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Pinnipeds (from Latin pinna, wing or fin, and ped-, foot) or fin-footed mammals are a widely distributed and diverse group of semi-aquatic marine mammals comprising the families Odobenidae (the walrus), Otariidae (eared seals, including sea lions and fur seals), and Phocidae (earless 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 Puijila darwini, of about 23 million years ago. Pujilla had heavy limbs, indicative of upright movement on land, and flattened phalanges, indicating that they were probably webbed, but not yet flippers. The discovery of Pujilla in northern Canadamarker strongly suggests that pinnipeds originated in the Arctic. The reference to Charles Darwin is in honor of his contention made in On the Origin of Species (1859) that
A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly aquatic as to brace the open ocean
The otter-like mustelid Potamotherium may actually represent an even earlier phase in the evolution of pinnipeds.

Another (more advanced) early pinniped 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

Comparative anatomy of an otariid seal and a phocid seal.
A mother seal voicing its displeasure at human intrusion.
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. The smallest pinniped, the Galápagos Fur Seal, weighs about 30 kg (65 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). In the Phocidae the two back flippers have evolved into a tail-like structure which can no longer be used for walking on land.

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 so 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 maneuverability 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, have more streamlined snouts, and are generally more aquatically adapted than otariids. 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. These mammals 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 mollusks that differentiated the original walrus ancestor from the other pinniped lineages. There remains debate as to whether the walrus diverged from the eared seals before or after the true seals.

Adaptations

Flippers

Pinnipeds' limbs, which have evolved into flippers, are proportionally shorter than those of most other mammals. Pinnipeds' 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 water has a much higher density than air, their flippers can be much smaller proportionately in relation to their size than the wings of a bird or bat. Additionally, pinnipeds are essentially weightless in the water, allowing them to come to a standstill, and perform aquabatic feats in water that would be impossible for flying creatures.

Oxygen conservation

Pinnipeds can conserve oxygen for long periods of time underwater. When the animal starts diving its heart rate slows to about one-tenth of the normal rate. The arteries squeeze shut and the sense organ 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. Pinnipeds can hold their breath for nearly two hours underwater.

Thermoregulation

Pinnipeds use several strategies to conserve body heat while foraging in cold waters. Most pinnipeds primarily rely on a thick layer of blubber under their skin, which also provides buoyancy, hydrodynamic shape, and stored energy. Fur seals have both blubber and a specially adapted fur coat, including outer guard hairs that repel water and a layer of insulating underfur, the reason they were particularly prized by sealers and why many species were nearly hunted to extinction. Additionally, the pinniped circulatory system is uniquely adapted so that blood can be directed away from body surface areas to prevent heat loss.

Pinnipeds living in warmer climes, such as Galapagos or Australian sea lions, must keep cool while they are hauled out on land to rest, breed, and nurse their pups. Strategies include resting in the shade or in tide pools, covering themselves in a thin layer of sand ("sand-flipping"), and waving their flippers in the air. Their unique circulatory system also allows for blood to be shunted to the surface of their flippers, which can then be rapidly cooled by waving or dipping in pools.



Molting

For most pinniped species molting is an annual process of replacing worn fur (and in some cases, skin) that ties them to land for a period of time. While molting, pinnipeds' thermoregulation can be compromised, so some species, such as Elephant seals, fast and remain onshore for a month or more. In many species, pups are born with a natal coat of a different length ,texture and/or color than adults of the same species. This coat is adapted for the terrestrial, pre-weaning period, either a thick pelage to keep them warm in arctic environments, or a thin layer of fur to keep them cool on summer sands. During their first molt (in about 11 days after birth) the pups will replace this with an adult coat better suited to life at sea. Until this age, pups face an increased risk of hypothermia and drowning if they spend much time in the ocean.

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 whisker to help navigate and sensors in their skull to absorb sounds underwater and transmit them to the cochlea.

Feeding

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 penguins 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 the polar bear diet.

Reproduction

Males of many species, (e.g. Elephant seals, South American Sea Lions, and Northern Fur Seals) aggressively defend groups of specific females, referred to as harem. Males of other species (e.g. most sea lions and Brown Fur Seals) 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, otariid feeding grounds are generally closer to shore and females go on foraging trips. Fat content of otariid milk is lower than that of the phocids, owing to the protracted lactation period (typically 25–50%). Protracted nursing also leads to the formation of social bonds.

Taxonomy

Image:Pinniped-phylogeny.gif|803pxrect 684 1 800 11 Canidaerect 684 12 800 22 Ursidaerect 684 23 800 33 Odobenus rosmarusrect 684 34 800 44 Callorhinus ursinusrect 684 45 800 55 Neophoca cinerearect 684 56 800 66 Otaria byroniarect 684 67 800 77 Arctocephalus pusillusrect 684 78 800 88 Phocarctos hookerirect 684 89 800 99 Arctocephalus forsterirect 230 191 300 202 Arctocephalus forsterirect 684 100 800 110 Arctocephalus australisrect 230 203 300 213 Arctocephalus australisrect 684 111 800 121 Arctocephalus galapagoensisrect 230 214 300 224 Arctocephalus galapagoensisrect 684 122 800 132 Arctocephalus gazellarect 230 225 300 235 Arctocephalus gazellarect 684 133 800 143 Arctocephalus tropicalisrect 230 236 300 246 Arctocephalus tropicalisrect 684 144 800 154 Arctocephalus philippiirect 230 247 300 257 Arctocephalus philippiirect 684 155 800 165 Arctocephalus townsendirect 230 258 300 268 Arctocephalus townsendirect 684 166 800 176 Eumetopias jubatusrect 684 177 800 187 Zalophus californianusrect 684 188 800 198 Erignathus barbatusrect 684 199 800 209 Cystophora cristatarect 684 210 800 220 Pusa hispidarect 230 285 300 295 Pusa hispidarect 684 221 800 231 Pusa sibiricarect 230 296 300 306 Pusa sibiricarect 684 232 800 242 Halichoerus grypusrect 230 307 300 317 Halichoerus grypusrect 684 243 800 253 Pusa caspicarect 230 218 300 328 Pusa caspicarect 684 254 800 264 Phoca largharect 230 269 300 339 Phoca largharect 684 265 800 275 Phoca vitulinarect 230 280 300 350 Phoca vitulinarect 684 276 800 286 Histriophoca fasciatarect 684 287 800 297 Pagophilus groenlandicusrect 684 298 800 308 Lobodon carcinophagusrect 684 309 800 319 Ommatophoca rossiirect 684 320 800 330 Hydrurga leptonyxrect 684 331 800 341 Leptonychotes weddelliirect 684 342 800 352 Mirounga angustirostrisrect 684 353 800 363 Mirounga leoninarect 684 364 800 375 Monachus monachusrect 684 376 800 387 Monachus schauinslandirect 684 388 800 398 Monachus tropicalisdesc noneAbove: Phylogeny determined from parsimony analysis of 50 maximum likelihood gene trees



See also



References

  1. "'Missing link' fossil seal walked", BBC News, 22 April 2009
  2. Rybczynski, N., Dawson, M.R & Tedford, R.H. (2009): A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia. Nature no 458, pp 1021–1024. Full text
  3. 1
  4. Encarta article on Seals
  5. Pinniped Seal



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