Fossil range: Early Eocene - Recent
|humpback whale breaching|
humpback whale breaching
|Around 88 species; see list of cetaceans or below.|
The order Cetacea (IPA: /sɪˈteɪʃiə/, L. cetus, whale) includes whales, dolphins and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal," was more general. It comes from Ancient Greek κῆτος (kētos), meaning "whale" or "any huge fish or sea monster". Cetology is the branch of marine science associated with the study of cetaceans.
Cetaceans are the mammals most fully adapted to aquatic life. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial; they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Cetaceans are nearly hairless, and are insulated by a thick layer of blubber. As a group, they are noted for their high intelligence.
The order Cetacea contains ninety species, all marine except for five species of freshwater dolphins. The order is divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises).
Respiration, vision, hearing and echolocation
As mammals, cetaceans need to breathe air. Because of this, they need to come to the water's surface to exhale carbon dioxide and inhale a fresh supply of oxygen. During diving, a muscular action closes the blowholes (nostrils), which remain closed until the cetacean next breaks the surface; when it surfaces, the muscles open the blowholes and warm air is exhaled.
Cetaceans' blowholes have evolved to a position on top of the head, allowing more time to expel stale air and inhale fresh air. When the stale air, warmed from the lungs, is exhaled, it condenses as it meets the cold air outside. As with a terrestrial mammal breathing out on a cold day, a small cloud of 'steam' appears. This is called the 'blow' or 'spout' and is different in terms of shape, angle and height, for each cetacean species. Cetaceans can be identified at a distance, using this characteristic, by experienced whalers or whale-watchers.
Cetaceans can go underwater for much longer periods of time than other mammals. Their duration under water varies greatly between speicies due to large physiological differences between many members of this Order. There are two studied advantages of cetacean physiology that let this Order (and other marine mammals) forage underwater for extended periods of time without breathing at the water surface.
Myoglobin concentrations in skeletal muscle of mammals have much variation. A New Zealand white rabbit has 0.08+/-0.06 g (in a 100 g Wet muscle) of myoglobin, whereas a Northern bottlenose whale has 6.34 g (in a 100 g Wet muscle) of myoglobin. Myoglobin, by nature, has a higher affinity to oxygen than haemoglobin. That is, myoglobin retains oxygen molecules better than hemoglobin. Therefore, it is useful to have higher concentrations of myoglobin when needed and there is no oxygen available for re-uptake. The higher the myoglobin concentration in cetacean skeletal muscle, the longer they can stay underwater and forage.
Increased body size is another way of elongating dive duration of large cetaceans. This is true because of two considered aspects. An increase in body size means that there is increase in muscle mass, therefore, increase in muscle oxygen stores. Another aspect is the universal correlation of mass and metabolic rate (Kleiber's law). In layman’s terms Kleiber’s law states that the metabolic rate of a large animal is slower than a small animal per unit mass. From this we can conclude that larger animals will use up less oxygen than smaller animals (per mass unit).
The cetacean's eyes are set well back and to either side of its huge head. This means that cetaceans with pointed 'beaks' (such as dolphins) have good binocular vision forward and downward but others, with blunt heads (such as the Sperm Whale), can see either side but not directly ahead or directly behind. Tear glands secrete greasy tears, which protect the eyes from the salt in the water. Cetaceans also have an almost spherical lens in their eyes, which is most efficient at focusing what little light there is in the deep waters. Cetaceans make up for their generally quite poor vision (with the exception of the dolphin) with excellent hearing.
As with the eyes, the cetacean's ears are also small. Life in the sea accounts for the cetacean's loss of its external ears, whose function is to collect airborne sound waves and focus them in order for them to become strong enough to hear well. However, water is a better conductor of sound than air, so the external ear was no longer needed: It is no more than a tiny hole in the skin, just behind the eye. The inner ear, however, has become so well developed that the cetacean can not only hear sounds dozens of miles away, but it can also discern from which direction the sound comes.
Some cetaceans are capable of echolocation. Many toothed whales emit clicks similar to those in echolocation, but it has not been demonstrated that they echolocate. Mysticeti have little need of echolocation, as they prey upon small fish that would be impractical to locate with echolocation. Some members of Odontoceti, such as dolphins and porpoises, do perform echolocation. These cetaceans use sound in the same way as bats - they emit a sound (called a click), which then bounces off an object and returns to them. From this, cetaceans can discern the size, shape, surface characteristics and movement of the object, as well as how far away it is. With this ability cetaceans can search for, chase and catch fast-swimming prey in total darkness. Echolocation is so advanced in most Odontoceti that they can distinguish between prey and non-prey (such as humans or boats); captive cetaceans can be trained to distinguish between, for example, balls of different sizes or shapes.
Cetaceans also use sound to communicate, whether it be groans, moans, whistles, clicks or the complex 'singing' of the Humpback Whale.
When it comes to food and feeding, cetaceans can be separated into two distinct groups. The toothed whales, Odontoceti like sperm whales, beluga whales, dolphins and porpoises, usually have lots of teeth that they use for catching fish, squid or other marine life. They do not chew their food, but swallow it whole. In the rare cases that they catch large prey, as when Orca (Orcinus orca) catch a seal, they tear chunks off it that in turn are swallowed whole.
The baleen whales or Mysticeti do not have teeth. Instead they have plates made of keratin (the same substance as human fingernails) which hang down from the upper jaw. These plates act like a giant filter, straining small animals (such as krill and fish) from the seawater. Cetaceans included in this group include the Blue Whale, the Humpback Whale, the Bowhead Whale and the Minke Whale.
Not all Mysticeti feed on plankton: the larger whales tend to eat small shoaling fish, such as herrings and sardine, called micronecton. One species of Mysticeti, the Gray Whale (Eschrichtius robustus), is a benthic feeder, primarily eating sea floor crustaceans.
As mammals, cetaceans have characteristics that are common to all mammals: They are warm-blooded, breathe in air through their lungs, bear their young alive and suckle them on their own milk, and have hair, although very little of it.
Another way of discerning a cetacean from a fish is by the shape of the tail. The tail of a fish is vertical and moves from side to side when the fish swims. The tail of a cetacean – called a fluke – is horizontal and moves up and down, as cetaceans' spines bend in the same manner as a human spine.
The classification here closely follows Dale W. Rice, Marine Mammals of the World: Systematics and Distribution (1998), which has become the standard taxonomy reference in the field. There is very close agreement between this classification and that of Mammal Species of the World: 3rd Edition (Wilson and Reeder eds., 2005). Any differences are noted using the abbreviations "Rice" and "MSW3" respectively. Further differences due to recent discoveries are also noted.
Discussion of synonyms and subspecies are relegated to the relevant genus and species articles.
- ORDER CETACEA
- Suborder Mysticeti: Baleen whales
- Family Balaenidae: Right whales and Bowhead Whale
- Family Balaenopteridae: Rorquals
- Subfamily Balaenopterinae
- Genus Balaenoptera
- Common Minke Whale, Balaenoptera acutorostrata
- Antarctic Minke Whale, Balaenoptera bonaerensis
- Sei Whale, Balaenoptera borealis
- Bryde's Whale, Balaenoptera brydei
- Eden's Whale Balaenoptera edeni - Rice lists this as a separate species, MSW3 does not
- Balaenoptera omurai - MSW3 lists this is a synonym of Bryde's Whale but suggests this may be temporary.
- Blue Whale, Balaenoptera musculus
- Fin Whale, Balaenoptera physalus
- Genus Balaenoptera
- Subfamily Megapterinae
- Subfamily Balaenopterinae
- † Genus Eobalaenoptera
- † Harrison's Whale, Eobalaenoptera harrisoni
- Family Eschrichtiidae
- Family Neobalaenidae: Pygmy Right Whale
- Suborder Odontoceti: toothed whales
- Family Delphinidae: Dolphin
- Genus Cephalorhynchus
- Genus Delphinus
- Genus Feresa
- Pygmy Killer Whale, Feresa attenuata
- Genus Globicephala
- Genus Grampus
- Risso's Dolphin, Grampus griseus
- Genus Lagenodelphis
- Fraser's Dolphin, Lagenodelphis hosei
- Genus Lagenorhynchus
- Genus Lissodelphis
- Genus Orcaella
- Genus Orcinus
- Killer Whale, Orcinus orca
- Genus Peponocephala
- Melon-headed Whale, Peponocephala electra
- Genus Pseudorca
- False Killer Whale, Pseudorca crassidens
- Genus Sotalia
- Tucuxi, Sotalia fluviatilis, but see the species article for a discussion
- Genus Sousa
- Genus Stenella
- Genus Steno
- Rough-toothed Dolphin, Steno bredanensis
- Genus Tursiops - Rice and MSW3 tentatively agree on this classification but see species article for more detail.
- Family Monodontidae
- Family Phocoenidae: Porpoises
- Family Physeteridae: Sperm Whale family
- Family Kogiidae - MSW3 treats Kogia as a member of Physeteridae
- Superfamily Platanistoidea: River dolphins
- Family Iniidae
- Family Lipotidae - MSW3 treats Lipotes as a member of Iniidae
- Family Pontoporiidae - MSW3 treats Pontoporia as a member of Iniidae
- Family Platanistidae
- Family Ziphidae, Beaked whales
- Genus Berardius
- Subfamily Hyperoodontidae
- Genus Hyperoodon
- 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
- Layard's Beaked Whale, Mesoplodon layardii
- True's Beaked Whale, Mesoplodon mirus
- Perrin's Beaked Whale, Mesoplodon perrini. This species was recognised in 2002 and as such is listed by MSW3 but not Rice.
- Pygmy Beaked Whale, Mesoplodon peruvianus
- Stejneger's Beaked Whale, Mesoplodon stejnegeri
- Spade Toothed Whale, Mesoplodon traversii
- Genus Tasmacetus
- Tasman Beaked Whale (Shepherd's Beaked Whale), Tasmacetus shepherdi
- Genus Ziphius
- Cuvier's Beaked Whale, Ziphius cavirostris
- Family Delphinidae: Dolphin
- Suborder Mysticeti: Baleen whales
- Beached whale
- Cetacean Conservation Center
- Cetacean intelligence
- Evolution of cetaceans
- Famous cetaceans
- List of cetaceans
- List of dolphin species
- List of extinct cetaceans
- List of porpoise species
- List of whale species
- Vocal learning
- Castellini and Somero, 1981Template:Citation broken
- Scholander, 1940Template:Citation broken
- University Of Michigan (2001, September 20). "New Fossils Suggest Whales And Hippos Are Close Kin". ScienceDaily. Retrieved 2007-12-21.
- Northeastern Ohio Universities Colleges of Medicine and Pharmacy (2007, December 21). "Whales Descended From Tiny Deer-like Ancestors". ScienceDaily. Retrieved 2007-12-21.
|40x40px||Look up Cetacea in Wiktionary, the free dictionary.|
- ARKive - images and movies of Whales, Dolphins and Porpoises
- Tree of Life illustration showing how Cetacea relates to other lifeforms
- images and movies of Whales and Dolphins. Text is in French
- http://www.acsonline.org/ American Cetacean Society
- British Cetacean Site especially interesting is taxonomy
- Cetacea.org homepage
- Walker's Mammals of the World Online - Cetaceans
- Tursiops.org: Current Cetacean-related news
- Dolphins and Whales on Science Daily
- All About Dolphins: Information and News Concerning Dolphins and other Cetacea
- http://www.sonsdemar.eu/ Sounds of the cetaceans, bioindicators, acoustic trauma, acoustic signals
- Cetacea Evolution - by Douglas J. Futuyma
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