Polar Bears In Depth
Description
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FIGURE 27.4. Rare unpigmented polar bear claws, on a polar bear captured in the southern Beaufort Sea. SOURCE: Photo by Steven C. Amstrup. Click image to enlarge.
Claws. The claws of polar bears are shorter and more strongly curved than those of brown bears. They also are larger and heavier than those of black bears (Ursus americanus). They appear to be very well adapted to clambering over blocks of ice and snow and especially to securely gripping prey animals. The claws are normally black (Fig. 27.3), but rarely may, like polar bear fur, lack pigment (Fig. 27.4).
Skull and Dentition. Polar bears share the general ursid dental formula : I 3/3, C 1/1, P 4/4, M 2/3. The first premolars are vestigial and occur in a long diastema or gap between the functional canine and molariform teeth. That gap allows the powerful canines to penetrate deeply into the bodies of seals and other prey without interference from adjacent cheek teeth. Although polar bears apparently evolved from brown bears <250,000 years ago, their teeth have changed significantly from the brown bear form. The cheek teeth are greatly reduced in size and surface area, and the carnassials are more pronounced than in brown bears, reflecting the predatory lifestyle. The teeth of polar bears are well suited to the tasks of grabbing and holding prey and shearing meat and hide. They no longer are as suited to grinding grasses and other vegetation as are those of brown bears. The canine teeth of males are larger and heavier, relative to the size of the jaw, than those of females (Kurt´en 1955), and the molar arcade of males is longer than in females (Larsen 1971). The proportionately larger canines coincide with the pronouced sexual dimorphism which is more accentuated in polar bears than it is in any other ursid (Stirling and Derocher 1990).
The skull of the polar bear shares the principal characteristics of the skulls of other ursids. The largest brown bear skulls are larger than the largest polar bear skulls. Polar bear skulls are proportionately narrower across the palate between second molars than skulls of brown bears (Kurt´en 1964). The ratio of condylobasal length to zygomatic width (L/W) also is larger in the polar bear, accentuating the narrower skull. The L/W for 279 brown bears taken by hunters was 1.59, whereas the L/W for 150 polar bears was 1.63 (calculated from Nesbitt and Parker 1977). The difference in actual measurements is not as pronounced as the visual impression suggests. This is because of the more strongly developed and overhanging occiput and significantly greater height in skulls of brown bears (Kurt´en 1964). In lateral view, the lower height, combined with absence of the pronounced brow ridge that tends to give brown bears a "dish-faced" appearance, yields a smooth curve from canines across the maxillary bones to the cranium (Fig. 27.5). These features combine to give the polar bear a "Roman nose" appearance.
Skull and Dentition. Polar bears share the general ursid dental formula : I 3/3, C 1/1, P 4/4, M 2/3. The first premolars are vestigial and occur in a long diastema or gap between the functional canine and molariform teeth. That gap allows the powerful canines to penetrate deeply into the bodies of seals and other prey without interference from adjacent cheek teeth. Although polar bears apparently evolved from brown bears <250,000 years ago, their teeth have changed significantly from the brown bear form. The cheek teeth are greatly reduced in size and surface area, and the carnassials are more pronounced than in brown bears, reflecting the predatory lifestyle. The teeth of polar bears are well suited to the tasks of grabbing and holding prey and shearing meat and hide. They no longer are as suited to grinding grasses and other vegetation as are those of brown bears. The canine teeth of males are larger and heavier, relative to the size of the jaw, than those of females (Kurt´en 1955), and the molar arcade of males is longer than in females (Larsen 1971). The proportionately larger canines coincide with the pronouced sexual dimorphism which is more accentuated in polar bears than it is in any other ursid (Stirling and Derocher 1990).
The skull of the polar bear shares the principal characteristics of the skulls of other ursids. The largest brown bear skulls are larger than the largest polar bear skulls. Polar bear skulls are proportionately narrower across the palate between second molars than skulls of brown bears (Kurt´en 1964). The ratio of condylobasal length to zygomatic width (L/W) also is larger in the polar bear, accentuating the narrower skull. The L/W for 279 brown bears taken by hunters was 1.59, whereas the L/W for 150 polar bears was 1.63 (calculated from Nesbitt and Parker 1977). The difference in actual measurements is not as pronounced as the visual impression suggests. This is because of the more strongly developed and overhanging occiput and significantly greater height in skulls of brown bears (Kurt´en 1964). In lateral view, the lower height, combined with absence of the pronounced brow ridge that tends to give brown bears a "dish-faced" appearance, yields a smooth curve from canines across the maxillary bones to the cranium (Fig. 27.5). These features combine to give the polar bear a "Roman nose" appearance.
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