The walrus (Odobenus rosmarus; plural walrus or walruses) is a large pinniped marine mammal with discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. It is the only extant species in the family Odobenidae and genus Odobenus. This species is subdivided into two subspecies: the Atlantic walrus (O. r. rosmarus), which lives in the Atlantic Ocean, and the Pacific walrus (O. r. divergens), which lives in the Pacific Ocean.
Adult walrus are characterised by prominent tusks and whiskers, and considerable bulk; adult males in the Pacific can weigh more than and, among pinnipeds, are exceeded in size only by the two species of elephant seals. Walrus live mostly in shallow waters above the continental shelves, spending significant amounts of their lives on the sea ice looking for benthic bivalve molluscs. Walruses are relatively long-lived, social and intelligent animals, and are considered a keystone species of the Arctic marine region.
The walrus has played a prominent role in the cultures of many indigenous Arctic peoples, who have hunted it for meat, fat, skin, tusks, and bone. During the 19th century and the early 20th century, walrus were widely hunted for their blubber, walrus ivory, leather, and meat; in this period, the walrus population dropped rapidly all around the Arctic region. It has rebounded somewhat since, though the populations of Atlantic and Laptev walruses remain fragmented and at low levels compared with the time before human interference.
Etymology
Despite seeming to have a Latin shape, the origin of the word walrus derives from a Germanic language, and it has been attributed largely to either Dutch or Old Norse in some form. Its first part derives from the Old Norse ('whale'), and the second part is from the Old Norse ('horse'). For example, the Old Norse word means 'horse-whale' and is thought to have been passed in an inverted form to both Dutch and the dialects of northern Germany as and . An alternative theory is that it comes from the Dutch words 'shore' and 'giant'.
The species name rosmarus is Scandinavian. The Norwegian manuscript Konungs skuggsjá, thought to date from around AD 1240, refers to the walrus as in Iceland and in Greenland (walruses were by now extinct in Iceland and Norway, while the word evolved in Greenland). Several place names in Iceland, Greenland and Norway may originate from walrus sites: Hvalfjord, Hvallatrar and Hvalsnes to name some, all being typical walrus breeding grounds.
The archaic English word for walrus—morse—is widely thought to have come from the Slavic languages, which in turn borrowed it from Finno-Ugric languages, and ultimately (according to Ante Aikio) from an unknown Pre-Finno-Ugric substrate language of Northern Europe. Compare () in Russian, in Finnish, in Northern Saami, and in French. Olaus Magnus, who depicted the walrus in the in 1539, first referred to the walrus as the , probably a Latin derivation of , and this was adopted by Linnaeus in his binomial nomenclature.
The coincidental similarity between morse and the Latin word ('a bite') supposedly contributed to the walrus's reputation as a "terrible monster".
The compound Odobenus comes from (Greek for 'teeth') and (Greek for 'walk'), based on observations of walruses using their tusks to pull themselves out of the water.
The Inuttitut term for the creature is aivik, similar to the Inuktitut word: aiviq ᐊᐃᕕᖅ.
Taxonomy and evolution
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 accepted; 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.
The modern walrus is mostly known from Arctic regions, but a substantial breeding population occurred on isolated Sable Island, southeast of Nova Scotia and due east of Portland, Maine, until the early Colonial period. Abundant walrus remains have also been recovered from the southern North Sea dating to the Eemian interglacial period, when that region would have been submerged as it is today, unlike the intervening glacial lowstand when the shallow North Sea was dry land. Fossils known from San Francisco, Vancouver, and the Atlantic US coast as far south as North Carolina have been referred to glacial periods.
An isolated population in the Laptev Sea was 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), but has since been determined to be of Pacific walrus origin.
Anatomy
While some outsized Pacific males can weigh as much as , most weigh between . An occasional male of the Pacific subspecies far exceeds normal dimensions. In 1909, a walrus hide weighing was collected from an enormous bull in Franz Josef Land, while in August 1910, Jack Woodson shot a walrus, harvesting its 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 . However, in a 2015 study examining the sizes of large marine animals, McClain and colleagues found that both of the former records were difficult to verify, and that the maximum weight recorded was a male measuring in length and , and a maximum length of were considered the most reliable and accurately measured individuals. The Atlantic subspecies weighs about 10–20% less than the Pacific subspecies. Male Atlantic walrus weigh an average of . The Atlantic walrus also tends to have relatively shorter tusks and somewhat more of a flattened snout. Females weigh about two-thirds as much as males, with the Atlantic females averaging , sometimes weighing as little as , and the Pacific female averaging . Length typically ranges from . Newborn walruses are already quite large, averaging in weight and in length across both sexes and subspecies. Overall, the walrus is the third largest pinniped species, surpassed in size only by the southern and northern 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. 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.
Tusks and dentition
Although some extinct species of walrus did not have long tusks, they are the most prominent feature of the living species. Tusks are elongated canines that are present in both male and female walruses and can reach a length of 1 m (3 ft 3 in) and weigh up to 5.4 kg (12 lb). They are slightly longer and thicker in males, who use them for fighting, dominance, and display. The strongest males with the largest tusks typically dominate social groups. Tusks are also used to form and maintain holes in the ice and aid the walrus in climbing out of water onto ice. Tusks were once thought to be used to dig out prey from the seabed, but analyses of abrasion patterns on the tusks indicate they are dragged through the sediment while the upper edge of the snout is used for digging. The dentition of walruses is highly variable, but they normally have relatively few teeth other than their tusks. The maximal number of teeth is 38 with dentition formula: , but over half of the teeth are rudimentary and occur with less than 50% frequency, such that a typical dentition includes only 18 teeth
Vibrissae (whiskers) Surrounding the tusks is a broad mat of stiff bristles ("mystacial vibrissae"), giving the walrus a characteristic whiskered appearance, bestowing the namesake of the walrus moustache. There can be 400 to 700 vibrissae in 13 to 15 rows reaching 30 cm (12 in) in length, though in the wild they are often worn to much shorter lengths due to constant use in foraging. The vibrissae are attached to muscles and are supplied with blood and nerves, making them highly sensitive organs capable of differentiating shapes thick and wide.
Skin Aside from the vibrissae, the walrus is sparsely covered with fur and appears bald. Its skin is highly wrinkled and thick, up to around the neck and shoulders of males. The blubber layer beneath is up to thick. Young walruses are deep brown and grow paler and more cinnamon-colored as they age. Old males, in particular, become nearly pink. Because skin blood vessels constrict in cold water, the walrus can appear almost white when swimming. As a secondary sexual characteristic, males also acquire significant nodules, called "bosses", particularly around the neck and shoulders.
The walrus has an air sac under its throat which acts like a flotation bubble and allows it to bob vertically in the water and sleep. The males possess a large baculum (penis bone), up to in length, the largest of any land mammal, both in absolute size and relative to body size.
Life history
Reproduction Walruses live to about 20–30 years old in the wild. Comment The oldest-living walrus on record was Slowpoke, a captive walrus at SeaWorld Orlando, who lived to be 49 years old when he died. The males reach sexual maturity as early as 7 years, but do not typically mate until fully developed at around 15 years of age. They rut from January through April, decreasing their food intake dramatically. The females begin ovulating as soon as 4-6 years old. Females reach physical maturity at 10–12 years of age, and males at 15–16 years of age. The females are diestrous, coming into heat in late summer and around February, yet the males are fertile only around February; the potential fertility of this second period is unknown. Breeding occurs from January to March, peaking in February. Males aggregate in the water around ice-bound groups of estrous females and engage in competitive vocal displays. The females join them and copulate in the water.