Species

Walrus

April 19, 2011, 4:00 pm
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The Walrus (scientific name: Odobenus rosmarus) is a marine mammal found in the northern waters of the Arctic and sub-Arctic seas and most notable for its large tusks and whiskers. It is the sole species in it's family (Odobenidae) and genus (Odobenus). Together with the families of eared seals and true seals (or "earless seals"), walruses form the group of marine mammals known as pinnipeds.

The walrus is one of the larger pinnipeds, only the two species of Elephant seals being bigger. 

Three distinct subspecies are recognized and commonly named after the region in which they are found:

  • Atlantic walrus (Odobenus rosmarus rosmarus Linnaeus, 1758)
  • Pacific walrus (Odobenus rosmarus divergens Illiger, 1815)
  • Laptev walrus (Odobenus rosmarus laptevi  Chapskii, 1940)

Some taxonomists do not recognize the Laptev Sea population as a separate subspecies and consider it to be the western-most population of the Pacific walrus.

Walruses were heavily hunted until the early twentieth century. Since then, their numbers have revived considerably. Continued uncertainty about the population of Atlantic walruses results in the species not having been assigned a conservation status. 

 
caption Walrus at Togiak National Wildlife Refuge. Source: U.S. Fish and Wildlife Service
 Conservation Status
Data Deficient

Scientific Classification

Kingdom: Animalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Carnivora (Carnivores)
Family:-------- Odobenidae (Walrus)
Genus:---------- Odobenus
Species:---------- Odobenus rosmarus (Linnaeus, 1758)

Physical Description

The most obvious physical characteristic of the walrus is the presence of large tusks in both the male and female. These tusks, which are canine teeth, can reach lengths of one meter (m) with an average size of 0.5 m; these structures are typically longer and heavier in the males (bulls) than in the females (cows).

Accompanying the tusks are stiff beard bristles, called "vibrissae", and, although individual variation in length is great, the bristles can extend to 0.3 m. The bristles are replaced yearly. In natural environments these bristles are often quite worn. Bulls are physically larger than the cows, growing to lengths of three m compared to 2.6 m for cows.

Aside from the conspicuous beards, both males and females appear almost completely bald. In fact, they are covered with short coarse hair that becomes less dense as the animal ages.

Their skin, which lies in many folds and wrinkles, can be four centimeters thick, which depth is accentuated most on the neck and shoulders of adult males. As walruses age their skin becomes paler. When the animals enter the water they become even paler as blood flow to the skin is restricted. Conversely, when walruses are warm their skin is flushed with blood and they appear to be very red, almost sunburned. Walruses have no external ears and their eyes are small and piglike.

The walrus is one of the larger pinnipeds, only the two species of Elephant seals exceeding its size.  Male Pacific walruses grow to about 3.6 m in length and 880 to 1557 kilograms (kg) in body mass, while the females achieve about three meters in length and 580 to 1039 kg in mass. Atlantic walruses are slightly smaller.

Walrus pups are born approximately 1.0 to 1.4 m in length and 33 to 85 kg in mass.

Distinguishing characteristics: long tusks, stout whiskers, large size, square face, brown skin colour with sparse red hair. This species may appear white or red at times.

Behavior

Walruses are extremely gregarious. Often they haul out on land or ice floes in herds of up to several thousand individuals lying in close physical contact. During the nonbreeding season, these groups are sexually segregated, and there are dominance hierarchies based on both body and tusk size. When an individual hauls out of the water and seeks a resting spot occupied by a smaller animal, it may throw back its head and point its tusks at the smaller animal. If this does not frighten the smaller walrus, the larger may strike with its tusks, and frequently there is bloodshed. Apparently, walruses prefer to lie in the middle of these large congregations.

Walruses do not typically dive to as great depths as many other pinnipeds, reaching 200 m or more in search of food.

Reproduction

Because walruses breed during the harsh Arctic winters, little is known about their mating systems. The walrus species is believed to exhibit a female-defense polygyny. Dominant mature males have exclusive access to a herd of females for one to five days at a time.

Courtship behavior usually consists of several mature bulls competing for the exclusive mating rights to a herd of cows. Groups of males gather in large herds and use a variety of tactics to attract females. These tactics include extensive singing and vocal displays, as well as male-male combat. Males make a variety of clicking and bell-like sounds underwater. They also raise their heads above water and emit a series of sharp clucks and whistles.

Occasionally, males engage in physical combat, trying to injure each other by stabbing their tusks into the neck region of the opponent. These fights generally do not last long and characteristically end with one bull leaving the vicinity. Males successful in attracting females usually retain their position for one to five days, after which they are often displaced by another male. 

Mating and birth are both presumed to occur underwater. Walruses are interesting because implantation of the blastocyst is delayed for four to five months, until June or July. Birth occurs 10 to 11 months later, from mid-April to mid-June, meaning that the total gestation period is 15 to 16 months.

When a female is almost ready to give birth, she often leaves her herd. Females give birth to a single, precocial offspring. The calf is grey in color and can swim at birth. The social bond between the mother and calf is very strong, and cows are extremely protective of their offspring. After giving birth, she joins a group of other mothers and their young.

Lactation generally lasts for two years, but calves are often able to find supplementary food before they are fully weaned. Female offspring remain in the mother's group, but young males disperse at around two to three years old to join a herd of males. Bulls provide no parental care for the young, but community care of offspring by other females has been observed. Adoptions of orphaned calves have also been documented.

Young bulls become sexually mature at eight to ten years of age, but are often unable to compete successfully for females until they are at least 15 years old. Females become sexually mature at age six to seven, and are full grown at age ten to twelve. Female fecundity is greatest when cows are nine to eleven years old, at which age they can produce a calf every other year. The interval between births is longer in older females. In the wild, walruses have been known to live for over 40 years.

 

Distribution

caption Walrus colony. Source: U.S. Fish and Wildlife Service
Walruses occupy a nearly circumpolar region of the Arctic.
  • Atlantic walrus:  eastern Canadian Arctic and Greenland, Barents, White and Pechora seas, Svalbard,  Franz Josef Land, and western Kara Sea. Historically,  the range extended south to the Gulf of St Lawrence and individuals reported from New England, Iceland and the Bay of Biscay.
  • Pacific walrus: Bering Sea, Chukchi Sea, western Beaufort Sea and adjacent Arctic Ocean. Individuals have been know to stray into the North Pacific Ocean as far as Japan and southcentral Alaska.
  • Laptev Sea: Laptev Sea, north of Siberia.

Habitat

Walruses prefer to inhabit areas with ice floes in the shallower regions near the coasts of Arctic waterways. Their seasonal migration patterns coincide with the changes in the ice. In the winter, walruses move south as the Arctic ice expands, and in the summer they retreat north as the ice recedes. This migration can cover distances of 3000 km. Individuals concentrate where the ice is relatively thin and dispersed in the winter. In the summer time, bulls may use isolated coastal beaches and rocky islets. Cows and young prefer to stay on ice floes in all seasons

Food Habits

Walruses feed on animals that reside on the surface of the ocean floor, or in the benthic sediments. Their chief diet includes mussels, snails, echinoderms, and crabs. Walrus foraging dives usually endure for two to ten minutes at depths of 10 to 50 m. They swim headfirst along the ocean bottom, rooting about with their stiff beard bristles in a piglike fashion. Softbodied organisms are swallowed whole. Two theories on how walruses eat bivalves have been proposed, and it appears that walruses employ both methods. Walruses crack mollusk shells between their flippers and then eat the soft part. More often, walruses hold the shelled organisms in their lips and ingest the fleshy parts by powerful suction, discarding the shells. Occasionally, walruses also prey on fish, seals, and young whales. Walruses are capable of holding down seals and small whales with their flippers and tearing them apart with their tusks. In the winter, when accompanying the females and young, the bulls apparently eat very little. It was once believed that walruses used their tusks to dig up food, but this notion is now considered false.

Conservation Status

Although walrus populations had declined drastically by the early 20th century, management programs have resulted in the dramatic rebound of the Pacific population. While the populations are not believed to be in danger of becoming [[causes of extinction|extinct\, the Atlantic and Laptev Sea populations still remain at low levels. Presently, estimates of the number of walruses in all populations is not sufficiently documented to warrant an IUCN listing.

Economic Importance for Humans

Walruses have been exploited by humans for millennia. Native peoples harvest them for their meat, skin (which they used for shelters and kayak coverings), and ivory for tools, weapons, and arts. In the early 10th century, Viking traders began taking large numbers, and this European decimation of walruses continued until the early 20th century. Walruses are now managed by governments, but continue to be killed in some regions. Northern cultures are allowed to hunt walruses for subsistence living, but poachers continue to take walruses illegally, mostly for their ivory tusks. Aside from human exploitation, walruses have little contact with people; consequently, walruses have little impact on human economies.

caption Walrus colony. Source: U.S. Fish and Wildlife Service

 

Further Reading

  • Odobenus rosmarus (Linnaeus, 1758). Encyclopedia of Life. (accessed July 22, 2010)
  • Odobenus rosmarus, Carling, M., 1999, Animal Diversity Web (accessed July 22, 2010)
  • WoRMS, World Registry of Marine Species (accessed July 22, 2010)
  • Odobenus rosmarus IUCN Red List of Threatened Species (accessed July 22, 2010)
  • Walrus, Seal Conservation Society (accessed April 23, 2009)
  • Odobenus rosmarus, MarineBio.org (accessed, April 23, 2009)
  • Marianne Riedman. 1991. The Pinnipeds: Seals, Sea Lions, and Walruses, University of California Press, ISBN: 0520064984 
  • Bernd Wursig. 2002. Encyclopedia of Marine Mammals, Academic Press, ISBN: 0125513402
  • Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, Suzanne Montgomery and Timothy J. Ragen, Johns Hopkins University Press, 2005 ISBN: 0801882559
  • Ronald M.Nowak. 1999. Walker's Mammals of the World, Johns Hopkins University Press,  ISBN: 0801857899
Glossary

Citation

Life, E. (2011). Walrus. Retrieved from http://www.eoearth.org/view/article/156905

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