Walrus

The walrus (Odobenus rosmarus) is a large flippered marine mammal with a discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. The walrus is the only living species in the family Odobenidae and genus Odobenus. This species is subdivided into three subspecies: the Atlantic walrus (O. r. rosmarus) which lives in the Atlantic Ocean, the Pacific walrus (O. r. divergens) which lives in the Pacific Ocean, and O. r. laptevi, which lives in the Laptev Sea of the Arctic Ocean.
Adult walrus are easily recognized by their prominent tusks, whiskers, and bulk. Adult males in the Pacific can weigh more than 2,000 kg (4,400 lb) and, among pinnipeds, are exceeded in size only by the two species of elephant seals. Walruses live mostly in shallow waters above the continental shelves, spending significant amounts of their lives on the sea ice looking for benthic bivalve mollusks to eat. Walruses are relatively long-lived, social animals, and they are considered to be a "keystone species" in the Arctic marine regions.
The walrus has played a prominent role in the cultures of many indigenous Arctic peoples, who have hunted the walrus for its meat, fat, skin, tusks, and bone. During the 19th century and the early 20th century, walruses were widely hunted and killed for their blubber, walrus ivory, and meat. The population of walruses dropped rapidly all around the Arctic region. Their population has rebounded somewhat since then, though the populations of Atlantic and Laptev walruses remain fragmented and at low levels compared with the time before human interference.
== Description ==
The walrus is a mammal in the order Carnivora. It is the sole surviving member of the family Odobenidae, one of three lineages in the suborder Pinnipedia along with true seals (Phocidae) and eared seals (Otariidae). While there has been some debate as to whether all three lineages are monophyletic, i.e. descended from a single ancestor, or diphyletic, recent genetic evidence suggests all three descended from a caniform ancestor most closely related to modern bears. Recent multigene analysis indicates the odobenids and otariids diverged from the phocids about 20–26 million years ago, while the odobenids and the otariids separated 15–20 million years ago. Odobenidae was once a highly diverse and widespread family, including at least twenty species in the subfamilies Imagotariinae, Dusignathinae and Odobeninae. The key distinguishing feature was the development of a squirt/suction feeding mechanism; tusks are a later feature specific to Odobeninae, of which the modern walrus is the last remaining (relict) species.
Two subspecies of walrus are widely recognized: the Atlantic walrus, O. r. rosmarus (Linnaeus, 1758) and the Pacific walrus, O. r. divergens (Illiger, 1815). Fixed genetic differences between the Atlantic and Pacific subspecies indicate very restricted gene flow, but relatively recent separation, estimated at 500,000 and 785,000 years ago. These dates coincide with the hypothesis derived from fossils that the walrus evolved from a tropical or subtropical ancestor that became isolated in the Atlantic Ocean and gradually adapted to colder conditions in the Arctic. From there, it presumably recolonized the North Pacific Oceanduring high glaciation periods in the Pleistocene via the Central American Seaway.
An isolated population in the Laptev Sea is considered by some authorities, including many Russian biologists and the canonical Mammal Species of the World, to be a third subspecies, O. r. laptevi(Chapskii, 1940), and is managed as such in Russia. Where the subspecies separation is not accepted, whether to consider it a subpopulation of the Atlantic or Pacific subspecies remained under debate until 2009, when multiple lines of molecular evidence showed it to represent the westernmost population of the Pacific walrus.
While some outsized Pacific males can weigh as much as 2,000 kg (4,400 lb), most weigh between 800 and 1,700 kg (1,800 and 3,700 lb). An occasional male of the Pacific subspecies far exceeds normal dimensions. In 1909, a walrus hide weighing 500 kg (1,100 lb) was collected from an enormous bull in Franz Josef Land, while in August 1910, Jack Woodson shot a 4.9 m (16 ft) long walrus, harvesting its 450 kg (1,000 lb) hide. Since a walrus's hide usually accounts for about 20% of its body weight, the total body mass of these two giants is estimated to have been at least 2,300 kg (5,000 lb). The Atlantic subspecies weighs about 10–20% less than the Pacific subspecies. Male Atlantic walrus weigh an average of 900 kg (2,000 lb). The Atlantic walrus also tends to have relatively shorter tusks and somewhat more flattened snout. Females weigh about two-thirds as much as males, with the Atlantic females averaging 560 kg (1,230 lb), sometimes weighing as little as 400 kg (880 lb), and the Pacific female averaging 800 kg (1,800 lb). Length typically ranges from 2.2 to 3.6 m (7.2 to 11.8 ft). Newborn walruses are already quite large, averaging 33 to 85 kg (73 to 187 lb) in weight and 1 to 1.4 m (3.3 to 4.6 ft) in length across both sexes and subspecies. All told, the walrus is the third largest pinniped species, after the two elephant seals. Walruses maintain such a high body weight because of the blubber stored underneath their skin. This blubber keeps them warm and the fat provides energy to the walrus.
The walrus's body shape shares features with both sea lions (eared seals: Otariidae) and seals (true seals: Phocidae). As with otariids, it can turn its rear flippers forward and move on all fours; however, its swimming technique is more like that of true seals, relying less on flippers and more on sinuous whole body movements.[4]Also like phocids, it lacks external ears.
The extraocular muscles of the walrus are well-developed. This and its lack of orbital roof allow it to protrude its eyes and see in both a frontal and dorsal direction. However, vision in this species appears to be more suited for short-range.
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