San Francisco State University
Department of Geography

Geography 316:  Biogeography

The Biogeography of  the Blue Whale (Balaenoptera musculus)

by Peter Finkbeiner, student in Geography 316, Fall 1999

Kingdom:    Animalia
    Phylum:      Mammalia
        Class:         Chordata
            Order:        Cetacea
                Suborder:   Mysticeti
                    Family:       Balaenopteridae
                        Subfamily:  Balaenopterinae
                            Genus:        Balaenoptera
                                Species:       Balaenoptera musculus
 

Description of Species:

     The blue whale, a baleen whale, is grayish-blue in color and is mottled with lighter spots that are mainly limited to the back and shoulders.  The whale’s underside is similarly colored and has smaller, light-colored spots on the pleated throat and sometimes in the genital area.  Blue whales can reach lengths of over 100 feet and may weigh as much as 200 tons (Ellis 1985).  Larger specimens are typically found in the Antarctic while northern hemisphere whales are usually limited to 24 to 26 meters in length.  The females tend to be slightly larger than the males of the same age (Leatherwood 1983).

Habitat and Diet:

    There is not a great deal known about the movements of blue whales in the world’s oceans aside from the fact that they appear to  summer and feed in arctic waters and return to warmer tropical areas to breed in the winter (Ellis 1985).  Because of their great size, blue whales need to consume large quantities of food, and it is believed that they travel so far into polar regions with cold, nutrient-rich waters to do just this (Leatherwood 1983).  It is also believed that blue whales fast for the remaining 7or 8 months of the year that they spend in the tropics, surviving off the blubber accumulated the previous summer (Ellis 1985).  This is evidenced by whaling catches of relatively thin specimens returning to Antarctic at the beginning of the summer season.  Whales caught in subtropical areas at this time usually have little or nothing in their stomachs (Ellis 1985).
     The blue whale’s diet almost exclusively consists of a few species of krill (euphausiids).  Whales in the southern hemisphere feed exclusively on Euphausia superba while northern whales consume other species including Euphausia pacifica  (Ellis 1985). They need to consume from 3.5 to 4 tons of food each day during the summer feeding months.  Baleen whales like the blue whale feed by gulping large concentrations of prey and then forcing out the seawater through their baleen plates with its tongue, thus trapping and swallowing the krill (see Fig. 1) (Evans 1987).

            Figure 1.  Filter-feeding apparatus of baleen whales.

 
 

Natural History:

Breeding

    Blue whales migrate to tropical latitudes in the winter months to breed and bare their young. It is believed they do this because newborn blue whales have insufficient blubber to withstand the cold polar waters in which the adults feed (Small 1971).  Both the male and the female of the species reach sexual maturity at about ten years of age.  Females give birth to one calf every 2 to 3 years and the gestation period is approximately 12 months. Pregnant females usually return to the same breeding area to bare the calf.  The calf is usually weaned after 8 months and is averages about 15 meters in length at that time (Leatherwood 1983).

Population and Whaling

    Like many species of whales, blue whales have been decimated by centuries of whaling.  Although protected by an international agreement since 1965, their numbers are still quite low.  There are estimated to be approximately 1,200 to 1,700 whales in the North Pacific and only a few hundred in the North Atlantic.  In the southern hemisphere, population estimates are around 9000 specimens (Leatherwood 1983).
 

Evolution:

    The earliest recognizable whales, or cetaceans, seem to have come onto the scene about 50 million years ago when the earth was rich in tropical vegetation and swamps.  Fossils of these elongated aquatic mammals have been found in rock layers from the Middle to Late Eocene ages and have been given the suborder name of Archaeoceti (Evans 1987).
     Archaeoceti is believed to have evolved from Mesonychids of the terrestrial order Condylarthra, an order that also gave rise to Artiodactyla (ancestor to modern ungulates).  It is thought that a variety of Mesonychids started evolving towards a aquatic existence, possibly taking over niches vacated by reptiles, at the end of the Cretaceous period (Evans 1987).  They began to develop elongated bodies, short necks, paddle-shaped front limbs, and a tail designed for up and down movement (as opposed to the side to side motion of fish) (Evans 1987).
     38 to 25 million years ago, during the Oligocene, four different families arose from the archaeocetes; Agorophiidae, Squalodontidae, Aetiocetidae, and Cetotheriidae.  It is from these last two, Aetiocetidae and Cetotheriidae, that modern mysticetes (baleen whales) evolved (Evans 1987).  It is believed that these whales lost their differentiated teeth and developed filter-feeding baleen structures during the Oligocene to take advantage of larger amounts of zooplankton in what would become the Pacific Ocean (Evans 1987).
     The four modern families of mysticetes are believed to have developed from these early baleen whales. The Balaenopteridae family, of which the blue whale is a member, appears to have developed from the Cetotheriidae sometime in the Miocene, although the fossil evidence is not complete (see Fig. 2) (Evans 1987).


 
 

Distribution

    The blue whale can be found in all of the world’s oceans.  They are primarily found along the edges of continental shelves and at the edges of ice packs, but they can also be found in deep ocean trenches and shallow areas as well.  Researchers have recognized three major populations of blue whales worldwide, with several subgroups each:  North Pacific, North Atlantic and southern hemisphere (Leatherwood 1983).
    The North Pacific population ranges from the Gulf of Alaska in the north to the waters off the central coast of California.  There are pods that venture farther north into the Bering Sea and south towards the equator, but the bulk of the population stays along the west coast of North America (Leatherwood 1983).
    The range of the North Atlantic population extends from the coast of Panama north to the ice pack, but their range is restricted.  They summer in northern waters from the Gulf of Saint Lawrence to the edge of the ice pack.  Their winter range is unknown but may extend into the tropics.  There are no recorded sightings off the coast of North America south of New Jersey, although strandings have been reported in the Gulf of Mexico (Leatherwood 1983).
    The southern hemisphere population generally stays south of 40o S in the summer months and moves north into the waters off Brazil and Ecuador in South America and along the coasts of South Africa in the winter (Leatherwood 1983).


 
 

Bibliography
Barnes, L.G., Domining, D. and Ray, C.  1985  “Status of studies on fossil marine
     mammals”, Marine Mammal Science, 1(1), 15-53.

Ellis, Richard.  1985.  The Book of Whales.  New York.  Alfred A. Knoff, Inc.

Evans, Peter G.H.  1987.  The Natural History of Whales and Dolphins.  New York.  Fact  On File Publications.

Leatherwood, Stephen and Randall R. Reeves.  1983.  The Sierra Club Handbook of
    Whales and Dolphins.  San Francisco.  Sierra Club Books.

Small, George L.  1971.  The Blue Whale.  New York & London.  Columbia University
     Press.

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