Toothed whale: Difference between revisions

Toothed whales; Temporal range: late Eocene–Present PreꞒ Ꞓ O S D C P T J K Pg N
	Bottlenose dolphin
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Infraorder:	Cetacea
Parvorder:	Odontoceti; Flower, 1869
Families
	See text
Diversity
	Around 73

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The toothed whales (systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales possessing teeth, such as the beaked whales and sperm whales. Seventy-three species of toothed whales (also called odontocetes) are described. They are one of two living groups of cetaceans, the other being the baleen whales (Mysticeti), which have baleen instead of teeth. The two groups are thought to have diverged around 34 million years ago (mya).

Toothed whales range in size from the 4.5 ft (1.4 m) and 120 lb (54 kg) vaquita to the 20 m (66 ft) and 55 t (61-short-ton) sperm whale. Several species of odontocetes exhibit sexual dimorphism, in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Some can travel at up to 20 knots. Odontocetes have conical teeth designed for catching fish or squid. They have well-developed hearing, that is well adapted for both air and water, so much so that some can survive even if they are blind. Some species are well adapted for diving to great depths. Almost all have a layer of fat, or blubber, under the skin to keep warm in the cold water, with the exception of river dolphins.

Toothed whales consist of some of the most widespread mammals, but some, as with the vaquita, are restricted to certain areas. Odontocetes feed largely on fish and squid, but a few, like the killer whale, feed on mammals, such as pinnipeds. Males typically mate with multiple females every year, but females only mate every two to three years, making them polygynous. Calves are typically born in the spring and summer, and females bear the responsibility for raising them, but more sociable species rely on the family group to care for calves. Many species, mainly dolphins, are highly sociable, with some pods reaching over a thousand individuals.

Once hunted for their products, cetaceans are now protected by international law. Some species are attributed with high levels of intelligence. 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 nonhuman persons. Besides whaling and drive hunting, they also face threats from bycatch and marine pollution. The baiji, for example, is considered functionally extinct by the IUCN, with the last sighting in 2004, due to heavy pollution to the Yangtze River. Whales occasionally feature in literature and film, as in the great white sperm whale of Herman Melville's Moby-Dick. Small odontocetes, mainly dolphins, are kept in captivity and trained to perform tricks. Whale watching has become a form of tourism around the world.

Taxonomy

Research history

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 Aristotle's time, the fourth 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.^[1] 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 harbor 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. Many of the findings that apply to all cetaceans were therefore first discovered in the porpoises.^[2] 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.^[3]^[4]

Evolution

Toothed whales, as well as baleen whales, are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are closely related to the hippopotamus, sharing a common ancestor that lived around 54 million years ago (mya).^[5] The primitive cetaceans, or archaeocetes, first took to the sea approximately 49 mya and became fully aquatic by 5–10 million years later.^[6]

The adaptation of echolocation occurred when toothed whales split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 mya.^[7]^[8]^[9] Modern toothed whales do not rely on their sense of sight, but rather on their sonar to hunt prey. Echolocation also allowed toothed whales to dive deeper in search of food, with light no longer necessary for navigation, which opened up new food sources.^[10]^[11] Toothed whales (Odontocetes) echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted through their melon-shaped foreheads, reflected off objects, and retrieved through the lower jaw. Skulls of Squalodon show evidence for the first hypothesized appearance of echolocation.^[12] Squalodon lived from the early to middle Oligocene to the middle Miocene, around 33-14 mya. Squalodon featured several commonalities with modern Odontocetes. The cranium was well compressed, the rostrum telescoped outward (a characteristic of the modern parvorder Odontoceti), giving Squalodon an appearance similar to that of modern toothed whales. However, it is thought unlikely that squalodontids are direct ancestors of living dolphins.^[13]

Classification

Parvorder Odontoceti: toothed whales
- Superfamily Delphinoidea: dolphins and relatives
  - Family Delphinidae: oceanic dolphins
    - Subfamily Delphininae
      - Genus Delphinus
        Short-beaked common dolphin, Delphinus delphis
        
        Long-beaked common dolphin, Delphinus capensis
        
        (Arabian common dolphin, Delphinus tropicalis)
      - Genus Lagenodelphis
        Fraser's dolphin, Lagenodelphis hosei
      - Genus Sousa
        Atlantic humpback dolphin, Sousa teuszi
        
        Indian humpback dolphin, Sousa plumbea
        
        Chinese white dolphin, Sousa chinensis
      - Genus Stenella (syn. Clymenia, Micropia, Fretidelphis, Prodelphinus)
        Pantropical spotted dolphin, Stenella attenuata
        
        Atlantic spotted dolphin, Stenella frontalis
        
        Spinner dolphin, Stenella longirostris
        
        Clymene dolphin, Stenella clymene
        
        Striped dolphin, Stenella coeruleoalba
      - Genus Tursiops
        Common bottlenose dolphin, Tursiops truncatus
        
        Indian Ocean bottlenose dolphin, Tursiops aduncus
        
        Burrunan dolphin, Tursiops australis
    - Subfamily Lissodelphininae
      - Genus Cephalorhynchus (syn. Eutropia)
        Commerson's dolphin, Cephalorhynchus commersonii
        
        Chilean dolphin, Cephalorhynchus eutropia
        
        Heaviside's dolphin, Cephalorhynchus heavisidii
        
        Hector's dolphin, Cephalorhynchus hectori
      - Genus Lissodelphis (syn. Tursio, Leucorhamphus)
        Northern right whale dolphin, Lissodelphis borealis
        
        Southern right whale dolphin, Lissodelphis peronii
    - Subfamily Orcininae
      - Genus Feresa
        Pygmy killer whale, Feresa attenuata
      - Genus Globicephala (syn. Sphaerocephalus, Globiceps, Globicephalus)
        Long-finned pilot whale, Globicephala melas
        
        Short-finned pilot whale, Globicephala macrorhyncus
      - Genus Grampus (syn. Grampidelphis, Grayius)
        Risso's dolphin, Grampus griseus
      - Genus Orcaella
        Irrawaddy dolphin, Orcaella brevirostris
        
        Australian snubfin dolphin, Orcaella heinsohni
      - Genus Orcinus (syn. Orca, Ophysia, Gladiator)
        Killer whale (orca), Orcinus orca
      - Genus Peponocephala
        Melon-headed whale, Peponocephala electra
      - Genus †Platalearostrum (blunt-snouted dolphin)
        †Hoekman's blunt-snouted dolphin, Platalearostrum hoekmani
      - Genus Pseudorca (syn. Neorca)
        False killer whale, Pseudorca crassidens
    - Subfamily Stenoninae
      - Genus Sotalia (syn. Tucuxa)
        Tucuxi, Sotalia fluviatilis
        
        Costero, Sotalia guianensis
      - Genus Steno (syn. Glyphidelphis, Stenopontistes)
        Rough-toothed dolphin, Steno bredanensis
    - Subfamily incertae sedis
      - Genus Lagenorhynchus
        White-beaked dolphin, Lagenorhynchus albirostris
        
        Atlantic white-sided dolphin, Lagenorhynchus acutus
        
        Pacific white-sided dolphin, Lagenorhynchus obliquidens
        
        Dusky dolphin, Lagenorhynchus obscurus
        
        Black-chinned dolphin, Lagenorhynchus australis
        
        Hourglass dolphin, Lagenorhynchus cruciger
  - Family Monodontidae
    - Subfamily Delphinapterinae
      - Genus Delphinapterus
        Beluga whale, Delphinapterus leucas
    - Subfamily Monodontinae
      - Genus Monodon
        Narwhal, Monodon monoceros
  - Family Phocoenidae: porpoises
    - Subfamily Phocoeninae
      - Genus Neophocaena (syn. Meomeris)
        Finless porpoise, Neophocaena phocaenoides
      - Genus Phocoena (syn. Australophocaena, Acanthodelphis)
        Harbour porpoise, Phocoena phocaena
        
        Vaquita, Phocoena sinus
        
        Spectacled porpoise, Phocoena dioptrica
        
        Burmeister's porpoise, Phocoena spinipinnis
    - Subfamily Phocoenoidinae
      - Genus Phocoenoides
        Dall's porpoise, Phocoenoides dalli
- Superfamily Inioidea, river dolphins
  - Family Iniidae
    - Genus Inia
      - Bolivian river dolphin, Inia boliviensis
      - Amazon river dolphin, Inia geoffrensis
      - Araguaian river dolphin, Inia araguaiaensis
  - Family Pontoporiidae
    - Genus Pontoporia
      - La Plata dolphin, Pontoporia blainvillei
- Superfamily Platanistoidea, river dolphins
  - Family Platanistidae
    - Genus Platanista
      - Ganges and Indus River dolphin, Platanista gangetica
  - Family †Squalodontidae
    - Genus †Eosqualodon
    - Genus †Macrophoca
    - Genus †Neosqualodon
    - Genus †Phoberodon
    - Genus †Phocodon
    - Genus †Smilocamptus
    - Genus †Squalodon (jr. synonyms Arionius, Crenidelphinus, Kelloggia, Rhizoprion)
    - Genus †Tangaroasaurus
- Superfamily Lipotoidea, river dolphins (potentially extinct)
  - Family Lipotidae
    - Genus Lipotes
      - Chinese river dolphin, Lipotes vexillifer
- Superfamily Physeteroidea, sperm whales
  - Family Kogiidae
    - Genus Kogia
      - Dwarf sperm whale, Kogia sima
      - Pygmy sperm whale, Kogia breviceps
  - Family Physeteridae: sperm whale family
    - Genus Physeter
      - Sperm whale, Physeter macrocephalus
- Superfamily Ziphioidea, beaked whales
  - Family Ziphidae, beaked whales
    - Subfamily Berardiinae
      - Genus Berardius, giant beaked whales
        Arnoux's beaked whale, Berardius arnuxii
        
        Baird's beaked whale (North Pacific bottlenose whale), Berardius bairdii
    - Subfamily Hyperoodontinae
      - Genus Hyperoodon
        Northern bottlenose whale, Hyperoodon ampullatus
        
        Southern bottlenose whale, Hyperoodon planifrons
      - Genus Indopacetus
        Tropical bottlenose whale (Longman's beaked whale), Indopacetus pacificus
      - Genus Mesoplodon, mesoplodont whales
        Hector's beaked whale, Mesoplodon hectori
        
        True's beaked whale, Mesoplodon mirus
        
        Gervais' beaked whale, Mesoplodon europaeus
        
        Sowerby's beaked whale, Mesoplodon bidens
        
        Gray's beaked whale, Mesoplodon grayi
        
        Pygmy beaked whale, Mesoplodon peruvianus
        
        Andrews' beaked whale, Mesoplodon bowdoini
        
        Bahamonde's beaked whale, Mesoplodon bahamondi
        
        Hubbs' beaked whale, Mesoplodon carlhubbsi
        
        Ginkgo-toothed beaked whale, Mesoplodon ginkgodens
        
        Stejneger's beaked whale, Mesoplodon stejnegeri
        
        Strap-toothed whale, Mesoplodon layardii
        
        Blainville's beaked whale, Mesoplodon densirostris
        
        Perrin's beaked whale, Mesoplodon perrini
        
        Deraniyagala's beaked whale, Mesoplodon hotaula
    - Subfamily Ziphiinae
      - Genus Tasmacetus
        Shepherd's beaked whale (Shepherd's beaked whale), Tasmacetus shepherdi
      - Genus Ziphius
        Cuvier's beaked whale, Ziphius cavirostris

Biology

Anatomy

Features of a sperm whale skeleton

Toothed whales have torpedo-shaped bodies with inflexible necks, limbs modified into flippers, nonexistent external ear flaps, a large tail fin, and bulbous heads (with the exception of sperm whales). Their skulls have small eye orbits, long beaks (with the exception sperm whales), and eyes placed on the sides of their heads. Toothed whales range in size from the 4.5 ft (1.4 m) and 120 lb (54 kg) vaquita to the 20 m (66 ft) and 55 t (61-short-ton) sperm whale. Overall, they tend to be dwarfed by their relatives, the baleen whales (Mysticeti). Several species have sexual dimorphism, with the females being larger than the males. One exception is with the sperm whale, which has males larger than the females.^[14]^[15]

Odontocetes, such as the sperm whale, possess teeth with cementum cells overlying dentine cells. Unlike human teeth, which are composed mostly of enamel on the portion of the tooth outside of the gum, whale teeth have cementum outside the gum. Only in larger whales, where the cementum is worn away on the tip of the tooth, does enamel show.^[14] Except for the sperm whale, most toothed whales are smaller than the baleen whales. The teeth differ considerably among the species. They may be numerous, with some dolphins bearing over 100 teeth in their jaws. At the other extreme are the narwhals with their single long tusks and the almost toothless beaked whales with tusk-like teeth only in males.^[16]Not all species are believed to use their teeth for feeding. For instance, the sperm whale likely uses its teeth for aggression and showmanship.^[14]

Breathing involves expelling stale air from their one blowhole, forming an upward, steamy spout, followed by inhaling fresh air into the lungs. Spout shapes differ among species, which facilitates identification. The spout only forms when warm air from the lungs meets cold air, so it does not form in warmer climates, as with river dolphins.^[14]^[17]^[18]

Almost all cetaceans have a thick layer of blubber, with the exception of river dolphins. In species that live near the poles, the blubber can be as thick as 11 in (28 cm). This blubber can help with buoyancy, protection to some extent as predators would have a hard time getting through a thick layer of fat, energy for fasting during leaner times, and insulation from the harsh climates. Calves are born with only a thin layer of blubber, but some species compensate for this with thick lanugos.^[14]^[19]

Toothed whales have a two-chambered stomach similar in structure to terrestrial carnivores. They have fundic and pyloric chambers.^[20]

Locomotion

Short-beaked common dolphin pod swimming

Cetaceans have two flippers on the front, and a tail fin. These flippers contain four digits. Although toothed whales do not possess fully developed hind limbs, some, such as the sperm whale, possess discrete rudimentary appendages, which may contain feet and digits. Toothed whales are fast swimmers in comparison to seals, which typically cruise at 5–15 knots, or 9–28 km/h (5.6–17.4 mph); the sperm whale, in comparison, can travel at speeds of up to 35 km/h (22 mph). The fusing of the neck vertebrae, while increasing stability when swimming at high speeds, decreases flexibility, rendering them incapable of turning their heads; river dolphins, however, have unfused neck vertebrae and can turn their heads. When swimming, toothed whales rely on their tail fins to propel them through the water. Flipper movement is continuous. They swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Some species log out of the water, which may allow then to travel faster. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin.^[14]^[19]

Most toothed whales are adapted for diving to great depths, porpoises are one exception. In addition to their streamlined bodies, they can slow their heart rate to conserve oxygen; blood is rerouted from tissue tolerant of water pressure to the heart and brain among other organs; haemoglobin and myoglobin store oxygen in body tissue; and they have twice the concentration of myoglobin than haemoglobin. Before going on long dives, many toothed whales exhibit a behaviour known as sounding; they stay close to the surface for a series of short, shallow dives while building their oxygen reserves, and then make a sounding dive.^[21]

Senses

Toothed whale eyes are relatively small for their size, yet they do retain a good degree of eyesight. As well as this, the eyes are placed on the sides of its head, so their vision consists of two fields, rather than a binocular view as humans have. When a beluga surfaces, its lenses and corneas correct the nearsightedness that results from the refraction of light; they contain both rod and cone cells, meaning they can see in both dim and bright light. They do, however, lack short wavelength-sensitive visual pigments in their cone cells, indicating a more limited capacity for colour vision than most mammals.^[22] Most toothed whales have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas, and a tapetum lucidum; these adaptations allow for large amounts of light to pass through the eye, and, therefore, a very clear image of the surrounding area. In water, a whale can see around 10.7 m (35 ft) ahead of itself, but they have a smaller range above water. They also have glands on the eyelids and outer corneal layer that act as protection for the cornea.^[14]^[23]^{: 505–519}

The olfactory lobes are absent in toothed whales, and unlike baleen whales, they lack the vomeronasal organ, suggesting they have no sense of smell.^[23]^{: 481–505}

Toothed whales are not thought to have a good sense of taste, as their taste buds are atrophied or missing altogether. However, some dolphins have preferences between different kinds of fish, indicating some sort of attachment to taste.^[23]^{: 447–455}

Sonar

Toothed whales are capable of making a broad range of sounds using nasal airsacs located just below the blowhole. Roughly three categories of sounds can be identified: frequency-modulated whistles, burst-pulsed sounds, and clicks. Dolphins communicate with whistle-like sounds produced by vibrating connective tissue, similar to the way human vocal cords function,^[24] and through burst-pulsed sounds, though the nature and extent of that ability is not known. The clicks are directional and are used for echolocation, often occurring in a short series called a click train. The click rate increases when approaching an object of interest. Toothed whale biosonar clicks are amongst the loudest sounds made by marine animals.^[25]

The cetacean ear has specific adaptations to the marine environment. In humans, the middle ear works as an impedance equalizer between the outside air's low impedance and the cochlear fluid's high impedance. In whales, and other marine mammals, no great difference exists between the outer and inner environments. Instead of sound passing through the outer ear to the middle ear, whales receive sound through the throat, from which it passes through a low-impedance, fat-filled cavity to the inner ear.^[26] The ear is acoustically isolated from the skull by air-filled sinus pockets, which allow for greater directional hearing underwater.^[27] Odontocetes send out high-frequency clicks from an organ known as a melon. This melon consists of fat, and the skull of any such creature containing a melon will have a large depression. The melon size varies between species, the bigger it is, the more dependent they are on it. A beaked whale, for example, has a small bulge sitting on top of its skull, whereas a sperm whale's head is filled mainly with the melon.^[14]^[23]^: 1–19^[28]^[29]

Bottlenose dolphins have been found to have signature whistles unique to a specific individual. These whistles are used for dolphins to communicate with one another by identifying an individual. It can be seen as the dolphin equivalent of a name for humans.^[30] Because dolphins are generally associated in groups, communication is necessary. Signal masking is when other similar sounds (conspecific sounds) interfere with the original acoustic sound.^[31] In larger groups, individual whistle sounds are less prominent. Dolphins tend to travel in pods, in which the groups of dolphins range from two to 1000.^[32]

Life history and behaviour

Intelligence

Cetaceans are known to communicate and therefore are able to teach, learn, cooperate, scheme, and grieve.^[33] The neocortex of many species of dolphins is home to elongated spindle neurons that, prior to 2007, were known only in hominids.^[34] In humans, these cells are involved in social conduct, emotions, judgement, and theory of mind. Dolphin spindle neurons are found in areas of the brain homologous to where they are found in humans, suggesting they perform a similar function.^[14]

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 around the two-thirds or three-quarters exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation 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, in comparison to the average human brain which averages 1,450 cm³ (88 in³) in mature males.^[35] The brain to body mass ratio in some odontocetes, such as belugas and narwhals, is second only to humans.^[36]

Researchers pushed a pole with a sponge attached along the substrate to simulate the sponging behavior by dolphins

Dolphins are known to engage in complex play behaviour, which includes such things as producing stable underwater toroidal air-core vortex rings or "bubble rings". 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. Dolphins are known to use this method during hunting.^[37] Dolphins have also been known to use tools. In Shark Bay, a population of Indo-Pacific bottlenose dolphins put sponges on their beak to protect them from abrasions and sting ray barbs while foraging in the seafloor.^[38] This behaviour is passed on from mother to daughter, and it is only observed in 54 female individuals.^[39]

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,^[40] possess self-awareness.^[41] 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 whether the animal shows signs of self-recognition is determined.^[41] In 1995, Marten and Psarakos used television to test dolphin self-awareness.^[42] 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.^[41]

Vocalisations

Spectrogram of dolphin vocalizations. Whistles, whines, and clicks are visible as upside down V's, horizontal striations, and vertical lines, respectively.

Dolphins are capable of making a broad range of sounds using nasal airsacs located just below the blowhole. Roughly three categories of sounds can be identified: frequency modulated whistles, burst-pulsed sounds and clicks. Dolphins communicate with whistle-like sounds produced by vibrating connective tissue, similar to the way human vocal cords function,^[24] and through burst-pulsed sounds, though the nature and extent of that ability is not known. The clicks are directional and are for echolocation, often occurring in a short series called a click train. The click rate increases when approaching an object of interest. Dolphin echolocation clicks are amongst the loudest sounds made by marine animals.^[43]

Bottlenose dolphins have been found to have signature whistles, a whistle that is unique to a specific individual. These whistles are used in order for dolphins to communicate with one another by identifying an individual. It can be seen as the dolphin equivalent of a name for humans.^[30] These signature whistles are developed during a dolphin's first year; it continues to maintain the same sound throughout its lifetime.^[44] An auditory experience influences the whistle development of each dolphin. Dolphins are able to communicate to one another by addressing another dolphin through mimicking their whistle. The signature whistle of a male bottlenose dolphin tends to be similar to that of his mother, while the signature whistle of a female bottlenose dolphin tends to be more identifying.^[45] Bottlenose dolphins have a strong memory when it comes to these signature whistles, as they are able to relate to a signature whistle of an individual they have not encountered for over twenty years.^[46] Research done on signature whistle usage by other dolphin species is relatively limited. The research on other species done so far has yielded varied outcomes and inconclusive results.^[47]^[48]^[49]^[50]

Sperm whales can produce three specific vocalisations: creaks, codas, and slow clicks. A creak is a rapid series of high-frequency clicks that sounds somewhat like a creaky door hinge. It is typically used when homing in on prey.^[51]^: 135 A coda is a short pattern of 3 to 20 clicks that is used in social situations to identify one another (like a signature whistle), but it is still unknown whether sperm whales possess individually specific coda repertoires or whether individuals make codas at different rates.^[52] Slow clicks are heard only in the presence of males (it is not certain whether females occasionally make them). Males make a lot of slow clicks in breeding grounds (74% of the time), both near the surface and at depth, which suggests they are primarily mating signals. Outside breeding grounds, slow clicks are rarely heard, and usually near the surface.^[51]^: 144

Characteristics of sperm whale clicks^[51]^: 135
Click type	Apparent source level (dB re 1µPa [Rms])	Directionality	Centroid frequency (kHz)	Inter-click interval (s)	Duration of click (ms)	Duration of pulse (ms)	Range audible to sperm whale (km)	Inferred function
Usual	230	High	15	0.5–1.0	15–30	0.1	16	searching for prey
Creak	205	High	15	0.005–0.1	0.1–5	0.1	6	homing in on prey
Coda	180	Low	5	0.1–0.5	35	0.5	~2	social communication
Slow	190	Low	0.5	5–8	30	5	60	communication by males

Foraging and predation

All whales are carnivorous and predatory. Odontocetes, as a whole, mostly feed on fish and cephalopods, and then followed by crustaceans and bivalves. All species are generalist and opportunistic feeders. Some may forage with other kinds of animals, such as other species of whales or certain species of pinnipeds.^[19]^[53] One common feeding method is herding, where a pod squeezes a school of fish into a small volume, known as a bait ball. Individual members then take turns plowing through the ball, feeding on the stunned fish.^[54] Coralling is a method where dolphins chase fish into shallow water to catch them more easily.^[54] Killer whales and bottlenose dolphins have also been known to drive their prey onto a beach to feed on it, a behaviour known as beach or strand feeding.^[55]^[56] The shape of the snout may correlate with tooth number and thus feeding mechanisms. The narwhal, with its blunt snout and reduced dentition, relies on suction feeding.^[57]

Sperm whales usually dive between 300 to 800 metres (980 to 2,620 ft), and sometimes 1 to 2 kilometres (3,300 to 6,600 ft), in search of food.^[51]^: 79 Such dives can last more than an hour.^[51]^: 79 They feed on several species, notably the giant squid, but also the colossal squid, octopuses, and fish like demersal rays, but their diet is mainly medium-sized squid.^[51]^: 43–55 Some prey may be taken accidentally while eating other items.^[51]^: 43–55 A study in the Galápagos found that squid from the genera Histioteuthis (62%), Ancistrocheirus (16%), and Octopoteuthis (7%) weighing between 12 and 650 grams (0.026 and 1.433 lb) were the most commonly taken.^[58] Battles between sperm whales and giant squid or colossal squid have never been observed by humans; however, white scars are believed to be caused by the large squid. A 2010 study suggests that female sperm whales may collaborate when hunting Humboldt squid.^[59]

The killer whale is known to prey on numerous other toothed whale species. One example is the false killer whale.^[60] To subdue and kill whales, orcas continuously ram them with their heads; this can sometimes kill bowhead whales, or severely injure them. Other times, they corral their prey before striking. They are typically hunted by groups of 10 or fewer killer whales, but they are seldom attacked by an individual. Calves are more commonly taken by killer whales, but adults can be targeted, as well.^[61] Groups even attack larger cetaceans such as minke whales, gray whales, and rarely sperm whales or blue whales.^[62]^[63] Other marine mammal prey species include nearly 20 species of seal, sea lion and fur seal.^[64]

These cetaceans are targeted by terrestrial and pagophilic predators. The polar bear is well-adapted for hunting Arctic whales and calves. Bears are known to use sit-and-wait tactics, as well as active stalking and pursuit of prey on ice or water. Whales lessen the chance of predation by gathering in groups. This, however, means less room around the breathing hole as the ice slowly closes the gap. When out at sea, whales dive out of the reach of surface-hunting killer whales. Polar bear attacks on belugas and narwhals are usually successful in winter, but rarely inflict any damage in summer.^[65]

For most of the smaller species of dolphins, only a few of the larger sharks, such as the bull shark, dusky shark, tiger shark, and great white shark, are a potential risk, especially for calves.^[66] Dolphins can tolerate and recover from extreme injuries (including shark bites) although the exact methods used to achieve this are not known. The healing process is rapid and even very deep wounds do not cause dolphins to hemorrhage to death. Even gaping wounds restore in such a way that the animal's body shape is restored, and infection of such large wounds are rare.^[67]

Lifecycle

Toothed whales are fully aquatic creatures, which means their birth and courtship behaviours are very different from terrestrial and semiaquatic creatures. Since they are unable to go onto land to calve, they deliver their young with the fetus positioned for tail-first delivery. This prevents the calf from drowning either upon or during delivery. To feed the newborn, toothed whales, being aquatic, must squirt the milk into the mouth of the calf. Being mammals, they have mammary glands used for nursing calves; they are weaned around 11 months of age. This milk contains high amounts of fat which is meant to hasten the development of blubber; it contains so much fat, it has the consistency of toothpaste.^[68] Females deliver a single calf, with gestation lasting about a year, dependency until one to two years, and maturity around seven to 10 years, all varying between the species. This mode of reproduction produces few offspring, but increases the survival probability of each one. Females, referred to as "cows", carry the responsibility of childcare, as males, referred to as "bulls", play no part in raising calves.

Interaction with humans

Threats

Sperm whaling

The head of the sperm whale is filled with a waxy liquid called spermaceti. This liquid can be refined into spermaceti wax and sperm oil. These were much sought after by 18th-, 19th-, and 20th-century whalers. These substances found a variety of commercial applications, such as candles, soap, cosmetics, machine oil, other specialized lubricants, lamp oil, pencils, crayons, leather waterproofing, rustproofing materials, and many pharmaceutical compounds.^[69] ^[70]^[71]^[72] Ambergris, a solid, waxy, flammable substance produced in the digestive system of sperm whales, was also sought as a fixative in perfumery.

Sperm whaling in the 18th century began with small sloops carrying only a pair of whaleboats (sometimes only one). As the scope and size of the fleet increased, so did the rig of the vessels change, as brigs, schooners, and finally ships and barks were introduced. In the 19th-century stubby, square-rigged ships (and later barks) dominated the fleet, being sent to the Pacific (the first being the British whaleship Emilia, in 1788),^[73] the Indian Ocean (1780s), and as far away as the Japan grounds (1820) and the coast of Arabia (1820s), as well as Australia (1790s) and New Zealand (1790s).^[74]^[75]

A sperm whale is killed and stripped of its blubber and spermaceti

Hunting for sperm whales during this period was a notoriously dangerous affair for the crews of the 19th-century whaleboats. Although a properly harpooned sperm whale generally exhibited a fairly consistent pattern of attempting to flee underwater to the point of exhaustion (at which point it would surface and offer no further resistance), it was not uncommon for bull whales to become enraged and turn to attack pursuing whaleboats on the surface, particularly if it had already been wounded by repeated harpooning attempts. A commonly reported tactic was for the whale to invert itself and violently thrash the surface of the water with its fluke, flipping and crushing nearby boats.

The estimated historic worldwide sperm whale population numbered 1,100,000 before commercial sperm whaling began in the early 18th century.^[76] By 1880, it had declined an estimated 29%.^[76] From that date until 1946, the population appears to have recovered somewhat as whaling pressure lessened, but after the Second World War, with the industry's focus again on sperm whales, the population declined even further to only 33%.^[76] In the 19th century, between 184,000 and 236,000 sperm whales were estimated to have been killed by the various whaling nations,^[77] while in the modern era, at least 770,000 were taken, the majority between 1946 and 1980.^[78] Remaining sperm whale populations are large enough so that the species' conservation status is vulnerable, rather than endangered.^[76] However, the recovery from the whaling years is a slow process, particularly in the South Pacific, where the toll on males of breeding age was severe.^[79]

Drive hunting

Dolphins and porpoises are hunted in an activity known as dolphin drive hunting. This is accomplished by driving a pod together with boats and 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.^[80] Despite the controversial nature of the hunt resulting in international criticism, and the possible health risk that the often polluted meat causes,^[81] thousands of dolphins are caught in drive hunts each year.^[82]

In Japan, the hunting is done by a select group of fishermen.^[83] When a pod of dolphins has been spotted, they are driven into a bay by the fishermen while banging on metal rods in the water to scare and confuse the dolphins. When the dolphins are in the bay, it is quickly closed off with nets so the dolphins cannot escape. The dolphins are usually not caught and killed immediately, but instead left to calm down over night. The following day, the dolphins are caught one by one and killed. The killing of the animals used to be done by slitting their throats, but the Japanese government banned this method, and now dolphins may officially only be killed by driving a metal pin into the neck of the dolphin, which causes them to die within seconds according to a memo from Senzo Uchida, the executive secretary of the Japan Cetacean Conference on Zoological Gardens and Aquariums.^[84] A veterinary team's analysis of a 2011 video footage of Japanese hunters killing striped dolphins using this method suggested that, in one case, death took over four minutes.^[85]

Since much of the criticism is the result of photos and videos taken during the hunt and slaughter, it is now common for the final capture and slaughter to take place on site inside a tent or under a plastic cover, out of sight from the public. The most circulated footage is probably that of the drive and subsequent capture and slaughter process taken in Futo, Japan, in October 1999, shot by the Japanese animal welfare organization Elsa Nature Conservancy.^[86] Part of this footage was, amongst others, shown on CNN. In recent years, the video has also become widespread on the internet and was featured in the animal welfare documentary Earthlings, though the method of killing dolphins as shown in this video is now officially banned. In 2009, a critical documentary on the hunts in Japan titled The Cove was released and shown amongst others at the Sundance Film Festival.^[87]

Other threats

Toothed whales can also be threatened by humans more indirectly. They are unintentionally caught in fishing nets by commercial fisheries as bycatch and accidentally swallow fishing hooks. Gillnetting and Seine netting are significant causes of mortality in cetaceans and other marine mammals.^[88] Porpoises are commonly entangled in fishing nets. Whales are also affected by marine pollution. High levels of organic chemicals accumulate in these animals since they are high in the food chain. They have large reserves of blubber, more so for toothed whales, as they are higher up the food chain than baleen whales. Lactating mothers can pass the toxins on to their young. These pollutants can cause gastrointestinal cancers and greater vulnerability to infectious diseases.^[89] They can also be poisoned by swallowing litter, such as plastic bags.^[90] Pollution of the Yangtze river has led to the extinction of the Baiji.^[91] Environmentalists speculate that advanced naval sonar endangers some whales. Some scientists suggest that sonar may trigger whale beachings, and they point to signs that such whales have experienced decompression sickness.^[92]^[93]^[94]^[95]

Conservation

Currently, no international convention gives universal coverage to all small whales, although the International Whaling Commission has attempted to extend its jurisdiction over them. ASCOBANS was negotiated to protect all small whales in the North and Baltic Seas and in the northeast Atlantic. ACCOBAMS protects all whales in the Mediterranean and Black Seas. The global UNEP Convention on Migratory Species currently covers seven toothed whale species or populations on its Appendix I, and 37 species or populations on Appendix II. All oceanic cetaceans are listed in CITES appendices, meaning international trade in them and products derived from them is very limited.^[96]^[97]

Numerous organisation are dedicated to protecting certain species that do not fall under any international treaty, such as the Committee for the Recovery of the Vaquita,^[98] and the Wuhan Institute of Hydrobiology (for the Yangtze finless porpoise).^[99]

In captivity

Species

A killer whale by the name of Ulises performing at SeaWorld, 2009

Various species of toothed whales, mainly 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 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. Killer whales are well known for their performances in shows, but the number kept in captivity is very small, especially when compared to the number of bottlenose dolphins, with only 44 captives being held in aquaria as of 2012.^[100] 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. Also, fewer than ten pilot whales, Amazon river dolphins, Risso's dolphins, spinner dolphins, or tucuxi are 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 common/bottlenose hybrids reside in captivity: one at Discovery Cove and the other at SeaWorld San Diego.^[101]

Controversy

Organisations such as Animal Welfare Institute and the Whale and Dolphin Conservation Society campaign against the captivity of dolphins and killer whales.^[102] SeaWorld faced a lot of criticism after the documentary Blackfish was released in 2013.^[103]

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.^[104] 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.^[105] 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.^[106] Tilikum had been associated with the deaths of two people previously.^[104]^[107] 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 US$12,000.^[108] Trainers were banned from making close contact with the killer whales.^[109] In April 2014, the US Court of Appeals for the District of Columbia denied an appeal by SeaWorld.^[110]

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, which has been involved in the deaths of three people.^[111] In the aftermath of the release of the film, Martina McBride, 38 Special, REO Speedwagon, Cheap Trick, Heart, Trisha Yearwood, and Willie Nelson cancelled scheduled concerts at SeaWorld parks.^[112] 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.^[113]

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^ ^a ^b ^c ^d Taylor, B.L.; Baird, R.; Barlow, J.; Dawson, S.M.; Ford, J.; Mead, J.G.; Notarbartolo di Sciara, G.; Wade, P.; Pitman, R.L. (2008). "Physeter macrocephalus". The IUCN Red List of Threatened Species. 2008. IUCN: e.T41755A10554884. doi:10.2305/IUCN.UK.2008.RLTS.T41755A10554884.en. Retrieved 13 January 2018. {{cite journal}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
^ Davis, L; Gallman, R.; Gleiter, K. (1997). In Pursuit of Leviathan: Technology, Institutions, Productivity, and Profits in American Whaling, 1816–1906 (National Bureau of Economic Research Series on Long-Term Factors in Economic Dev). University of Chicago Press. p. 135. ISBN 0-226-13789-9. {{cite book}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
^ Over 680,000 officially reported at "Whaling Statistics". Retrieved 2008-10-15.. In addition, studies have found that official reports understated USSR catches by at least 89,000 "Sperm Whale (Physeter macrocephalus) California/Oregon/Washington Stock" (PDF). Retrieved 2008-10-16.. Furthermore, other countries, such as Japan, have been found to have understated catches."The RMS – A Question of Confidence: Manipulations and Falsifications in Whaling" (PDF). Archived from the original (PDF) on 2008-10-07. Retrieved 2008-10-16. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
^ Whitehead, H. (2003). "Ghosts of Whaling Past". Sperm Whales Social Evolution in the Ocean. University of Chicago Press. pp. 360–362. ISBN 0-226-89518-1.
^ Wakatsuki, Yoko; Park, Madison (2014). "Japan officials defend dolphin hunting at Taiji Cove". {{cite newsgroup}}: |access-date= requires |url= (help)
^ World Health Organisation (2008). "Guidance for identifying populations at risk from mercury exposure" (PDF). p. 36. Retrieved 29 August 2013.
^ Izadi, Elahe (2014). "Why more than 15,000 dolphins have been killed in Solomon Islands drive hunts". Retrieved 25 November 2015.
^ Paul Kenyon (2004), reporter for the BBC. BBC's dining with the dolphin hunters, retrieved on June 21, 2008.
^ Duits, Kjeld (2005). "Activists Worldwide Protest Japan's Dolphin Slaughter". {{cite newsgroup}}: |access-date= requires |url= (help)
^ Butterworth, A.; Brakes, P.; Vail, C. S.; Reiss, D. (2013). "A Veterinary and Behavioral Analysis of Dolphin Killing Methods Currently Used in the "Drive Hunt" in Taiji, Japan". Journal of Applied Animal Welfare Science. 16 (2): 184–204. doi:10.1080/10888705.2013.768925. PMID 23544757.
^ Monroe, Mark (2009). The Cove (Motion picture). {{cite AV media}}: |access-date= requires |url= (help)
^ Japan Times (2009). "Dolphin slaughter film a hit at Sundance". Retrieved 25 November 2015.
^ NOAA Fisheries – Office of Protected Resources. "The Tuna-Dolphin Issue". noaa.gov. Retrieved 29 August 2015.
^ Metcalfe, C. (23 February 2012). "Persistent organic pollutants in the marine food chain". United Nations University. Retrieved 16 August 2013.
^ Tsai, Wen-Chu. "Whales and trash-bags". Taipei Times. Retrieved 5 August 2015.
^ World Wildlife Fund. "Threat of Pollution in the Yangtze". WWF.
^ Rommel, S. A.; et al. (2006). "Elements of beaked whale anatomy and diving physiology and some hypothetical causes of sonar-related stranding" (PDF). Journal of Cetacean Resource Management. 7 (3): 189–209. Retrieved 29 August 2015.
^ Schrope, Mark (2003). "Whale deaths caused by US Navy's sonar". Nature. 415 (6868): 106. doi:10.1038/415106a. ISSN 1476-4687. PMID 11805797.
^ Kirby, Alex (8 October 2003). "Sonar may cause Whale deaths". BBC News. Retrieved 14 September 2006.
^ Piantadosi, C. A.; Thalmann, E. D. (2004). "Pathology: whales, sonar and decompression sickness". Nature. 428 (6894): 716–718. doi:10.1038/nature02527a. PMID 15085881.
^ Convention on the Conservation of Migratory Species. "ACCOBAMS". CMS. Retrieved 25 November 2015.
^ Convention on International Trade of Endangered Species of Wild Flora, Fauna. "CITES". CITES.
^ Cetacean Specialist Group (March 2015). "Vaquita". International Union for Conservation of Nature. Retrieved 20 May 2016.
^ Wang, Ding; Zhang, Xiangfeng; Wang, Kexiong; Wei, Zhuo; Würsig, Bernd (2006). "Conservation of the Baiji: No Simple Solution" (PDF). Conservation Biology. 20 (3): 624. doi:10.1111/j.1523-1739.2006.00471.x.
^ "Orcas in Captivity - A look at killer whales in aquariums and parks". 23 November 2009. Retrieved 6 September 2015.
^ Captive Cetacean Database (2006). "Ceta-Base: Captive Cetaceans". ceta-base.org. Retrieved 25 November 2015.
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^ Hogenboom, Melissa (2016). "Why killer whales should not be in captivity". BBC–Earth. Retrieved 11 June 2016.
^ ^a ^b Parsons, E. C. M. (2012). "Killer Whale Killers". Tourism in Marine Environments. 8 (3): 153–160. doi:10.3727/154427312X13491835451494.
^ "Near Death At SeaWorld: Worldwide Exclusive Video". The Huffington Post. Retrieved July 12, 2015.
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^ Karlamangla, Soumya (November 13, 2013). "Killer whales: Court weighs lifting ban on trainer-orca contact".
^ SeaWorld appeal of OSHA citations denied
^ Whiting, Candace (2013). "In the Wake of Blackfish -- Is it Time to Retire the Last Killer Whale Whose Capture Was Shown in the Film?". Retrieved 29 November 2015.
^ "Martina McBride, 38 Special, cancel SeaWorld gig over 'Blackfish'". December 16, 2013.
^ Bazzle, Steph (December 20, 2013). "SeaWorld Tries to Combat Animal Abuse Allegations". Indyposted. Retrieved December 26, 2013.

External links

Media related to Odontoceti at Wikimedia Commons
Data related to Odontoceti at Wikispecies

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[iucn-76] Taylor, B.L.; Baird, R.; Barlow, J.; Dawson, S.M.; Ford, J.; Mead, J.G.; Notarbartolo di Sciara, G.; Wade, P.; Pitman, R.L. (2008). "Physeter macrocephalus". The IUCN Red List of Threatened Species. 2008. IUCN: e.T41755A10554884. doi:10.2305/IUCN.UK.2008.RLTS.T41755A10554884.en. Retrieved 13 January 2018. {{cite journal}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)

[77] Davis, L; Gallman, R.; Gleiter, K. (1997). In Pursuit of Leviathan: Technology, Institutions, Productivity, and Profits in American Whaling, 1816–1906 (National Bureau of Economic Research Series on Long-Term Factors in Economic Dev). University of Chicago Press. p. 135. ISBN 0-226-13789-9. {{cite book}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)

[78] Over 680,000 officially reported at "Whaling Statistics". Retrieved 2008-10-15.. In addition, studies have found that official reports understated USSR catches by at least 89,000 "Sperm Whale (Physeter macrocephalus) California/Oregon/Washington Stock" (PDF). Retrieved 2008-10-16.. Furthermore, other countries, such as Japan, have been found to have understated catches."The RMS – A Question of Confidence: Manipulations and Falsifications in Whaling" (PDF). Archived from the original (PDF) on 2008-10-07. Retrieved 2008-10-16. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)

[whiteheadghosts-79] Whitehead, H. (2003). "Ghosts of Whaling Past". Sperm Whales Social Evolution in the Ocean. University of Chicago Press. pp. 360–362. ISBN 0-226-89518-1.

[80] Wakatsuki, Yoko; Park, Madison (2014). "Japan officials defend dolphin hunting at Taiji Cove". {{cite newsgroup}}: |access-date= requires |url= (help)

[81] World Health Organisation (2008). "Guidance for identifying populations at risk from mercury exposure" (PDF). p. 36. Retrieved 29 August 2013.

[82] Izadi, Elahe (2014). "Why more than 15,000 dolphins have been killed in Solomon Islands drive hunts". Retrieved 25 November 2015.

[83] Paul Kenyon (2004), reporter for the BBC. BBC's dining with the dolphin hunters, retrieved on June 21, 2008.

[84] Duits, Kjeld (2005). "Activists Worldwide Protest Japan's Dolphin Slaughter". {{cite newsgroup}}: |access-date= requires |url= (help)

[85] Butterworth, A.; Brakes, P.; Vail, C. S.; Reiss, D. (2013). "A Veterinary and Behavioral Analysis of Dolphin Killing Methods Currently Used in the "Drive Hunt" in Taiji, Japan". Journal of Applied Animal Welfare Science. 16 (2): 184–204. doi:10.1080/10888705.2013.768925. PMID 23544757.

[86] Monroe, Mark (2009). The Cove (Motion picture). {{cite AV media}}: |access-date= requires |url= (help)

[87] Japan Times (2009). "Dolphin slaughter film a hit at Sundance". Retrieved 25 November 2015.

[88] NOAA Fisheries – Office of Protected Resources. "The Tuna-Dolphin Issue". noaa.gov. Retrieved 29 August 2015.

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[94] Kirby, Alex (8 October 2003). "Sonar may cause Whale deaths". BBC News. Retrieved 14 September 2006.

[95] Piantadosi, C. A.; Thalmann, E. D. (2004). "Pathology: whales, sonar and decompression sickness". Nature. 428 (6894): 716–718. doi:10.1038/nature02527a. PMID 15085881.

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