Cetacea

Cetacea Temporal range: 53.5–0  Ma PreꞒ Ꞓ O S D C P T J K Pg N Early Eocene – Present
Harbor porpoise
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Suborder:	Whippomorpha
Clade:	Cetaceamorpha
Infraorder:	Cetacea Brisson, 1762
Subgroups
†Pakicetidae †Ambulocetidae †Remingtonocetidae † paraphyletic ) Pelagiceti Uhen, 2008[1] †Basilosauridae (paraphyletic) †Kekenodontidae Neoceti Fordyce & de Muizon, 2001[2] Mysticeti Odontoceti (see text for families)
Diversity
Around 94 species

Cetacea (

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

Cetaceans are famous for their high intelligence, complex social behaviour, and the enormous size of some of the group's members. For example, the blue whale reaches a maximum confirmed length of 29.9 meters (98 feet) and a weight of 173 tonnes (190 short tons), making it the largest animal ever known to have existed.^[5][6][7]

There are approximately 89

.

Cetaceans have been extensively hunted for their meat, blubber and oil by commercial operations. Although the International Whaling Commission has agreed on putting a halt to commercial whaling, whale hunting is still going on, either under IWC quotas to assist the subsistence of Arctic native people or in the name of scientific research, although a large spectrum of non-lethal methods are now available to study marine mammals in the wild.^[9] Cetaceans also face severe environmental hazards from underwater noise pollution, entanglement in abandoned ropes and nets, collisions with ships, plastic and heavy metals build-up, to accelerating climate change,^[10][11] but how much they are affected varies widely from species to species, from minimally in the case of the southern bottlenose whale to the baiji (Chinese river dolphin) which is considered to be functionally extinct due to human activity.^[12]

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

Baleen whales have bristles made of

) have massive heads that can make up 40% of their body mass. Most mysticetes prefer the food-rich colder waters of the Northern and Southern Hemispheres, migrating to the Equator to give birth. During this process, they are capable of fasting for several months, relying on their fat reserves.

The parvorder of Odontocetes – the toothed whales – include sperm whales, beaked whales, orcas, dolphins and porpoises. Generally their teeth have evolved to catch fish, squid or other

Toothed whales have well-developed senses – their eyesight and hearing are adapted for both air and water, and they have advanced sonar capabilities using their melon. Their hearing is so well-adapted for both air and water that some blind specimens can survive. Some species, such as sperm whales, are well adapted for diving to great depths. Several species of toothed whales show sexual dimorphism, in which the males differ from the females, usually for purposes of sexual display or aggression.^[17]

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).^[18]

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

The male genitals and the mammary glands of females are sunken into the body.^[19][20] The male genitals are attached to a vestigial pelvis.^[21]

The body is wrapped in a thick layer of fat, known as blubber. This provides thermal insulation and gives cetaceans their smooth, streamlined body shape. In larger species, it can reach a thickness up to one-half meter (1.6 feet).^{[citation needed]}

Sexual dimorphism evolved in many toothed whales. Sperm whales, narwhals, many members of the beaked whale family, several species of the porpoise family, orcas, pilot whales, eastern spinner dolphins and northern right whale dolphins show this characteristic.^[22] 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.^[22][23] Hind legs are not present in cetaceans, nor are any other external body attachments such as a

Head

Whales have an elongated head, especially

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

In toothed whales, connective tissue exists in the

River dolphins, unlike most other cetaceans, can turn their head 90°. Most other cetaceans have fused neck vertebrae and are unable to turn their head at all.^{[citation needed]}

The baleen of baleen whales consists of long, fibrous strands of keratin. Located in place of the teeth, it has the appearance of a huge fringe and is used to sieve the water for plankton and krill.^[28]

Sperm whales have the largest brain mass of any animal on Earth, averaging 8,000 cm³ (490 in³) and 7.8 kg (17 lb) in mature males.

]

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.[31]

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. m_brain ∝ (m_body)^k where k is between two-thirds and three-quarters. Thus if Species B is twice the size of Species A, its brain size will typically be somewhere between 60% and 70% higher.^[32] 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.^[33]

The

Skeleton

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The cetacean skeleton is largely made up of

.

The number of

.

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

is completely absent.

Cetaceans have a

Physiology

Circulation

Cetaceans have powerful hearts. Blood oxygen is distributed effectively throughout the body. They are warm-blooded, i.e., they hold a nearly constant body temperature.[38]

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

The structure of the

, provides an effective bond. This additional oxygen storage is vital for deep diving, since beyond a depth around 100 m (330 ft), the lung tissue is almost completely compressed by the water pressure.

Abdominal organs

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

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.

Senses

Vision

Cetacean eyes are set on the sides rather than the front of the head. This means only species with pointed 'beaks' (such as dolphins) have good binocular vision forward and downward. Tear glands secrete greasy tears, which protect the eyes from the salt in the water. The lens is almost spherical, which is most efficient at focusing the minimal light that reaches deep water.^{[citation needed][clarification needed]}

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.^[46]

At least one species, the

Ears

Cetaceans are known to possess excellent hearing.[48]

Biosonar

The external ear has lost the

A bony structure of the middle and inner ear, the

Cetaceans use sound to communicate, using groans, moans, whistles, clicks or the 'singing' of the humpback whale.^[47]

While differences in ear structure associated with echolocating abilities are found amongst Cetacea, cranial asymmetry has also been found to be a factor in the ability to produce sounds used in echolocation. Mysticeti, who don't have the ability to echolocate, possess general symmetry of the skull and facial region, while Odontoceti display a nasofacial asymmetry that is linked to their echolocating abilities.[53] Differences in the level of asymmetry also seem to correlate with differences in the types of sounds produced.^[53]

Mysticeti have exceptionally thin, wide basilar membranes in their

Chromosomes

The initial

Ecology

Range and habitat

Cetaceans are found in many aquatic habitats. While many marine species, such as the blue whale, the humpback whale and the orca, have a distribution area that includes nearly the entire ocean, some species occur only locally or in broken populations. These include the vaquita, which inhabits a small part of the Gulf of California and Hector's dolphin, which lives in some coastal waters in New Zealand. Most river dolphin species live exclusively in fresh water.^[56]

Many species inhabit specific latitudes, often in tropical or subtropical waters, such as

]

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.^[57] Migratory species' reproductive sites often lie in the tropics and their feeding grounds in polar regions.

Thirty-two species are found in European waters, including twenty-five toothed and seven baleen species.^{[citation needed]}

Many species of whales migrate on a latitudinal basis to move between seasonal habitats. For example, the gray whale migrates 10,000 miles (16,000 km) round trip. The journey begins at winter birthing grounds in warm lagoons along Baja California, and traverses 5,000–7,000 miles (8,000–11,300 km) of coastline to summer feeding grounds in the Bering, Chuckchi and Beaufort seas off the coast of Alaska.^[58]

Conscious breathing cetaceans

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.[60]

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

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.[63]

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.^[63]

Males in some baleen species communicate via whale song, sequences of high pitched sounds. These "songs" can be heard for hundreds of kilometers. Each population generally shares a distinct song, which evolves over time. Sometimes, an individual can be identified by its distinctive vocals, such as the 52-hertz whale that sings at a higher frequency than other whales. Some individuals are capable of generating over 600 distinct sounds.^[63] In baleen species such as humpbacks, blues and fins, male-specific song is believed to be used to attract and display fitness to females.^[64]

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

Intelligence

Cetacea are known to teach, learn, cooperate, scheme and grieve.^[65]

Smaller cetaceans, such as dolphins and porpoises, engage in complex play behavior, including such things as producing stable underwater toroidal air-core vortex rings or "bubble rings". The two main methods of bubble ring production are rapid puffing of air into the water and allowing it to rise to the surface, forming a ring, or swimming repeatedly in a circle and then stopping to inject air into the helical vortex currents thus formed. They also appear to enjoy biting the vortex rings, so that they burst into many separate bubbles and then rise quickly to the surface. Whales produce bubble nets to aid in herding prey.^[66]

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.^[67] Humpback whales also display this behaviour.^[68]

Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning (thinking about thinking) that humans exploit. Dolphins appear to possess self-awareness.^[69] 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.^[70]

Critics claim that the results of these tests are susceptible to the

In 1995, Marten and Psarakos used video to test dolphin self-awareness.[71] 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.^[70]

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).^[72] Little is known about how these decisions work, though studies have found evidence messy consensus decisions in groups of sperm whales and leadership in other species like bottlenose dolphins and killer whales.^{[citation needed]}

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.^[63]

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

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

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

]

Suckling

Like other placental mammals, cetaceans give birth to well-developed calves and nurse them with milk from their

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

In several species of both whales and dolphins, alloparenting has been observed.^[74][75]

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

Among cetaceans, whales are distinguished by an unusual longevity compared to other higher mammals. Some species, such as the

Death

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

Deterioration of whale carcasses happens through three stages. Initially, organisms such as

Disease

Evolution

The direct ancestors of today's cetaceans are probably found within the

Transition from land to sea

The fossil record traces the gradual transition from terrestrial to aquatic life. The regression of the hind limbs allowed greater flexibility of the spine. This made it possible for whales to move around with the vertical tail hitting the water. The front legs transformed into flippers, costing them their mobility on land.^[82]

One of the oldest members of ancient cetaceans (Archaeoceti) is Pakicetus from the Middle Eocene of Pakistan. This is an animal the size of a wolf, whose skeleton is known only partially. It had functioning legs and lived near the shore. This suggests the animal could still move on land. The long snout had carnivorous dentition.^[80]

The transition from land to sea dates to about 49 million years ago, with the Ambulocetus ("running whale"), also discovered in Pakistan. It was up to 3 m (9.8 ft) long. The limbs of this archaeocete were leg-like, but it was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly.^[83] 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.^[84]

Dating from about 45 million years ago are species such as

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

Wadi Al-Hitan ("Whale Valley") in Egypt contains numerous skeletons of Basilosaurus, as well as other marine vertebrates.^[85]

External phylogeny

Tethys Sea deposits in northern India and Pakistan. The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian plate.^{[citation needed}

]

aquatic reptiles

.

Molecular and morphological evidence suggests that

Artiodactyla can be represented in the following cladogram:^{[89][90][91][92][93]}

Artiodactyla	Tylopoda (camels) Artiofabula Suina (pigs) Cetruminantia Ruminantia  (ruminants) Tragulidae (mouse deer) Pecora (horn bearers) Cetancodonta/Whippomorpha Hippopotamidae (hippopotamuses) Cetacea (whales) 54 mya

Internal phylogeny

Further information: List of cetaceans and List of extinct cetaceans

Within Cetacea, the two

Odontoceti), which have teeth shaped like cones, spades, pegs, or tusks, and can perceive their environment through biosonar.^{[citation needed}

]

Relationship of extinct and extant cetaceans[94]:

Raoellidae † Cetacea Pakicetidae † Ambulocetidae † Remingtonocetidae † Rodhocetus † Dorudon † Basilosaurus † Mysticeti Janjucetus † Balaenidae Right whales Neobalaenidae Pygmy right whale Balaenopteridae Rorquals Eschrichtiidae Gray whales ( baleen whales , 25 mya) Odontoceti Kogiidae Dwarf sperm whales Physeteridae Sperm whales Platanistidae South Asian river dolphins Ziphiidae Beaked whales Inioidea Lipotidae Yangtze river dolphin Iniidae Pontoporiidae (South American river dolphins) Delphinoidea Delphinidae Oceanic dolphins ("true" dolphins) Monodontidae Belugas & Narwhals Phocoenidae Porpoises ( toothed whales )

† Extinct taxa

The terms whale and dolphin are informal:

• Mysticeti
  :

• Balaenopteridae (rorquals), Eschrichtiidae
  (grey whales)

• Odontoceti
  :

• Ziphiidae
  (beaked whales)
• Delphinidae (oceanic dolphins), Platanistidae (South Asian river dolphins), Lipotidae (old world river dolphins) Iniidae (new world river dolphins), and Pontoporiidae (La Plata dolphins
  )
• Phocoenidae

The term 'great whales' covers those currently regulated by the International Whaling Commission:^[95] 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).^[96]

Threats

The primary threats to cetaceans come from people, both directly from whaling or drive hunting and indirect threats from fishing and pollution.^[97]

Whaling

Main articles:

History of Whaling, and Dolphin drive hunting

Methods of

drive hunt in Hvalba on the Faroe Islands

being taken away with a forklift.