San Francisco State University
Department of Geography

Geography 316:  Biogeography

The Biogeography of the Pacific Walrus
(Odobenus rosmarus divergens)

by Garrett Dunwoody, student in Geography 316, fall 1999

General Information
   
    The Walrus species (Odobenus rosmarus) is part of a group of mammals known as pinnipeds.  The walrus is the largest of the pinnipeds within the Arctic and subarctic seas. In general, the walrus tends to be found in shallower waters, close to ice or land (Alaska Department of Fish and Game 1999).  Two sub-species of walrus are recognized: the Atlantic Walrus (Odobenus rosmarus rosmarus) which tends to be smaller than its counterpart the Pacific Walrus (Odobenus rosmarus divergens)  (Wilson and Reeder 1993).
 

Total Taxonomic Classification of Odobenus rosmarus divergens

The following information was gathered at the web site (Smithsonian Natural History Home page 1999)
Kingdom-Animalia
Phylum-Chordata
Class-Mammalia
Order-Carnivora
Family-Pinnipedia
Genus-Odobenus
Species-Odobenus rosmarus
Sub-species-Odobenus rosmarus divergens
 
 

Evolution
   
    It is believed that walrus evolved during the Oligocene Era from bear or dog like ancestors of the North Pacific (Riedman 1990).  In relative terms the walrus is new to the North Pacific region.  The presence of these animals, in accordance with fossil record, date back to one hundred thousand years ago.  It is believed that walrus first entered this region from the northwest via the northern coast of Eurasia (USDI 1982).  The genus Odobenus constitutes a separate order Pinnipedia.  However, it is customary to consider them as a suborder within the family Carnivora (Gittleman 1989).  The origin of pinnipeds is not clear especially when referring to the walrus.  The modern view, however, is that they are monophyletic.  Biologists have been arguing over pinniped evolution for years.  If it is true that pinnipeds are monophyletic it would mean that walrus would be more closely related to other pinnipeds than to its relative the otter.  Studies do show that walrus are in fact monophyletic  and that they are more closely related to other pinnipeds than to mammals (Riedman 1990).  Perhaps this controversy about the origin and evolution of the walrus is largely due to the aggressive behavior  of the walrus and that its’ instincts more closely resemble that of the family carnivora.

Physical Appearance

    The male walrus tends to average a larger body and tusks than the female walrus (Renouf 1991).  The most distinguishing feature of all walrus are its tusks, which are carried by male and female.  These canines have been measured at lengths of one meter (Carling 1999).  Overall, walrus are considered to be cinnamon brown in color.  However, this will vary with sex and age.  In general younger walrus will be the darkest where their skin is highly pigmented and their hair will be almost black (Ridgeway et al. 1981).  In a comparison of standard lengths of males and females males are on the average of 50 cm longer than the average female.  However, there is a overlap in size.  Out of the specimens sampled  the  males range in size from 279-356 cm and females range from 230-312 cm.  Not only do males tend to be longer, on average they weigh 400 kg more than female walrus.  Where males weigh 1,210 kg (2667 lbs.) females will only weigh 832 kg (1834 lbs.) (USDI 1982).

Habitat
     Walrus are considered “pagophilic” pinnipeds, which means, “ice loving”. Pagophillic pinnipeds prefer to haul out on ice, however, they will also come ashore on rocky islands when ice is not available (Reidman 1990).  It is observed that walrus will not stray far from the ice edge.  The highest density of walrus lie within the nearest 37 km of the ice edge (Gilbert 1989).
 The Pacific walrus fulfills a particular niche within the ecology of the Bering and Chukchi Seas.  Walrus are bottom or benthic feeders that disturb and physically alter the sea floor (Fay et al. 1997).   Walrus need to inhabit seas and oceans in which the bottoms and banks are covered with an extensive variety of shellfish.  Due to seasonality and ice coverage the walrus is forced to follow food sources.  The walrus’ ability to locate food is largely dependent on the amount and type of ice present; this results in elaborate migratory patterns (Perry 1967).

Population
    Walrus populations are circumpolar, however, they are not uniformly distributed (Carling 1999).  There are many areas in the Arctic where there is low or no population of walrus.  The discontinuity of this species suggests that there are isolated populations with little or no chance to interbreed.  In many cases the populations of particular walrus species are separated by twelve hundred kilometers of unpopulated seas and oceans (USDI 1982).  Over the last one hundred and fifty years the population of walrus has fluctuated . The original population of walrus is unknown; however, estimates point in the direction of two hundred to two hundred and fifty thousand animals.   The walrus population was believed to reach its lowest in the mid 1950’s where population was estimated to be between fifty and one hundred thousand.  In the latest estimations walrus population is seeing a recovery with a recorded population of roughly two hundred to two hundred and thirty thousand (Fish and Wildlife Service 1998).

The Pacific Walrus (Odobenus rosmarus divergens)
     The Pacific Walrus is given its name due to the fact that they are found in the North Pacific Region of the Arctic, the Bering, and the Chukchi Seas (Fay 1981).  The Pacific Walrus inhabits the area from the eastern East Siberian Sea through the Chukchi Sea all the way to the western Beaufort Sea and southward to the waters of the Bering Sea. The majority of the population of the Pacific Walrus will congregate in the summer months in the southern regions of the Chukchi Sea.  The remaining population, which consists of mostly adult males, tends to remain further north in the Bering Sea and Bristol Bay.  In winter months these species will tend to follow areas that produce a thinner layer of pack ice, however, the exact migration and location will vary due to the magnitude of seasonality (Fish and Wildlife Services 1998).

Migration
    The following explanations are monthly records of The United States Department of the Interior (1982).

• January  In the month of January more than 90% of the northern seas are covered by heavy ice.  Due to this phenomenon very few walrus sightings occur in January.  This is mostly due to the fact that most have traveled southward prior to the development of the heavier ice.
• February  February is the coldest month where pack ice extends south to the edge of the continental shelf.  Walrus sightings are increased when compared to January.
• March  The number of walrus sightings is increased in March.  This is partly due to a fall in wind and a moderation in temperature.  March provides a very good model for the winter distribution of the walrus.
• April  During this month the water of the Bering Sea is much warmer largely due to the duration and angle of solar radiation.  The ice pack is diminishing.  This is evident with a larger percentage of the population present, along with a migration northward to cooler waters.
• May  By this time of year new ice formation has ceased.  Surface winds are warmer and weaker.  A large walrus population passes through the Bering Strait and northward.
• June  The ice pack of the Bering Sea continues to diminish.  By June what remains of  the population will move northward through the Bering Strait.
• July  With the warmer months the walrus moves northward to roughly 70 degrees N Latitude.  The walrus remaining south of the Bering Strait will remain there for the summer months.
• August  August usually begins with a slight recession of the ice pack.  At this time the walrus move north westwards towards Wrangell Island.  The geographic range of walrus at this time is from the Bear Islands east of Siberia to roughly 74 degrees N Latitude.
• September  At this time the majority of walrus population is above 70 degrees N Latitude.
• October  Due to the formation of new ice the walrus travels southward.  Much of the walrus population congregates at the Bering Strait.
• November  The ice pack moves south to the Bering Strait where the majority of the population of walrus is at rest between St. Lawrence Island and the ice pack of the North.
• December  The ice sheet has moved south past St. Lawrence Island where part of the population of walrus remains.  However, much like January very few sightings occur.

References Cited

Alaska Department of Fish and Game, 1999. Notebook Series. http://www.state.ak.us/local/akpages/FISH.GAME/notebook/marine/walrus.htm

Carling, M. 1999. University of Michigan. Museum of Zoology Animal Diversity Web. http://www.animaldiversity.ummz.umich.edu/accounts/odobenus/o._rosmarus$narrative.html

Fay, F.H. 1981. “Walrus Odobenus rosmarus.” pp.1-24. In Ridgway, S.H. and R.J. Harrison,eds. Handbook of Marine Mammals. London: Academic Press Inc.

Fay, F.H., L.L. Eberhardt, B.P. Kelly, J.J. Burns, and L.T. Quakenbush. 1997. “ Status of the Pacific Walrus Population, 1950-1989.” Marine Mammal Science13(4):537-565.

Fish and Wildlife Service,1998. Marine Mammals Management-Walrus Program. www.r7.fws.gov/mmm/w1.html

Gilbert,J.R.1989. “Aerial Census of Pacific Walruses in the Chukchi Sea.” Marine Mammal Science5(1):17-28.

Gittleman, J.L. 1989. Carnivore Behavior, Ecology, and Evolution. United States. Cornell University Press.

Perry, R. 1967. The World Of The Walrus. New York. Taplinger Publishing Company.

Renouf, D. 1991. Behaviour of Pinnipeds. Great Britain. Chapman and Hall.

Ridgway, S.H. Harrison, R.J. 1981. Handbook of Marine Mammals. London. Academic Press.

Riedman, M.1990. The Pinnipeds Seal, Sea Lions, and Walrus. Berkeley, Ca. University of California Press.

(USDI) United States Department of the Interior. 1982. Ecology and Biology of the Pacific Walrus, Odobenus Rosmarus Divergens Illiger. Washington D.C.

Wilson D.E. and D.M. Reeder.1993. Mammal Species of the World A Taxonomic and Geographic Reference.  Washington. Smithsonian Institution Press.
 
 
 

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