Editing Marine mammal (section)

==Adaptations==
{{Further|Aquatic locomotion}}
[[File:Dolphin Anatomy.svg|thumb|400px|left|The labelled, anatomy of a [[dolphin]] showing its skeleton, major organs, and shape of the body]]

Marine mammals have a number of [[physiological]] and [[anatomical]] features to overcome the unique challenges associated with aquatic living. Some of these features are very species-specific. Marine mammals have developed a number of features for efficient [[Animal locomotion|locomotion]] such as torpedo-shaped bodies to reduce drag; modified limbs for propulsion and [[steering]]; tail flukes and [[dorsal fin]]s for propulsion and balance.<ref name=berta/> Marine mammals are adept at [[thermoregulation]] using dense [[fur]] or [[blubber]], circulatory adjustments ([[counter-current heat exchange]]); and reduced appendages, and large size to prevent heat loss.<ref name=berta/>

Marine mammals are able to dive for long periods. Both pinnipeds and cetaceans have large and complex [[blood vessel]] systems pushing large volumes of [[blood]] rich in [[myoglobin]] and [[hemoglobin]], which serve to store greater quantities of [[oxygen]]. Other important reservoirs include [[muscle]]s and the [[spleen]] which all have the capacity to hold a high concentration of oxygen. They are also capable of [[bradycardia]] (reduced heart rate), and [[vasoconstriction]] (shunting most of the oxygen to vital organs such as the brain and heart) to allow extended diving times and cope with oxygen deprivation.<ref name=berta/> If oxygen is depleted ([[Hypoxia in fish|hypoxia]]), marine mammals can access substantial reservoirs of [[glycogen]] that support [[anaerobic glycolysis]].<ref>{{cite journal |title=Renal cellular and tissue specializations in the bottlenose dolphin (''Tursiops truncatus'') and beluga whale (''Delphinapterus leucas'') |journal=Aquatic Mammals |year=1997 |last=Pfeiffer |first=Carl J. |volume=23 |issue=2 |pages=75–84 |url=http://aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/1997/AquaticMammals_23-02/23-02_Pfeiffer.pdf |access-date=2014-04-25 |archive-date=2014-04-26 |archive-url=https://web.archive.org/web/20140426214722/http://aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/1997/AquaticMammals_23-02/23-02_Pfeiffer.pdf |url-status=dead }}{{open access}}</ref><ref>{{cite journal |title=Body composition of the sperm whale, ''Physeter cation'', with special reference to the possible functions of fat depots |journal=Journal of the Marine Research Institute |year=1991 |last=Lockyer |first=Christina |volume=12 |issue=2 |issn=0484-9019 |url=http://www.hafro.is/Bokasafn/Greinar/Rit-fisk/rit_fisk_12-2.pdf }}{{open access}}</ref><ref name="HochachkaStorey1975">{{cite journal|last1=Hochachka|first1=P.|last2=Storey|first2=K.|title=Metabolic consequences of diving in animals and man|journal=Science|volume=187|issue=4177|year=1975|pages=613–621|issn=0036-8075|doi=10.1126/science.163485|pmid=163485|bibcode=1975Sci...187..613H|s2cid=36151144}}</ref>

Sound travels differently through water, and therefore marine mammals have developed adaptations to ensure effective communication, prey capture, and predator detection.<ref name=white/> The most notable adaptation is the development of [[Animal echolocation|echolocation]] in whales and dolphins.<ref name=berta/> Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips.<ref name=cranford>{{cite book|last=Cranford|first=T. W.|year=2000|chapter=In Search of Impulse Sound Sources in Odontocetes|title=Hearing by Whales and Dolphins|series=Springer Handbook of Auditory Research|editor-first=W. W. L.|editor-last=Au|editor2-first=A. N.|editor2-last=Popper|editor3-first=R. R.|editor3-last=Fay|publisher=Springer-Verlag|location=New York|chapter-url={{Google books|plainurl=yes|id=SJ7eBwAAQBAJ|page=front}}|isbn=978-1-4612-7024-9|oclc=840278009}}</ref>{{rp|{{URL|1=https://books.google.com/books?id=SJ7eBwAAQBAJ&pg=PA112|2=p. 112}}}} These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities.<ref name=cranford/>{{rp|121}}<ref>{{cite journal |last=Nummela, Sirpa |title=Sound transmission in archaic and modern whales: Anatomical adaptations for underwater hearing |journal=The Anatomical Record |volume=290 |issue=6 |pages=716–733 |year=2007 |doi=10.1002/ar.20528 |last2=Thewissen |first2=J.G.M |last3=Bajpai |first3=Sunil |last4=Hussain |first4=Taseer |last5=Kumar |first5=Kishor |pmid=17516434|doi-access=free }}</ref>

Marine mammals have evolved a wide variety of features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinnipeds and odontocetes are specifically adapted to capture fish and squid. In contrast, [[baleen whale]]s have evolved [[baleen plate]]s to filter feed [[plankton]] and small fish from the water.<ref name=berta/>

Polar bears, otters, and [[fur seal]]s have long, oily, and waterproof fur in order to trap air to provide insulation. In contrast, other marine mammals—such as whales, dolphins, porpoises, manatees, dugongs, and walruses—have lost long fur in favor of a thick, dense [[epidermis (skin)|epidermis]] and a thickened fat layer (blubber) to prevent [[drag (physics)|drag]]. Wading and bottom-feeding animals (such as manatees) need to be heavier than water in order to keep contact with the floor or to stay submerged. Surface-living animals (such as sea otters) need the opposite, and free-swimming animals living in open waters (such as dolphins) need to be neutrally buoyant in order to be able to swim up and down the water column. Typically, thick and dense bone is found in bottom feeders and low bone density is associated with mammals living in deep water. Some marine mammals, such as polar bears and otters, have retained four weight-bearing limbs and can walk on land like fully terrestrial animals.<ref name="Reidenberg">{{cite journal|first=Joy S.|last=Reidenberg|year=2007|title=Anatomical Adaptations of Aquatic Mammals|journal=The Anatomical Record|volume=290|issue=6|pages=507–513|doi=10.1002/ar.20541|pmid=17516440|oclc=255630658|doi-access=free}}{{open access}}</ref>

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