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 walruses are easily recognized by their prominent tusks, whiskers, and bulkiness. 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.

Tusks and Dentition[]

WalrusBreathing — Walrus using its tusks to hang on a breathing hole in the ice near St. Lawrence Island, Bering Sea

The most prominent feature of the walrus 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: 3.1.4.23.1.3.2, but over half of the teeth are rudimentary and occur with less than 50% frequency, such that a typical dentition includes only 18 teeth 1.1.3.00.1.3.0

Vibrissae[]

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 3 mm (0.12 in) thick and 2 mm (0.079 in) 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 10 cm (3.9 in) around the neck and shoulders of males. The blubber layer beneath is up to 15 cm (5.9 in) 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 floatation bubble and allows it to bob vertically in the water and sleep. The males possess a large baculum (penis bone), up to 63 cm (25 in) in length, the largest of any land mammal, both in absolute size and relative to body size.

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 north shore of Alaska, 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 Pacific walruses ranged that far south during the last ice age. There were roughly 200,000 Pacific walruses according to the most recent (1990) census-based estimate.

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 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 Cape Cod, Massachusetts, and as late as the eighteenth 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 northwest Atlantic walrus in Quebec, New Brunswick, Nova Scotia, Newfoundland and Labrador as having been eradicated in Canada.

The isolated population of Laptev 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. The limited diving abilities of walruses brings them to depend on shallow waters (and the nearby ice floes) for reaching their food supply.

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; their deepest recorded dives are around 80 m (260 ft). They can remain submerged for as long as half an hour.

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, 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.

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.

Seal tissue has been observed in fairly significant proportion of walrus stomachs in the Pacific, but the importance of seals in the walrus diet is under debate. There have been isolated observations of walruses preying on seals up to the size of a 200 kg (440 lb) bearded seal. Rarely, incidents of walruses preying on seabirds, particularly the Brünnich's guillemot (Uria lomvia), have been documented. Walruses may occasionally prey on ice-entrapped narwhals and scavenge on whale carcasses but there is little evidence to prove this.

Predation[]

Due to its great size and tusks, the walrus has only two natural predators: the killer whale (orca) and the polar bear. The walrus does not, however, comprise a significant component of either predator's diets. Both the orca and the polar bear are also most likely to prey on walrus calves. The polar bear often hunts the walrus by rushing at beached aggregations and consuming the individuals crushed or wounded in the sudden exodus, typically younger or infirm animals. The bears also isolate walruses when they overwinter and are unable to escape a charging bear due to inaccessible diving holes in the ice. However, even an injured walrus is a formidable opponent for a polar bear, and direct attacks are rare. Walruses have been known to fatally injure polar bears in battles if the latter follows the other into the water where the bear is at a disadvantage. Polar bear–walrus battles are often extremely protracted and exhausting, and bears have been known to forgo the attack after injuring a walrus. Orcas regularly attack walrus, although walruses are believed to have successfully defended themselves via counterattack against the larger cetacean. However, orcas have been observed successfully attacking walruses with few or no injuries.