Walrus



The walrus (Odobenus rosmarus) is a large flippered marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and sub-Arctic seas of the Northern Hemisphere. The walrus is the only living species in the Odobenidae family and Odobenus genus. It is subdivided into three subspecies:[1]the Atlantic Walrus (O. rosmarus rosmarus) found in the Atlantic Ocean, the Pacific Walrus (O. rosmarus divergens) found in the Pacific Ocean, and O. rosmarus laptevi, found in the Laptev Sea.

The walrus is immediately recognized by its prominent tusks, whiskers and great bulk. Adult Pacific males can weigh up to 2,000 kilograms (4,400 lb) and, among pinnipeds, are exceeded in size only by the two species of elephant seals.[3] It resides primarily in shallow oceanic shelf habitat, spending a significant proportion of its life on sea ice in pursuit of its preferred diet of benthic bivalve mollusks. It is a relatively long-lived, social animal and is considered a keystone species in Arctic marine ecosystems.

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 andbone. In the 19th and early 20th centuries, the walrus was the object of heavy commercial exploitation for blubber and ivory and its numbers declined rapidly. Its global population has since rebounded, though the Atlantic and Laptev populations remain fragmented and at historically depressed levels.

Evolution
The walrus is a mammal in the order Carnivora. It is the sole surviving member of the familyOdobenidae, 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 that all three descended from a caniform ancestor most closely related to modern bears.[8] There remains uncertainty as to whether the odobenids diverged from the otariids before or after the phocids,[8]though the most recent molecular data synthesis suggests that phocids were the first to diverge.[9] What is known, however, is that Odobenidae was once a highly diverse and widespread family, including at least twenty species in the Imagotariinae, Dusignathinae and Odobeninae subfamilies.[10] 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 are commonly 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.[11] These dates coincide with the fossil-derived hypothesis that the walrus evolved from a tropical or sub-tropical ancestor that became isolated in the Atlantic Ocean and gradually adapted to colder conditions in the Arctic.[11] From there, it presumably re-colonized the North Pacific during high glaciation periods in the Pleistocene via the Central American Seaway.[9] An isolated population in the Laptev Sea is considered by some, including Russian biologists and the canonical Mammal Species of the World,[1] to be a third subspecies, O. r. laptevi (Chapskii, 1940), and is managed as such in Russia.[12] Where the subspecies separation is not accepted, whether to consider it a subpopulation of the Atlantic or Pacific subspecies remains under debate.