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Cetaceans[1]
Fossil range: 55–0 Ma
Early Eocene - Present
Humpback Whale breaching
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Eutheria
Superorder: Laurasiatheria
(unranked) Cetartiodactyla
(unranked) Whippomorpha
Order: Cetacea
Brisson, 1762
Suborders

Mysticeti
Odontoceti
Archaeoceti (extinct)
(see text for families)

Diversity
Around 88 species; see list of cetaceans or below.

The order Cetacea (pronounced /sɨˈteɪʃ(i)ə/, L. cetus, whale, from Greek) includes the marine mammals commonly known as 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". In Greek mythology the monster Perseus defeated was called Ceto, which is depicted by the constellation of Cetus. Cetology is the branch of marine science associated with the study of cetaceans.

Cetaceans are the mammals best 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 from the cooler water they inhabit by a thick layer of blubber. Some species are noted for their high intelligence.

Contents

Respiration

Overhead photo of whale at surface
Cetacean in Strait of Gibraltar

Cetaceans breathe air. They surface periodically 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 at the top of the head, simplifying breathing in sometimes rough seas. 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 'blow' or 'spout' and varies by species in terms of shape, angle and height. Species can be identified at a distance using this characteristic.

Cetaceans can remain under water for much longer periods than other mammals, (approximately 7-30 minutes, varying by species) due to large physiological differences. Two studied advantages of cetacean physiology let this Order (and other marine mammals) forage underwater for extended periods without breathing:

  • Mammalian myoglobin concentrations in skeletal muscle have much variation. New Zealand white rabbits have 0.08 grams (0.0028 oz) 0.08 g (0.0028 oz) +/-0.06 (0.0021 oz) in a 100 grams (3.5 oz) wet muscle of myoglobin,[2] whereas a Northern Bottlenose Whale has 6.34 grams (0.224 oz).[3] Myoglobin, by nature, has a higher oxygen affinity than hemoglobin. The higher the myoglobin concentration in skeletal muscle, the longer the animal can stay underwater.
  • Increased body size also increases maximum dive duration. Greater body size implies increased muscle mass and in oxygen stores. Cetaceans also obey Kleiber's law, which states that mass and metabolic rate are inversely related. I.e., larger animals consume less oxygen than smaller animals per unit mass.

Vision, hearing and echolocation

Cetacean eyes are set on the side rather than the front of the head. This means that 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 (with the exception of the dolphin) with excellent hearing.

As with the eyes, cetacean ears are also small. Life in the sea accounts for the cetacean's loss of its external ears, whose function is to collect and focus airborne sound waves. However, water conducts sound better than air, so the external ear is unneeded: it is a tiny hole in the skin, just behind the eye. The highly developed inner ear can detect sounds from dozens of miles away and discern from which direction the sound comes.

Odontoceti are generally capable of echolocation.[4] From this, Odontoceti 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 Odontoceti can be trained to distinguish between, for example, balls of different sizes or shapes. Mysticeti have little need of echolocation, because they prey upon tiny fish such as krill that are impractical to locate with echolocation.

Cetaceans also use sound to communicate, whether it be groans, moans, whistles, clicks or the complex 'singing' of the Humpback Whale.

Feeding

The toothed whales such as the Sperm Whale, Beluga, dolphins and porpoises, have teeth that they use for catching fish, squid or other marine life. They do not chew but swallow prey whole. When they catch large prey, such as when the Orca (Orcinus orca) catches a seal, they bite off and swallow one chunk at a time.

Mysticeti instead have baleen plates made of keratin (the same substance as human fingernails) which hang from the upper jaw. These plates filter small animals (such as krill and fish) from the seawater. Cetaceans included in this group include the Blue, Humpback, Bowhead and Minke whales.

Not all Mysticeti feed on plankton: the larger species eat small shoaling fish, such as herring and sardine, called micronecton. The Gray Whale (Eschrichtius robustus), is a benthic feeder, primarily eating sea floor crustaceans.

Mammalian nature

Cetaceans are mammals, that is, members of the class Mammalia. The closest living relatives of cetaceans are the even-toed ungulates, such as the hippopotamus and deer.[5][6]

Mammalian characteristics include warm-bloodedness, breathing air through their lungs, and suckling their young, and growing hair, although very little of it.

Another way of distinguishing a cetacean from a fish is by the shape of the tail. Fish tails are vertical and move from side to side when the fish swims. Cetacea tails—called a fluke—are horizontal and move up and down, because cetacea spines bend in the same manner as a human spine.

Taxonomy

Size comparison of all known extant cetacean species. Note the human diver at lower right for scale.

The order Cetacea contains about ninety species, all marine except for four species of freshwater dolphins. The order contains two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises). The species range in size from Commerson's Dolphin, smaller than a human, to the Blue Whale, the largest mammal known to have lived.

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Mysticeti vs Odontoceti

Fossils indicate that before evolving baleen, Mysticeti also had teeth, so defining the Odontoceti via teeth alone is problematic, and paleontologists have instead identified other features uniting fossil and modern odontocetes that are not shared by Mysticetes.

Characteristic Odontoceti Mysticeti
Feeding Echolocation, no fast Filter feeder, summer fast
Size Smaller (except Sperm whale) Larger
Blowhole One Two
Dentition Teeth Baleen plates
Melon Ovoid, in anterior facial region Vestigial or none
Skull and facial tissue Dorsally asymmetric Symmetric
Sexual dimorphism Some species have larger males Females always larger
Mandible Symphyseal Nonsymphyseal
Pan bone of lower jaw Yes No
Maxillae projection Outward over expanded supraorbital processes Under eye orbit, with bony protuberance anterior to eye orbit
Tympanic bulla and periotic bone Fused, equal sized Larger, separate tympanic

[7]

Tree

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"[8] and "MSW3"[1] respectively. Further differences due to recent discoveries are also noted.

Discussion of synonyms and subspecies are relegated to the relevant genus and species articles.

†Extinct

See also

References

  1. ^ a b Mead, James G. and Robert L. Brownell, Jr (November 16, 2005). Wilson, D. E., and Reeder, D. M. (eds). ed. Mammal Species of the World (3rd edition ed.). Johns Hopkins University Press. pp. 723–743. ISBN 0-801-88221-4. http://www.bucknell.edu/msw3/browse.asp?id=14300001. 
  2. ^ Castellini and Somero, 1981
  3. ^ Scholander, 1940
  4. ^ Hooker, Sascha K.. Perrin, William F.; Wursig, Bernd; Thewissen, J. G. M.. eds. Encyclopedia of Marine Mammals (2 ed.). 30 Corporate Drive, Burlington Ma. 01803: Academic Press. p. 1176. ISBN 978-0-12-3733553-9. http://www.elsevier.com/wps/find/bookdescription.cws_home/716899/description#description. 
  5. ^ University Of Michigan (2001, September 20). "New Fossils Suggest Whales And Hippos Are Close Kin". ScienceDaily. http://www.sciencedaily.com/releases/2001/09/010920072245.htm. Retrieved 2007-12-21. 
  6. ^ Northeastern Ohio Universities Colleges of Medicine and Pharmacy (2007, December 21). "Whales Descended From Tiny Deer-like Ancestors". ScienceDaily. http://www.sciencedaily.com/releases/2007/12/071220220241.htm. Retrieved 2007-12-21. 
  7. ^ Hooker, Sascha K.. Perrin, William F.; Wursig, Bernd; Thewissen, J. G. M.. eds. Encyclopedia of Marine Mammals (2 ed.). 30 Corporate Drive, Burlington Ma. 01803: Academic Press. p. 1174. ISBN 978-0-12-3733553-9. http://www.elsevier.com/wps/find/bookdescription.cws_home/716899/description#description. 
  8. ^ Rice, Dale W. (1998). "Marine mammals of the world: systematics and distribution". Society of Marine Mammalogy Special Publication Number 4: 231pp. 

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