Marine mammal
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Papa Lima Whiskey
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[[Image:Humpback Whale underwater shot.jpg|thumb|A [[Humpback whale]] (''Megaptera novaeangliae''), a member of order [[Cetacea]]]]
[[Image:Hydrurga leptonyx edit1.jpg|thumb|A [[Leopard seal]] (''Hydrurga leptonyx''), a member of suborder [[Pinniped]]ia of order [[Carnivora]]]]
[[Image:FL fig04.jpg|thumb|A [[West Indian Manatee]] (''Trichechus manatus''), a member of order [[Sirenia]]]]
[[Image:Sea_otter_cropped.jpg|thumb|A [[Sea Otter]] (''Enhydra lutris''), a member of family [[Mustelidae]]]]
[[Image:Ursus maritimus 4 1996-08-04.jpg|thumb|A [[Polar bear]] (''Ursus maritimus''), a member of family [[Ursidae]]]]
'''Marine mammals''' are a diverse group of roughly 120 species of [[mammal]] that are primarily [[ocean]]-dwelling or depend on the ocean for food. They include the [[cetaceans]] ([[whale|whales]], [[dolphin|dolphins]], and [[porpoise|porpoises]]), the [[sirenians]] ([[manatee|manatees]] and [[dugong]]), the [[pinnipeds]] ([[true seals]], [[eared seals]] and [[walrus]]), and several [[otters]] (the [[sea otter]] and [[marine otter]]). The [[polar bear]] is also usually grouped with the marine mammals.
Marine mammals evolved from land dwelling ancestors and share several adaptive features for life at sea such as generally large size, [[hydrodynamic]] body shapes, modified [[appendages]] and various [[Thermoregulation|thermoregulatory]] adaptations. Different species are, however, adapted to marine life to varying degrees. The most fully adapted are the cetaceans and the sirenians, whose entire life cycle takes place under water, whereas the other groups spend at least some time on land.
Despite the fact that marine mammals are highly recognizable [[charismatic megafauna]], many populations are vulnerable or [[endangered]] due to a history of commercial exploitation for [[blubber]], [[meat]], [[ivory]] and [[fur]]. Most species are currently protected from commercial exploitation.
==Groups==
There are some 120 extant species of marine mammals, generally subdivided into the five groups bold-faced below.<ref> Hoelzel, A. R. (Ed.) 2002. Marine mammal biology: an evolutionary approach. Oxford: Blackwell Publishing. ISBN 0632 05232 5 </ref>
* '''[[Order (biology)|Order]] [[Sirenia]]''': Sirenians
** family ''Trichechidae'': manatees (3 species)
** family ''Dugongidae'': dugong (1 species)
* '''Order [[Cetacea]]''': Cetaceans
** Suborder [[Mysticeti]]: Baleen whales (14 or 15 species)
** Suborder [[Odontoceti]]: Toothed whales (around 73 species)
* '''Order [[Carnivora]]''',
** '''superfamily [[Pinniped]]ia'''
*** family ''Phocidae'': [[true seals]] (around 20 species)
*** family ''Otariidae'': [[eared seals]] (around 16 species)
*** family ''Odobenidae'': [[walrus]] (1 species)
** '''family [[Mustelidae]]'''
*** [[sea otter]] (''Enhydra lutris'')
*** [[marine otter]] (''Lontra felina'')
** '''family [[Ursidae]]'''
*** [[polar bear]] (''Ursus maritimus'')
The sirenians and cetaceans are thought to be descendent from an [[ungulate]] ancestor, while the pinnipeds, otters and polar bear are descended from a [[caniform]] ancestor. The [[morphology (biology)|morphological]] similarities between these diverse groups are a result of [[convergent evolution|convergent]] and [[parallel evolution]].
==Adaptations==
Since mammals originally evolved on land, their [[vertebral column|spine]]s are optimized for running, allowing for up-and-down but only little sideways motion. Therefore, marine mammals typically swim by moving their spine up and down. By contrast, fish normally swim by moving their spine sideways. For this reason, fish mostly have vertical caudal (tail) fins, while marine mammals have horizontal caudal fins.
Some of the primary differences between marine mammals and other marine life are:
* Marine mammals breathe air, while most other marine animals extract oxygen from water.
* Marine mammals have hair. [[Cetacea]]ns have little or no hair, usually a very few bristles retained around the head or mouth. All members of the [[Carnivora]] have a coat of fur or hair, but it is far thicker and more important for [[thermoregulation]] in [[sea otter|sea otters]] and [[polar bear|polar bears]] than in [[Pinniped|seals]] or [[sea lions]]. Thick layers of fur contribute to drag while swimming, and slow down a swimming mammal, giving it a disadvantage in speed.
* Marine mammals have thick layers of [[blubber]] used to insulate their bodies and prevent heat loss. Sea otters and [[polar bear|polar bears]] are exceptions, relying more on fur and behavior to stave off hypothermia.
* Marine mammals give birth. Most marine mammals give birth to one calf or pup at a time.
* Marine mammals feed off milk as young. Maternal care is extremely important to the survival of offspring that need to develop a thick insulating layer of [[blubber]]. The milk from the [[mammary gland]]s of marine mammals often exceeds 40-50% fat content to support the development of blubber in the young.
* Marine mammals maintain a high internal body temperature. Unlike most other marine life, marine mammals carefully maintain a core temperature much higher than their environment. [[Blubber]], thick coats of [[fur]], bubbles of air between skin and water, [[countercurrent exchange]], and behaviors such as hauling out, are all [[adaptation]]s that aid marine mammals in retention of body heat.
The [[polar bear]] spends a large portion of its time in a marine environment, albeit a frozen one. When it does swim in the open sea it is extremely proficient and has been shown to cover 74 km in a day. For these reasons, some scientists regard it as a marine mammal.
==Research==
Considerable research has been conducted on the incidence of diseases that afflict marine mammals in the marine environment. This work has addressed [[leptospirosis]], phocine [[herpesvirus]], [[neurological]] diseases, toxicology<ref>Lavery, TJ, Butterfield, N, Kemper, CM, Reid, RJ, Sanderson, K. 2008. Metals and selenium in the liver and bone of three dolphin species from South Australia, 1988 - 2004. Science of the Total Environment, 390: 77 - 86.</ref> and other pathologies affecting marine mammals. Entire research organizations have developed such as the [[Marine Mammal Center]] to focus upon the rehabilitation and research functions of marine mammals.
==References==
{{reflist|1}}
==See also==
*[[Institute for Marine Mammal Studies]]
==External links==
*[http://digital.library.unt.edu/govdocs/crs/search.tkl?q=marine+mammal&search_crit=title&search=Search&date1=Anytime&date2=Anytime&type=form Read Congressional Research Service (CRS) Reports regarding Marine mammals]
*[http://www.nap.edu/books/0309094496/html A 2005 Report] by the [[United States National Academy of Sciences|National Academy of Sciences]] entitled ''Marine Mammal Populations and Ocean Noise'', is available for free online reading and research
*University of Washington Libraries Digital Collections -- [http://content.lib.washington.edu/cgi-bin/queryresults.exe?CISOOP=adv&CISORESTMP=%2Fsite-templates%2Fsearch_results-sub.html&CISOVIEWTMP=%2Fsite-templates%2Fitem_viewer.html&CISOMODE=thumb&CISOGRID=thumbnail%2CA%2C1%3Btitle%2CA%2C1%3Bsubjec%2CA%2C0%3Bdescri%2C200%2C0%3B0%2CA%2C0%3B10&CISOBIB=title%2CA%2C1%2CN%3Bsubjec%2CA%2C0%2CN%3Bdescri%2CK%2C0%2CN%3B0%2CA%2C0%2CN%3B0%2CA%2C0%2CN%3B10&CISOTHUMB=3%2C5&CISOTITLE=10&CISOPARM=%2Ffishimages%3Asubjec%3Amammals&x=29&y=4 Freshwater and Marine Image Bank -- Aquatic Mammals] An ongoing digital collection of images related to marine and aquatic mammals.
*[http://www.aquaticmammals.org Marine Mammal Resource]
[[Category:Marine mammals| ]]
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