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The northern elephant seal (''Mirounga angustirostris'') is one of two species of elephant seal (the other is the southern elephant seal). It is a member of the family'' Phocidae'' ("true seals"). Elephant seals derive their name from their great size and from the male's large proboscis, which is used in making extraordinarily loud roaring noises, especially during the mating competition. Sexual dimorphism in size is great: The males can grow to and , while the females grow to and . Correspondingly, the mating system is highly polygynous; a successful male is able to impregnate up to 50 females in one season. ==Description== The huge male northern elephant seal typically weighs and measures , although some males can weigh up to .〔Beer, ''Encyclopedia of North American Mammals: An Essential Guide to Mammals of North America''. Thunder Bay Press (2004), ISBN 978-1-59223-191-1.〕 Females are much smaller and can range from in weight, or roughly a third of the male's bulk, and measure from .〔(''Mirounga angustirostris''. Northern Elephant Seal ). Smithonian National Museum of Natural History〕 The bull Southern Elephant Seals are, on average, larger than those in the Northern species but the females in both are around the same size, indicating the even higher level of sexual dimorphism in the Southern species.〔Novak, R. M. (1999). ''Walker's Mammals of the World.'' 6th edition. Johns Hopkins University Press, Baltimore. ISBN 0-8018-5789-9〕 Both adult and juvenile elephant seals are bar-skinned and black before molting. After molting, they generally have a silver to dark gray coat that fades to brownish-yellow and tan. Adult males have hairless necks and chests speckled with pink, white and light brown. Pups are mostly black at birth and molt to a silver gray after weaning. The eyes are large, round and black. The width of the eyes and a high concentration of low light pigments suggest sight plays an important role in the capture of prey. Like all seals, elephant seals have atrophied hind limbs whose underdeveloped ends form the tail and tail fin. Each of the "feet" can deploy five long, webbed fingers. This agile, dual palm is used to propel water. The pectoral fins are used little while swimming. While their hind limbs are unfit for locomotion on land, elephant seals use their fins as support to propel their bodies. They are able to propel themselves quickly (as fast as 8 km/h) in this way for short-distance travel, to return to water, catch up with a female or chase an intruder. Like other seals, elephant seals' bloodstreams are adapted to the cold in which a mixture of small veins surrounds arteries capturing heat from them. This structure is present in extremities such as the hind legs. A unique characteristic of the northern elephant seal is that it has developed the ability to store oxygenated red blood cells within its spleen. In a 2004 study researchers used MRI to observe physiological changes of the spleen of 5 seal pups during simulated dives. By 3 minutes, the spleens on average contracted to a fifth of their original size, indicating a dive-related sympathetic contraction of the spleen. Also, a delay was observed between contraction of the spleen and increased hematocrit within the circulating blood, and attributed to the hepatic sinus. This fluid-filled structure is initially expanded due to the rush of RBC from the spleen and slowly releases the red blood cells into the circulatory system via a muscular vena caval sphincter found on the cranial aspect of the diaphragm. This ability to slowly introduce RBC into the blood stream is likely to prevent any harmful effects caused by a rapid increase in hematocrit. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「northern elephant seal」の詳細全文を読む スポンサード リンク
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