Editing Sirenia (section)

===Adaptations===
{{See also|Aquatic locomotion}}
{{Multiple image|width1=200|width2=221|image1=FL fig04.jpg|image2=Dugong dugon fin egypt.jpg|footer=The paddle-shaped fluke of a manatee (left) vs. that of a dugong (right)}}
The [[tail fluke]] of a dugong is notched and similar to those of [[dolphin]]s, whereas the tail fluke of manatee is paddle-shaped.{{sfn|Berta|2005|loc=[http://cetus.ucsd.edu/sio133/PDF/BertaChap5.pdf pp. 89–100]}} The fluke is pumped up and down in long strokes to move the animal forward, or twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.<ref name=berta2012/>{{sfn|Berta|2005|loc=p. 250}} Unlike manatees, the dugong lacks [[nail (anatomy)|nails]] on its flippers, which are only 15% of a dugong's body length.<ref name=AussieFauna/> Manatees generally glide at speeds of {{convert|8|kph|0}}, but can reach speeds of {{convert|24|kph}} in short bursts.<ref name=natgeomanatee>{{cite magazine|url=http://animals.nationalgeographic.com/animals/mammals/manatee/|archive-url=https://web.archive.org/web/20100114005408/http://animals.nationalgeographic.com/animals/mammals/manatee/|url-status=dead|archive-date=January 14, 2010|title=Manatee|magazine=National Geographic|access-date=16 January 2017}}</ref> The body is [[wikt:fusiform|fusiform]] to reduce [[Parasitic drag|drag]] in the water. Like those of cetaceans, the hind limbs are internal and [[vestigial]]. The [[rostrum (anatomy)|snout]] is angled downwards to aid in [[Bottom feeder|bottom-feeding]].<ref name=feldhamer2015>{{cite book|url={{google books|plainurl=yes|id=Ugq5BgAAQBAJ|page=412}}|first1=G. A.|last1=Feldhamer|first2=L. C.|last2=Drickamer|first3=S. H.|last3=Vessey|first4=J. F.|last4=Merritt|first5=Carey|last5=Krajewski|year= 2015|title=Mammalogy: Adaptation, Diversity, Ecology|edition=4th|location=Baltimore|publisher=Johns Hopkins University Press|pages=402–418|isbn=978-1-4214-1588-8}}</ref> Sirenians typically make two- to three-minute dives,<ref>{{Cite journal|last1=Chilvers|first1=B. Louise|author-link=Louise Chilvers|last2=Delean|first2=S.|last3=Gales|first3=N. J.|last4=Holley|first4=D. K.|last5=Lawler|first5=I. R.|last6=Marsh|first6=H.|last7=Preen|first7=A. R.|year=2004|title=Diving behaviour of dugongs, ''Dugong dugon''|url=|journal=Journal of Experimental Marine Biology and Ecology|volume=304|issue=2|page=203|doi=10.1016/j.jembe.2003.12.010|bibcode=2004JEMBE.304..203L }}</ref> but manatees can hold their breath for up to 15 minutes while resting<ref name=natgeomanatee/> and dugongs up to six minutes. They may stand on their tails to hold their heads above water.<ref name=natgeodugong>{{cite magazine|url= http://animals.nationalgeographic.com/animals/mammals/dugong/|archive-url= https://web.archive.org/web/20100112221900/http://animals.nationalgeographic.com/animals/mammals/dugong/|url-status= dead|archive-date= January 12, 2010|title=Dugong|magazine=National Geographic|access-date=16 January 2017}}</ref>

{{Multiple image
| image1 = West Indian Manatee Skull.jpg
| image2 = Crâne de Dugong dugon-Musée zoologique de Strasbourg (2).jpg
| footer = A manatee skull (left) vs. a dugong skull (right)
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Much like elephants, manatees are [[polyphyodont]]s, continuously replacing their teeth from the back of the jaw. Adults lack incisors, canines, and premolars, and instead have eight to ten [[cheek teeth]]. Manatees have an unlimited supply of teeth moving in from the back and shedding in the front; these are continuously formed by a dental capsule behind the tooth row. These teeth are constantly worn down by the abrasive vascular plants they forage, particularly aquatic grasses. Unlike those of manatees, the dugong's teeth do not continually grow back via horizontal tooth replacement.<ref name="evolutionpdf">{{Citation|title=Evolution of Sirenia |url=http://www.sirenian.org/sirenianevolution.pdf |author=Self-Sullivan, Caryn |publisher=sirenian.org |access-date=10 March 2007 |url-status=dead |archive-url=https://web.archive.org/web/20061231005452/http://www.sirenian.org/sirenianevolution.pdf |archive-date=31 December 2006 }}</ref> The dugong has two tusks which emerge in males during puberty, and sometime later in life for females after reaching the base of the [[premaxilla]].<ref name=AussieFauna>{{cite book|last=Marsh |first=Helene |chapter=Chapter 57: Dugongidae |title=Fauna of Australia |volume=1B |publisher=CSIRO |isbn=978-0-644-06056-1 |chapter-url=http://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/57-ind.pdf |url-status=bot: unknown |archive-url=https://web.archive.org/web/20130511221756/http://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/57-ind.pdf |archive-date=2013-05-11 |year=1987 }}</ref> The number of growth layer groups in a tusk indicates the age of a dugong.<ref name="IUCNpdf"/>

Sirenians exhibit [[pachyostosis]], a condition in which the ribs and other long bones are solid and contain little or no [[bone marrow]]. They have among the densest bones in the [[animal]] kingdom. These may act as [[ballast tank|ballast]], countering the buoyancy of their blubber and helping them remain suspended slightly below the water's surface.<ref name=SeaLife>{{cite book|last1=Waller|first1=Geoffrey|last2=Dando|first2=Marc|year=1996|title=Sealife: A Complete Guide to the Marine Environment|publisher=Smithsonian Institution|isbn=978-1-56098-633-1|pages=[https://archive.org/details/isbn_9781560986331/page/413 413]–420|url=https://archive.org/details/isbn_9781560986331|url-access=registration}}</ref> Manatees do not possess blubber per se, but rather have thick skin and consequently are sensitive to temperature changes. They often migrate to warmer waters whenever the water temperature dips below {{convert|20|C}}. The [[lung]]s of sirenians are unlobed;<ref name=Eldredge/> along with the [[Thoracic diaphragm|diaphragm]], these extend the entire length of the vertebral column, helping the animals control their buoyancy and reducing tipping in the water.<ref>{{cite journal| doi = 10.1111/j.1748-7692.1991.tb00111.x| last = Domning| first = Daryl|author2=Vivian Buffrenil | title = Hydrostasis in the Sirenia: Quantitative Data and Functional Interpretations| journal =Marine Mammal Science| volume = 7| issue = 4 | pages = 331–368| year = 1991| bibcode = 1991MMamS...7..331D}}</ref><ref>{{cite journal| last = Rommel| first = Sentiel|author2=John E. Reynolds | title = Diaphragm structure and function in the Florida manatee (''Trichechus manatus latirostris'')| journal =The Anatomical Record| volume = 259| issue = 1 | pages = 41–51| publisher =Wiley-Liss, Inc.| year = 2000| pmid = 10760742| doi = 10.1002/(SICI)1097-0185(20000501)259:1<41::AID-AR5>3.0.CO;2-Q| doi-access = free}}</ref>

Extant sirenians grow to between {{convert|2.5|and|4|m|ft}} in length and can weigh up to {{convert|1,500|kg}}. Steller's sea cow was the largest known sirenian to have lived, and could reach lengths of {{convert|9|m}}<ref name=Eldredge>{{cite book | author=Eldredge, Neal|year=2002|title= Life on Earth: An Encyclopedia of Biodiversity, Ecology and Evolution| url=https://archive.org/details/lifeonearthencyc00eldr| url-access=limited|publisher= ABC-CLIO| pages= [https://archive.org/details/lifeonearthencyc00eldr/page/n548 532]|isbn= 978-1-57607-286-8}}</ref> and weight of {{convert|8|to|10|t|ST|abbr=off}}.<ref>{{cite journal |last=Scheffer |first=Victor B. |title=The Weight of the Steller Sea Cow |journal=Journal of Mammalogy |volume=53 |issue=4 |date=November 1972 |pages=912–914 |doi=10.2307/1379236 |jstor=1379236}}</ref> A dugong's brain weighs a maximum of {{convert|300|g|abbr=off}}, about 0.1% of the animal's body weight.<ref name=AussieFauna/> The bodies of sirenians are sparsely covered in short hair ([[vibrissae]]), except that it becomes denser on the muzzle, which may allow for [[tactition|tactile]] interpretation of their environment.<ref name="BBE">{{cite journal|doi=10.1159/000064161 |last1=Reep |first1=R.L. |year=2002 |title=Tactile Hairs on the Postcranial Body in Florida Manatees: A Mammalian Lateral Line? |url=http://www.tamug.edu/marb/Marshall_Publications/mammalian%20lateral%20line.pdf |journal=Brain, Behavior and Evolution |volume=59 |issue=3 |pages=141–154 |last2=Marshall |first2=C.D. |last3=Stoll |first3=M.L. |url-status=dead |archive-url=https://web.archive.org/web/20120111093402/http://www.tamug.edu/marb/Marshall_Publications/mammalian%20lateral%20line.pdf |archive-date=11 January 2012 |pmid=12119533 |s2cid=17392274 }}</ref> Manatees are the only known organism with uniformly vascularized corneas. This may be the result of irritation from or protection against their [[hypotonic]] freshwater environment.<ref>{{Cite journal| last1 = Ambati | first1 = B. K. | last2 = Nozaki | first2 = M. | last3 = Singh | first3 = N. | last4 = Takeda | first4 = A. | last5 = Jani | first5 = P. D. | last6 = Suthar | first6 = T. | last7 = Albuquerque | first7 = R. J. C. | last8 = Richter | first8 = E. | last9 = Sakurai | first9 = E. | last10 = Newcomb | doi = 10.1038/nature05249 | first10 = M. T. | last11 = Kleinman | first11 = M. E. | last12 = Caldwell | first12 = R. B. | last13 = Lin | first13 = Q. | last14 = Ogura | first14 = Y. | last15 = Orecchia | first15 = A. | last16 = Samuelson | first16 = D. A. | last17 = Agnew | first17 = D. W. | last18 = St Leger | first18 = J. | last19 = Green | first19 = W. R. | last20 = Mahasreshti | first20 = P. J. | last21 = Curiel | first21 = D. T. | last22 = Kwan | first22 = D. | last23 = Marsh | first23 = H. | last24 = Ikeda | first24 = S. | last25 = Leiper | first25 = L. J. | last26 = Collinson | first26 = J. M. | last27 = Bogdanovich | first27 = S. | last28 = Khurana | first28 = T. S. | last29 = Shibuya | first29 = M. |last30 = Baldwin | first30 = M. E. | title = Corneal avascularity is due to soluble VEGF receptor-1 | journal = Nature | volume = 443 | issue = 7114 | pages = 993–997 | year = 2006 | pmid = 17051153 | pmc =2656128 | bibcode = 2006Natur.443..993A }}</ref>