Whales have an elongated head, especially [[baleen whales]], due to the wide overhanging jaw. Bowhead whale plates can be {{convert|9|m|ft}} long. Their nostril(s) make up the [[Blowhole (anatomy)|blowhole]], with one in toothed whales and two in baleen whales.<ref>{{Cite journal|last1=Buono|first1=Mónica R.|last2=Fernández|first2=Marta S.|last3=Fordyce|first3=R. Ewan|last4=Reidenberg|first4=Joy S.|date=2015|title=Anatomy of nasal complex in the southern right whale, Eubalaena australis (Cetacea, Mysticeti)|journal=Journal of Anatomy|language=en|volume=226|issue=1|pages=81–92|doi=10.1111/joa.12250|issn=1469-7580|pmc=4313901|pmid=25440939}}</ref>

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.<ref name="Klima1999">{{cite book|author=Milan Klima |title=Development of the Cetacean Nasal Skull|url=https://books.google.com/books?id=kOHAz9SCL54C |date=29 January 1999 |publisher=Springer |isbn=978-3-540-64996-0 }} {{page needed|date=April 2018}}</ref>

==== Brain ====
Sperm whales have the largest brain mass of any animal on Earth, averaging {{convert|8,000|cm3|in3|abbr=on}} and {{convert|7.8|kg|lb|abbr=on}} in mature males.<ref>{{cite web | url=http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/spermwhale.htm |title=Sperm Whales brain size| publisher=NOAA Fisheries – Office of Protected Resources |access-date=9 August 2015}}</ref> The [[brain to body mass ratio]] in some odontocetes, such as belugas and narwhals, is second only to humans.<ref>{{cite magazine |url=http://blogs.scientificamerican.com/news-blog/are-whales-smarter-than-we-are/ |author=Fields, R. Douglas |title=Are whales smarter than we are? | magazine=Scientific American |access-date=9 August 2015}}</ref> 

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.<ref name="Gerussi Graïc Peruffo Behroozi 2023 pp. 1963–1976">{{cite journal |last1=Gerussi |first1=Tommaso |last2=Graïc |first2=Jean-Marie |last3=Peruffo |first3=Antonella |last4=Behroozi |first4=Mehdi |last5=Schlaffke |first5=Lara |last6=Huggenberger |first6=Stefan |last7=Güntürkün |first7=Onur |last8=Cozzi |first8=Bruno |date=2023 |title=The prefrontal cortex of the bottlenose dolphin (Tursiops truncatus Montagu, 1821): a tractography study and comparison with the human |journal=Brain Structure and Function |volume=228 |issue=8 |pages=1963–1976 |doi=10.1007/s00429-023-02699-8 |issn=1863-2661 |pmc=10517040 |pmid=37660322}}</ref> 

[[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.
<big>''m''<sub>''brain''</sub> ∝ (''m''<sub>''body''</sub>)<sup>''k''</sup></big>
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.<ref>{{cite web |url=http://pages.ucsd.edu/~jmoore/courses/allometry/allometry.html |title=Allometry |first=Jim |last=Moore| publisher=University of California San Diego |access-date=9 August 2015}}</ref> 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.<ref>{{cite book|last1=Pontarotti|first1=Pierre|title=Evolutionary Biology: Convergent Evolution, Evolution of Complex Traits|date=2016|publisher=Springer|isbn=978-3-319-41324-2|pages=74}}</ref> 

The [[neocortex]] of many cetaceans is home to elongated [[spindle neurons]] that, prior to 2019, were known only in [[Hominidae|hominids]].<ref>{{cite journal |author=Watson, K.K. |title=Dendritic architecture of the Von Economo neurons |journal=Neuroscience |volume=141 |issue=3 |pages=1107–1112 |year=2006 |doi=10.1016/j.neuroscience.2006.04.084 |last2=Jones |first2=T. K. |last3=Allman |first3=J. M. |pmid=16797136|s2cid=7745280 }}<!--|access-date=29 August 2015--></ref> In humans, these cells are thought to be involved in social conduct, emotions, judgment and theory of mind.<ref>{{cite journal |author=Allman, John M. |title=Intuition and autism: a possible role for Von Economo neurons |journal=Trends Cogn Sci |volume=9 |issue=8 |pages=367–373 |year=2005 |doi=10.1016/j.tics.2005.06.008 |last2=Watson |first2=Karli K. |last3=Tetreault |first3=Nicole A. |last4=Hakeem |first4=Atiya Y. |pmid=16002323|s2cid=14850316 }}</ref> Cetacean [[spindle neuron]]s are found in areas of the brain homologous to where they are found in humans, suggesting they perform a similar function.<ref>{{cite journal |author=Hof, Patrick R. |title=Structure of the cerebral cortex of the humpback whale, ''Megaptera novaeangliae'' (Cetacea, Mysticeti, Balaenopteridae) |journal=The Anatomical Record |volume=290 |issue=1 |pages=1–31 |year=2007 |doi=10.1002/ar.20407 |last2=Van Der Gucht |first2=Estel |pmid=17441195|s2cid=15460266 |doi-access=free }}<!--|access-date=29 August 2015--></ref>

=== Fluke ===
<!--[[Fluke (tail)]] redirects directly here.-->
Cetaceans have a [[cartilage|cartilaginous]] fluke at the end of their tails that is used for propulsion. The [[Tail fluke|fluke]] is set horizontally on the body and used with vertical movements, unlike fish and ichthyosaurs, which have vertical tails which move horizontally.<ref>{{Cite web|title=Why do whale and dolphin tails go up and down?|url=https://us.whales.org/whales-dolphins/why-do-whale-and-dolphin-tails-go-up-and-down/|access-date=2021-12-23|website=Whale & Dolphin Conservation USA|language=en-US}}</ref>

== Physiology ==

=== 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 [[Histology|histologically]]. The [[liver]] is large and separate from the [[gall bladder]].<ref>{{cite book|author1=C. Edward Stevens|author2=Ian D. Hume|title=Comparative Physiology of the Vertebrate Digestive System|publisher=University of Cambridge|year=1995|access-date=5 September 2015|page=51|isbn=978-0-521-44418-7|url= https://books.google.com/books?id=DZuAsci2apAC&q=cetacean+digestive+system&pg=PR15}}</ref>

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.<ref>{{cite journal|author=Clifford A. Hui|title=Seawater Consumption and Water Flux in the Common Dolphin ''Delphinus delphis''|journal=Chicago Journals|volume=54|issue=4|pages=430–440|year=1981|jstor= 30155836}}</ref> The [[urinary bladder]] is proportionally smaller in cetaceans than in land mammals.<ref name="Hunter2015">{{cite book|url=https://books.google.com/books?id=TNAIBwAAQBAJ&pg=PR35|title=The Works of John Hunter, F.R.S.|date=26 March 2015|publisher=Cambridge University|isbn=978-1-108-07960-0|page=35|author=John Hunter}}</ref> The [[testicles|testes]] are located internally, without an external [[scrotum]].<ref name="WürsigThewissen2017">{{cite book|author1=Bernd Würsig|author2=J.G.M. Thewissen|author3=Kit M. Kovacs|title=Encyclopedia of Marine Mammals|url=https://books.google.com/books?id=T3zGDgAAQBAJ&q=testes+OR+testis+OR+testicles|date=27 November 2017|publisher=Elsevier Science|isbn=978-0-12-804381-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161516/https://books.google.com/books?id=T3zGDgAAQBAJ&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="rommel2007">{{cite book | last1 = Rommel | first1 = S.A. | first2 = D.A. | last2 = Pabst | first3 = W.A. | last3 = McLellan | chapter = Functional anatomy of the cetacean reproductive system, with comparisons to the domestic dog | title = Reproductive Biology and Phylogeny of Cetacea: Whales, Porpoises and Dolphins | date = 2007 | pages = 127–145 | editor-last = Miller | editor-first = D.L. | chapter-url = https://www.researchgate.net/publication/286334625 | doi = 10.1201/b11001| isbn = 9780429063626 }}</ref><ref name="rommel1998">{{cite magazine | last1 = Rommel | first1 = S.A. | first2 = D.A. | last2 = Pabst | first3 = W.A. | last3 = McLellan | title = Reproductive Thermoregulation in Marine Mammals | journal = American Scientist | volume = 86 | number = 5 | date = 1998 | pages = 440–448 | jstor = 27857097 | url = https://uncw.edu/mmsp/documents/rommeletal1998reproductivethermoregulation.pdf | url-status = live | archive-date = 22 November 2021 | archive-url = https://web.archive.org/web/20211122202231/https://uncw.edu/mmsp/documents/rommeletal1998reproductivethermoregulation.pdf}}</ref><ref name="Pabst, D. Ann 1998">{{cite book | last1 = Pabst | first1 = D.A. | last2 = Sentiel | first2 = A.R | first3 = W.A. | last3 = McLellan | chapter = Evolution of thermoregulatory function in cetacean reproductive systems | title = The Emergence of Whales | publisher = Springer US | date = 1998 | pages = 379–397 | series = Advances in Vertebrate Paleobiology | doi = 10.1007/978-1-4899-0159-0_13 | isbn = 978-1-4899-0161-3 | editor-last = Thewissen | editor-first = J.G.M.}}</ref> The uterus is [[Uterus#Other_animals|bicornuate]].<ref name="rommel2007" />

=== Ears ===
The external ear has lost the [[Pinna (anatomy)|pinna]] (visible ear), but still retains a narrow [[ear canal]]. The three small bones or [[ossicles]] that transmit sound within each ear are [[Pachyosteosclerosis|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.<ref name="Thewissen-Hearing">{{cite book |last=[[Hans Thewissen|Thewissen]] |first=J. G. M. |title=Encyclopedia of Marine Mammals |publisher=Academic Press |year=2002 |isbn=978-0-12-551340-1 |editor1-last=Perrin |editor1-first=William R. |pages=[https://archive.org/details/encyclopediaofma2002unse/page/570 570–572] |chapter=Hearing |editor2-last=Wiirsig |editor2-first=Bernd |editor3-last=[[Hans Thewissen|Thewissen]] |editor3-first=J. G. M. |chapter-url=https://archive.org/details/encyclopediaofma2002unse/page/570}}</ref>

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 [[Physeteroidea|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 [[bat]]s.<ref>{{cite book |last=Ketten |first=Darlene R. |title=The Evolutionary Biology of Hearing |publisher=Springer |year=1992 |editor1-last=Webster |editor1-first=Douglas B. |pages=717–750 |chapter=The Marine Mammal Ear: Specializations for Aquatic Audition and Echolocation |editor2-last=Fay |editor2-first=Richard R. |editor3-last=Popper |editor3-first=Arthur N. |chapter-url=https://www.researchgate.net/publication/230691464}} Pages 725–727 used here.</ref>

Cetaceans use sound to [[communication|communicate]], using groans, moans, whistles, clicks or the 'singing' of the humpback whale.<ref name=":0" />

=== 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. [[Lacrimal gland|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]].<ref>{{Cite journal|last1=Godfrey|first1=Stephen J.|last2=Geisler|first2=Jonathan|last3=Fitzgerald|first3=Erich M. G.|date=2013|title=On the Olfactory Anatomy in an Archaic Whale (Protocetidae, Cetacea) and the Minke Whale Balaenoptera acutorostrata (Balaenopteridae, Cetacea)|journal=The Anatomical Record|language=en|volume=296|issue=2|pages=257–272|doi=10.1002/ar.22637|pmid=23233318|s2cid=25260840|issn=1932-8494|doi-access=free}}</ref> Cetaceans are known to possess excellent hearing.<ref>{{cite web |last1=Mead |first1=James |title=Cetacea |url=https://school.eb.com/levels/high/article/cetacean/105982 |website=Britannica School High |publisher=Encyclopædia Britannica, Inc. |access-date=3 June 2019}}</ref>

At least one species, the [[tucuxi]] or Guiana dolphin, is able to use [[electroreception]] to sense prey.<ref name=":0">{{cite web |last=Morell |first=Virginia |title=Guiana Dolphins Can Use Electric Signals to Locate Prey |work=Science |publisher=American Association for the Advancement of Science (AAAS) |date=July 2011 |url=http://news.sciencemag.org/sciencenow/2011/07/guiana-dolphins-can-use-electric.html?ref=hp |url-status=dead |archive-url=https://web.archive.org/web/20130530040210/http://news.sciencemag.org/sciencenow/2011/07/guiana-dolphins-can-use-electric.html?ref=hp |archive-date=2013-05-30 }}</ref>

==== Echolocation ====
[[Odontoceti]] are generally capable of [[Animal echolocation|echolocation]].<ref name="emm">{{cite book |editor1-last=Perrin |editor1-first=William F. |editor2-last=Wursig |editor2-first=Bernd |editor3-last=[[Hans Thewissen|Thewissen]]|editor3-first=J. G. M. |title=Encyclopedia of Marine Mammals |last=Hooker |first=Sascha K. |edition=2 |publisher=Academic Press |page=1176 |year=2009 |isbn=978-0-12-373553-9 |url=https://books.google.com/books?id=2rkHQpToi9sC&q=Encyclopedia%20of%20Marine%20Mammals&pg=PP1}}</ref> 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.<ref name="Obaldia2015Estimating">{{cite journal |author = de Obaldia, C., Simkus, G. & and Zölzer, U. | title = Estimating the number of sperm whale (Physeter macrocephalus) individuals based on grouping of corresponding clicks
 | journal = 41. Jahrestagung für Akustik (DAGA 2015), Nürnberg | year = 2015 | doi = 10.13140/RG.2.1.3764.9765 | url= https://www.researchgate.net/publication/277009691 }}</ref>

Mysticeti have exceptionally thin, wide basilar membranes in their [[cochlea]]e without stiffening agents, making their ears adapted for processing low to [[infrasonic]] frequencies.<ref>{{cite journal | last = Ketten | first = Darlene R. | title = Structure and function in whale ears | year = 1997 | journal = The International Journal of Animal Sound and Its Recording | volume = 8 | issue = 1–2 | pages = 103–135 | url = http://csi.whoi.edu/sites/default/files/literature/Full%20Text_29.pdf | doi = 10.1080/09524622.1997.9753356 | bibcode = 1997Bioac...8..103K | access-date = 2013-12-21 | archive-date = 2014-08-01 | archive-url = https://web.archive.org/web/20140801161859/http://csi.whoi.edu/sites/default/files/literature/Full%20Text_29.pdf | url-status = dead }}</ref>

=== Chromosomes ===
The initial [[karyotype]] includes a set of [[chromosome]]s 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 whale]]s and [[pygmy sperm whale]]s, which have 2n = 42.<ref>{{cite journal|author=Ulfur Anarson|title=Comparative chromosome studies in Cetacea|journal=Institute of Genetics|year=1974|volume=77|issue=1|doi=10.1111/j.1601-5223.1974.tb01351.x|pages=1–36|pmid=4137586|doi-access=free}}</ref>

== Ecology ==

=== Range and habitat ===
{{see also|Cetaceans of the Caribbean}}
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. Most [[river dolphin]] species live exclusively in fresh water.<ref name="Cassens2000">{{cite journal|last1=Cassens|first1=I.|last2=Vicario|first2=S.|last3=Waddell|first3=V.G.|last4=Balchowsky|first4=H.|last5=Van Belle|first5=D.|last6=Ding|first6=W.|last7=Fan|first7=C.|last8=Mohan|first8=R.S.|last9=Simões-Lopes|first9=P.C.|last10=Bastida|first10=R.|last11=Meyer|first11=A.|last12=Stanhope|first12=M.J.|last13=Milinkovitch|first13=M.C.|title=Independent adaptation to riverine habitats allowed survival of ancient cetacean lineages|journal=Proceedings of the National Academy of Sciences of the United States of America|date=2000|volume=97|issue=21|pages=11343–11347|doi=10.1073/pnas.97.21.11343|pmid=11027333|pmc=17202|bibcode=2000PNAS...9711343C|doi-access=free}}</ref>

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]].<ref>{{cite journal|author=AR Hoelzel|title=Genetic structure of cetacean populations in sympatry, parapatry, and mixed assemblages: implications for conservation policy|journal=Journal of Heredity|year=1998|doi=10.1093/jhered/89.5.451|volume=89|issue=5|pages=451–458|doi-access=free}}</ref>

==== Migration ====
Many species of whales migrate on a latitudinal basis to move between seasonal habitats. For example, the gray whale migrates {{convert|10,000|mi}} round trip. The journey begins at winter birthing grounds in warm lagoons along Baja California, and traverses {{convert|5,000|-|7,000|mi}} of coastline to summer feeding grounds in the Bering, Chuckchi and Beaufort seas off the coast of Alaska.<ref>{{cite web |url=https://journeynorth.org/tm/gwhale/annual/map.html |title=Gray Whale Migration |website=journeynorth.org |access-date=3 July 2021 |url-status=live|archive-url=https://web.archive.org/web/20190609231527/https://journeynorth.org/tm/gwhale/annual/map.html |archive-date=2019-06-09 }}</ref>

== Behaviour ==

=== 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.<ref>{{cite journal | vauthors = Sekiguchi Y, Arai K, Kohshima S | title = Sleep behaviour: sleep in continuously active dolphins | journal = Nature | volume = 441 | issue = 7096 | pages = E9-10; discussion E11 | date = June 2006 | pmid = 16791150 | doi = 10.1038/nature04898 | s2cid = 4406032 | bibcode = 2006Natur.441E...9S }}</ref>

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.<ref>{{cite journal | vauthors = Miller PJ, Aoki K, Rendell LE, Amano M | title = Stereotypical resting behavior of the sperm whale | journal = Current Biology | volume = 18 | issue = 1 | pages = R21-3 | date = January 2008 | pmid = 18177706 | doi = 10.1016/j.cub.2007.11.003 | s2cid = 10587736 | doi-access = free | bibcode = 2008CBio...18..R21M }}</ref>

=== Diving ===
{{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. Some [[rorqual]]s can dive for up to 40 minutes, [[sperm whales]] between 60 and 90 minutes and [[bottlenose whale]]s for two hours. Diving depths average about {{convert|100|m|ft|abbr=on}}. Species such as sperm whales can dive to {{convert|3,000|m|ft|abbr=on}}, although more commonly {{convert|1,200|m|ft}}.<ref>{{cite journal|last=Scholander|first=Per Fredrik|title=Experimental investigations on the respiratory function in diving mammals and birds|year=1940|journal=Hvalraadets Skrifter|volume=22}}</ref><ref>{{cite journal|author1=Bruno Cozzi|author2=Paola Bagnoli|author3=Fabio Acocella|author4=Maria Laura Costantino|title=Structure and biomechanical properties of the trachea of the striped dolphin Stenella coeruleoalba: Evidence for evolutionary adaptations to diving|journal=The Anatomical Record|volume=284|issue=1|pages=500–510|year=2005|doi=10.1002/ar.a.20182|pmid=15791584}}</ref>

=== 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.<ref name="mann">{{cite book|editor1=Janet Mann|editor2=Richard C. Connor|editor3=Peter L. Tyack|editor4=Hal Whitehead| display-editors = 3|title=Cetacean Societies: Field Study of Dolphins and Whales|publisher=University of Chicago}}</ref>

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.<ref name="mann" />

==== Whale song ====
Males in some baleen species communicate via [[Whale vocalization|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.<ref name="mann" /> In baleen species such as humpbacks, blues and fins, male-specific song is believed to be used to attract and display fitness to females.<ref>{{cite journal |last1=Janik |first1=Vincent |title=Cetacean vocal learning and communication |journal=Current Opinion in Neurobiology |date=2014 |volume=28 |pages=60–65 |doi= 10.1016/j.conb.2014.06.010|pmid=25057816 |s2cid=40334723 }}</ref>

==== 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). <ref>{{cite journal |vauthors=Zwamborn E, Boon N, Whitehead H |date=October 2023 |title=Collective Decision-making in Aquatic Mammals |journal=Mammal Review |volume=53 |issue=4 |pages=238–253 |doi=10.1111/mam.12321 |s2cid=261141293 |doi-access=free}}</ref>

=== 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 net|bubble carpets]] to herd krill or plankton into bait balls before lunging at them.<ref name="mann" />

=== Intelligence ===
{{Main|Cetacean intelligence}}
Cetacea are known to teach, learn, cooperate, scheme and grieve.<ref name="wwwu">{{cite news |last=Siebert |first=Charles |title=Watching Whales Watching Us |work=[[The New York Times]] Magazine |date=8 July 2009 |access-date=29 August 2015 |url=https://www.nytimes.com/2009/07/12/magazine/12whales-t.html?pagewanted=all}}</ref>

Smaller cetaceans, such as dolphins and porpoises, engage in complex play behavior, including such things as producing stable underwater [[toroid]]al air-core [[vortex]] rings or "[[bubble ring]]s". 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 [[Helix|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.<ref>{{cite journal |author1=Wiley, David|s2cid=55168063 |title=Underwater components of humpback whale bubble-net feeding behaviour |journal=Behaviour |volume=148 |issue=5 |pages=575–602 |year=2011 |doi=10.1163/000579511X570893 |display-authors=etal}}</ref>

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.<ref>{{cite book |last1=Carwardine |first1=M. H. |last2=Hoyt |first2=E. |year=1998 |title=Whales, Dolphins and Porpoises |journal=NSW: Reader's Digest |isbn=978-0-86449-096-4}}</ref>

[[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 [[metacognition|metacognitive reasoning]] (thinking about thinking) that humans exploit. Dolphins appear to possess self-awareness.<ref>{{cite web | url=http://www.livescience.com/4272-elephant-awareness-mirrors-humans.html | title=Elephant Self-Awareness Mirrors Humans |publisher=Live Science |date=30 October 2006 |access-date=29 August 2015}}</ref> 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.<ref name="Mirror test">{{cite news |url=https://www.nytimes.com/2001/05/01/science/brainy-dolphins-pass-the-human-mirror-test.html |title=Mirror test |author=Derr, Mark |newspaper=New York Times |access-date=3 August 2015|date=May 2001 }}</ref>

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.<ref name="Mirror test" />

In 1995, Marten and Psarakos used video to test dolphin self-awareness.<ref>{{cite journal|last1=Marten|first1=Ken|last2=Psarakos|first2=Suchi|title=Using Self-View Television to Distinguish between Self-Examination and Social Behavior in the Bottlenose Dolphin (''Tursiops truncatus'')|journal=Consciousness and Cognition|volume=4|number=2|date=June 1995|doi=10.1006/ccog.1995.1026|pages=205–224|pmid=8521259|s2cid=44372881}}<!--|access-date=5 September 2015--></ref> 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.<ref name="Mirror test" />

== Life history ==

=== Reproduction and development ===
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.<ref name="mann" /> 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.<ref name="mann" />

=== 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.<ref>{{cite journal|author1=John C George|author2=Jeffrey Bada|author3=Judith Zeh|author3-link= Judy Zeh |author4=Laura Scott|author5=Stephen E Brown|author6=Todd O'Hara|author7=Robert Suydam|title=Age and growth estimates of bowhead whales (''Balaena mysticetus'') via aspartic acid racemization|journal=Canadian Journal of Zoology|year=1999|volume=77|issue=4|doi=10.1139/z99-015|pages=571–580}}<!--|access-date=5 September 2015--></ref>

=== Death ===
{{Main|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 other [[neritic]] biodiversity hotspots, such as [[cold seep]]s and [[hydrothermal vent]]s.<ref name="Smith&Baco2003">{{cite book |url=http://www.soest.hawaii.edu/oceanography/faculty/csmith/Files/Smith%20and%20Baco%202003.pdf |archive-url=https://web.archive.org/web/20060817174541/http://www.soest.hawaii.edu/oceanography/faculty/csmith/Files/Smith%20and%20Baco%202003.pdf |archive-date=2006-08-17 |url-status=live |title=Ecology of Whale Falls at the Deep-Sea Floor |author1=Smith, Craig R. |author2=Baco, Amy R. |volume=41 |date=2003 |pages=311–354 |access-date=23 August 2014 |doi=10.1201/9780203180594.ch6|series=Oceanography and Marine Biology - an Annual Review |isbn=978-0-415-25462-5 }}</ref>

Deterioration of whale carcasses happens through three stages. Initially, organisms such as [[shark]]s 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.<ref name="Smith&Baco2003" /><ref>{{cite journal |author1=Fujiwara, Yoshihiro|title=Three-year investigations into sperm whale-fall ecosystems in Japan |journal=Marine Ecology |volume=28 |issue=1 |pages=219–230|date=16 February 2007|display-authors=etal |doi=10.1111/j.1439-0485.2007.00150.x|bibcode=2007MarEc..28..219F|doi-access=free}}</ref>

=== 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 [[miscarriage]]s, [[male infertility]], neurobrucellosis, [[cardiopathy|cardiopathies]], bone and skin [[lesion]]s, [[Cetacean stranding|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.<ref>{{cite journal|first1=Guzmán-Verri|last1=Caterina| first2=Rocío|last2=González-Barrientos|first3=Gabriela|last3=Hernández-Mora|first4=Juan-Alberto|last4=Morales|first5=Elías|last5=Baquero-Calvo|first6=Esteban|last6=Chaves-Olarte|first7=Edgardo|last7=Moreno|title=''Brucella ceti'' and Brucellosis in Cetaceans|journal=Frontiers in Cellular and Infection Microbiology|volume=2|pages=3|year=2012|doi=10.3389/fcimb.2012.00003|pmid=22919595|pmc=3417395|doi-access=free}}<!--|access-date=30 October 2015--></ref>

== Evolution ==

{{main|Evolution of cetaceans}}

=== Fossil history ===

==== Transition from land to sea ====

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 [[Carnivore|carnivorous]] dentition.<ref name="Thewissen" />

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 {{convert|3|m|ft|abbr=on}} 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.<ref>{{Cite journal |last1=Ando |first1=Konami |last2=Fujiwara |first2=Shin-ichi |date=2016-07-10 |title=Farewell to life on land - thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals |journal=Journal of Anatomy |volume=229 |issue=6 |pages=768–777 |doi=10.1111/joa.12518 |issn=0021-8782 |pmc=5108153 |pmid=27396988}}</ref> 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.<ref>{{cite journal |last=Thewissen |first=Hans |author-link=Hans Thewissen |year=1994 |title=Phylogenetic aspects of Cetacean origins: A morphological perspective |journal=Journal of Mammalian Evolution |pages=157–184 |doi=10.1007/bf01473527 |volume=2 |issue=3 |s2cid=27675176}}</ref>

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 [[Pinniped|seal]].<ref name="Thewissen" />

==== 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.<ref>{{cite web |title=First intact fossil of prehistoric whale discovered in Wadi Al-Hitan |url=https://www.iucn.org/content/first-intact-fossil-prehistoric-whale-discovered-wadi-al-hitan |publisher=[[International Union for Conservation of Nature]] |date=9 June 2015}}</ref>

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.<ref name="Thewissen" />

The two parvorders, [[baleen whale]]s (Mysticeti) and [[toothed whale]]s (Odontoceti), are thought to have diverged around thirty-four million years ago.<ref>{{Cite journal |last1=Cerchio |first1=Salvatore |last2=Tucker |first2=Priscilla |date=1998-06-01 |title=Influence of Alignment on the mtDNA Phylogeny of Cetacea: Questionable Support for a Mysticeti/Physeteroidea Clade |journal=Systematic Biology |volume=47 |issue=2 |pages=336–344 |doi=10.1080/106351598260941 |issn=1076-836X |pmid=12064231 |s2cid=16270218 |doi-access=free}}</ref>

{{anchor|Classification}}

=== Phylogeny ===

{{Further|List of cetaceans|List of extinct cetaceans}}

[[Molecular biology]], [[immunology]], and fossils show that cetaceans are phylogenetically closely related with the [[even-toed ungulate]]s (Artiodactyla). Whales' direct lineage began in the early [[Eocene]], around 55.8 million years ago, with early artiodactyls.<ref name="Thewissen" /> Most molecular biological evidence suggests that [[Hippopotamidae|hippos]] are the closest living relatives. Common anatomical features include similarities in the [[Morphology (biology)|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.<ref name="Thewissen">{{cite journal |last1=Thewissen|first1=J. G. M. |last2=Cooper |first2=Lisa Noelle|last3=Clementz |first3=Mark T. |last4=Bajpai |first4=Sunil |last5=Tiwari |first5=B. N. |title=Whales originated from aquatic artiodactyls in the Eocene epoch of India |year=2007 |journal=Nature |volume=450 |issue=7173 |pages=1190–4 |url=http://repository.ias.ac.in/4642/1/316.pdf |archive-url=https://web.archive.org/web/20131224104529/http://repository.ias.ac.in/4642/1/316.pdf |archive-date=2013-12-24 |url-status=live |doi=10.1038/nature06343 |pmid=18097400 |bibcode=2007Natur.450.1190T |s2cid=4416444 |author1-link=Hans Thewissen }}</ref> 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.<ref name="DNA">{{cite journal|title = More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen|author = Gatesy, J.|journal = [[Molecular Biology and Evolution]]|date=1 May 1997| volume = 14|pages = 537–543|pmid = 9159931|issue = 5|doi = 10.1093/oxfordjournals.molbev.a025790|doi-access = free}}</ref><ref name="Cetartiodactyla">{{cite journal |author1=Boisserie, Jean-Renaud |author2=Lihoreau, Fabrice |author3=Brunet, Michel |year= 2005|title= The position of Hippopotamidae within Cetartiodactyla|journal= [[Proceedings of the National Academy of Sciences]] |volume= 102 |issue= 5|pages= 1537–1541|doi= 10.1073/pnas.0409518102|pmid= 15677331 |pmc=547867|bibcode= 2005PNAS..102.1537B|doi-access=free }}</ref><ref name="ScienceNews">{{cite web|title = Scientists find missing link between the dolphin, whale and its closest relative, the hippo|date = 2005-01-25|access-date = 2011-01-08|url = http://www.innovations-report.com/html/reports/life_sciences/report-39309.html|work = Science News Daily}}</ref>

Molecular and morphological evidence suggests that [[artiodactyl]]s 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]]:<!--why have we got a heap of refs for a single cladogram?--><ref>
{{cite journal
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{{cite journal
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 |last13= Perini      |first13= F.A.       |last14= Randall |first14= Z.S.
 |last15= Rougier     |first15= G.W.       |last16= Sargis  |first16= E.J.
 |last17= Silcox      |first17= M.T.       |last18= Simmons |first18= N.B.
 |last19= Spaulding   |first19= M.         |last20= Velazco |first20= P.M.
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</ref><ref name="Song2012">
{{cite journal
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</ref><ref name="dos Reis2012">
{{cite journal
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</ref><ref name="Upham2019">
{{cite journal
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 |last2= Esselstyn |first2= J.A.
 |last3= Jetz      |first3= W.
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}} (see e.g. Fig S10)
</ref>

{{Clade
   |style=font-size:100%;line-height:100%;width:820px;
   |label1=[[Artiodactyla]]&nbsp;
   |1={{Clade
     |1=[[Tylopoda]] (camels)[[File:Cladogram of Cetacea within Artiodactyla (Camelus bactrianus).png|50 px]]
     |label2=&nbsp;[[Artiofabula]]&nbsp;
     |2={{Clade
           |1=&nbsp;[[Suina]] (pigs)[[File:Recherches pour servir à l'histoire naturelle des mammifères (Pl. 80) (white background).jpg|50 px]]
           |label2=&nbsp;[[Cetruminantia]]&nbsp;
           |2={{Clade
               |label1=&nbsp;[[Ruminantia]] (ruminants)&nbsp;
               |1={{Clade
                  |1=&nbsp;[[Chevrotain|Tragulidae]] (mouse deer)[[File:Tragulus napu - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|50 px]]
                  |2=&nbsp;[[Pecora]] (horn bearers)[[File:Walia ibex illustration white background.png|50 px]]
                  }}
               |label2=&nbsp;[[Cetancodonta]]/[[Whippomorpha]]&nbsp;
               |sublabel2=54 mya
               |2={{Clade
                  |1=&nbsp;[[Hippopotamidae]] (hippopotamuses)[[File:Voyage en Abyssinie Plate 2 (white background).jpg|50 px]]
                  |2=&nbsp;'''Cetacea''' (whales)[[File:Bowhead-Whale1 (16273933365).jpg|50 px]]
                  }}
              }}
           }}
        }}
     }}

{{Cladogram
|title=<small>Relationship of extinct and extant cetaceans<ref>
{{cite journal
 |author1=Gatesy, John       |author2=Geisler, Jonathan H. 
 |author3=Chang, Joseph      |author4=Buell, Carl 
 |author5=Berta, Annalisa    |author6=Meredith, Robert W. 
 |author7=Springer, Mark S.  |author8=McGowen, Michael R. 
 |year=2012
 |title=A phylogenetic blueprint for a modern whale
 |journal=[[Molecular Phylogenetics and Evolution]]
 |volume=66  |issue=2  |pages=479–506
 |doi=10.1016/j.ympev.2012.10.012  |pmid=23103570
 |url=https://www.montclair.edu/profilepages/media/5008/user/Gatesy_et_al._2012_A_phylogenetic_blueprint_for_a_modern_whale.pdf |archive-url=https://web.archive.org/web/20130227091508/https://www.montclair.edu/profilepages/media/5008/user/Gatesy_et_al._2012_A_phylogenetic_blueprint_for_a_modern_whale.pdf |archive-date=2013-02-27 |url-status=live
 |access-date=4 September 2015
}}
</ref></small>: 
|caption={{center|† Extinct taxa}}
|align=left
|cladogram=
 {{Clade
 |style=font-size:85%;line-height:100%;width:580px;
 |label1=&nbsp;
 |1={{Clade
    |1=&nbsp;[[Raoellidae]] †
    |label2=&nbsp;'''Cetacea'''&nbsp;
    |2={{Clade
       |1=&nbsp;[[Pakicetidae]] †
       |2={{Clade
          |1=&nbsp;[[Ambulocetidae]] † [[File:Ambulocetus BW.jpg|50 px]]
          |2={{Clade
             |1=&nbsp;[[Remingtonocetidae]] † [[File:Remingtonocetus cropped.png|50 px]]
             |2={{Clade
                |1=&nbsp;''[[Rodhocetus]]'' †
                |2={{Clade
                   |1=&nbsp;''[[Dorudon]]'' †
                   |2={{Clade
                      |1=&nbsp;''[[Basilosaurus]]'' † [[File:Basilosaurus cropped.png|50 px]]
                      |2={{Clade
                         |label1=&nbsp;[[Mysticeti]]&nbsp;
                         |sublabel1=([[baleen whales]], 25 mya)
                         |1={{Clade
                            |1=&nbsp;''[[Janjucetus]]'' †
                            |2={{Clade
                               |label1=&nbsp;[[Balaenidae]]&nbsp;
                               |1=&nbsp;[[Balaenidae|Right whales]] [[File:Balaena mysticetus NOAA.jpg|50 px]]
                               |2={{Clade
                                  |label1=&nbsp;[[Neobalaenidae]]&nbsp;
                                  |1=&nbsp;[[Pygmy right whale]] <span style="{{MirrorH}}">[[File:Caperea marginata 3.jpg|50 px]]</span>
                                  |2={{Clade
                                     |label1=&nbsp;[[Balaenopteridae]]&nbsp;
                                     |1=&nbsp;[[Rorqual]]s [[File:Megaptera novaeangliae NOAA.jpg|50 px]]
                                     |label2=&nbsp;[[Eschrichtiidae]]&nbsp;
                                     |2=&nbsp;[[Gray whale]]s [[File:Eschrichtius robustus NOAA.jpg|50 px]]
                                     }}
                                  }}
                               }}
                            }}
                         |label2=&nbsp;[[Odontoceti]]&nbsp;
                         |sublabel2=([[toothed whales]])
                         |2={{Clade
                            |1={{Clade
                               |label1=&nbsp;[[Kogiidae]]&nbsp;
                               |1=&nbsp;[[Dwarf sperm whale]]s [[File:Kogia sima (transparent background).png|50 px]]
                               |label2=&nbsp;[[Physeteridae]]&nbsp;
                               |2=&nbsp;[[Sperm whale]]s [[File:Physeter macrocephalus NOAA.jpg|50 px]]
                               }}
                            |2={{Clade
                               |label1=&nbsp;[[Platanistidae]]&nbsp;
                               |1=&nbsp;[[South Asian river dolphin]]s [[File:Ganges.png|50 px]]
                               |2={{Clade
                                  |label1=&nbsp;[[Ziphiidae]]&nbsp;
                                  |1=&nbsp;[[Beaked whale]]s [[File:Ziphius cavirostris NOAA.jpg|50 px]]
                                  |2={{Clade
                                     |label1=&nbsp;[[Inioidea]]&nbsp;
                                     |sublabel1=(South American<br/>river dolphins)
                                     |1={{Clade
                                        |label1=&nbsp;[[Lipotidae]]&nbsp;
                                        |1=[[Yangtze river dolphin]] [[File:Lipotes vexillifer.png|50 px]]
                                        |2={{Clade
                                           |1=&nbsp;[[Iniidae]] [[File:Inia geoffrensis.jpg|50 px]]
                                           |2=&nbsp;[[Pontoporiidae]] [[File:Pontoporia blainvillei.jpg|50 px]]
                                           }}
                                        }}
                                     |label2=&nbsp;[[Delphinoidea]]&nbsp;
                                     |2={{Clade
                                        |label1=&nbsp;[[Delphinidae]]&nbsp;
                                        |sublabel1=("true" dolphins)
                                        |1=&nbsp;[[Oceanic dolphins]] [[File:Orcinus orca NOAA 2.jpg|50 px]]
                                        |2={{Clade
                                           |label1=&nbsp;[[Monodontidae]]&nbsp;
                                           |1=&nbsp;[[beluga whale|Belugas]] & [[Narwhal]]s [[File:Delphinapterus leucas NOAA.jpg|50 px]]
                                           |label2=&nbsp;[[Phocoenidae]]&nbsp;
                                           |2=&nbsp;[[Porpoise]]s [[File:Vaquita swimming through the void. (Phocoena Sinus).png|50 px]]
                                           }}
                                        }}
                                     }}
                                  }}
                               }}
                            }}
                         }}
                      }}
                   }}
                }}
             }}
          }}
       }}
    }}
 }}
}}
{{clear}}

The terms ''whale'' and ''dolphin'' are informal:

*[[Mysticeti]]:
:*[[Whale]]s, with four families: [[Balaenidae]] (right and bowhead whales), [[Cetotheriidae]] (pygmy right whales), [[Balaenopteridae]] (rorquals), [[Eschrichtiidae]] (grey whales)
*[[Odontoceti]]:
:*[[Whale]]s: with four families: [[Monodontidae]] (belugas and narwhals), [[Physeteridae]] (sperm whales), [[Kogiidae]] (dwarf and pygmy sperm whales), and [[Ziphiidae]] (beaked whales)
:*[[Dolphin]]s, with five families: [[Delphinidae]] (oceanic dolphins), [[Platanistidae]] ([[South Asian river dolphin]]s), [[Lipotidae]] (old world river dolphins) [[Iniidae]] (new world river dolphins), and [[Pontoporiidae]] ([[La Plata dolphin]]s)
:*[[Porpoise]]s, with one family: [[Phocoenidae]]
The term 'great whales' covers those currently regulated by the [[International Whaling Commission]]:<ref name="iwcjuris">{{cite web |title=Small cetaceans |website=iwc.int |publisher=[[International Whaling Commission]] |url=https://iwc.int/smallcetacean |access-date=2018-04-08}}</ref> 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).<ref name="iwcgreat">{{Cite web |title=Lives of Whales |website=iwc.int |publisher=[[International Whaling Commission]] |url=https://iwc.int/lives |access-date=2018-04-08}}</ref>

== Threats ==
The primary threats to cetaceans come from people, both directly from whaling or [[Dolphin drive hunting|drive hunting]] and indirect threats from fishing and pollution.<ref name="wdcs">{{cite book|author=Cara E. Miller|title=Current State of Knowledge of Cetacean Threats, Diversity, and Habitats in the Pacific Islands Region|publisher=Whale and Dolphin Conservation Society|year=2007|access-date=5 September 2015|isbn=978-0-646-47224-9|url=http://uk.whales.org/sites/default/files/whales-and-dolphins-in-the-pacific-islands.pdf|archive-url=https://web.archive.org/web/20150908003502/http://uk.whales.org/sites/default/files/whales-and-dolphins-in-the-pacific-islands.pdf|archive-date=8 September 2015|url-status=dead}}</ref>

=== Whaling ===
{{Main|Whaling|History of Whaling|Dolphin drive hunting}}
The first efforts to protect whales came in 1931. Some particularly endangered species, such as the [[humpback whale]] (which then numbered about 100 animals).<ref>{{Cite web |title=A History of the International Whaling Commission (IWC) |url=https://wwf.panda.org/discover/knowledge_hub/endangered_species/cetaceans/cetaceans/iwc/history |access-date=2024-08-08 |website=wwf.panda.org |language=en}}</ref>

[[Aboriginal whaling]] is still permitted. About 1,200 pilot whales were taken in the [[Faroe Islands]] in 2017.<ref name="faroe">{{Cite web |url=http://heimabeiti.fo/default.asp?menu=400 |title=Hagar & seyðamark |website=heimabeiti.fo |access-date=2018-04-07 |archive-date=2014-09-24 |archive-url=https://web.archive.org/web/20140924041544/http://www.heimabeiti.fo/default.asp?menu=400 |url-status=dead }}</ref>

==== Bycatch ====
{{See also|Cetacean bycatch}}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.<ref name="wdcs" />

=== Strandings ===
{{Main|Cetacean stranding}}
The causes are not clear. Possible reasons for mass beachings are:<ref name="wdcs" />
* 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#Wildlife|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.<ref>{{cite journal |author=Schrope, Mark. |title=Whale deaths caused by US Navy's sonar |journal=Nature |volume=415 |issue=106 |pages=106 |year=2003  |doi=10.1038/415106a|bibcode = 2002Natur.415..106S |pmid=11805797|s2cid=52827761 |doi-access=free }}</ref>

[[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.<ref>{{Cite journal |last1=Evans |first1=K |last2=Thresher |first2=R |last3=Warneke |first3=R.M |last4=Bradshaw |first4=C.J.A |last5=Pook |first5=M |last6=Thiele |first6=D |last7=Hindell |first7=M.A |date=2005-06-22 |title=Periodic variability in cetacean strandings: links to large-scale climate events |journal=Biology Letters |volume=1 |issue=2 |pages=147–150 |doi=10.1098/rsbl.2005.0313 |issn=1744-9561 |pmc=1626231 |pmid=17148151}}</ref>

=== Environmental hazards ===
{{See also|Marine mammals and sonar}}
{{Quote box
| quote = 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.
| source = —Rear Admiral Lawrence Rice
| align = left
| width = 30%
}}

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.<ref name="wdcs" />

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.<ref name="wdcs" />

Food supplies are also reduced long-term by [[ocean acidification]] due to increased absorption of increased atmospheric carbon dioxide. The CO<sub>2</sub> 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.<ref name="wdcs" />

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.<ref name="Nowacek">{{cite journal|last1=Nowacek|first1=Douglas|last2=Donovan|first2=Greg|last3=Gailey|first3=Glenn|last4=Racca|first4=Roberto|last5=Reeves|first5=Randall|last6=Vedenev|first6=Alexander|last7=Weller|first7=David|last8=Southall|first8=Brandon|title=Responsible Practices for Minimizing and Monitoring Environmental Impacts of Marine Seismic Surveys with an Emphasis on Marine Mammal|journal=Aquatic Mammals|date=2013|volume=39|issue=4 |pages=356–377|doi=10.1578/am.39.4.2013.356}}<!--|access-date=April 2, 2016--></ref> 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]].<ref>{{Cite book|author1=M. Andre|title=Europe Oceans 2005|author2=T. Johansson|author3=E. Delory|author4=M. van der Schaar|publisher=Oceans 2005–Europe|year=2005|volume=2|pages=1028–1032 Vol. 2|doi=10.1109/OCEANSE.2005.1513199|chapter=Cetacean biosonar and noise pollution|isbn=978-0-7803-9103-1|s2cid=31676969}}</ref>

== Relationship with humans ==
{{See also|Cryptid whale|Whale#In myth, literature and art}}

=== Antiquity ===
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.<ref>{{Cite book |last=Aristotle |url=https://penelope.uchicago.edu/aristotle/histanimals8.html |title=The History of Animals, Book VIII |language=en |translator-last=Thompson |translator-first=D'Arcy Wentworth |chapter=Chapter 2 |access-date=April 16, 2022 |archive-url=https://archive.today/20220416102355/https://penelope.uchicago.edu/aristotle/histanimals8.html |archive-date=April 16, 2022}}</ref>  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 (Pliny)|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 [[cetology|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.<ref>{{cite book|author= Conrad Gesner|url=http://etext.lib.virginia.edu/toc/modeng/public/AriHian.html|title=Historiae animalium |date=6 September 2008 |archive-url= https://web.archive.org/web/20080906090248/http://etext.lib.virginia.edu/toc/modeng/public/AriHian.html |archive-date=6 September 2008 |access-date=4 September 2015}}</ref> 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.<ref>{{cite journal|author=J. Ray|title=An account of the dissection of a porpess|journal=Philosophical Transactions of the Royal Society of London|volume=6|issue=69–80|year=1671|pages=2274–2279|bibcode = 1671RSPT....6.2274R|doi=10.1098/rstl.1671.0048|s2cid=186210473|doi-access=free}}</ref><ref>{{cite journal|author=Susanne Prahl|title=Studies for the construction of epicranialen airway when porpoise (Phocoena phocoena Linnaeus, 1758)|journal=Dissertation for the Doctoral Degree of the Department of Biology of the Faculty of Mathematics, Computer Science and Natural Sciences at the University of Hamburg|year=2007|page=6}}</ref>

{{Blockquote|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 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<ref>{{Bibleverse|Psalms|104:26}}</ref> and should again be destroyed by him.<ref>{{Bibleverse|Psalms|74:14}}</ref><ref>{{Bibleverse|Isaiah|27:1}}.</ref> In the [[Book of Job]], the leviathan is described in more detail.<ref>{{Bibleverse|Job|40:25}}</ref><ref>{{Bibleverse|Job|41:26}}</ref>

=== Middle Ages to the 19th century ===
{{See also|History of Whaling|Cryptid whales}}In the Canadian Arctic (east coast) in Punuk and [[Thule]] culture (1000–1600 C.E.),<ref>{{cite journal|title=The circumpolar zone|last1=Cunliffe|first1=B.|last2=Gosden|first2=C.|last3=Joyce|first3=R.|journal=The Oxford Handbook of Archaeology}}</ref> 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.<ref>{{cite journal|author=J. Savelle| title=The Role of Architectural utility in the formation of archaeological Whale Bone Assemblages|journal=Journal of Archaeological Science|volume=24| issue=10|year=1997|pages=869–885|doi=10.1006/jasc.1996.0167| bibcode=1997JArSc..24..869S}}<!--|access-date=4 September 2015--></ref>

=== Modern culture ===
{{Further|Whale#Interactions with humans}}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 (1995 TV series)|Flipper]]'' was a bottle-nose dolphin. The TV series ''[[SeaQuest DSV]]'' (1993–1996), the movies ''[[Free Willy]]'' and ''[[Star Trek IV: The Voyage Home]]'', and the book series ''[[The Hitchhiker's Guide to the Galaxy]]'' by [[Douglas Adams]] are examples.<ref>{{cite web|url=http://www.movieretriever.com/videohound_lists/90895/Whales|title=Movie Retriever: Whales|author=unknown|work=movieretriever.com|url-status=dead|archive-url=https://web.archive.org/web/20151015214719/http://www.movieretriever.com/videohound_lists/90895/Whales|archive-date=2015-10-15}}</ref>

== Captivity ==

=== Belugas ===
{{Main|Beluga whale#Captivity}}

[[Beluga whale]]s 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 American Museum|Barnum's Museum]] in [[New York City]] in 1861.<ref name="New York Tribune">{{cite web | url=http://chnm.gmu.edu/lostmuseum/lm/190/ | title=The Whales, New York Tribune, August 9, 1861 | access-date=5 December 2011 | date=9 August 1861 | work=New York Tribune}}</ref> For most of the 20th century, Canada was the predominant source.<ref name="cmeps" /> They were taken from the [[Saint Lawrence River|St. Lawrence River]] estuary until the late 1960s, after which they were predominantly taken from the [[Churchill River (Hudson Bay)|Churchill River]] estuary until capture was banned in 1992.<ref name="cmeps">{{cite web|url=http://webpages.charter.net/hrynyshyn/pdfs/Beluga_Report_web2006.pdf |title=Beluga Whales in Captivity: Hunted, Poisoned, Unprotected |access-date=26 December 2014 |year=2006 |work=Special Report on Captivity 2006 |publisher=Canadian Marine Environment Protection Society |url-status=dead |archive-url=https://web.archive.org/web/20141226215306/http://webpages.charter.net/hrynyshyn/pdfs/Beluga_Report_web2006.pdf |archive-date=26 December 2014 }}</ref> Russia then became the largest provider.<ref name="cmeps" /> Belugas are caught in the [[Amu Darya|Amur Darya]] delta and their eastern coast and are transported domestically to aquaria or [[dolphinarium|dolphinaria]] in [[Moscow]], [[St. Petersburg]] and [[Sochi]], or exported to countries such as Canada.<ref name="cmeps" /> They have not been domesticated.<ref name="zoos">{{cite web | url=http://www.waza.org/en/zoo/visit-the-zoo/aquatic-mammals-1254385523/delphinapterus-leucas | title=Beluga (''Delphinapterus leucas'') Facts – Distribution – In the Zoo | access-date=5 December 2011 | publisher=World Association of Zoos and Aquariums | archive-url=https://web.archive.org/web/20120210101419/http://www.waza.org/en/zoo/visit-the-zoo/aquatic-mammals-1254385523/delphinapterus-leucas | archive-date=10 February 2012 | url-status=dead }}</ref>

As of 2006, 30 belugas lived in Canada and 28 in the United States. 42 deaths in captivity had been reported.<ref name="cmeps" /> A single specimen can reportedly fetch up to US$100,000 ([[STG pound|£]]64,160). The beluga's popularity is due to its unique color and its [[facial expression]]s. 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.<ref name="Bonner, W.N. 17, 23–24">{{cite book |author=Bonner, Nigel |title=Whales |publisher=Facts on File |isbn=978-0-7137-0887-5 |pages=[https://archive.org/details/whales0000bonn/page/17 17, 23–24] |year=1980 |url=https://archive.org/details/whales0000bonn/page/17 }}</ref>

=== Orcas ===
{{Main|Captive killer whales}}

{{See also|SeaWorld#Criticism and resulting impact|Incidents at SeaWorld parks}}
The orca's [[animal intelligence|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.<ref>{{cite web |publisher=[[National Marine Fisheries Service]] (NMFS) Northwest Regional Office |author=NMFS |year=2005 |url=http://www.nwr.noaa.gov/Marine-Mammals/Whales-Dolphins-Porpoise/Killer-Whales/Conservation-Planning/upload/SRKW-propConsPlan.pdf |title=Conservation Plan for Southern Resident Killer Whales (''Orcinus orca'') |location=[[Seattle]], U.S. |access-date=January 2, 2009 |archive-url=https://web.archive.org/web/20080626121719/http://www.nwr.noaa.gov/Marine-Mammals/Whales-Dolphins-Porpoise/Killer-Whales/Conservation-Planning/upload/SRKW-propConsPlan.pdf |archive-date=June 26, 2008 |url-status=dead |pages=43–44}}</ref>

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.<ref>{{cite web|author=Rose, N. A.|year=2011|url=http://www.hsi.org/assets/pdfs/orca_white_paper.pdf |archive-url=https://web.archive.org/web/20111026123839/http://www.hsi.org/assets/pdfs/orca_white_paper.pdf |archive-date=2011-10-26 |url-status=live|title=Killer Controversy: Why Orcas Should No Longer Be Kept in Captivity|publisher=Humane Society International and the Humane Society of the United States|access-date=December 21, 2014}}</ref>

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 {{convert|160|km|mi|-1}} in a day and critics say the animals are too big and intelligent to be suitable for captivity.<ref name="cbs2010">{{cite news|url= https://www.cbsnews.com/news/whale-attack-renews-captive-animal-debate/|title=Whale Attack Renews Captive Animal Debate|newspaper=CBS News|date=March 1, 2010|access-date=6 September 2015}}</ref> Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of [[stress (medicine)|stress]].<ref name="armstrong">{{cite book|author=Susan Jean Armstrong|title=Animal Ethics Reader| isbn=978-0-415-27589-7|year=2003|publisher=Psychology Press }}</ref> 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 [[List of captive orcas|captive orcas]] being held in aquaria as of 2012.<ref>{{cite web|url=http://www.orcahome.de/orcastat.htm|title=Orcas in Captivity – A look at killer whales in aquariums and parks|date=23 November 2009|access-date=6 September 2015|archive-url=https://web.archive.org/web/20070602050516/http://www.orcahome.de/orcastat.htm|archive-date=2 June 2007|url-status=dead}}</ref>

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 §&nbsp;3.104, under the ''Specifications for the Humane Handling, Care, Treatment and Transportation of Marine Mammals''.<ref>{{cite journal|url=http://www.ecfr.gov/cgi-bin/text-idx?SID=c7a201bd3f7d31d5f8218167efcd49ba&node=9:1.0.1.1.3.5.31.5&rgn=div8|title=Chapter I: Space requirements|journal=Electronic Code of Federal Regulation|volume= 1|access-date=6 September 2015}}</ref>

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 (film)|Blackfish]]'', which documents the history of [[Tilikum (orca)|Tilikum]], an orca at SeaWorld Orlando, who had been involved in the deaths of three people.<ref>Whiting, Candace Calloway. [https://www.huffpost.com/entry/blackfish-killer-whales_b_4166923 In the Wake of Blackfish &ndash; Is it Time to Retire the Last Killer Whale Whose Capture Was Shown in the Film?"], HuffPost, October 29, 2013. Retrieved October 29, 2013.</ref> 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.<ref>{{Cite web|url=https://www.forbes.com/sites/dalebuss/2016/03/24/it-was-a-losing-battle-seaworlds-ceo-on-its-abrupt-change-and-what-comes-next/#5754041a223b|title=Shamu Goes Out With the Tide: SeaWorld CEO On Its Abrupt Change &ndash; And What Comes Next|last=Buss|first=Dale|date=2016-03-24|website=Forbes|access-date=2016-03-26}}</ref>

=== Others ===
In repeated attempts in the 1960s and 1970s, [[narwhal]]s kept in captivity died within months. A breeding pair of [[pygmy right whale]]s 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 [[Ojo de Liebre Lagoon|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 {{convert|680|kg|lbs}} calf was a popular attraction and behaved normally, despite separation from his mother. A year later, the then {{convert|8,164.7|kg|lbs}} 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.<ref name="Whales">{{cite book|url=https://portals.iucn.org/library/sites/library/files/documents/RD-1991-001.pdf |archive-url=https://web.archive.org/web/20150509184324/https://portals.iucn.org/library/sites/library/files/documents/RD-1991-001.pdf |archive-date=2015-05-09 |url-status=live|last1=Klinowska|first1=Margaret|last2=Cooke|first2=Justin|year=1991| title=Dolphins, Porpoises, and Whales of the World: the IUCN Red Data Book|access-date=6 September 2015}}</ref><ref>{{cite journal| url=http://www.aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/2001/AquaticMammals_27-03/27-03_Goff.pdf |archive-url=https://web.archive.org/web/20151015214719/http://www.aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/2001/AquaticMammals_27-03/27-03_Goff.pdf |archive-date=2015-10-15 |url-status=live |title=Growth of two gray whale calves|author1=J. L. Sumich|author2=T. Goff|author3=W. L. Perryman|journal=Aquatic Mammals|pages=231–233|year=2001|access-date=6 September 2015}}</ref>

== Images ==
{{col-begin|width=70%}}
{{col-3}}
{{multiple image
| align             = right
| perrow            = 1
| total_width       = 250
| image1            = Eubalaena glacialis (North Atlantic right whale) 1 (30986325841).jpg
| caption1          = Skull of the [[North Atlantic right whale]] (Mysticeti)
| image2            = Orca Schaedel Senckenberg.jpg
| caption2          = Skull of the [[orca]] (Odontoceti)
}}
{{col-3}}
{{multiple image
| align             = right
| perrow            = 2
| total_width       = 400
| image1            = Beluga premier.gov.ru-1.jpeg
| caption1          = Head profile of a [[beluga whale]], featuring the large "melon" region
| image2            = SBNMS - feeding humpback whale (31319806616).jpg
| caption2          = Close up of a feeding [[humpback whale]], showing baleen
}}
{{col-3}}
{{multiple image
| direction         = vertical <!-- horizontal (default), vertical -->
<!-- Header -->| header            = Methods of [[whaling]]
| header_align      = center <!-- center (default), left, right -->
<!-- Images -->| width             = 220
| caption_align     = center
<!-- Essential parameters -->| image1            = Japan Factory Ship Nisshin Maru Whaling Mother and Calf.jpg <!-- filename only, i.e. without "File:" or "Image:" prefix -->
| alt1              = Mother and calf minke whales are pulled through the rear of a Japanese research ship.
| caption1          = Japanese research ship whaling mother and calf [[minke whales]].
<!--image 2-->| image2            = Hvalba 26-08-06 (3).jpg <!-- filename only, i.e. without "File:" or "Image:" prefix -->
| alt2              = A dolphin caught in a drive hunt is transported away by a forklift.
| caption2          = An [[Atlantic white-sided dolphin]] caught in a [[dolphin drive hunt|drive hunt]] in [[Hvalba]] on the [[Faroe Islands]] being taken away with a forklift.
| align             = 
| total_width       = 
}}
{{col-end}}

<gallery mode="packed" heights="160">
File:Preserved sperm whale brain.jpg|Brain of the [[sperm whale]], considered the largest brain in the world
File:Humpback whale fluke (2).jpg|Humpback whale fluke
File:Delfinekko.gif|[[Biosonar]] 
File:Whales Bubble Net Feeding-edit1.jpg|Bubble net feeding
File:Orca porpoising.jpg|Killer whale [[Cetacean surfacing behaviour#Porpoising|porpoising]]
File:Adult female and fetal Maiacetus.jpg|Fossil of a ''Maiacetus'' (red, beige skull) with fetus (blue, red teeth) shortly before the end of gestation <ref name="Gingerich PD">{{cite journal |author1=Gingerich PD |author2=ul-Haq M |author3=von Koenigswald W |author4=WJ Sanders |author5=Smith BH |year=2009 |title=New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and sexual dimorphism |journal=PLOS ONE |volume=4 |issue=2 |pages=e4366 |bibcode=2009PLoSO...4.4366G |doi=10.1371/journal.pone.0004366 |pmc=2629576 |pmid=19194487 |doi-access=free}}</ref>
File:Ichthyosaur vs dolphin.svg|Cetaceans [[convergent evolution|evolved convergently]] as [[Streamlining theory|streamlined]] swimmers with fish and [[Ichthyosaur|aquatic reptiles]]. 
File:Domino whale-bone hg.jpg|[[Dominoes]] made of baleen
File:Ecomare - gestrande bultrug op Razende Bol (bultrug2012-razende-bol-412-sw).jpg|Beached humpback whale
File:Trolual (Gessner).jpg|A whale as depicted by Conrad Gesner, 1587, in ''Historiae animalium''
File:Whales caught recently.png|Whales caught 2010–2014, by country
File:Destruction of Leviathan.png|''Destruction of Leviathan''; engraving by [[Gustave Doré]], 1865
File:Tarentum.jpg|Silver coin with [[Taras (mythology)|Taras]] riding a dolphin
File:La Baleine.jpg|Depiction of baleen whaling, 1840
File:Im Februar 1598 an der holländischen Küste gestrandeter Walfisch.jpg|Stranded sperm whale engraving, 1598
File:Sea World1.jpg|Sea World show featuring [[bottlenose dolphin]]s and [[false killer whale]]s
File:2009-Seaworld-Shamu.jpg|Ulises the orca, 2009
File:Dawn Brancheau - Riders on the Storm.jpg|Dawn Brancheau doing a show four years before [[Incidents at SeaWorld parks#SeaWorld Orlando|the incident]]
File:Short-finned Pilot Whale (8793172995).jpg|[[SeaWorld San Diego|SeaWorld]] [[pilot whale]] with trainers
File:Dolphin Anatomy.svg|upright=1.5|Dolphin anatomy
</gallery>

== References ==
{{reflist}}

== External links ==
{{Wiktionary|Cetacea}}
{{Wikispecies|Cetacea}}
{{Commons category|Cetacea}}
* {{Wikibooks inline|Dichotomous Key|Cetacea}}
* {{cite EB1911|wstitle=Cetacea |volume=5 |short=x}}
* {{cite web |url=http://www.eoearth.org/article/Cetaceans?topic=49540 |publisher=Encyclopedia of Earth |title=Cetaceans}}
* [http://www.crru.org.uk/ Scottish Cetacean Research & Rescue] – see page on [http://www.crru.org.uk/taxonomy.asp Taxonomy]
* {{cite web |url=https://www.sciencedaily.com/news/plants_animals/dolphins_and_whales/ |website=Science Daily |title=Dolphin and Whale News}}
* {{cite web |first=Douglas J. |last=Futuyma |year=1998 |url=http://www.stephenjaygould.org/library/futuyma_cetacea.html |title=Cetacea Evolution |access-date=2007-03-23 |archive-date=2008-05-29 |archive-url=https://web.archive.org/web/20080529032651/http://www.stephenjaygould.org/library/futuyma_cetacea.html |url-status=dead}}
* [https://web.archive.org/web/20110227051045/http://www.eia-international.org/campaigns/species/cetaceans/ EIA Cetacean campaign]: Reports and latest info.
* [https://web.archive.org/web/20101121062046/http://www.eia-global.org/species_in_peril/ EIA in USA]: reports etc.

{{Mammals}}
{{Cetacea}}
{{Portal bar|Cetaceans|Mammals|Marine life|Oceans}}
{{Taxonbar|from1=Q160|from2=Q37496661|from3=Q21075082}}
{{Authority control}}

[[Category:Cetaceans| 
[[Category:Extant Ypresian first appearances
[[Category:Mammal infraorders
[[Category:Taxa named by Mathurin Jacques Brisson
[[Category:Taxa described in 1762