Pacific 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 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 their 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. 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.

Etymology

The origin of the word ''walrus'' derives from a Germanic language, and it has been attributed largely to either the Dutch language or Old Norse. Its first part is thought to derive from a word such as Old Norse ('whale') and the second part has been hypothesized to come from the Old Norse word ('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 ''Carta Marina'' in 1539, first referred to the walrus as the , probably a Latinization 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 term in Latin means 'turning apart', referring to their tusks.

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 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 Ocean during high glaciation periods in the Pleistocene via the Central American Seaway.

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 . 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 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. 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. 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

While this was not true of all extinct walruses, the most prominent feature of the living species is its long tusks. These are elongated canines, which 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). Tusks are slightly longer and thicker among males, which 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. While the dentition of walruses is highly variable, they generally have relatively few teeth other than the 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. 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. The males reach sexual maturity as early as seven 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 four to six years old. The females are diestrous, coming into heat in late summer and also 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.

Gestation lasts 15 to 16 months. The first three to four months are spent with the blastula in suspended development before it implants itself in the uterus. This strategy of delayed implantation, common among pinnipeds, presumably evolved to optimize both the mating season and the birthing season, determined by ecological conditions that promote newborn survival. Calves are born during the spring migration, from April to June. They weigh at birth and are able to swim. The mothers nurse for over a year before weaning, but the young can spend up to five years with the mothers. Walrus milk contains higher amounts of fats and protein compared to land animals but lower compared to phocid seals. This lower fat content in turn causes a slower growth rate among calves and a longer nursing investment for their mothers. Because ovulation is suppressed until the calf is weaned, females give birth at most every two years, leaving the walrus with the lowest reproductive rate of any pinniped.

Migration

The rest of the year (late summer and fall), walruses tend to form massive aggregations of tens of thousands of individuals on rocky beaches or outcrops. The migration between the ice and the beach can be long-distance and dramatic. In late spring and summer, for example, several hundred thousand Pacific walruses migrate from the Bering Sea into the Chukchi Sea through the relatively narrow Bering Strait.

Ecology

Range and habitat

The majority of the population of the Pacific walrus spends its summers north of the Bering Strait in the Chukchi Sea of the Arctic Ocean along the northern coast of eastern Siberia, around Wrangel Island, in the Beaufort Sea along the northern shore of Alaska south to Unimak Island, and in the waters between those locations. Smaller numbers of males summer in the Gulf of Anadyr on the southern coast of the Siberian Chukchi Peninsula, and in Bristol Bay off the southern coast of Alaska, west of the Alaska Peninsula. In the spring and fall, walruses congregate throughout the Bering Strait, reaching from the western coast of Alaska to the Gulf of Anadyr. They winter over in the Bering Sea along the eastern coast of Siberia south to the northern part of the Kamchatka Peninsula, and along the southern coast of Alaska. A 28,000-year-old fossil walrus was dredged up from the bottom of San Francisco Bay, indicating that Pacific walruses ranged that far south during the last Ice Age.

Commercial harvesting reduced the population of the Pacific walrus to between 50,000 and 100,000 in the 1950s-1960s. Limits on commercial hunting allowed the population to increase to a peak in the 1970s-1980s, but subsequently, walrus numbers have again declined. Early aerial censuses of Pacific walrus conducted at five-year intervals between 1975 and 1985 estimated populations of above 220,000 in each of the three surveys.

In 2006, the population of the Pacific walrus was estimated to be around 129,000 on the basis of an aerial census combined with satellite tracking. There were roughly 200,000 Pacific walruses in 1990.

The much smaller population of Atlantic walruses ranges from the Canadian Arctic, across Greenland, Svalbard, and the western part of Arctic Russia. There are eight hypothetical subpopulations of Atlantic walruses, based largely on their geographical distribution and movements: five west of Greenland and three east of Greenland. The Atlantic walrus once ranged south to Sable Island, Nova Scotia, and as late as the 18th century was found in large numbers in the Greater Gulf of St. Lawrence region, sometimes in colonies of up to 7,000 to 8,000 individuals. This population was nearly eradicated by commercial harvest; their current numbers, though difficult to estimate, probably remain below 20,000. In April 2006, the Canadian Species at Risk Act listed the population of the northwestern Atlantic walrus in Quebec, New Brunswick, Nova Scotia, Newfoundland and Labrador as having been eradicated in Canada. A genetically distinct population existed in Iceland that was wiped out after Norse settlement around 1213–1330 AD.

The isolated population of Laptev Sea walruses is confined year-round to the central and western regions of the Laptev Sea, the eastmost regions of the Kara Sea, and the westmost regions of the East Siberian Sea. The current population of these walruses has been estimated to be between 5,000 and 10,000.

Even though walruses can dive to depths beyond 500 meters, they spend most of their time in shallow waters (and the nearby ice floes) hunting for food.

In March 2021, a single walrus, nicknamed Wally the Walrus, was sighted at Valentia Island, Ireland, far south of its typical range, potentially due to having fallen asleep on an iceberg that then drifted south towards Ireland. Days later, a walrus, thought to be the same animal, was spotted on the Pembrokeshire coast, Wales.
In June 2022, a single walrus was sighted on the shores of the Baltic Sea - at Rügen Island, Germany, Mielno, Poland and Skälder Bay, Sweden. In July 2022, there was a report of a lost, starving walrus (nicknamed as Stena) in the coastal waters of the towns of Hamina and Kotka in Kymenlaakso, Finland, that, despite rescue attempts, died of starvation when the rescuers tried to transport it to the Korkeasaari Zoo for treatment.

Diet

Walruses prefer shallow shelf regions and forage primarily on the sea floor, often from sea ice platforms. They are not particularly deep divers compared to other pinnipeds; the deepest dives in a study of Atlantic walrus near Svalbard were only 31±17 m (102 ft) but a more recent study recorded dives exceeding 500 m (1640 ft) in Smith Sound, between NW Greenland and Arctic Canada - in general peak dive depth can be expected to depend on prey distribution and seabed depth.

The walrus has a diverse and opportunistic diet, feeding on more than 60 genera of marine organisms, including shrimp, crabs, tube worms, soft corals, tunicates, sea cucumbers, various mollusks (such as snails, octopuses, and squid), some types of slow-moving fish, and even parts of other pinnipeds. However, it prefers benthic bivalve mollusks, especially clams, for which it forages by grazing along the sea bottom, searching and identifying prey with its sensitive vibrissae and clearing the murky bottoms with jets of water and active flipper movements. The walrus sucks the meat out by sealing its powerful lips to the organism and withdrawing its piston-like tongue rapidly into its mouth, creating a vacuum. The walrus palate is uniquely vaulted, enabling effective suction. The diet of the Pacific walrus consist almost exclusively of benthic invertebrates (97 percent).

Aside from the large numbers of organisms actually consumed by the walrus, its foraging has a large peripheral impact on benthic communities. It disturbs ( bioturbates) the sea floor, releasing nutrients into the water column, encouraging mixing and movement of many organisms and increasing the patchiness of the benthos

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