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) which lives in the Atlantic Ocean, the Pacific walrus (O. rosmarus divergens) which lives in the Pacific Ocean, and O. rosmarus laptevi, which lives in the Laptev Sea.

The walrus is easily recognized by its prominent tusks, whiskers and great bulk. Adult Pacific males can weigh more than 1,700 kilograms (3,700 lb)^[3] and, among pinnipeds, are exceeded in size only by the two species of elephant seals.^[4] 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 and bone. 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.

Etymology

A walrus, labeled "Ros marus piscis", depicted in а 16th century map of Scandinavia (the Carta Marina).

The origin of walrus is thought to derive from a Germanic language, such as English, and is variously attributed to Dutch or Old Norse. The first part is thought to derive from a word such as English "whale", Dutch "walvis" meaning whale. And the second part could come from a Norse word for "horse".^[5] For example, the Old Norse word hrossvalr, means "horse-whale", and is thought to have been passed in an inverted form to Dutch and the North-German dialects as walros and Walross.^[6] An alternative theory is from Dutch wal ("shore") and reus ("giant").^[7]

The now archaic English word for walrus—morse—is widely supposed to have come from the Slavic.^[8] Thus морж (morž) in Russian, mursu in Finnish, moršâ in Saami, and morse in French. Olaus Magnus, who depicted the walrus in the carta marina in 1539, first referred to it as the ros marus, likely a Latinization of morž, and this was adopted by Linnaeus in the binomial nomenclature.^[9] The coincidental similarity between "morse" and the Latin word morsus meaning "a bite" supposedly contributed to the walrus's reputation as a "terrible monster".^[9]

The compound Odobenus comes from odous (Greek for "tooth") and baino (Greek for "walk"), based on observations of walruses using their tusks to pull themselves out of the water. The term divergens in Latin means "turning apart", referring to the 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 that all three descended from a caniform ancestor most closely related to modern bears.^[10] Recent multigene analysis indicates that the odobenids and otariids diverged from the phocids approximately 20–26 million years ago, while the odobenids and the otariids separated 15–20 million years ago.^[11][12] Odobenidae was once a highly diverse and widespread family, including at least twenty species in the Imagotariinae, Dusignathinae and Odobeninae subfamilies.^[13] 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.^[14] 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.^[14] From there, it presumably re-colonized the North Pacific during high glaciation periods in the Pleistocene via the Central American Seaway.^[11] 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.^[15] Where the subspecies separation is not accepted, whether to consider it a subpopulation of the Atlantic or Pacific subspecies remains under debate.^[4][16]

Young male Pacific Walruses on Cape Pierce in Alaska. Note the variation in the curvature and orientation of the tusks and the bumpy skin (bosses), typical of males.

Anatomy

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

Skeleton

While some outsized Pacific males can weigh as much as 2,000 kg (4,400 lb), most weigh between 800 and 1,680 kg (1,800 and 3,700 lb). The Atlantic subspecies weighs about 10–20% less than the Pacific subspecies.^[4] The Atlantic Walrus also tends to have relatively shorter tusks and somewhat more flattened snout. Females weigh about two-thirds as much, with the Atlantic females averaging 560 kg (1,200 lb), sometimes weighing as little as 400 kg (880 lbs), and the Pacific female averaging 794 kg (1,750 lb).^[17] Length ranges from 2.2 to 3.6 m (7.2–12 ft).^[18][19] It is the second largest pinniped, after the elephant seals.

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.

Tusks and dentition

The most prominent feature of the walrus is its long tusks. These are elongated canines, which are present in both sexes and can reach a length of 1 m (3 ft 3 in) and weigh up to 5.4 kg (12 lb).^[20] Tusks are slightly longer and thicker among 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.^[21] It was previously assumed that tusks were used to dig out prey from the seabed, but analyses of abrasion patterns on the tusks indicate that they are dragged through the sediment while the upper edge of the snout is used for digging.^[22] 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 ^[4]

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 a much shorter length due to constant use in foraging.^[23] The vibrissae are attached to muscles and are supplied with blood and nerves making them a highly sensitive organ capable of differentiating shapes 3 mm (0.12 in) thick and 2 mm (0.079 in) wide.^[23]

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.^[21]

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.^[4]

Life history

Walruses fighting

Reproduction

Walruses live to about 20–30 years old in the wild.^[24] The males reach sexual maturity as early as 7 years, but do not typically mate until fully developed around 15 years of age.^[4] They rut from January through April, decreasing their food intake dramatically. The females begin ovulating as soon as 4–6 years old.^[4] The females are polyestrous, 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.^[25] The females join them and copulate in the water.^[21]

Gestation lasts 15 to 16 months. The first 3 to 4 months are spent with the blastula in suspended development before it implants itself in the placenta. 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.^[26] Calves are born during the spring migration, from April to June. They weigh 45–75 kg (99–170 lb) at birth and are able to swim. The mothers nurse for over a year before weaning, but the young can spend up to 3 to 5 years with the mothers.^[21] 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.^[27]

Migration

The rest of the year (late summer and fall) the walrus tends 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.^[21]

Ecology

Range and habitat

The majority of the Pacific Walrus population summers north of the Bering Strait in the Chukchi Sea along the north shore 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 south shore of Siberia's Chukchi Peninsula and in Bristol Bay off the south shore of southern Alaska west of the Alaska Peninsula. In the spring and fall they congregate throughout the Bering Strait, reaching from the west shores of Alaska to the Gulf of Anadyr. They winter in the Bering Sea along the eastern shore of Siberia south to the northern part of the Kamchatka Peninsula, and along Alaska's southern shore.^[4] A 28,000 year old fossil walrus specimen was dredged out of San Francisco Bay, indicating that the Pacific Walrus ranged far south during the last ice age.^[28]

The much smaller Atlantic population ranges from the Canadian Arctic, Greenland, Svalbard and the western portion of the Russian Arctic. There are eight presumed sub-populations based largely on geographical distribution and movement data, five to the west of Greenland and three to the east.^[29] The Atlantic Walrus once ranged south to Cape Cod and occurred in large numbers in the Gulf of St. Lawrence. In April 2006, the Canadian Species at Risk Act listed the Northwest Atlantic Walrus population (Québec, New Brunswick, Nova Scotia, Newfoundland and Labrador) as being extirpated in Canada.^[30]

The isolated Laptev population is confined year-round to the central and western regions of the Laptev Sea, the easternmost regions of the Kara Sea, and the westernmost regions of the East Siberian Sea. Current populations are estimated to be between 5,000 and 10,000 individuals.^[31]

Their limited diving ability brings them to depend on shallow waters (and appropriate nearby ice coverage) to enable them to reach their preferred benthic prey.

Population

There were roughly 200,000 Pacific Walruses according to the most recent (1990) census-based estimate.^[32][33]

The Atlantic Walrus was nearly eradicated by commercial harvest and has a much smaller population. Good estimates are difficult to obtain, but the total population is probably below 20,000.^[34][35]

Diet

Vibrissae of captive walrus (Japan)

Walruses leaving the water

Walruses prefer shallow shelf regions and forage primarily on the sea floor, often from sea ice platforms.^[4] They are not particularly deep divers compared to other pinnipeds: the deepest recorded dives are around 80 m (260 ft). They can remain submerged for as long as half an hour.^[36]

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.^[37] 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.^[38] The walrus sucks the meat out by sealing its powerful lips to the organism and withdrawing its tongue, piston-like, 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.^[22]

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.^[39] There have been isolated observations of walruses preying on seals up to the size of a 200 kg (440 lb) Bearded seal.^[40] There have been rare documented incidents of walruses preying on seabirds, particularly the Brünnich's Guillemot Uria lomvia.^[41]

Predation

Due to its great size and tusks, the walrus has only two natural predators: the orca and the polar bear. The walrus does not, however, comprise a significant component of either predator's diet. 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 individuals that are crushed or wounded in the sudden exodus, typically younger or infirm animals.^[42] The bears also isolate walruses when the walrus overwinter and are unable to escape a charging bear due to inaccessible diving holes in the ice.^[43] However, even an injured walrus is a formidable opponent for a polar bear, and direct attacks are rare. 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 it is believed that walruses have successfully defended themselves via counterattack against the larger cetacean.^[44]

Relation to humans

Conservation

Siberian Yupik woman holding walrus tusks

Walrus tusk engraving made by Chukchi artisans depicting polar bears attacking walruses. On display in the Magadan Regional Museum, Magadan, Russia

In the 18th and 19th centuries, the walrus was heavily exploited by American and European sealers and whalers, leading to the near extirpation of the Atlantic population.^[45] Commercial walrus harvesting is now outlawed throughout its range, although Chukchi, Yupik and Inuit peoples,^[46] continue to kill small numbers towards the end of each summer.

Traditional hunters used all parts of the walrus.^[47] The meat, often preserved, is an important winter nutrition source; the flippers are fermented and stored as a delicacy until spring; tusks and bone were historically used for tools as well as material for handicrafts; the oil was rendered for warmth and light; the tough hide made rope and house and boat coverings; the intestines and gut linings made waterproof parkas; etc. While some of these uses have faded with access to alternative technologies, walrus meat remains an important part of local diets,^[48] and tusk carving and engraving remain a vital art form.

Walrus hunts are regulated by resource managers in Russia, the United States, Canada and Denmark and representatives of the respective hunting communities. An estimated 4–7,000 Pacific Walruses are harvested in Alaska and Russia, including a significant portion (approx. 42%) of struck and lost animals.^[49] Several hundred are removed annually around Greenland.^[50] The sustainability of these levels of harvest is difficult to determine given uncertain population estimates and parameters such as fecundity and mortality.

The effects of global climate change are another element of concern. The extent and thickness of the pack ice has reached unusually low levels in several recent years. The walrus relies on this ice while giving birth and aggregating in the reproductive period. Thinner pack ice over the Bering Sea has reduced the amount of resting habitat near optimal feeding grounds. This more widely separates lactating females from their calves, increasing nutritional stress for the young and lower reproductive rates.^[51] Reduced coastal sea ice has also been implicated in the increase of stampeding deaths crowding the shorelines of the Chukchi Sea between eastern Russia and western Alaska.^[52][53] However, there is insufficient climate data to make reliable predictions on population trends.^[54]

Currently, two of the three walrus subspecies are listed as "least-concern" by the IUCN, while the third is "data deficient".^[2] The Pacific Walrus is not listed as "depleted" according to the Marine Mammal Protection Act nor as "threatened" or "endangered" under the Endangered Species Act. The Russian Atlantic and Laptev Sea populations are classified as Category 2 (decreasing) and Category 3 (rare) in the Russian Red Book.^[31] Global trade in walrus ivory is restricted according to a CITES Appendix 3 listing.

Culture

Walrus Ivory masks made by Yupik in Alaska

John Tenniel's illustration for Lewis Carroll's poem The Walrus and the Carpenter

The walrus plays an important role in the religion and folklore of many Arctic peoples. Skin and bone are used in some ceremonies and the animal appears frequently in legends. For example, in a Chukchi version of the widespread myth of the Raven, in which Raven recovers the sun and the moon from an evil spirit by seducing his daughter, the angry father throws the daughter from a high cliff and, as she drops into the water, she turns into a walrus — possibly the original walrus. According to various legends, the tusks are formed either by the trails of mucus from the weeping girl or her long braids.^[55] This myth is possibly related to the Chukchi myth of the old walrus-headed woman who rules the bottom of the sea, who is in turn linked to the Inuit goddess Sedna. Both in Chukotka and Alaska, the aurora borealis is believed to be a special world inhabited by those who died by violence, the changing rays representing deceased souls playing ball with a walrus head.^[55][56]

Because of its distinctive appearance, great bulk and immediately recognizable whiskers and tusks, the walrus also appears in the popular cultures of peoples with little direct experience with the animal, particularly in English children's literature. Perhaps its best-known appearance is in Lewis Carroll's whimsical poem The Walrus and the Carpenter that appears in his 1871 book Through the Looking-Glass. In the poem, the eponymous anti-heroes use trickery to consume a great number of oysters. Although Carroll accurately portrays the biological walrus's appetite for bivalve mollusks, oysters, primarily nearshore and intertidal inhabitants, in fact comprise an insignificant portion of its diet, even in captivity.^[57]

Another appearance of the walrus in literature is in the story The White Seal in Rudyard Kipling's The Jungle Book, where it is the "old Sea Vitch—the big, ugly, bloated, pimpled, fat-necked, long-tusked walrus of the North Pacific, who has no manners except when he is asleep".^[58]

The walrus is also mentioned in the songs "I Am the Walrus", "Glass Onion" and "Come Together" by The Beatles.

References

Further reading