Pinniped

Pinnipeds ("fin-feet", lit. "winged feet") or fin-footed mammals are a widely distributed and diverse group of semi-aquatic marine mammals comprising the families Odobenidae (walruses), Otariidae (eared seals, including sea lions and fur seals), and Phocidae (true seals). Formerly classified as a separate biological suborder, Pinnipedia is now sometimes considered a superfamily within Caniformia, a suborder in the Carnivora order.

Evolution
Recent molecular evidence suggests that pinnipeds evolved from a bearlike ancestor about 23 million years ago during the late Oligocene or early Miocene epochs, a transitional period between the warmer Paleogene and cooler Neogene period. The earliest fossil pinniped that has been found is Enaliarctos, which lived 24 – 22 million years ago. It is believed to have been a good swimmer, but to have been able to move on land as well as in water, more like an otter than like modern pinnipeds. There has been longstanding debate as to whether walruses diverged from a common otariid-phocid ancestor, or whether the phocids diverged before a common otariid-odobenid ancestor. The most recent evidence suggest that the latter hypothesis is more likely.

Overview
Pinnipeds are typically sleek-bodied, barrel-shaped and can be rather large. Their bodies are well adapted to the aquatic habitat where they spend most of their lives. Their limbs have evolved into short, wide, flat flippers whence the name (derived from the Latin pinna = "feather", "wing" or "fin", and ped = foot). The smallest pinniped, the Galapagos fur seal, weighs about 30 kg (66 lb) when full-grown and is 1.2 m (4 ft) long; the largest, the male southern elephant seal, is over 4 m (13 ft) long and weighs up to 2,200 kg (4,850 lb, more than 2 tons).

Otariidae
Eared seals, also called "walking seals" and "otariids", include the animals commonly known as sea lions and fur seals. These are vocal, social animals that are somewhat better adapted to terrestrial habitats with rear flippers that can turn forward such that they can move on all fours on land. Their foreflippers are larger than those of earless seals and are used as a primary source of maneuverablity in the water. Eared seals have external ears, as their name suggests, and more dog-like snouts, further distinguishing them from the true seals. While sea lions are generally larger than fur seals and lack the dense underfur of the latter, the long-standing division into subfamilies (Arctocephalinae and Otariinae for fur seals and sea lions respectively) has been shown to be unjustified in light of recent genetic evidence suggesting that several fur seal species are more closely related to some sea lions than other fur seals. The iconic ball-balancing circus seal is generally some species of sea lion, most commonly a California sea lion.

Phocidae
Earless seals, also called “true seals" or "phocids" are the most diverse and widespread of the pinnipeds. They lack external ears and more streamlined snouts and are generally more aquatically adapted. They swim with efficient undulating whole body movements using their more developed rear flippers.  The efficiency of their swimming and an array of other physiological adaptations make them better built for deep and long diving and long distance migrations. They are, however, very cumbersome on land, moving by wriggling their front flippers and abdominal muscles. True seals generally communicate by slapping the water and grunting, rather than vocalizing.

Odobenidae
The walrus is an exclusively Arctic species - the sole surviving member of the once diverse and widespread Odobenidae family. They are easily recognized by their long tusks and great bulk (up to 2000 kg). While they share with otariids the ability to turn their rear flippers forward, their swimming is more reminiscent of that of true seals, relying more on sinuous whole body movements. They also lack external ears. Unlike eared seals and true seals, which feed primarily by hunting fish and squid in the water column, walrus generally prefer benthic invertebrates, in particular clams. It is the development of the unique squirt and suck method of feeding on molluscs that differentiated the original walrus ancestor from the other pinniped lineages. There remains debate as to whether the the walrus diverged from the eared seals before or after the true seals.

Flippers
Pinnipeds have shorter limbs than most other mammals. As noted above, their limbs have evolved into flippers with true seals having more developed hind flippers and eared seals having more developed fore flippers. The walrus is intermediate between the two. A pinniped’s fingers and toes are bound together by a web of skin. They also have claws that are found either on the front flippers (earless seals) or back flippers (eared seals). Because pinnipeds are essentialy weightless in the water, their flippers are much smaller proportionately in relation to their size than the wings of a bird or bat.

Oxygen conservation
Pinnipeds can conserve oxygen for long period of time underwater. When the animal starts diving its heart rate slows to about one-tenth of the norm. The arteries squeeze shut and the sense organs and nervous system are the only organs to continue to receive a normal flow of blood. Pinnipeds are able to resist more pain and fatigue caused by lactic acid accumulation than other mammals. However once they return to the water surface, they need time to recover and bring their body chemistry back to normal.

Warmth
To keep warm in cold waters, pinnipeds have a layer of blubber under their skin (which also provides buoyancy). The thickness depends on the species. Their blubber can also provide food for the animal. Newborn pinnipeds have no blubber. Some pinnipeds can also can get warm from their fur. The white coat of the infant harp seal, may trap the energy of sunlight as heat near the skin. As noted above, fur seal have underfur.

Moulting
Like other mammals, pinnipeds have to shed their fur once in a while. Eared seals shed more slowly than earless seals. Most earless seals spend time in the water while moulting.

Other adaptations
A pinniped’s eyes are well adapted for seeing both above and below the water. When diving the animal has a clear membrane that covers and protects its eyes. In addition, its nostrils close automatically. Testicles and mammary glands are located in slits under the skin to keep the pinniped’s streamlined shape. They also have whiskers to help navigate and sensors in their skull to absorb sounds underwater and trasmit them to the cochlea.

Feeding
All pinnipeds are carnivorous, eating fish, shellfish, squid, and other marine creatures. Most are generalist feeders, but some are specialists. For example; Ross Seals and Southern elephant seals mainly feed on squid. Crabeater seals eat mostly krill and Ringed seals feed almost exclusively on crustaceans. Additionally, the Walrus consumes molluscan prey items by sucking the soft parts from the shell.

Some seals will even eat warm-blooded prey including other seals. The leopard seal, which is probably the most carnivorous and predatory of all the pinnipeds, will eat penguins as well as Crabeater and Ross Seals. The South American sea lion also eats penguin as well as flying seabirds and young South American fur seals. Steller sea lions have been recorded eating Northern fur seal pups, Common seal pups and birds.

Almost all pinnipeds are potential prey for orcas and larger sharks. Arctic species are an important component of polar bear diet.

Reproduction
Pinnipeds often come ashore or haul out on ice to breed, often travelling long distances from their feeding grounds to suitable mating grounds with a high level of reproductive synchrony. Almost all pinnipeds are polygynous, i.e. males breed with up to several dozen females in a season. Males of many species, (e.g. elephant seals and Northern fur seals) aggressively defend groups of specific females, referred to as harems. Males of other species (e.g. most sea lions) defend territories on reproductive rookeries while females move freely between them. Some form of competition, either for females or territories, some of which can be violent, is an integral part of the male breeding strategy among most pinnipeds. Otariids, which are generally more land-adapted, tend to form major aggregations in the summer months on beaches or rocky outcrops. Consequently, their reproductive behavior is easier to observe and well studied. Walruses and many phocids, on the other hand, tend to form smaller aggregations, often in remote locations or on ice, and copulate in the water. Their reproductive behavior is therefore generally less well known.

Females have a postpartum oestrus allowing them to mate soon after giving birth. Subsequent implantation of the embryo is delayed (embryonic diapause) thus removing the need to come ashore (haul-out) twice, once to give birth and again later to mate. After giving birth mothers suckle their young for a variable length of time. Amongst the phocids, lactation varies from 4 to 50 days, whereas the otarids may lactate from 4 to 36 months. This reflects the fact that phocid feeding grounds tend to be a long way off-shore so lactation is associated with maternal fasting. To compensate for the short lactation period, the fat content of phocid milk is higher than in any other species of marine mammal (45 – 60% fat). After lactation most female phocids make extensive migratory movements to feeding grounds for intensive foraging to recoup depleted energy reserves. On the other hand, otarid feeding grounds are generally closer to shore and females go on foraging trips to maintain lactation. Fat content of otarid milk is lower than that of the phocids owing to the protracted lactatory period (typically 25 – 50%). Protracted nursing also leads to the formation of social bonds.

Taxonomy

 * Family Odobenidae
 * Walrus, Odobenus rosmarus
 * Imagotaria downsi (extinct)
 * Family Otariidae
 * Genus Arctocephalus
 * Antarctic Fur Seal, A. gazella
 * Guadalupe Fur Seal, A. townsendi
 * Juan Fernandez Fur Seal, A. philippii
 * Galapagos Fur Seal, A. galapagoensis
 * Cape Fur Seal, A. pusillus
 * South African Fur Seal, A. pusillus pusillus
 * Australian Fur Seal, A. pusillus doriferus
 * New Zealand Fur Seal or Southern Fur Seal, A. forsteri
 * Subantarctic Fur Seal, A. tropicalis
 * South American Fur Seal, A. australis
 * Genus Callorhinus
 * Northern Fur Seal, C. ursinus
 * Genus Eumetopias
 * Steller Sea Lion, E. jubatus
 * Genus Neophoca
 * Australian Sea Lion, N. cinerea
 * Genus Otaria
 * South American Sea Lion, O. flavescens
 * Genus Phocarctos
 * New Zealand Sea Lion or Hooker's Sea Lion, P. hookeri
 * Genus Zalophus
 * California Sea Lion, Z. californianus
 * Japanese Sea Lion, Z. japonicus - extinct (1950s)
 * Galapagos Sea Lion, Z. wollebaeki
 * Family Phocidae
 * Subfamily Monachinae
 * Tribe Monachini
 * Monachopsis (extinct)
 * Pristiphoca (extinct)
 * Properiptychus (extinct)
 * Messiphoca (extinct)
 * Mesotaria (extinct)
 * Callophoca (extinct)
 * Pliophoca (extinct)
 * Pontophoca (extinct)
 * Hawaiian Monk Seal, Monachus schauinslandi
 * Mediterranean Monk Seal, Monachus monachus
 * Caribbean Monk Seal, Monachus tropicalis (probably extinct around 1950)
 * Tribe Miroungini
 * Northern Elephant Seal, Mirounga angustirostris
 * Southern Elephant Seal, Mirounga leonina
 * Tribe Lobodontini
 * Monotherium wymani (extinct)
 * Ross Seal, Ommatophoca rossi
 * Crabeater Seal, Lobodon carcinophagus
 * Leopard Seal, Hydrurga leptonyx
 * Weddell Seal, Leptonychotes weddellii
 * Swan-necked Seal, Acrophoca longirostris (extinct)
 * Piscophoca pacifica (extinct)
 * Homiphoca capensis (extinct)
 * Subfamily Phocinae
 * Kawas benegasorum (extinct)
 * Leptophoca lenis (extinct)
 * Preapusa (extinct)
 * Cryptophoca (extinct)
 * Bearded Seal, Erignathus barbatus
 * Hooded Seal, Cystophora cristata
 * Tribe Phocini
 * Common Seal or Harbor Seal, Phoca vitulina
 * Spotted Seal or Larga Seal, Phoca largha
 * Ringed Seal, Pusa hispida (formerly Phoca hispida)
 * Nerpa or Baikal Seal, Pusa sibirica (formerly Phoca sibirica)
 * Caspian Seal, Pusa caspica (formerly Phoca caspica)
 * Harp Seal, Pagophilus groenlandica (formerly Phoca groenlandicus)
 * Ribbon Seal, Histriophoca fasciata (formerly Phoca fasciata)
 * Phocanella (extinct)
 * Platyphoca (extinct)
 * Gryphoca (extinct)
 * Grey Seal, Halichoerus grypus

In culture
Seals and sea lions are popular animals in the media. They are often portrayed balancing beach balls on their noses and clapping with their flippers.

Notable fictional seals include:
 * Lou Seal: mascot for the San Francisco Giants
 * Kotick: the main character in Rudyard Kipling's short story The White Seal, later made into a cartoon by Chuck Jones
 * Salty a seal that appears in the Disney cartoons Pluto's Playmate and Mickey and the Seal and later in Mickey's Mouseworks and House of Mouse.
 * The title character of Andre
 * Slappy the sea lion from Slappy and the Stinkers
 * Whiskers from Manta and Moray
 * Esmeralda the sea lion from the Disney version of 20,000 Leagues Under the Sea
 * The San-X company characters Mamegoma
 * The main characters of Romeo & Juliet: Sealed with a Kiss
 * Sparky, an escaped seal in the episode "Love and Sandy" from the 1964 television series Flipper.