Cetacea

Cetacea (/sᵻˈteɪʃə/), (from Latin cetus "large sea creature" and Greek ketos "sea-monster"^[2]) are a widely distributed and diverse infraorder of carnivorous, aquatic, marine mammals. They comprise the families Balaenidae (right whales), Balaenoptera (rorqual), Eschrichtiidae (the gray whale), Delphinidae (oceanic dolphins), Monodontidae (Arctic whales), Phocoenidae (porpoises), Physeteridae (the sperm whale), Kogiidae (lesser sperm whales), Platanistidae (Old World river dolphins), Iniidae (New World river dolphins), Pontoporiidae (the La plata dolphin), and Ziphidae (beaked whales). There are currently 88 species of cetacean. While cetaceans were historically thought to have descended from mesonychids, molecular evidence supports them as descendants of Artiodactyla (even-toed ungulates). Cetaceans belong to the order Cetartiodactyla (derived from Cetacea and Artiodactyla), and their closest living relatives are hippopotamuses, having diverged about sixty million years ago.

Cetaceans range in size from the 1.3 m (4.3 ft) and 50 kg (110 lb) Maui's dolphin (a subspecies of Hector's dolphin) to the 34 m (112 ft) and 190 t (210 short tons) blue whale, which is also the largest creature. Several species exhibit sexual dimorphism. They have streamlined bodies and two limbs that are modified into flippers. Though not as flexible and agile as seals, cetaceans can swim very fast. Baleen whales use their plates to filter out food from the water, while toothed whales use their teeth to catch fast prey. River dolphins, unlike most other cetaceans, have the ability to turn their head 90°. By comparison, other cetaceans have fused neck vertebrae, and are unable to turn their head at all. Baleen whales have two blowholes, while toothed whales have only one. 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. A lot of species are well adapted for diving to great depths. Most have a layer of fat, or blubber, under the skin to keep warm in the cold water.

Although cetaceans are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend their lives in the water, mating, giving birth, molting completely submerged, coming to the surface to breathe. Toothed whales feed largely on fish and marine invertebrates; but a few, like the killer whale, feed on large vertebrates, such as penguins and seals. Grey whales are specialized for feeding on bottom-dwelling mollusks. Males typically mate with more than one female (polygyny), although the degree of polygyny varies with the species. Male pinniped strategies for reproductive success vary between performing ritual displays or lek mating. Calves are typically born in the spring and summer months and females bear almost all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long period. Cetaceans produce a number of vocalisations, notably the songs of the humpback whale, and the clicks of dolphins.

The meat, blubber and oil of cetaceans have traditionally been used by indigenous peoples of the Arctic. Cetaceans have been depicted in various cultures worldwide. Dolphins are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Once relentlessly hunted by commercial industries for their products, cetaceans are now protected by international law. The Yangtze river dolphin has become functionally extinct (though sometimes ranked critically endangered) in the past century, while the North Atlantic right whale and Yangtze finless porpoise are ranked critically endangered by the International Union for Conservation of Nature. Besides hunting, cetaceans also face threats from accidental trapping and marine pollution.

Baleen whales and toothed whales

The two parvorders of cetaceans, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have split up around 34 million years ago. The baleen whales have bristles made of keratin instead of teeth. The bristles are used to filter out water when feeding on krill and other small invertebrates. Grey whales are specialized for feeding on bottom-dwelling mollusks. The rorqual family (balaenopterids) use throat pleats to expand their mouths to take in food and sieve out the water. Balaenids (right whales and bowhead whales) have massive heads that can make up 40% of their body mass. Most mysticetes prefer the food-rich colder waters of the Northern and Southern Hemispheres, migrating to the Equator to give birth. During this process, they are capable of fasting for several months, relying on their fat reserves.

The parvorder of odontocetes - the toothed whales - include the sperm whale, the beaked whales, the killer whales, dolphins, and porpoises. They have conical teeth designed for catching fish or squid. A few, like the killer whale (orca), feed on mammals, such as pinnipeds and other whales. They have well-developed hearing − so well adapted for both air and water that some can survive even if they are blind. Some species, such as sperm whales, are well adapted for diving to great depths. Several species of odontocetes show sexual dimorphism, in which the males differ from the females, usually for purposes of sexual display or aggression.

Biology

Cetaceans are fully aquatic marine mammals. They are not able to survive on land. If cetaceans are stranded, their body weight compresses their lungs or breaks their ribs, as they sometimes can weigh 90 metric tons (99 short tons). Smaller whales can die of heatstroke because of their well-developed thermal insulation. The build of cetaceans is well adapted to their habitat, yet they still share essential characteristics with all other higher mammals (Eutheria):^[3]

• Cetaceans have lungs, meaning they are air-breathers. The time an individual can last without a breath varies from a few minutes to over two hours depending on the species.
• Cetaceans have especially powerful hearts. Likewise, the oxygen in the blood is distributed very effectively throughout the body.
• Cetaceans are warm-blooded animals, i.e., they hold a nearly constant, independent body temperature, as opposed to the cold-blooded animals.
• Like other placental mammals, cetaceans give birth to well-developed calves and nurse them with milk from their mammary glands. The embryonic development takes place in the body of the mother. During this time, the embryo is fed by a special nutritive tissue, the placenta.

Cetaceans include the largest animals on Earth. The blue whale (Balaenoptera musculus) can grow to a length of 33.5 m (110 ft) and weigh up to 180 metric tons (200 short tons), making it the largest known creature that has ever lived. The sperm whale (Physeter macrocephalus) is the largest of all toothed predatory animals, and possesses the largest brain. By contrast, the smallest cetacean species may reach a maximum body length of only about 1.5 m (4.9 ft), such as the La Plata dolphin, Hector's dolphin, and the vacquita.^[4]

Among cetaceans, whales are also distinguished by an unusual longevity compared to other higher mammals. Some species, such as the bowhead whale (Balaena mysticetus), can reach an age 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.^[5]

External anatomy

The body shape of cetaceans is similar to that of fish generally, which can be attributed to their lifestyle and the special conditions of the habitat. They have a streamlined shape, and their forelimbs are transformed into flippers. Almost all have a dorsal fin on their backs which can take on many forms depending on the species. In a few species, such as the beluga whale, it is completely absent. Both the flipper and the fin are for stabilization and steering in the water. They have a cartilaginous fluke at the end of their tails which is used for propulsion. The fluke is set horizontally instead of vertically on the body, as opposed to fish, which have vertical tails.

The hind legs are completely missing in cetaceans, as well as any other external body attachments that could hinder the streamlined shape, like the pinna and the hair. The male genitals and mammary glands of females are sunk into the body.^[6][7]

All whales have an elongated head, especially baleen whales, due to the extremely wide overhanging jaw. Their nostril(s) make up the blowhole, with one in toothed whales, and two in baleen whales. The blowhole lies on the top of the head, so that the rest of the body can remain submerged while surfacing for air. When the 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 between species in terms of shape, angle, and height. Species can be identified at a distance using this characteristic. In toothed whales, a connective tissue exists in the melon as a head buckle. This is filled with air sacs and fat which aids in buoyancy and biosonar. The sperm whale has a particularly pronounced melon; this is called the spermaceti organ and contains the eponymous spermaceti, hence the name "sperm whale". Even the long tusk of the narwhal is a vice-formed tooth. 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 among others.

The body is wrapped in a thick layer of fat, known as blubber, used for thermal insulation and gives cetaceans their smooth, streamlined body shape. In larger species, it can reach a thickness up to half a meter (1.6 ft).

Sexual dimorphism evolved in many toothed whales, with mating systems in which males compete to monopolize access to females. Sperm whales, narwhals, many members of the beaked whale family, several species of the porpoise family, killer whales, pilot whales, eastern spinner dolphins and northern right whale dolphins have all developed varying degrees and types of sexual dimorphism.^[8] Males in these species developed external features absent in females which are advantageous in combat or display. For example, sperm whale males are up to 63% percent larger than females and many beaked whales possess tusks used in combat competitions between males.^[8][9]

Skeleton

The cetacean skeleton is largely made up of cortical bone, which stabilizes the animal in the water. For this reason, the usual terrestrial compact bones, which are finely woven through cancellous bone, are replaced with lighter and more elastic bones. In many places, bone elements are replaced by cartilage and even fat, thereby improving their hydrostatic qualities. The ear and the muzzle contain a bone shape that is exclusive to cetaceans with an extremely high density, resembling porcelain. This has special acoustic properties and conducts sound better than other bones, thus aiding biosonar. In many toothed whales, the depression in their skull is due to the formation of a large melon and multiple air bags which are formed asymmetrically.

The skull of all cetaceans is extended, which can be clearly seen in baleen whales. The nostrils are located on top of the head above the eyes. The back of the skull is significantly shortened and deformed. By shifting the nostrils to the top of the head, the nasal passages extend perpendicularly through the skull. The teeth or baleen in the upper jaw sit exclusively on the maxilla. The braincase is concentrated through the nasal passage to the front and is correspondingly higher, with individual cranial bones that overlap. The bony otic capsule, the petrosal, is only cartilaginous when connected to the skull, so that it can swing independently.^[10]

The number of vertebrae that make up the spine varies between species, anywhere between 40 and 93 individual vertebrae. The cervical spine, found in all mammals, consists of seven vertebrae which, however, are greatly reduced or fused together. This gives stability during swimming at the expense of mobility. The fins are carried by the thoracic vertebrae, ranging from 9 to 17 individual vertebrae. The sternum is only cartilaginous, but nonetheless strong. The last two to three pairs of ribs are not connected at all and hang freely in the body wall. Behind it is the stable lumbar and tail part of the spine which includes all other vertebrae. Below the caudal vertebrae is the chevron bone; the vortex developed provides additional attachment points for the tail musculature.^[10]

The front limbs are paddle-shaped with shortened arms and elongated finger bones, to support the movement. They are united by cartilage. It also leads to a proliferation of the finger members, a so-called hyperphalangy, on the second and third fingers. The only functional joint is the shoulder joint in all cetaceans except for the Amazon river dolphin. The collarbone is completely absent. The movement of cetaceans on land is no longer necessary nor possible, due to the great body weight and the atrophied hindlimbs. In fact the rear limbs have become a rudimentary internal appendage without connections to the spine.^[10]

Internal anatomy and physiology

The structure of the respiratory and circulatory systems are of particular importance for the life of marine mammals. The oxygen balance of cetaceans is highly effective. Each breath can replace up to 90% of the total volume of air in their lungs. For land mammals, in comparison, this value is usually about 15%. During inhalation, about twice as much oxygen is withdrawn by the lung tissue as in a land mammal. As with all mammals, the oxygen is stored in the blood and the lungs, but in cetaceans, it is also stored in various tissues, mainly in the muscles. The muscle pigment, myoglobin, provides an effective bond. This additional oxygen storage is vital for deep diving, since from a depth around 100 m (330 ft), the lung tissue is almost completely compressed by the water pressure. During the diving process, the animals massively reduce their oxygen consumption by lowering the heart activity and blood circulation; individual organs are not supplied with oxygen during this time. Some rorquals can dive for up to 40 minutes, sperm whales between 60 and 90 minutes, and bottlenose whales for even two hours. Diving depths average about 100 m (330 ft), but some species, like sperm whales, can dive to 3,000 m (9,800 ft) deep. However they usually dive to 1,200 metres (3,900 ft).^[11][12]

The stomach consists of three chambers. The first region is formed by a loose gland and a very muscular forestomach (missing in beaked whales), which is then followed by the main stomach and the pylorus. Both are equipped with glands to help digestion. A bowel adjoins the stomachs, whose individual sections can only be distinguished histologically. The liver is very large and separate from the gall bladder.^[13]

The kidneys are markedly flattened and very long. As the salt concentration in cetaceans' blood is lower than that in seawater, the kidneys are also used to excrete salt. This allows the animals to drink seawater.^[14]

Senses

Cetacean eyes are set on the sides rather than the front of the head. This means only cetaceans 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. Cetaceans make up for their generally poor vision (except dolphins) with excellent hearing.

The external ear of cetaceans has lost the pinna (visible ear), but still retains an extremely narrow external auditory meatus. To register sounds, instead, the posterior part of the mandible has a thin lateral wall (the pan bone) behind which a concavity houses a large fat pad. The fat pad passes anteriorly into the greatly enlarged mandibular foramen to reach in under the teeth, and posteriorly to reach the thin lateral wall of the ectotympanic. The ectotympanic only offers a reduced attachment area for the tympanic membrane and the connection between this auditory complex and the rest of the skull is reduced in cetaceans — to a single, small cartilage in oceanic dolphins. In odontocetes, the complex is surrounded by spongy tissue filled with air spaces, while in mysticetes, it is integrated into the skull as with land mammals. In odontocetes, the tympanic membrane (or ligament) has the shape of a folded-in umbrella that stretches from the ectotympanic ring and narrows off to the malleus (quite unlike the flat, circular membrane found in land mammals.) In mysticetes, it also forms a large protrusion (known as the "glove finger"), which stretches into the external meatus, and the stapes are larger than in odontocetes. In some small sperm whales, the malleus is fused with the ectotympanic. The ear ossicles are pachyosteosclerotic (dense and compact) and very different in shape compared to land mammals (other aquatic mammals, such as sirenians and earless seals, have also lost their pinnae). In modern cetaceans, the semicircular canals are much smaller relative to body size than in other mammals.^[15]

In modern cetaceans, the auditory bulla is separated from the skull and composed of two compact and dense bones (the periotic and tympanic) referred to as the tympanoperiotic complex. This complex is located in a cavity in the middle ear, which, in the Mysticeti, is divided by a bony projection and compressed between the exoccipital and squamosal, but in the odontoceti, is large and completely surrounds the bulla (hence called "peribullar"), which is, therefore, not connected to the skull except in physeterids. In the Odontoceti, the cavity is filled with a dense foam in which the bulla hangs suspended in five or more sets of ligaments. The pterygoid and peribullar sinuses that form the cavity tend to be more developed in shallow water and riverine species than in pelagic (oceanic) Mysticeti. In Odontoceti, the composite auditory structure is thought to serve as an acoustic isolator, analogous to the lamellar construction found in the temporal bone in bats.^[16]

The Odontoceti (toothed whales, which includes dolphins and porpoises) are generally capable of echolocation.^[17] From this, the Odontoceti can discern the size, shape, surface characteristics, distance, and movement of an object. With this ability, cetaceans can search for, chase, and catch fast-swimming prey in total darkness.^[18] Echolocation is so advanced in most Odontoceti, they 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. Mysticeti (baleen whales) have exceptionally thin, wide basilar membranes in their cochleae without stiffening agents, making their ears adapted for processing low to infrasonic frequencies.^[19] 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.^[20]

Cetaceans also use sound to communicate, whether it be groans, moans, whistles, clicks, or the complex 'singing' of the humpback whale. Besides hearing and vision, at least one species, the tucuxi or Guiana dolphin, is able to use electroreception to sense prey.^[21]

Sleep

Unlike most animals, cetaceans are conscious breathers. All animals sleep, but cetaceans cannot afford to become 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.^[22]

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.^[23]

Similarities in the chromosome genetics

The initial karyotype of cetaceans includes a set of chromosomes from 2n = 44. They have four pairs of telocentric chromosomes (whose centromeres sit at one of the telomeres), two to four pairs of subtelocentric, and one or two large pairs of submetacentric chromosomes. The remaining chromosomes are metacentric - the centromere is approximately in the middle - and are rather small. The sperm whales (Physeteridae), the beaked whales (Ziphiidae) and the right whales (Balaenidae) converge to a reduction in the number of chromosomes to 2n = 42nd.^[24]

Distribution and habitat

Cetaceans are generally marine animals and are found in all oceans of the world. By contrast, only a few species live exclusively in fresh water, the river dolphins. While many marine species of cetaceans, such as the blue whale, the humpback whale and the killer whale, have a distribution area that includes nearly the entire ocean, some species occur only locally or in broken populations. These include the vacquita, which inhabits a small part of the Gulf of California, and Hector's dolphin, which lives in some coastal waters in New Zealand. In the oceans, both species prefer the deeper marine areas and species that live frequently or exclusively in coastal and shallow water areas.

The distribution of habitats arises normally along certain temperature limits in the oceans, which means many species exist along certain latitudes. Many species live accordingly in tropical or subtropical waters, such as Bryde's whale or Risso's dolphin, while others are found only in the Southern Ocean, like the southern right whale dolphin or the hourglass dolphin, or in the Arctic Ocean, like the narwhal and the beluga. This vertical expansion is interrupted primarily by land masses as natural barriers. Many cosmopolitan species and individual populations occur in the Pacific, Atlantic, and Indian Oceans. Some species, conversely, live only in one of those oceans or in small clusters. For example, Sowerby's beaked whale and the Clymene dolphin exist only in the Atlantic, and the Pacific white-sided dolphin and the northern straight dolphin live only in the North Pacific. For migratory species, their reproductive sites often lie in tropics and their feeding grounds in polar regions, but the two populations, one in the Arctic and the other in the Antarctic, are genetically separated by their long distance apart from each other. In some species, this separation of the populations leads eventually to the formation of new species, such as the southern right whale, North Pacific right whale, and North Atlantic right whale.^[25]

A total of 32 species of cetaceans are found in European waters, including 25 species of toothed whales and seven species of baleen whales.

Lifestyle

Social behavior

Most whales are extremely social animals with highly developed social behaviors. Only a few species live in pairs or have solitary lives. A group of cetaceans, known as a pod, usually consists of 10 to 50 animals, but on certain occasions, such as mass occurrence of food or mating season, may also include more than a thousand individuals in the groups. Also, interspecies socialization may occur.^[26]

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 extreme stress such as lack of food, but usually it is peaceful. This contact swimming, mutual fondling, and nudging play a major role. 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.^[26]

Data communication in certain baleen whale species between the singing males gives similar tones and melodies, which can be heard for hundreds of kilometers in the water. This activity is known as whale song. Probably, every whale population developed its own typical singing. Sometimes, an individual can be identified by its distinctive vocals, such as the 52-hertz whale which sings at a higher frequency than other whales. Some whales are capable of generating up to 622 different sounds. Comparisons of older and more recent recordings show that the composition of the sounds over the years has developed significantly.^[26] In certain baleen whales such as humpback whales, blue whales, and fin whales, male-specific song is believed to be used to attract and display fitness to females.^[8][27][28]

Pod groups also hunt, often with other species. Many species of dolphins hunt accompany large tunas on hunting expeditions, following large schools of fish. The killer whale hunts in pods and targets belugas and even larger whales. Humpback whales, among others, form in collaboration bubble carpets to herd krill or plankton into bait balls before lunging at them.^[26]

Intelligence

The Cetacea are highly intelligent, and are known to teach, learn, cooperate, scheme, and grieve.^[29] The neocortex of many cetaceans is home to elongated spindle neurons that, prior to 2007, were known only in hominids.^[30] In humans, these cells are involved in social conduct, emotions, judgment, and theory of mind.^[31] Cetacean spindle neurons are found in areas of the brain homologous to where they are found in humans, suggesting they perform a similar function.^[32] At the 2012 meeting of the American Association for the Advancement of Science, support was reiterated for a cetacean bill of rights, listing cetaceans as non-human persons.^[33]

Brain size was previously considered a major indicator of the intelligence of an animal. 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 more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the two-thirds or three-quarter exponent of the body mass.^[34] Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient that can be used as another indication of animal intelligence. 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.^[35] The brain to body mass ratio in some odontocetes, such as belugas and narwhals, is second only to humans.^[36] In some whales, however, it is less than half that of humans: 0.9% versus 2.1%. This comparison seems more favorable if the large amount of blubber that some whales require for insulation is omitted.

Smaller cetaceans, such as dolphins and porpoises, are known to engage in complex play behavior, which includes 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 a burst 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 are also known to produce bubble nets for the purpose of foraging.^[37]

Larger whales are also thought, to some degree, to engage in play. The southern right whale, for example, elevates its tail fluke above the water, remaining in the same position for a considerable amount of 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.^[38] Humpback whales, among others, are also known to display this behaviour.

Self-awareness is seen, by some, to be a sign of highly developed, abstract thinking. Self-awareness, though not well-defined scientifically, is believed to be the precursor to more advanced processes like metacognitive reasoning (thinking about thinking) that are typical of humans. Research in this field has suggested that cetaceans, among others, possess self-awareness.^[39] 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; then researchers deetermine if the animal shows signs of self-recognition.^[40]

Some disagree with these findings, arguing that the results of these tests are open to human interpretation and susceptible to the Clever Hans effect. This test is much less definitive than when used for primates, because primates can touch the mark or the mirror, while cetaceans cannot, making their alleged self-recognition behavior less certain. Skeptics argue that behaviors that are said to identify self-awareness resemble existing social behaviors, so researchers could be misinterpreting self-awareness for social responses to another individual. The researchers counter that the behaviors shown are evidence of self-awareness, as they are very different from normal responses to another individual. Whereas apes can merely touch the mark on themselves with their fingers, cetaceans show less definitive behavior of self-awareness; they can only twist and turn themselves to observe the mark.^[40]

In 1995, Marten and Psarakos used television to test dolphin self-awareness.^[41] 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 repeated since then, dolphins have since "passed" the mirror test.^[40]

Reproduction and development

For most species, a seasonal reproductive cycle occurs, in which the ovulation of the females coincides with the males' fertility. This cycle is usually coupled with seasonal movements, which can be observed in many species. Most toothed whales have no fixed bonds. In many species, the females have several partners during a season. The baleen whales are, though, largely monogamous within each reproductive period.

The gestation period of cetaceans ranges from 9 to 16 months, with the duration not necessarily dependent on the size. Porpoises, like blue whales, gestate for about 11 months. Cetaceans usually bear only one calf; with the occurrence of twins, one usually dies because the mother cannot afford sufficient milk for both juveniles. The fetus is positioned for a tail-first delivery, so that the risk of drowning while being delivered is minimal. After parturition, the young animal is carried quickly to the surface for its first breath. The young animals are, at birth, about one-third of their adult length and tend to be independently active, comparable to the terrestrial mammals. When suckling, the mother splashes milk actively using the muscles of the mammary glands into the mouth of the calf, as it has no lips to suck. This milk usually has a very high fat content, ranging from 16 to 46%, causing the calf to increase rapidly in size and weight.^[26]

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 the mother and her offspring. In all cetaceans, the mother is solely responsible for rearing her young. In some species, so-called "aunts" also occasionally suckle the young. Most cetaceans mature late, typically at seven to 10 years. This reproductive strategy provides a few offspring that have a high survival rate. An exception to this is the La Plata dolphin, which is already sexually mature at two years, but lives to be only about 20 years old. The sperm whale reaches sexual maturity within about 20 years, but has a lifespan between 50 and 100 years.^[26]

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 seeps and hydrothermal vents.^[42]

Deterioration of whale carcasses happens though a series of three stages. Initially, moving organisms such as sharks and hagfish scavenge the soft tissues at a rapid rate over a period of months, and as long as two years. This is followed by the colonisation 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 other organisms such as mussels, clams, limpets, and sea snails. This stage may last for decades and supports a rich assemblage of species, averaging 185 species per site.^[42][43]

Disease

Brucellosis affects almost all mammals. While it may be distributed worldwide, fishing and pollution have caused porpoise population density pockets, which further risks infection and the spreading of disease. Brucella ceti, most prevalent in dolphins, has been shown to cause chronic disease, thus increasing the chance of failed birth and miscarriages, male infertility, neurobrucellosis, cardiopathies, bone and skin lesions, strandings, and death. Until 2008, no case had ever been reported in porpoises, but isolated populations have an increased risk, and consequentially a high mortality rate.^[44]

Evolution

Relationships and phylogenetic development

For a long time, paleontologists thought the ancestors of whales were the mesonychid, a group of carnivorous ungulates with a controversial systematic position, because of the similar nature of the skull and teeth. Later studies of molecular biology and immunology showed that cetaceans are phylogenetically closely related with the even-toed ungulates (Artiodactyla). The lineage of whales, therefore, began in the early Eocene, more than 50 million years ago, with early artiodactyls.^[46]

Fossil discoveries at the beginning of the 21st century have confirmed this. The most striking common feature of cetaceans and even-toed ungulates is the talus, a bone in the upper ankle joint. Early cetaceans, archaeocetes, show double castors which only occur in even-toed ungulates. Corresponding findings are from deposits of Tethys Sea in northern India and Pakistan. The Tethys Sea extended, during this time, as a shallow sea between the Asian continent and northward-bound Indian plate.

Most molecular biological evidence suggests that hippos are the closest living relatives, a sister group basically, of cetaceans. Some common anatomical features include similarities in the morphology of the posterior molars. The fossil record, however, cannot provide evidence to this assumed relationship, because the hippo lineage only dates back about 15 million years. The oldest cetaceans, however, date back to about 50 million years ago.^{[47][48][49][50]}

In 2007, a group led by paleontologist Hans Thewissen created an alternative pedigree. Accordingly, the sister group of archaeocetes were the Raoellidae, an extinct group of even-toed ungulates. Both taxa form accordingly together the sister group of the remaining artiodactyls including hippos.

Cetartiodactyla   other Artiodactyls  N.N.   Raoellidae (Indohyus, Khirharia...)  Cetacea

The presumed close relationship based on characteristics according Thewissen are primarily the bony ring on the temporal bone (bulla), and the involucre, a skull feature which was previously associated only with cetaceans, as well as other features of the premolars and the bone structure.^[51]

Use of the fossil record can trace the gradual transition from terrestrial life to aquatic life. The regression of the hind limbs allowed greater flexibility of the spine. This has made it possible for whales to move around with the vertical tail hitting the water. The front legs were transformed into flippers, but they lost their original mobility on land.

The ears of today's cetaceans are no longer external; the nostrils moved from the snout upwards, and transformed into a blowhole, so cetaceans do not have to surface completely to breathe. While the teeth are divided into incisors, canines, and molars among the terrestrial archaeocetes, the teeth of the modern cetaceans are brought into line with each other, which can be seen among the fish-eating odontocetes (transition from heterodont to homodont). A special and relatively late development occurred in the mysticetes: they evolved baleen around 25 million years ago to replace teeth. They are fringe-like structures of a horny protein (keratin) used for filtering food from water.

The transition from land to sea

One of the oldest members of ancient cetaceans (Archaeoceti) is Pakicetus from the Middle Eocene 50 million years ago. This animal is as big as a wolf, whose skeleton is known only partially. It still had functioning legs and lived near the shore. This suggests the animal could still move well on land. The long snout had a carnivorous dentition.^[46]

The important transition from land to sea is considered to date to about 49 million years ago, with the Ambulocetus ("running whale"), discovered in Pakistan. It was up to 3 m (9.8 ft) long. The limbs of this archaeocete were adapted to swimming, but terrestrial locomotion was still possible. It probably crawled like a seal or crocodile. The snout was elongated with overhead nostrils and eyes. The tail of the animal was very 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.^[52]

Dating from about 45 million years ago are more species such as Indocetus, Kutchicetus, Rodhocetus, and Andrewsiphius, all of which were clearly adapted to life in water. The hind limbs of these species were already highly regressed, and their body shapes are reminiscent of modern whales. Rodhocetus, a member of the Protocetidae family, is considered the first fully aquatic cetacean. The body was streamlined and delicate with extended hand and feet bones. The merged pelvic lumbar spine was still present, making it possible to support the floating movement of the tail. It was therefore a good swimmer, but could probably move only relatively clumsily on land, much like a modern seal.^[46]

Residents of the oceans

Since the late Eocene, about 40 million years ago, cetaceans populated the subtropical oceans of the world and no longer had any connection to land. An example is the 18-m-long Basilosaurus, sometimes referred to as Zeuglodon. The transition from land to water was therefore 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.

The direct ancestors of today's cetaceans are probably found within the Dorudontidae whose most famous member, Dorudon atrox, lived at the same time as Basilosaurus. Both groups had already developed the typical anatomical features of today's whales, such as hearing. Life in the water for a formerly terrestrial creature required significant adjustments such as the fixed bulla, which replaces the eardrum of land mammals, as well as sound-conducting elements for directional hearing under water. The wrists of these animals were stiffened and probably contributed to the typical build of flippers. The hind legs still existed, however, but significantly reduced in size and with a vestigial connected pelvis.^[46]

Classification

Baleen whales (Mysticeti) owe their name to their baleen, the comb-like, frayed structures at the ends of horny plates; the whales with these plates filter micro-organisms, such as plankton, from the seawater by taking a large amount of sea water into their mouth and squeezing it through the baleen. The plates of bowhead whales can be 4 metres (13 ft) long. This group includes the largest living animals. Toothed whales (Odontoceti), which include the dolphins and porpoises, have a number of conical teeth in both jaws, as with dolphins, or only in the lower jaw, for example, the sperm whale or the beaked whales. Porpoises, however, have spade-shaped teeth. Toothed whales are characterized by their ability perceive their environment through means of biosonar.

Relationship of extinct and extant cetaceans[53]:

Cetacea   Toothed whales   Delphinoidea   Belugas (Monodontidae)  Porpoise (Phocoenidae)  Oceanic dolphins (Delphinidae)  Iniidae  Pontoporiidae  Beaked whales (Ziphiidae)  River dolphins (Platanistidae)  Dwarf sperm whales (Kogiidae)  Sperm whales (Physeteridae)  Baleen whales   Rorquals (Balaenopteridae)  Gray whales (Eschrichtiidae)  Neobalaeninae  Right whales (Balaenidae)  Janjucetus †  Basilosaurus †  Dorudon †  Rodhocetus †  Remingtonocetidae †  Ambulocetidae †  Pakicetidae †  Raoellidae †

†Recently extinct

• INFRAORDER CETACEA
  • Parvoder Mysticeti: Baleen whales
    • Superfamily Balaenoidea: Right whales
      • Family Balaenidae
        • Genus Balaena
          • Bowhead whale, Balaena mysticetus
        • Genus Eubalaena
          • North Atlantic right whale, Eubalaena glacialis
          • North Pacific right whale, Eubalaena japonica
          • Southern right whale, Eubalaena australis
      • Family Cetotheriidae ^[54]
      • Genus Caperea
        • Pygmy right whale, Caperea marginata
    • Superfamily Balaenopteroidea
      • Family Balaenopteridae: Rorquals
        • Subfamily Balaenopterinae
          • Genus Balaenoptera: slender rorquals
            • Common minke whale, Balaenoptera acutorostrata
            • Antarctic minke whale, Balaenoptera bonaerensis
            • Sei whale, Balaenoptera borealis
            • Bryde's whale, Balaenoptera brydei
            • Eden's whale Balaenoptera edeni
            • Omura's whale – Balaenoptera omurai
            • Blue whale, Balaenoptera musculus
            • Fin whale, Balaenoptera physalus
        • Subfamily Megapterinae
          • Genus Megaptera
            • Humpback whale, Megaptera novaeangliae
      • Family Eschrichtiidae: Gray whales
        • Genus Eschrichtius
          • Gray whale, Eschrichtius robustus
  • Parvorder Odontoceti: Toothed whales
    • Superfamily Delphinoidea: Oceanic dolphins
      • Family Delphinidae
        • Genus Cephalorhynchus: blunt-nosed dolphins
          • Commerson's dolphin, Cephalorhyncus commersonii
          • Chilean dolphin, Cephalorhyncus eutropia
          • Heaviside's dolphin, Cephalorhyncus heavisidii
          • Hector's dolphin, Cephalorhyncus hectori
        • Genus Delphinus: common dolphins
          • Long-beaked common dolphin, Delphinus capensis
          • Short-beaked common dolphin, Delphinus delphis
          • Arabian common dolphin, Delphinus tropicalis
        • Genus Feresa
          • Pygmy killer whale, Feresa attenuata
        • Genus Globicephala: pilot whales
          • Short-finned pilot whale, Globicephala macrorhyncus
          • Long-finned pilot whale, Globicephala melas
        • Genus Grampus
          • Risso's dolphin, Grampus griseus
        • Genus Lagenodelphis
          • Fraser's dolphin, Lagenodelphis hosei
        • Genus Lagenorhynchus: false bottlenose dolphins
          • Atlantic white-sided dolphin, Lagenorhynchus acutus
          • White-beaked dolphin, Lagenorhynchus albirostris
          • Peale's dolphin, Lagenorhynchus australis
          • Hourglass dolphin, Lagenorhynchus cruciger
          • Pacific white-sided dolphin, Lagenorhynchus obliquidens
          • Dusky dolphin, Lagenorhynchus obscurus
        • Genus Lissodelphis: right whale dolphin
          • Northern right whale dolphin, Lissodelphis borealis
          • Southern right whale dolphin, Lissodelphis peronii
        • Genus Orcaella: Irrawaddy dolphins
          • Irrawaddy dolphin, Orcaella brevirostris
            • Australian snubfin dolphin, Orcaella heinsohni
        • Genus Orcinus
          • Killer whale, Orcinus orca
        • Genus Peponocephala
          • Melon-headed whale, Peponocephala electra
        • Genus Pseudorca
          • False killer whale, Pseudorca crassidens
        • Genus Sotalia: northern South American dolphins
          • Tucuxi, Sotalia fluviatilis
        • Guiana dolphin, Sotalia guianensis
        • Genus Sousa: humpback dolphins
          • Pacific humpback dolphin, Sousa chinensis
          • Indian humpback dolphin, Sousa plumbea
          • Atlantic humpback dolphin, Sousa teuszii
        • Genus Stenella: spotted dolphins
          • Pantropical spotted dolphin, Stenella attenuata
          • Clymene dolphin, Stenella clymene
          • Striped dolphin, Stenella coeruleoalba
          • Atlantic spotted dolphin, Stenella frontalis
          • Spinner dolphin, Stenella longirostris
        • Genus Steno
          • Rough-toothed dolphin, Steno bredanensis
        • Genus Tursiops: true bottlenose dolphins
          • Indian Ocean bottlenose dolphin, Tursiops aduncus
          • Burrunan dolphin, Tursiops australis
          • Common bottlenose dolphin, Tursiops truncatus
      • Family Monodontidae: Arctic whales
        • Genus Delphinapterus
          • Beluga, Delphinapterus leucas
        • Genus Monodon
          • Narwhal, Monodon monoceros
      • Family Phocoenidae: Porpoises
        • Genus Neophocaena
          • Finless porpoise, Neophocaena phocaenoides
        • Genus Phocoena
          • Spectacled porpoise, Phocoena dioptrica
          • Harbour porpoise, Phocoena phocaena
          • Vaquita, Phocoena sinus
          • Burmeister's porpoise, Phocoena spinipinnis
        • Genus Phocoenoides
          • Dall's porpoise, Phocoenoides dalli
    • Superfamily Physeteroidea: Sperm whales
      • Family Physeteridae
        • Genus Physeter
          • Sperm whale, Physeter catodon (syn. P. macrocephalus)
      • Family Kogiidae: lesser sperm whales
        • Genus Kogia
          • Pygmy sperm whale, Kogia breviceps
          • Dwarf sperm whale, Kogia sima
    • Superfamily Platanistoidea: Indian river dolphins
      • Family Platanistidae
        • Genus Platanista
          • South Asian river dolphin, Platanista gangetica
    • Superfamily Inioidea: South American river dolphins
      • Family Iniidae
        • Genus Inia
          • Amazon river dolphin, Inia geoffrensis
            • Bolivian river dolphin, Inia boliviensis
          • Araguaian river dolphin. Inia araguaiaensis
      • Family Pontoporiidae
        • Genus Pontoporia
          • La Plata dolphin, Pontoporia blainvillei
    • Superfamily Ziphioidea: Beaked whales
      • Family Ziphidae
        • Genus Berardius: giant beaked whales
          • Arnoux's beaked whale, Berardius arnuxii
          • Baird's beaked whale (North Pacific bottlenose whale), Berardius bairdii
        • Subfamily Hyperoodontidae
          • Genus Hyperoodon: bottlenose whales
            • Northern bottlenose whale, Hyperoodon ampullatus
            • Southern bottlenose whale, Hyperoodon planifrons
          • Genus Indopacetus
            • Indo-Pacific beaked whale (Longman's beaked whale), Indopacetus pacificus
          • Genus Mesoplodon, Mesoplodont whale
            • Sowerby's beaked whale, Mesoplodon bidens
            • Andrews' beaked whale, Mesoplodon bowdoini
            • Hubbs' beaked whale, Mesoplodon carlhubbsi
            • Blainville's beaked whale, Mesoplodon densirostris
            • Gervais' beaked whale, Mesoplodon europaeus
            • Ginkgo-toothed beaked whale, Mesoplodon ginkgodens
            • Gray's beaked whale, Mesoplodon grayi
            • Hector's beaked whale, Mesoplodon hectori
            • Strap-toothed whale, Mesoplodon layardii
            • True's beaked whale, Mesoplodon mirus
            • Perrin's beaked whale, Mesoplodon perrini
            • Pygmy beaked whale, Mesoplodon peruvianus
            • Stejneger's beaked whale, Mesoplodon stejnegeri
            • Spade-toothed whale, Mesoplodon traversii
            • Deraniyagala's beaked whale, Mesoplodon hotaula
        • Genus Tasmacetus
          • Shepherd's beaked whale, Tasmacetus shepherdi
        • Genus Ziphius
          • Cuvier's beaked whale, Ziphius cavirostris

Interactions with humans

Research history

In Aristotle's time, the 4th century BCE, whales were regarded as fish due to their superficial similarity. Aristotle, however, could already see many physiological and anatomical similarities with the terrestrial vertebrates, such as blood (circulation), lungs, uterus and fin anatomy.^[55] His detailed descriptions were assimilated by the Romans, but mixed with a more accurate knowledge of the dolphins, as mentioned by Pliny the Elder in his “Natural history”. In the art of this and subsequent periods, dolphins are portrayed with a high-arched head (typical of porpoises) and a long snout. The harbour porpoise is one of the most accessible species for early cetologists, because it could be seen very close to land, inhabiting shallow coastal areas of Europe. Much of the findings that apply to all cetaceans were therefore first discovered in the porpoises.^[56] One of the first anatomical descriptions of the airways of the whales on the basis of a harbor porpoise dates from 1671 by John Ray. It nevertheless referred to the porpoise as a fish.^[57][58]

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

Threats

The primary threats to cetaceans come from people, both the direct threat of hunting by whaling or drive hunting and indirect threats from sources such as fisheries and pollution.^[59]

Whaling

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

In the Middle Ages, people had many reasons for whaling, including their huge amounts of meat, the oil usable as fuel, and the jawbone, which was used in house construction. At the end of the Middle Ages, whole whaling fleets developed, mainly aimed at baleen whales, such as the bowhead whales. For example, in the 16th and 17th centuries, the Dutch fleet had about 300 whaling ships with 18,000 crewmen.

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

Large ships were transformed into mother ships for the whale handlers, and were supplied by fleets with steam drive. In the first half of the 20th century, whales were of great importance as a supplier of raw materials for industries. Whales were intensively hunted during this time; in the 1930s, 30,000 whales were killed. A further increase to over 40,000 animals per year was made up to the 1960s, which especially collapsed the stocks of large baleen whales.

Most hunted whales are now threatened, with some great whale populations exploited to the brink of extinction. Today, some are decimated, but a slow increase is possible. The Atlantic and the Korean gray whale populations were completely eradicated, and the North Atlantic right whales have an estimated population of 300 to 600 animals, while the blue whale probably has a population of around 14,000 animals.

The first efforts to protect whales were made in 1931. Some particularly endangered species, such as the humpback whale (which then numbered about 100 animals), were placed under international protection, and the first protected areas were established. In 1946, the International Whaling Commission was established, to monitor and secure the stocks of whales. The killing of whales for commercial purposes was prohibited worldwide by this organization from 1985 to 2005. The stocks of some species such as humpbacks and blue whales have staged a dramatic recovery in recent years, though still under threat from other causes. Whales are still hunted today. Japanese whaling ships are allowed to hunt whales of different species for supposedly scientific purposes. Native Greenlandics and other indigenous peoples of the world are allowed to continue their traditional whaling practices. Iceland and Norway do not recognize the ban, and openly operate commercial whaling. Countries like Norway and Japan are committed to ending the ban.

Dolphins and other smaller cetaceans are also hunted in an activity known as dolphin drive hunting. This is accomplished by driving a pod together with boats, usually into a bay or onto a beach. Their escape is prevented by closing off the route to the ocean with other boats or nets. Dolphins are hunted this way in several places around the world, including the Solomon Islands, the Faroe Islands, Peru, and Japan, the most well-known practitioner of this method. By numbers, dolphins are mostly hunted for their meat, though some end up in dolphinariums. Despite the controversial nature of the hunt resulting in international criticism, and the possible health risk that the often polluted meat causes, thousands of dolphins are caught in drive hunts each year.

Fishing

Dolphin pods often reside near large shoals of tuna. This is also known to fishermen, which is why they often look for dolphins to catch tuna. Dolphins are much easier to spot from a distance than tuna, since they have to take a breath at the surface regularly. The fishermen pull their nets hundreds of meters wide in a circle around the dolphin groups, in the expectation that they also include a tuna shoal. When the nets are pulled together, the dolphins become entangled under water and drown. Line fishery in larger rivers are threats especially to river dolphins.

A far greater threat than by-catch arises, however, for small cetaceans: targeted hunting. In Southeast Asia, they are sold in poorer countries as fish-replacement to the native population, since the actual edible fish of the region promise higher revenues from exports. In the Mediterranean, small cetaceans are pursued as competitors for food, since the metabolism of marine mammals has a disproportionately higher energy needs than in predatory fish, they are deliberately destroyed to maintain the population of edible fish without having to share with them.^[59]

Strandings

A stranding is the unintentional emergence of a cetacean to beach itself. The best known are mass strandings of pilot whales and sperm whales. The causes of cetacean strandings are not clear. Possible reasons for mass beachings are:^[59]

• toxic contaminants in the food chain
• debilitating parasites in the respiratory tract, brain or middle ear
• bacterial or viral infections
• panic flight from predators (incl. humans)
• strong social bonds within a group, so that all individuals follow a stranded animal
• disturbance of their magnetic senses by natural anomalies in the Earth's magnetic field
• Injuries
• Underwater noise pollution by shipping traffic, seismic surveys and military sonar experiments.

In the last 15 years, whale strandings frequently occurred in the context of 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 co-author of her interim report concludes that the animals killed by the active sonar of some Navy ships were injured. Generally, underwater noise, which is still on the increase, is made increasingly responsible for strandings because it impairs communication and sense of direction of the animals.^[60]

Climate change also appears by influencing the major wind systems to lead the world and thus the course of ocean currents which lead to cetacean strandings. Mark Hindell and his team from the Tasmanian University in Hobart studied cetacean strandings on the coast of Tasmania from 1920-2002 and found that greater strandings occurred at certain time intervals. Years with a tenfold number of strandings was accompanied by the occurrence of severe storms, which initiated the cold water flows increasingly close to the coast. In nutrient-rich cold water, cetaceans find particularly large prey animals, so they followed the cold water currents and thus, in these exceptional meteorological years, came into shallower waters than usual, where the risk is higher for strandings. Since many whales and dolphins live in pods and follow or accompany the sick or debilitated animals into shallow water, mass stranding occur often at low tide. Once stranded, large whales are crushed by their own body weight, if they cannot get into deeper water. In addition, the regulation of body temperature is no longer guaranteed in a stranded whale and there is a risk of overheating for smaller whales.

Environmental hazards

The increasing pollution for the marine mammals is also a serious problem. 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 diseases and have fewer offspring.^[59]

The destruction of the ozone layer affects baleen whales, because plankton is highly sensitive to radiation and multiplies less. This shrinks the food supply for many marine animals, but the baleen whales are most impacted by this. Even the Nekton is, in addition to the intensive exploitation, damaged by the intense UV irradiation and is limited in quantity and quality as a food source.^[59]

Similar effects can be, at least in a long-term sense, acidification of the oceans due to the increased uptake of carbon dioxide, an effect that global warming counteracts because it heats up the atmosphere while decreasing the amount of carbon. CO2 reacts with water to form carbonic acid. The acidic water interferes with the construction of the calcium carbonate skeletons of various algae and micro-organisms. This in turn decreases the amount of plankton that baleen whales depend on, as it represents the main food source for many species.^[59]

Above all, the military and the geology employ strong sonar and produce blasting operations along with vessel traffic which increases noise in the oceans. Marine mammals that are characterized for their use of biosonar for orientation and communication are not only hindered, but are also induced to panic surface. This leads to bubble out of bound in blood gases, and the animal then dies because the tubes are blocked, so-called decompression accidents (known in humans as a "serious diving accident").^[61]

Naval exercises with sonar regularly results in fallen cetaceans that are washed up with gas bubbles in the blood vessels. The sound is very extensive and develops its disastrous effects in more than 100 kilometres (62 mi) radius. Depending on the frequencies used, different species are more or less affected than others.

Cultural significance

Cetaceans play a big role in the culture of residents near sea areas and islands. These are mainly small cetaceans such as dolphins and porpoises, which could be observed intensively and thus were able to enter into the mythology of these people. However, great whales were known primarily from whale strandings (especially sperm whales) or from descriptions by mariners.

Prehistoric

Petroglyphs of the Stone Age, such as those in Roddoy and Reppa (Norway), show that the animals were known early in these cultures. Whale bones were used for numerous purposes. In the Neolithic settlement of Skara Brae on Orkney is made from vertebrae saucepans. Some of the vessels contain traces of color, Childe treated them as paint buckets. A shell of baleen comes from Foshigarry, Scotland.

Antiquity

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

In the Bible especially, the leviathan plays a role as a sea monster. The essence, which features a giant crocodile or a dragon and a whale, was created according to the Bible by God (Psalm 104, 26) and should again be destroyed by him (Psalm 74.14 and Isaiah 27.1). In the Book of Job, the leviathan is described in more detail (Job 40.25 to 41.26).

In Jonah 2.1 to 11 is a clearer, more recognizable description of a whale alongside the prophet Jonah, who, on his flight from the city of Nineveh is swallowed by a whale.

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

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

Dolphins occupy some space in Greek mythology. Because of their intelligence, they rescued multiple people from drowning. They were said to have a special love for music - probably not least because of their own song - they saved, in the legends, very famous musicians like Arion of Lesbos from Methymna or Kairanos from Miletus. Because of the mental faculties, dolphins were considered for the god Dionysus.

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

In ancient art, there are often dolphin representations. Even the Cretan Minoans represented them. Later they often found them on reliefs, gems, lamps, coins, mosaics, grave stones, etc. A particularly popular representation is that of Arion or the Taras riding on a dolphin. In early Christian art, the dolphin is a popular motif, not least because it was partially, next to the fish, used as a symbol of Christ.

Middle Ages to the 19th century

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

Most descriptions of large whales from this time to the whaling era, starting in the 17th century, came from beached whales, which resembled no other known animal. This was particularly true for the sperm whale, the most frequently stranded in larger groups. Raymond Gilmore in 1959 to 1723 documented 17 sperm whales in the estuary of the Elbe and in 1784 documented 31 animals on the coast of Great Britain. In 1827, a blue whale with a length of 28.5 metres (94 ft) beached itself off the coast of Ostend, whose bones were circulated throughout Europe for seven years. During this time, whales were used as attractions in museums and traveling exhibitions.

The sailors in the whaling fleets of the 17th to 19th century provided more graphic representations of whales, and the tales of whale watching led to stories that, to a large extent, can be regarded as sailors' yarns. Although they knew that most whales were harmless giants, they described many battles with harpooned animals. With the intensification of whaling, there were increasing descriptions of sea monsters, including huge whales, sharks, sea snakes, giant squid, and octopuses.

Among the first whalers who described their experiences on whaling trips was Captain William Scoresby from Great Britain, who published the book Northern Whale Fishery, describing the hunt for the great baleen whales of the northern seas. This was followed by Thomas Beale, a British surgeon, with the book Some observations on the natural history of the sperm whale in 1835; and Frederick Debell Bennett's The tale of a whale hunt in 1840. Also, whales were described in narrative literature and paintings, most famously in the novels Moby Dick by Herman Melville and 20,000 Leagues Under the Sea by Jules Verne. In the 1882 children's book Adventures of Pinocchio by Carlo Collodi, the wooden figures Pinocchio and Geppettos' creators were swallowed by a whale.

Even in historical times, baleen was used as a raw material. Individual vessel components made of baleen partly existed, like the bottom of a bucket in the Scottish National Museum Edinburgh. From Howmae comes a chair seat made from baleen. The Norse crafted ornamented plates from baleen, sometimes interpreted as ironing boards.

In the Canadian Arctic (east coast) in Punuk and Thule culture (1000-1600 C.E.),^[62] I baleen was used to construct houses. It was used 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 skins, which were covered with soil and moss.^[63]

Modern culture

Unlike in the past centuries, whales have not been seen in the 20th century as sea monsters and dangerous beasts. With its increasing research, they are gradually more and more discovered to be as an intelligent and peaceful group of animals that are hunted and killed for no reason by people. Above all others, the dolphins in particular received this role, which is also reflected in films and novels of the 1960s and 1990s. For example, the protagonist of the series Flipper was a bottlenose dolphin; from 1962 alongside other animal heroes like Rin Tin Tin, Lassie and Fury to a symbol of animal intelligence. This design was also used in the series SeaQuest DSV (1993-1996), the Warner Bros. movie Free Willy, the movie Star Trek IV: The Voyage Home, and the book series The Hitchhiker's Guide to the Galaxy by Douglas Adams.^[64]

The prestige of the great whales, until then mainly known by Moby-Dick, also changed dramatically. The animals are, since the 20th century, partly glorified as "gentle giants" who peacefully roam the seas. Above all, the study of whale song also led to an increasingly strong position in the field of esotericism. Today, their songs are used as a relaxing meditation music .

In captivity

Belugas

Belugas were the first whales to be kept in captivity. Other species were too rare, too shy, or too big. The first beluga was shown at Barnum's Museum in New York City in 1861.^[65] For most of the 20th century, Canada was the predominant source of wild belugas.^[66] They were taken from the St. Lawrence River estuary until the late 1960s, after which they were predominantly taken from the Churchill River estuary until capture was banned in 1992.^[66] Russia has become the largest provider since it had been banned in Canada.^[66] Belugas are caught in the Amur River delta and their eastern coast, and then are either transported domestically to aquariums or dolphinariums in Moscow, St. Petersburg, and Sochi, or exported to other countries, such as Canada.^[66] Most captive belugas are caught in the wild, since captive-breeding programs are not very successful.^[67]

As of 2006, 30 belugas were in Canada and 28 in the United States, and 42 deaths in captivity had been reported up to that time.^[66] A single specimen can reportedly fetch up to US$100,000 (UK£64,160) on the market. The beluga's popularity is due to its unique colour and its facial expressions. The latter is possible because while most cetacean "smiles" are fixed, the extra movement afforded by the beluga's unfused cervical vertebrae allows a greater range of apparent expression.^[68]

Killer whales

The killer whale's intelligence, trainability, striking appearance, playfulness in captivity and sheer size have made it a popular exhibit at aquaria and aquatic theme parks. From 1976 to 1997, 55 whales were taken from the wild in Iceland, 19 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 48 animals on display in the world were captive-born.^[69]

Organizations such as World Animal Protection and the Whale and Dolphin Conservation Society campaign against the practice of keeping them in captivity. In captivity, they often develop pathologies, such as the dorsal fin collapse seen in 60–90% of captive males. Captives have vastly reduced life expectancies, on average only living into their 20s, although there are examples of killer whales living longer, including several over 30 years old, and two captive orcas, Corky II and Lolita, are in their mid-40s. In the wild, females who survive infancy live 46 years on average, and up to 70–80 years in rare cases. Wild males who survive infancy live 31 years on average, and up to 50–60 years.^[70] 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 killer whale behavior. Wild killer whales may travel up to 160 kilometres (100 mi) in a day, and critics say the animals are too big and intelligent to be suitable for captivity.^[71] Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of stress.^[72] Between 1991 and 2010, the bull orca known as Tilikum was involved in the death of three people, and was featured in the critically acclaimed 2013 film, Blackfish.^[73] Tilikum has lived at SeaWorld since 1992.^[74][75][76]

In July 2015, the Journal of Mammalogy published a study comparing life-history parameters of free-ranging killer whale populations with orca managed in captive facilities by SeaWorld Parks. The article coauthored by staff at SeaWorld and the Minnesota Zoo, indicates that there is no significant difference in survivorship between free-ranging and captive killer whales. The authors speculate about the future utility of studying captive populations for the purposes of understanding orca biology and the implications of such research of captive animals in the overall health of both wild and marine park populations.^[77] Each country has their own tank requirements; in the US, the minimum enclosure size is set by the Code of Federal Regulations, 9 CFR E § 3.104, under the Specifications for the Humane Handling, Care, Treatment, and Transportation of Marine Mammals.^[78]

Other cetaceans

Various species of dolphins are kept in captivity as well as several other species of porpoise such as harbour porpoises and finless porpoises. These small cetaceans are more often than not kept in theme parks, such as SeaWorld, commonly known as a dolphinarium. Bottlenose dolphins are the most common species of dolphin kept in dolphinariums as they are relatively easy to train, have a long lifespan in captivity and have a friendly appearance. Hundreds if not thousands of bottlenose dolphins live in captivity across the world, though exact numbers are hard to determine. Orcas are well known for their performances in shows, but the number of orcas kept in captivity is very small, especially when compared to the number of bottlenose dolphins, with only 44 captive orcas being held in aquaria as of 2012.^[79] Other species kept in captivity are spotted dolphins, false killer whales and common dolphins, Commerson's dolphins, as well as rough-toothed dolphins, but all in much lower numbers than the bottlenose dolphin. There are also fewer than ten pilot whales, Amazon river dolphins, Risso's dolphins, spinner dolphins, or tucuxi in captivity. Two unusual and very rare hybrid dolphins, known as wolphins, are kept at the Sea Life Park in Hawaii, which is a cross between a bottlenose dolphin and a false killer whale. Also, two xommon/bottlenose hybrids reside in captivity: one at Discovery Cove and the other at SeaWorld San Diego.

Aquariums have tried housing species of whales in captivity, besides the successful beluga whale. The success of belugas turned attention to maintaining their relative, the narwhal, in captivity. However, in repeated attempts in the 1960s and 1970s, all narwhals kept in captivity died within months. A breeding pair of pygmy right whales were retained in an enclosed area (with nets); they were eventually released in South Africa. In 1971, SeaWorld captured a California gray whale calf in Mexico at Scammon's Lagoon. The calf, later named Gigi, was separated from her mother using a form of lasso attached to her flukes. Gigi was displayed at SeaWorld San Diego for a year. Gigi was released off shore with a radio beacon affixed to her back, however, contact was lost after three weeks. Gigi was the first baleen whale to be kept in captivity. JJ, another gray whale calf, was kept at SeaWorld San Diego. JJ was an orphaned calf that beached itself on April 1997, and was transported two miles to SeaWorld. The 680 kilograms (1,500 lb) calf was a popular attraction, and behaved normally, despite being separated from his mother. A year later, the 8,164.7 kilograms (18,000 lb) whale grew too big to keep in captivity, though smaller than average, and was released on April 1, 1998; he was the second and probably the last baleen whale to be kept in captivity. There is a captive Amazon river dolphin housed at Acuario de Valencia, and it is the only river dolphin in captivity.^[80][81]

Controversy

Organizations such as World Animal Protection and the Whale and Dolphin Conservation Society campaign against the captivity of dolphins and killer whales; SeaWorld, which holds most of the world's captive killer whales, is cited for its role.

Aggression among captive killer whales is common. In August 1989, a dominant female killer whale, Kandu V, attempted to rake a newcomer whale, Corky II, with her mouth during a live show, and smashed her head into a wall. Kandu V broke her jaw which severed an artery and then bled to death.^[83] In November 2006, a dominant female killer whale, Kasatka, repeatedly dragged experienced trainer, Ken Peters, to the bottom of the stadium pool during a show after hearing her calf crying for her in the back pools.^[84] In February 2010, an experienced female trainer at SeaWorld Orlando, Dawn Brancheau, was killed by killer whale Tilikum shortly after a show in Shamu Stadium.^[85] Tilikum had been associated with the deaths of two people previously.^[83][86] In May 2012 Occupational Safety and Health Administration administrative law judge Ken Welsch cited SeaWorld for two violations in the death of Dawn Brancheau and fined the company a total of $12,000.^[87] Trainers were banned from making close contact with the orcas.^[88] In April 2014 the US Court of Appeals for the District of Columbia denied an appeal by SeaWorld.^[89]

In 2013, SeaWorld's treatment of killer whales in captivity was the basis of the movie Blackfish, which documents the history of Tilikum, a killer whale captured by SeaLand of the Pacific, later transported to SeaWorld Orlando who has been involved in the deaths of three people.^[90] In the aftermath of the release of the film, Martina McBride, 38 Special, REO Speedwagon, Cheap Trick, Heart, Barenaked Ladies, Trisha Yearwood, and Willie Nelson canceled scheduled concerts at SeaWorld parks.^[91][92] SeaWorld disputes the accuracy of the film and in December 2013 released an ad countering the allegations and emphasizing its contributions to the study of cetaceans and their conservation.^[93]

In August 2014, SeaWorld announced it was building new killer whale tanks that would be almost double the size of the existing ones to provide more space for its them, scheduled for completion in 2018.^[94] The company maintained the move was not in response to the release of the Blackfish documentary. Wild killer whales may travel up to 160 kilometres (100 mi) in a day, and critics say the animals are too big and intelligent to be suitable for captivity.^[95] A "larger prison is still a prison," was the response of PETA which opposes orcas being kept in captivity and has proposed the theme park operator base its orcas in seaside sanctuaries.^[96] The company also pledged $10 million in matching funds for killer whale research.^[94] In November 2014, SeaWorld announced that attendance at the parks had dropped 5.2% from the previous year and profits had fallen 28% over that quarter. As of November 2014, the company's stock was down 50% from the previous year.^[97][98][99]

See also

References

External links

Wikispecies has information related to: Cetacea

Wikimedia Commons has media related to Cetacea.

The Wikibook Dichotomous Key has a page on the topic of: Cetacea

Look up Cetacea in Wiktionary, the free dictionary.

• "Cetaceans". Encyclopedia of Earth. Retrieved February 2011. 
• Scottish Cetacean Research & Rescue – see page on Taxonomy
• "Dolphin and Whale News". Science Daily. Retrieved March 2010. 
• Futuyma, Douglas J. (1998). "Cetacea Evolution". Retrieved 2010. 
• EIA Cetacean campaign: Reports and latest info.
• EIA in USA: reports etc.

Extant mammal orders by infraclass Kingdom Animalia Phylum Chordata Subphylum Vertebrata (unranked) Amniota Yinotheria Australosphenida Monotremata (Platypus and echidnas) Metatheria (Marsupial inclusive) Ameridelphia Paucituberculata (Shrew opossums) Didelphimorphia (Opossums) Australidelphia Microbiotheria (Monito del monte) Notoryctemorphia (Marsupial moles) Dasyuromorphia (Quolls and dunnarts) Peramelemorphia (Bilbies and bandicoots) Diprotodontia (Kangaroos and relatives) Eutheria (Placental inclusive) Xenarthra Cingulata (Armadillos) Pilosa (Anteaters and sloths) Afrotheria Afrosoricida (Tenrecs and golden moles) Macroscelidea (Elephant shrews) Tubulidentata (Aardvark) Hyracoidea (Hyraxes) Proboscidea (Elephants) Sirenia (Dugongs and manatees) Boreoeutheria Laurasiatheria Eulipotyphla (Hedgehogs, shrews, moles and relatives) Chiroptera (Bats) Pholidota (Pangolins) Carnivora (Dogs, cats and relatives) Perissodactyla (Odd-toed ungulates) Artiodactyla (Even-toed ungulates and cetaceans) Euarchontoglires Rodentia (Rodents) Lagomorpha (Rabbits and pikas) Scandentia (Treeshrews) Dermoptera (Colugos) Primates

Extant Cetacea species Kingdom Animalia Phylum Chordata Class Mammalia Infraclass Eutheria Superorder Laurasiatheria (unranked) Cetartiodactyla (unranked) Whippomorpha   Suborder Mysticeti (Baleen whales) Balaenidae Balaena Bowhead whale (B. mysticetus) Eubalaena (Right whales) Southern right whale (E. australis) North Atlantic right whale (E. glacialis) North Pacific right whale (E. japonica) Balaenopteridae (Rorquals) Balaenoptera Common minke whale (B. acutorostrata) Antarctic minke whale (B. bonaerensis) Sei whale (B. borealis) Bryde's whale (B. brydei) Pygmy Bryde's whale (B. edeni) Blue whale (B. musculus) Omura's whale (B. omurai) Fin whale (B. physalus) Megaptera Humpback whale (M. novaeangliae) Eschrichtiidae Eschrichtius Gray whale (E. robustus) Neobalaenidae Caperea Pygmy right whale (C. marginata)   Suborder Odontoceti (Toothed whales) (cont. below) Delphinidae (Oceanic dolphins) Cephalorhynchus Commerson's dolphin (C. commersonii) Chilean dolphin (C. eutropia) Haviside's dolphin (C. heavisidii) Hector's dolphin (C. hectori) Delphinus Long-beaked common dolphin (D. capensis) Short-beaked common dolphin (D. delphis) Feresa Pygmy killer whale (F. attenuata) Globicephala (Pilot whales) Short-finned pilot whale (G. macrorhynchus) Long-finned pilot whale (G. melas) Grampus Risso's dolphin (G. griseus) Lagenodelphis Fraser's dolphin (L. hosei) Lagenorhynchus Atlantic white-sided dolphin (L. acutus) White-beaked dolphin (L. albirostris) Peale's dolphin (L. australis) Hourglass dolphin (L. cruciger) Pacific white-sided dolphin (L. obliquidens) Dusky dolphin (L. obscurus) Lissodelphis (Right whale dolphins) Northern right whale dolphin (L. borealis) Southern right whale dolphin (L. peronii) Orcaella Irrawaddy dolphin (O. brevirostris) Australian snubfin dolphin (O. heinsohni) Orcinus Killer whale (O. orca) Peponocephala Melon-headed whale (P. electra) Pseudorca False killer whale (P. crassidens) Sotalia Tucuxi (S. fluviatilis) Costero (S. guianensis) Sousa Indo-Pacific humpback dolphin (S. chinensis) Atlantic humpback dolphin (S. teuszii) Stenella Pantropical spotted dolphin (S. attenuata) Clymene dolphin (S. clymene) Striped dolphin (S. coeruleoalba) Atlantic spotted dolphin (S. frontalis) Spinner dolphin (S. longirostris) Steno Rough-toothed dolphin (S. bredanensis) Tursiops Indo-Pacific bottlenose dolphin (T. aduncus) Burrunan dolphin (T. australis) Common bottlenose dolphin (T. truncatus)   Suborder Odontoceti (Toothed whales) (cont. above) Monodontidae Delphinapterus Beluga (D. leucas) Monodon Narwhal (M. monoceros) Phocoenidae (Porpoises) Neophocaena Finless porpoise (N. phocaeniodes) Phocoena Spectacled porpoise (P. dioptrica) Harbor porpoise (P. phocoena) Vaquita (P. sinus) Burmeister's porpoise (P. spinipinnis) Phocoenoides Dall's porpoise (P. dalli) Physeteridae Physeter Sperm whale (P. macrocephalus) Kogiidae Kogia Pygmy sperm whale (K. breviceps) Dwarf sperm whale (K. simus) Iniidae Inia Amazon river dolphin (I. geoffrensis) Lipotidae Lipotes Baiji (L. vexillifer) Platanistidae Platanista Ganges and Indus River dolphin (P. gangetica) Pontoporiidae Pontoporia La Plata dolphin (P. blainvillei) Ziphiidae (Beaked whales) Berardius Arnoux's beaked whale (B. arnuxii) Baird's beaked whale (B. bairdii) Hyperoodon Northern bottlenose whale (H. ampullatus) Southern bottlenose whale (H. planifrons) Indopacetus Tropical bottlenose whale (I. pacificus) Mesoplodon (Mesoplodont whales) Sowerby's beaked whale (M. bidens) Andrews' beaked whale (M. bowdoini) Hubbs' beaked whale (M. carlhubbsi) Blainville's beaked whale (M. densirostris) Gervais' beaked whale (M. europaeus) Ginkgo-toothed beaked whale (M. ginkgodens) Gray's beaked whale (M. grayi) Hector's beaked whale (M. hectori) Strap-toothed whale (M. layardii) True's beaked whale (M. mirus) Perrin's beaked whale (M. perrini) Pygmy beaked whale (M. peruvianus) Stejneger's beaked whale (M. stejnegeri) Spade-toothed whale (M. traversii) Tasmacetus Shepherd's beaked whale (T. sheperdi) Ziphius Cuvier's beaked whale (Z. cavirostris)

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