The polar bear (Ursus maritimus) is distributed throughout the artic region, or the area surrounding the North Pole (Nowak, 1991). The species has a unique habitat of pack ice and sea ice (Ramsay and Stirling, 1988; Nowak, 1991), and the front paws of polar bears are specialized for swimming. Polar bears are typically two to 2.5 m in length with a shoulder height of about 1.5 m. Males have been documented to weigh substantially more than females: females weigh between 150 kg and 300 kg, while males can weigh between 300 kg and 800 kg. Polar bears are typically white, although their coat can turn a yellowish shade during the summer (Nowak, 1991).

Polar bears belong to the mammalian group Ursidae. Ursidae includes three genera and eight species of extant bears (Nowak, 1991). Bears are cosmopolitan except for Antarctica and Australia (Ramsay and Stirling, 1988). Individuals are mostly solitary, although females are often accompanied by their young. Bears are typically inactive during the winter, spending the winter months sleeping in a den and living off stored fat. They are not usually aggressive, however individuals will engage in aggressive behavior if they feel threatened or when their cubs or food source is threatened (Nowak, 1991). Birth occurs between November and February, typically when bears are in their dens. Sexual maturity is reached between the ages of 2.5 and 6. Litters range in size from one to four young (Nowak, 1991). Females have long inter-birth intervals and are unable to enter estrous with unweaned cubs. Due to slow reproductive rates, bears have a difficult time recovering from population decreases. Additionally, populations have been documented to have a lack of, or substantial decrease in, reproduction during periods when resources are low (Ramsay and Stirling, 1988).

Unlike other bears that are omnivorous or herbivorous, polar bears are generally carnivorous (Ramsay and Stirling, 1988). Their diet primarily consists of ring and bearded seals (Ramsay and Stirling, 1988), although they will also eat reindeer, fish, and occasionally vegetation and berries (Nowak, 1991). During the spring, polar bears hunt for seals by breaking through the snow covers and breathing holes of seal dens (Ramsay and Stirling, 1988).

Polar bears also differ from other bears in having higher activity levels during the winter. Only pregnant females spend the winter months inside a den (Nowak, 1991). However, all individuals will use a den for shelter during particularly severe weather (Nowak, 1991; Gunderson, 1976). Dens can occur in a variety of places including caves, icebergs, dry river beds, and snow bridges. Although many dens are simple one-room structures, the dens of pregnant females can be complex with long entrances, multiple rooms, and even multiple levels (Gunderson, 1976). Pregnant females will enter their den between October and November and remain until March or April.

The polar bear mating season occurs between March and June. However, implantation is delayed until the fall months (Ramsay and Stirling, 1998; Nowak, 1991). Females give birth to young while they are in their dens, typically between November and January (Nowak, 1991). Females give birth every two to four years (Nowak, 1991).

Additional Information on the Skull

Click on the thumbnails below for labeled images of the skull in standard anatomical views.

Dorsal view	Lateral view	Ventral view

This specimen, a male, was collected from Eastern Shore, North Cornwall by members of the Crocker Land Expedition in 1916. It was made available to The University of Texas High-Resolution X-ray CT Facility for scanning courtesy of Dr. Blaire Van Valkenburgh of the Department of Ecology and Evolutionary Biology, University of California, Los Angeles. Funding for scanning and image processing was provided by Dr. Van Valkenburgh and by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin. The polar bear is one of several carnivorans included in ongoing research of respiratory turbinates by Dr. Van Valkenburgh.

This specimen, a male, was scanned by Matthew Colbert on 22 February 2006 along the coronal axis for a total of 540 1024x1024 pixel slices. Each slice is 0.75 mm thick, with an interslice spacing of 0.7 mm and a field of reconstruction of 255 mm.

Literature

Gunderson, H. L. 1976. Mammalogy. McGraw-Hill Book Company, New York, NY. 483 pp.

Nowak, R. M. 1991. Walker's Mammals of the World, Volume 2. The John Hopkins University Press, Baltimore, MD. 1629 pp.

Ramsay, M. A., and I. Stirling. 1988. Reproductive biology and ecology of female polar bears (Ursus maritimus). Journal of Zoology 214:601-634 Part 4.

Ursus maritimus page on the Animal Diversity Web (University of Michigan Museum of Zoology)

U. maritimus page on Wikipedia

U. maritimus page at Polar Bears International