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Walrus
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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: 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 recognizable due to its prominent tusks, whiskers and great bulk. Adult Pacific males can weigh up to , and, among pinnipeds, are exceeded in size only by the two species of elephant seals. 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.
EtymologyThe origins of the word "walrus" has variously been attributed to combinations of the Dutch words walvis ("whale") and ros ("horse") or wal ("shore") and reus ("giant"). However, the most likely origin of the word is the Old Norse hrossvalr, meaning "horse-whale", which was passed in a juxtaposed form to Dutch and the North-German dialects of the Hanseatic League as walros and Walross.
The now archaic English word for walrus morse is widely supposed to have come from the Slavic. Thus ???? (morž) in Russian, mors in Polish, also mursu in Finnish, moršâ in Saami, later morse in French, morsa in Spanish, etc.
The compound Odobenus comes from odous and baino (Greek for "walk"), based on observations of walruses using their tusks to pull themselves out of the water. Divergens in Latin means "turning apart", referring to the tusks.
Taxonomy and evolutionThe walrus is a mammal in the order Carnivora. It is the sole surviving members of the family Odobenidae, one of three lineages in the suborder Pinnipedia along with true seals, and eared seals. 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. There remains uncertainty as to whether the odobenids diverged from the otariids before or after the phocids, though the most recent synthesis of the molecular data suggests that the phocids were the first to diverge. What is known, however, is that Odobenidae was once a highly diverse and widespread family, including at least twenty known species in the Imagotariinae, Dusignathinae and Odobeninae subfamilies. 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 species.
Two subspecies of the walrus are commonly recognized: the Atlantic Walrus, O. r. rosmarus (Illiger, 1815) and the Pacific Walrus, O. r. divergens (Linnaeus, 1758). Fixed genetic differences between the Atlantic and Pacific subspecies indicate very restricted gene flow, but relatively recent separation, estimated to have occurred 500,000 and 785,000 years ago. 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. From there, it presumably re-colonized the North Pacific during high glaciation periods in the Pleistocene via the Central American Seaway. A third, isolated population of the walrus in the Laptev Sea is considered by some, including Russian biologists and the canonical Mammal Species of the World, to be a third subspecies, O. r. laptevi (Chapskii, 1940), when it was described and is managed as such in Russia. Where the subspecies separation is not accepted, there remains debate as to whether it should be considered a subpopulation of the Atlantic or Pacific subspecies.
Range and populationThere were roughly 200,000 Pacific Walruses according to the last census-based estimation in 1990. The majority of the Pacific Walrus population spends the summer 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 the Chukchi Peninsula of Siberia 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 to the south in the Bering Sea along the eastern shore of Siberia south to the northern part of the Kamchatka Peninsula, and along the southern shore of Alaska. A 28,000 year old fossil walrus specimen was dredged out of the San Francisco Bay, indicating that the Pacific Walrus ranged as far south as Northern California during the last ice age.
The Atlantic Walrus, which was nearly decimated by commercial harvest, is much smaller. Good estimates are difficult to obtain, but the total number is probably below 20,000. It ranges from the Canadian Arctic, Greenland, Svalbard and the western portion of the Russian Arctic. There are eight presumed sub-populations of the Atlantic Walrus based largely on geographical distribution and movement data, five to the west and three to the east of Greenland. The Atlantic Walrus once enjoyed a range that extended 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 as being extirpated in Canada.
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.
Description
While isolated Pacific males can weigh as much as , most weigh between and . Females weigh about two thirds as much as males, and the Atlantic subspecies is about 90% as massive as the Pacific subspecies. The Atlantic Walrus also tends to have relatively shorter tusks and somewhat more flattened snouts. The body shape of the walrus is in several ways intermediate between that of eared seals (Otariidae) and true seals (Phocidae). As with otariids, it has a prominent thick neck and the ability to turn its rear flippers forward and move on all fours; however, its swimming technique is more similar to that of true seals, relying less on flippers and more on sinuous whole body movements. Like phocids, it also lacks external ears.
The most prominent physical feature of the walrus is its long tusks, actually elongated canines, which are present in both sexes and can reach a length of and . These are slightly longer and thicker among males, who use them for fighting, dominance and display; the strongest males with the largest tusks typically dominating social groups. Tusks are also used to form and maintain holes in the ice and haul out onto ice. It was previously assumed that tusks were used to dig out prey items 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. The walrus has relatively few teeth other than the great canine tusks, and typically has a dental formula of:
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 in length, though in the wild they are often worn to a much shorter length due to constant use in foraging. The vibrissae are attached to muscles and are supplied with blood and nerves making the vibrissal array a highly sensitive organ capable of differentiating shapes thick and wide.
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 the blood vessels in the skin 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 the walrus to bob vertically in the water and sleep. The males possess a large baculum (penis bone), up to in length, the largest of any mammal both absolutely and relative to body size.
Life cycle
The walrus lives around 50 years. The males reach sexual maturity as early as 7 years, but do not typically mate until fully developed around 15 years of age. They go into a rut in January through April, decreasing their food intake dramatically. The females can begin ovulating as soon as 4–6 years old. The females are polyestrous, coming into heat in late summer and also around February, yet the males are only fertile around February; the potential fertility of this second period of estrous is unknown. Breeding occurs from January to March with peak conception 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 copulation occurs in the water.
Total gestation lasts 15 to 16 months, though 3 to 4 of those months are spent with the blastula in suspended development before finally implanting itself in the placenta. This strategy of delayed implantation, common among other pinnipeds, presumably evolved to optimize both the season when females select their mates and the season when the birth itself occurs, determined by ecological conditions that promote survival of the young. The calves are born during the spring migration from April to June. They weigh to 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. Because ovulation is suppressed until the calf is weaned, females give birth at most once every two years, resulting in the walrus having the lowest reproductive rate of any pinniped.
In the non-reproductive season (late summer and fall) the walrus tends to migrate away from the ice and form massive aggregations of tens of thousands of individuals on rocky beaches or outcrops. The nature of the migration between the reproductive period and the summer period can be a rather 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.
Exploitation and status
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. Commercial harvest of the walrus is now outlawed throughout its range, though a traditional subsistence hunt continues among Chukchi, Yupik and Inuit peoples. The walrus hunt occurs towards the end of the summer. Traditionally, all parts of the walrus was used. The meat, often preserved, is an important source of nutrition through the winter; 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 is used for rope and house and boat coverings; the intestines and gut linings are used for making waterproof parkas; etc. While some of these uses have faded with access to alternative technologies, walrus meat remains an important part of local diets, and tusk carving and engraving remain a vital art form among many communities.
Walrus hunts are regulated by resource managers in Russia, the U.S., Canada and Denmark and representatives of the respective walrus hunting communities. An estimated four to seven thousand Pacific Walruses are harvested in Alaska and Russia, including a significant portion (approx. 42%) of struck and lost animals. Several hundred are removed annually around Greenland. The sustainability of these levels of harvest are difficult to determine since there is considerable uncertainty in the population estimates themselves and in the population parameters such as fecundity and mortality.
The effects of global climate change on the walrus populations is another element of concern. In particular, there have been well-documented reductions on the extent and thickness of the pack ice which the walrus relies on as a substrate for giving birth and aggregating in the reproductive period. It is hypothesized that thinner pack ice over the Bering Sea has reduced the amount of suitable resting habitat near optimal feeding grounds. This causes greater separation of lactating females from their calves leading to nutritional stress for the young or lower reproductive rates for the females. However, there is as yet little data to make reliable predictions on the impacts of changing climate conditions on total population trends.
Currently, two of the three walrus subspecies are listed as "least-concern" by the IUCN, while the third is "data deficient". 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. Global trade in walrus ivory is restricted according to a CITES Appendix 3 listing.
See also
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