Editing Pinniped (section)

==Anatomy and physiology==
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Pinnipeds have streamlined, spindle-shaped bodies with small or non-existent ear flaps, rounded heads, short muzzles, flexible necks, limbs modified into flippers and small tails.<ref name=overview/><ref>{{cite book|last1=Karleskin |first1=G. |last2=Turner |first2=R. L. |last3=Small |first3=J. W. |year=2009 |title=Introduction to Marine Biology |publisher=Cengage Learning |page=328 |isbn=978-0-495-56197-2}}</ref>{{sfn|Berta|Sumich|Kovacs|2006|p=165}} The [[mammary gland]]s and genitals can withdraw into the body.<ref name=overview/> Seals are unique among carnivorans in that their orbital walls are mostly shaped by the maxilla and are not contained by certain facial bones.<ref name="Berta"/> Compared to land carnivores, pinnipeds have fewer teeth, which are pointed and cone-shaped. They are adapted for holding onto slippery prey rather than shearing meat like the [[carnassial]]s of other carnivorans. The walrus has unique tusks which are long upper canines.{{sfn|Riedman|1990|p=162, 164}}

Pinnipeds range in size from the {{convert|1|m|abbr=on}} and {{convert|45|kg|-1|abbr=on}} [[Baikal seal]] to the {{convert|5|m|abbr=on}} and {{convert|3200|kg|abbr=on}} [[southern elephant seal]]. Overall, they tend to be larger than other carnivores.<ref name=overview>Berta, A. "Pinnipedia, overview" in {{harvnb|Perrin|Würsig|Thewissen|2009|pp=881–884}}</ref> Several species have male-biased sexual dimorphism that depends on how [[Polygyny in nature|polygynous]] a species is: highly polygynous species like elephant seals are extremely sexually dimorphic, while less polygynous species have males and females that are closer in size, or, in the case of Antarctic seals, females are moderately bigger. Males of sexually dimorphic species also tend to have [[secondary sex characteristic]]s, such as larger or more prominent heads, necks, chests, [[Sagittal crest|crests]], noses/[[proboscis]]es and canine teeth as well as thicker fur and manes.<ref name=dimorphism>Ralls, K.; Mesnick, S. "Sexual dimorphism" in {{harvnb|Perrin|Würsig|Thewissen|2009|pp=1005–1011}}</ref>{{sfn|Berta|2012|pp=73–74}} Though more polygynous species tend to be sexually dimorphic, some evidence suggests that size differences between the sexes originated due to ecological differences, with polygyny developing later.<ref name="GonzalezSuarezCassini2014">{{cite journal |last1=Gonzalez-Suarez |first1=M. |last2=Cassini |first2=M. H. |title=Variance in male reproductive success and sexual size dimorphism in pinnipeds: testing an assumption of sexual selection theory |journal=Mammal Review |date=2014 |volume=44 |issue=2 |pages=88–93 |doi=10.1111/mam.12012|url=https://digital.csic.es/bitstream/10261/94542/1/Gonzalez-Suarez_%26_Cassini_CSIC_digital.pdf |hdl=10261/94542 |hdl-access=free }}</ref><ref name="Krugeretal2014">{{cite journal |last1=Kruger |first1=O. |last2=Wolf|first2=J. B. W. |last3=Jonker |first3=R. M. |last4=Hoffman |first4=J. I. |last5=Trillmich |first5=F. |title=Disentangling the contribution of sexual selection and ecology to the evolution of size dimorphism in pinnipeds |journal=Evolution |date=2014 |volume=68 |issue=5 |pages=1485–1496 |doi=10.1111/evo.12370 |pmid=24475921|s2cid=37919557 }}</ref>
[[File:Southern Sea Lions.jpg|thumb|upright=0.8|Male and female [[South American sea lion]]s, showing [[sexual dimorphism]]]]
Almost all pinnipeds have fur coats, the exception being the walrus, which is only sparsely covered. Even some fully furred species (particularly sea lions) are less furry than land mammals. Fur seals have lush coats consisting of an [[Fur#Composition|undercoat]] and [[guard hair]]s.{{sfn|Riedman|1990|pp=3, 68–70}} In [[ice seal|species that live on ice]], young pups have thicker coats than adults. The individual hairs on the coat, known collectively as [[lanugo]], can trap heat from sunlight and keep the pup warm.{{sfn|Riedman|1990|p=16}} Pinnipeds are typically [[Countershading|countershaded]], and are darker colored [[Dorsum (anatomy)|dorsally]] and lighter colored [[Abdomen|ventrally]], which serves to counter the effects of self-shadowing caused by light shining over the ocean water. The pure white fur of [[harp seal]] pups conceals them in their Arctic environment.{{sfn|Berta|2012|p=62}} Several species have clashing patterns of light and dark pigmentation.<ref name=overview/>{{sfn|Berta|2012|p=62}} All fully furred species [[molt]]; the process of which may be quick or gradual depending on the species.{{sfn|Riedman|1990|pp=253–255}} Seals have a layer of [[Panniculus adiposus|subcutaneous fat]], known as [[blubber]], that is particularly thick in phocids and walruses.<ref name=overview/>{{sfn|Riedman|1990|p=16}} Blubber serves both to keep the animals warm and to provide energy and nourishment when they are [[fasting]]. It can constitute as much as 50% of a pinniped's mass. Newborn pups have a thin layer of blubber, but some species compensate for this with thick lanugos.{{sfn|Riedman|1990|p=16}}

The simple stomach of pinnipeds is typical of carnivores. Most species have neither a [[cecum]] nor a clear demarcation between the [[small intestine|small]] and [[large intestine]]s; the large intestine is comparatively short and only slightly wider than the small intestine. Small intestine lengths range from 8&nbsp;times ([[California sea lion]]) to 25&nbsp;times (elephant seal) the body length. The length of the intestine may be an adaptation to frequent deep diving, allowing for more room in the digestive tract for partially digested food. An [[appendix (anatomy)|appendix]] is absent in seals.{{sfn|Berta|Sumich|Kovacs|2006|p=317}} As in most marine mammals, the [[reniculate kidney|kidneys]] are divided into lobes and filter out excess salt.{{sfn|Riedman|1990|p=31}}

===Locomotion===
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 | caption2 = [[Harbor seal]] (top) and California sea lion swimming. The former swims with its hind-flippers, the latter with its fore-flippers.
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Pinnipeds have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. Their elbows and ankles are not externally visible.{{sfn|Berta|2012|p=62}} Pinnipeds are not as fast as [[cetacean]]s, typically swimming at {{convert|5|–|15|kn|0|abbr=on|lk=in}} compared to around {{convert|20|kn|0|abbr=on}} for several species of [[dolphin]]. Seals are more agile and flexible,{{sfn|Riedman|1990|pp=5}} and some otariids, such as the California sea lion, can make dorsal turns as the back of their heads can touch their hind flippers.<ref name="Fish 2003">{{Cite journal |author=Fish, F. E. |title=Maneuverability by the sea lion ''Zalophus californianus'': Turning performance of an unstable body design |doi=10.1242/jeb.00144 |journal=Journal of Experimental Biology |volume=206 |issue=4 |pages=667–74 |year=2003 |pmid=12517984|doi-access=free }}</ref> Pinnipeds have several adaptions for reducing [[Drag (physics)|drag]]. In addition to their streamlined bodies, they have smooth networks of [[Muscle fascicle|muscle bundles]] in their skin that may increase [[laminar flow]] and cut through the water. The [[Arrector pili muscle|hair erector muscles]] are absent, so their fur can be streamlined as they swim.{{sfn|Riedman|1990|pp=3–4}}

When swimming, otariids rely on their fore-flippers for locomotion in a wing-like manner similar to [[penguin]]s and [[sea turtles]]. Fore-flipper movement is not continuous, and the animal glides between each stroke.<ref name=Feldkamp>{{cite journal |last=Feldkamp |first=S.D. |year=1987 |title=Swimming in the California sea lion: morphometrics, drag and energetics |journal=[[The Journal of Experimental Biology]] |volume=131 |pages=117–135 |pmid=3694112 |url=http://jeb.biologists.org/content/131/1/117.full.pdf |issue=1|doi=10.1242/jeb.131.1.117|doi-access=free }}</ref><ref name="Fish1996">{{Cite journal |author=Fish, F. E. |title=Transitions from drag-based to lift-based propulsion in mammalian swimming |doi=10.1093/icb/36.6.628 |journal=Integrative and Comparative Biology |volume=36 |issue=6 |pages=628–41 |year=1996|doi-access=free }}</ref> Compared to terrestrial carnivorans, the fore-limb bones of otariids are reduced in length, giving them less resistance at the elbow joint as the flippers flap;{{sfn|Berta|2012|p=62–64}} the hind-flippers maneuver them.{{sfn|Riedman|1990|p=7}} Phocids and walruses swim by moving their hind-flippers and lower body from side to side, while their fore-flippers are mainly used for maneuvering.<ref name="Fish1996"/>{{sfn|Berta|2012|p=63}}<ref name=Kastelein/> Some species [[Whale surfacing behaviour#Breaching, lunging, and porpoising|leap]] out of the water, and "ride" waves.{{sfn|Riedman|1990|pp=7–8}}

Pinnipeds can move around on land, though not as well as terrestrial animals. Otariids and walruses are capable of turning their hind-flippers forward and under the body so they can "walk" on all fours.{{sfn|Riedman|1990|p=11}} The fore-flippers move along a [[Transverse plane|transverse]] plane, rather than the [[Sagittal plane|sagittal]] plane like the limbs of land mammals.<ref name=English>{{Cite journal |author=English, A. W. |title=Limb movements and locomotor function in the California sea lion (''Zalophus californianus'') |doi=10.1111/j.1469-7998.1976.tb02274.x |journal=Journal of Zoology |volume=178 |issue=3 |pages=341–364 |year=2009}}</ref> Otariids create momentum by laterally swaying their heads and necks.{{sfn|Riedman|1990|pp=11–12}}<ref name=English/> Sea lions have been recorded climbing up flights of stairs. Phocids lack the ability to walk on their hind-flippers, and must flop and wriggle their bodies forward as their fore-flippers keep them stable. In some species, the fore-flippers may act like [[Rowing (sport)|oars]] pushing against the ground. Phocids can move faster on ice, as they are able to slide.{{sfn|Riedman|1990|p=12}}

===Senses===
[[File:Reflection in a seal eye.jpg|thumb|right|Light reflection on an elephant seal eye]]
The eyes of pinnipeds are relatively large for their size and are positioned near the front of the head. Only the smaller eyes of the walruses are located on each side of the head;{{sfn|Riedman|1990|p=43}}{{sfn|Berta|2012|pp=67}} since they forage at the bottom for [[Sedentary lifestyle|sedentary]] mollusks.{{sfn|Riedman|1990|p=43}} A seal's eye is suited for seeing both underwater and in air. Most of [[retina]] is [[equidistant]] around the spherical [[lens (anatomy)|lens]]. The [[cornea]] has a flattened center where [[refraction]] does not change between air and water. The vascular [[iris (anatomy)|iris]] has a strong [[Iris dilator muscle|dilator muscle]]. A [[Miosis|contracted]] pupil is typically pear-shaped, although the [[bearded seal]]'s is more horizontal. Compared to deep-diving elephant seals, the iris of shallower species, such as [[harbor seal]]s and California sea lions, does not change much in size between contraction and [[Mydriasis|expansion]].<ref>{{Cite journal |author1=Mass, A. M. |author2=Supin, A. Y. |doi=10.1002/ar.20529 |pmid=17516421 |title=Adaptive features of aquatic mammals' eye |journal=The Anatomical Record |volume=290 |issue=6 |pages=701–15 |year=2007 |s2cid=39925190 |doi-access=free }}</ref> Seals are able to see in relative darkness with a ''[[tapetum lucidum]]'', a reflecting layer that increases sensitivity by reflecting light back through the [[Rod cells|rods]].{{sfn|Riedman|1990|p=45}} 
[[File:Fur Seal at Cape Cross, Namibia (3045707919).jpg|thumb|left|Frontal view of [[brown fur seal]] head]]
On land, pinnipeds are [[Myopia|near-sighted]] in dim light. This is reduced in bright light as the contracted pupil decreases the ability of the lens and the cornea to refract (bend) light.{{sfn|Riedman|1990|p=46}} Polar living seals like the harp seal have corneas that can withstand the bright light that reflects off snow and ice, which would otherwise cause [[snow blindness]].<ref>Lavigne, D. M. "Harp seal" in {{harvnb|Perrin|Würsig|Thewissen|2009|pp=542–546}}</ref>{{sfn|Riedman|1990|p=46}} Color vision requires at least two types of visual pigments with different spectral sensitivities but since pinnipeds lack short-wavelength-sensitive [[cone cell]]s, they are generally considered to be color-blind.<ref>{{cite book|last1=Davis|first1=Randall W.|title=Marine mammals. Adaptations for an aquatic life|publisher=Springer|year=2019|ISBN=978-3-319-98278-6|page=182|chapter=Sensory Systems}}</ref> Flexible eye movement has been documented in seals.<ref>{{Cite journal |author1=Hanke, F. D. |author2=Hanke, W. |author3=Scholtyssek, C. |author4=Dehnhardt, G. |title=Basic mechanisms in pinniped vision |doi=10.1007/s00221-009-1793-6 |journal=Experimental Brain Research |volume=199 |issue=3–4 |pages=299–311 |year=2009 |pmid=19396435|s2cid=23704640 }}</ref> The walrus can project its eyes out from its sockets in both a forward and upward direction due to its advanced [[extraocular muscles]] and absence of an orbital roof.<ref name=Kastelein>Kastelein, R. A. "Walrus" in {{harvnb|Perrin|Würsig|Thewissen|2009|pp=1212–1216}}</ref> The seal eye is durable as the [[corneal epithelium]] is hardened by [[keratin]], and the [[sclera]] is thick enough to withstand the pressures of diving. Seals also secrete [[mucus]] from the [[lacrimal gland]] to protect their eyes. As in many mammals and birds, pinnipeds possess [[nictitating membrane]]s.{{sfn|Riedman|1990|pp=3, 49}}

The pinniped ear is adapted for hearing underwater, where it can hear sound frequencies of up to 70,000&nbsp;[[hertz|Hz]]. In air, hearing is somewhat reduced in pinnipeds compared to many terrestrial mammals. While their airborne hearing sensitivity is generally weaker than humans', they still have a wide frequency range.{{sfn|Riedman|1990|p=39}} One study of three species—the harbor seal, California sea lion and [[northern elephant seal]]—found that the sea lion was best adapted for airborne hearing, the elephant seal for underwater hearing and the harbor seal was equally adapted for both.<ref>{{Cite journal |author1=Kastak, D. |author2=Schusterman, R. J. |title=Low-frequency amphibious hearing in pinnipeds: Methods, measurements, noise, and ecology |doi=10.1121/1.421367 |journal=The Journal of the Acoustical Society of America |volume=103 |issue=4 |pages=2216–2228 |year=1998 |pmid=9566340|bibcode=1998ASAJ..103.2216K }}</ref> Although pinnipeds have a fairly good sense of smell on land,<ref>{{Cite journal |author1=Kowalewsky, S. |author2=Dambach, M. |author3=Mauck, B. |author4=Dehnhardt, G. |title=High olfactory sensitivity for dimethyl sulphide in harbour seals |doi=10.1098/rsbl.2005.0380 |journal=Biology Letters |volume=2 |issue=1 |pages=106–09 |year=2006 |pmid=17148339 |pmc=1617201}}</ref> it is useless under water as their nostrils are closed.{{sfn|Riedman|1990|p=40}}
[[File:Walrus - Kamogawa Seaworld - 1.jpg|thumb|right|[[Vibrissa]]e of walrus|alt=Photo of walrus head in profile showing one eye, nose, tusks, and "mustache"]]
The [[whiskers]] or [[vibrissa]] are normally smooth in otariids and walruses, while those of most phocids are wavey.<ref>{{cite journal|last1=Morgenthal|first1=K|last2=Krüger|first2=Y|last3=Rogers|first3=T|last4=Dehnhardt|first4=G|last5=Hanke|first5=F. D.|year=2025|title=Characterization of pinniped vibrissal type and number|journal=Marine Mammal Science|volume=41|issue=1|page=e13166|doi=10.1111/mms.13166|doi-access=free}}</ref> The whiskers of some otariids grow quite long—those of the [[Antarctic fur seal]] can reach {{convert|41|cm|in|abbr=on}}–<ref>{{cite book|author=Renouf, D.|editor-last=Renouf, D.|year=1991|chapter=Sensory reception and processing in Phocidae and Otariidae|title=Behaviour of Pinnipeds |publisher=Chapman and Hall |page=373|isbn=978-0-412-30540-5}}</ref> while Walruses have the most vibrissae, at 600–700&nbsp;individual hairs.{{sfn|Riedman|1990|p=42}} Compared to terrestrial mammals, the vibrissae of pinnipeds have ten times more nerve connections, allowing them to effectively detect vibrations in the water.<ref>{{cite journal |last1=Schusterman |first1=R. J. |last2=Kastak |first2=D. |last3=Levenson |first3=D. H. |last4=Reichmuth |first4=C. J. |last5=Southall |first5=B. L. |title=Why pinnipeds don't echolocate |doi=10.1121/1.428506 |journal=The Journal of the Acoustical Society of America |volume=107 |issue=4 |pages=2256–64 |year=2000 |pmid=10790051|bibcode=2000ASAJ..107.2256S |doi-access=free }}</ref> These vibrations are generated, for example, when a fish swims through water. Detecting vibrations is useful when the animals are foraging, and may add to or even replace vision, particularly in darkness.<ref name=whiskers>{{cite journal |last1=Miersch |first1=L. |last2=Hanke |first2=W. |last3=Wieskotten |first3=S. |last4=Hanke |first4=F. D. |last5=Oeffner |first5= J. |last6=Leder |first6=A. |last7=Brede |first7= M. |last8=Witte |first8=M. |last9=Dehnhardt |first9=G. |doi=10.1098/rstb.2011.0155 |title=Flow sensing by pinniped whiskers |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=366 |issue=1581 |pages=3077–84 |year=2011 |pmid=21969689 |pmc=3172597}}</ref><ref name=blindseals>{{Cite journal |author=Hyvärinen H. |doi=10.1111/j.1469-7998.1989.tb05008.x |title=Diving in darkness: whiskers as sense organs of the ringed seal (Phoca hispida saimensis) |journal=Journal of Zoology |volume=218 |issue=4 |pages=663–678 |year=1989}}</ref> 

Harbor seals can follow hydrodynamic paths made minutes earlier, similar to a dog following a scent trail,<ref name=dehnhardt>{{cite journal |title=Hydrodynamic trail-following in harbor seals (Phoca vitulina) |journal=Science |volume=293 |issue=5527 |pages=102–104 |doi=10.1126/science.1060514 | year=2001 | last1=Dehnhardt | first1=G. | pmid=11441183|s2cid=9156299 }}</ref><ref name=schulte-pelkum>{{cite journal | title=Tracking of biogenic hydrodynamic trails in harbour seals (Phoca vitulina) |journal=Journal of Experimental Biology |volume= 210 |issue=Pt 5 |pages=781–787 |year=2007 |doi=10.1242/jeb.02708 | pmid=17297138 |vauthors=Schulte-Pelkum N, Wieskotten S, Hanke W, Dehnhardt G, Mauck B |doi-access=free }}</ref> and can even discriminate the size and type of object responsible for the trail.<ref name=sealdiscrim>{{Cite journal |vauthors=Grant R, Wieskotten S, Wengst N, Prescott T, Dehnhardt G |title= Vibrissal touch sensing in the harbor seal (Phoca vitulina): how do seals judge size? |journal=Journal of Comparative Physiology A |volume=199 |issue= 6 |pages=521–531 |year=2013 |doi=10.1007/s00359-013-0797-7|pmid= 23397461 |s2cid= 14018274 }}</ref> Unlike terrestrial mammals, such as [[rodents]], pinnipeds do not sweep their whiskers over an object when examining it, but can protract the hairs forward while holding them steady, maximizing their detection.<ref name=whiskers/><ref name=angle>{{Cite journal |author1=Murphy, T.C. |author2=Eberhardt, W.C. |author3=Calhoun, B.H. |author4=Mann, K.A. |author5=Mann, D.A. |doi=10.1371/journal.pone.0069872 |pmid=23922834 |title=Effect of Angle on Flow-Induced Vibrations of Pinniped Vibrissae |journal=PLOS ONE |volume=8 |issue=7 |page=e69872 |year=2013 |pmc=3724740|bibcode=2013PLoSO...869872M |doi-access=free }}</ref> The vibrissa's angle relative to the flow seems to be the most important contributor to detection ability.<ref name=angle /> Whiskers may also play a role in navigation; [[spotted seal]]s appear to use them to detect breathing holes in the ice.{{sfn|Riedman|1990|p=42}}

===Diving adaptations===
{{see also|Physiology of underwater diving#Pinnipeds}}
[[File:Weddell seal swims underwater in McMurdo Sound (Image 3).jpg|right|thumb|[[Weddell seal]] underwater]]
To dive, a pinniped must first exhale much of the air out of its lungs and shut its nostrils and throat cartilages to protect the [[vertebrate trachea|trachea]].{{sfn|Riedman|1990|p=25}}{{sfn|Berta|2012|p=69}} The airways are supported by [[cartilage|cartilaginous rings]] and [[smooth muscle]], and the chest muscles and [[Pulmonary alveolus|alveoli]] can completely deflate during deeper dives.{{sfn|Berta|Sumich|Kovacs|2006|p=245}}<ref name="miller2006b"/> While land mammals generally cannot empty their lungs, pinnipeds can reinflate their lungs even after alveolar collapse.<ref name="miller2006b">{{cite journal |last1=Miller|first1=N. J. |last2=Postle|first2=A. D. |last3=Orgeig|first3=S. |last4=Koster|first4=G. |last5=Daniels|first5=C. B. |year=2006b |title=The composition of pulmonary surfactant from diving mammals |journal=Respiratory Physiology & Neurobiology |volume=152 |issue=2 |pages=152–68 |doi=10.1016/j.resp.2005.08.001|pmid=16140043 |s2cid=23633245 }}</ref> The middle ear contains [[sinus (anatomy)|sinus]]es that probably fill with blood during dives, preventing [[Ear clearing|middle ear squeeze]].<ref name="Costa"/> The heart of a seal is moderately flattened to allow the lungs to deflate. The trachea is flexible enough to collapse under pressure.{{sfn|Riedman|1990|p=25}} During deep dives, any remaining air in their lungs is shifted to the [[bronchiole]]s and trachea, which stops gas exchange with the blood, and thereby prevents them from developing [[decompression sickness]], [[oxygen toxicity]] and [[nitrogen narcosis]]. In addition, seals can tolerate large amounts of [[lactic acid]], which reduces skeletal muscle fatigue during intense physical activity.<ref name="Costa">{{cite book |author=Costa, D. P. |year=2007 |chapter=Diving physiology of marine vertebrates |title=Encyclopedia of Life Sciences |doi=10.1002/9780470015902.a0004230 |url=http://www.ucmp.berkeley.edu/about/shortcourses/costa_divingphysiology2007.pdf |isbn=978-0-470-01617-6}}</ref>

The [[circulatory system]] of pinnipeds is large and elaborate; [[rete mirabile|retia mirabilia]] line the inside of the trunk and limbs, allowing for greater oxygen storage during diving.{{sfn|Berta|Sumich|Kovacs|2006|p=241}} As with other diving mammals, pinnipeds have large amounts of [[hemoglobin]] and [[myoglobin]] stored in their blood and muscles respectively. This provides enough oxygen storage for them to stay submerged for long periods. Deep-diving species such as elephant seals have [[blood volume]]s that represent up to 20% of their body weight. When diving, they reduce their heart rate, and blood flow is mostly restricted to the heart, brain and lungs.<ref name="Costa"/> Pinnipeds have bulb-shaped [[ascending aorta]]s which are largest in deeper and longer diving species, allowing them to better maintain their [[blood pressure]].<ref>{{cite journal|last1=Storlund|first1=R. L.|last2=Rosen|first2=D. A. S.|last3=Trites|first3=A. W.|year=2024|title=Pinnipeds with proportionally wider aortic bulbs make longer dives|journal=Marine Mammal Science|volume=40|issue=4|page=e13145|doi=10.1111/mms.13145|doi-access=free}}</ref>

===Thermoregulation===
[[File:N elephant seal resting.JPG|thumb|right|Northern elephant seal resting in water]]
Pinnipeds keep warm by having large, thick bodies, insulating blubber and fur, and fast metabolism. Their idle body temperature is around {{convert|38|C|F|0|abbr=on}} against the {{convert|0|–|5|C|F|0|abbr=on}} ocean water. Metabolic rates of different species vary between 1.5 and 3 times that of land mammals.{{sfn|Riedman|1990|pp=14, 18}} Also, the blood vessels in their flippers are adapted for [[countercurrent exchange]]; small veins surround arteries transporting blood from the body core, capturing heat from them.{{sfn|Berta|2012|p=65}} While blubber and fur keep the seal warm in water, they can also overheat the animal when it is on land. To counteract overheating, many species cool off by covering themselves in sand. Monk seals may even dig up the cooler layers. The northern fur seal cools off by [[wikt:pant|panting]].{{sfn|Berta|Sumich|Kovacs|2006|p=220}}

===Sleep===
Pinnipeds spend many months at a time at sea, so they must sleep in the water. Scientists have recorded them sleeping for minutes at a time while slowly drifting downward in a belly-up orientation.<ref name=sleep/> Like other marine mammals, seals sleep in water with [[Unihemispheric slow-wave sleep|half of their brain awake]] so that they can detect and escape from predators, as well as surface for air without fully waking. When they are asleep on land, both sides of their brain go into sleep mode.<ref>{{cite journal|author1=Lapierre, J. L. |author2=Kosenko, P. O. |author3=Kodama, T. |author4=Peever, J. H. |author5=Mukhametov, L. M. |author6=Lyamin, O. I. |author7=Siegel, J. M. |year=2013|title=Symmetrical serotonin release during asymmetrical slow-wave sleep: Implications for the neurochemistry of sleep–waking states |journal=The Journal of Neuroscience |volume=33 |issue=6 |pages=2555–2561 |doi=10.1523/JNEUROSCI.2603-12.2013 |pmid=23392683 |pmc=3711592}}</ref>