FIGURE 27.2. In winter, polar bear foot pads may be densely furred. This mayprovide a better purchase on the slippery ice surface than naked pads. SOURCE: Photo by Steven C. Amstrup. Click image to enlarge.
Size and Weight. The polar bear is the largest of the extant bears (DeMaster and Stirling 1981). In Hudson Bay, the mean scale weight of 94 males >5 years of age was 489 kg. The largest bear in that group was a 13-year-old, which weighed 654 kg (Kolenosky et al. 1992). The heaviest bear we have weighed in Alaska was 610 kg, and several animals were heavy enough that we could not raise them with our helicopter or weighing tripod. Some animals too heavy to lift have been estimated to weigh 800 kg (DeMaster and Stirling 1981). Females are smaller, with peak weights usually not exceeding 400 kg. Total lengths of males in the Beaufort Sea of Alaska ranged up to 285 cm. Such an animal may reach nearly 4 m when standing on its hind legs and is 1.7 m shoulder height when standing on all four legs. Chest girth for large males is close to 200 cm. Although smaller, females in the Beaufort sea were as long as 247 cm with chest girths up to 175 cm. Only prehistoric polar bears and the giant short-faced bear (Arctodus spp.) of the Pleistocene were of greater stature than today's polar bears (Kurt�en 1964; Stirling and Derocher 1990).
Manning (1971) suggested there is a cline in size of polar bears across the Arctic. Size increases, he suggested, with distance from east Greenland across the Nearctic to the Chukchi Sea between Alaska and Russia. Manning (1971) also suggested that polar bears from Svalbard may be larger than those from east Greenland. A cline in size across the Palearctic also might occur, but samples from the Russian Arctic are inadequate to confirm it (Manning 1971).
The hypothesized cline was based on measurements made from skulls housed in museums around the world. Unfortunately, the sources of skulls in the various collections were not similar. Of particular note was that many of the skulls originating in the Chukchi Sea may have been donated by trophy hunters. These hunters worked over the ice in teams of aircraft (Tovey and Scott 1957) and were quite effective in killing a great number of the largest polar bears (Amstrup et al. 1986). Another potential problem is that ages of bears in the sample were estimated only by class or life stage. Hence, older bears from one locale might have been compared to younger bears (of the same age class) in another.
Potentially nonstandardized collection methods prevent any meaningful conclusions about relative sizes of polar bears from different locales. Also, if there is a cline in skull sizes around the world, it appears that body sizes and weights of polar bears do not follow a similar cline. The largest bears for which actual scale weights are known have come from the Hudson and James Bay areas of Canada and from the Beaufort Sea of Alaska, not from the Chukchi Sea. That observation, too, may be subject to some bias, as the most prolonged and intensive polar bear studies have been conducted in Hudson Bay and the Beaufort Sea. Greater numbers of captures in those locations may have increased the probability that very large bears were included in the sample.
Despite their large adult sizes, the young of polar bears are among the most altricial (undeveloped) of eutherian mammals (Ramsay and Dunbrack 1986). Newborn polar bears weigh only 600 �700 g. They are blind, only lightly furred, and totally helpless (Blix and Lentfer 1979). Mother polar bears when giving birth commonly weigh over 300 kg, and can weigh 400 kg (Ramsay 1986). If only a single cub is born, the ratio of maternal to neonate weights could be between 400 and 500 to 1. Even with the more common two-cub litter, the ratio of maternal to neonate mass is extraordinarily large (Ramsay and Dunbrack 1986). Cubs grow very fast after birth. In Alaska, they average 13 kg on emergence from the den in late March or early April, with maximum weights of 22 kg. Cubs continue to grow rapidly through their first summer on the sea-ice and some weigh over 100 kg as they approach 1 year of age.
FIGURE 27.3. Normal front and rear claws of a female polar bear from the Beaufort Sea. Note strong curve and sharp points for clinging to blocks of ice and for capturing prey. SOURCE: Photo by Steven C. Amstrup. Click image to enlarge.
Pelage. Polar bears are completely furred except for the tip of the nose. Pelage density is more even than in other ursids, which are often more sparsely furred ventrally and in axillary and groin areas. Even the pads of the feet of polar bears may be covered with hair, especially in late winter (Fig. 27.2). Furred foot pads may provide a more secure purchase on the slippery sea ice surface and add another layer of insulation between the bear's foot and the substrate of ice and snow. Under the fur, pads of the feet of polar bears are made up of the same cornified epidermis characteristic of the pads of other bears (Storer and Tevis 1955; Ewer 1973).
The skin of polar bears is uniformly black. Hence, if polar bears lose hair due to physical trauma or disease, they appear from a distance to have black patches on their bodies. Polar bear fur appears white when it is clean and in even sunlight. Because it actually is without pigment, however (�ritsland and Ronald 1978; Grojean et al. 1980), bears may take on the yellow-orange hues of the setting and rising sun and the blue of sunlight filtered through clouds and fog. They appear the whitest right after molting. In spring and late winter, however, many polar bears are "off-white" or yellowish because of oils from their prey and other impurities that have attached to and been incorporated into their hair.
The molt appears to be somewhat variable, but begins by late April and May. The molt appears to be complete by late summer, and bears captured in autumn have notably shorter coats than those captured in spring. The pelt is thick with a dense underfur and guard hairs of various lengths. Polar bear fur may have a high propensity to take on the colors of environmental impurities because the guard hairs have a hollow medulla (or core) where impurities may lodge. In zoo environments, some species of algae can enter the hollow cores of guard hairs and result in a pronounced "greening" of the fur (Lewin and Robinson 1979).
Lavigne and Oritsland (1974) noted that polar bears effectively absorb ultraviolet (UV) light, and suggested that could be useful in remote-sensing surveys to enumerate them. The discovery that polar bears appear to absorb UV light led to much speculation about their ability to capture the energy in that light. Popular and scientific reports claimed that the ability to absorb energy in the UV spectrum was an adaptation to help maintain body heat in the rigorous Arctic environment (Anonymous 1978; Grojean et al. 1980; Lopez 1986; Mirsky 1988). Suddenly, the hollow hairs of polar bears, adept at catching algae and other contaminants (Lewin and Robinson 1979) also were endowed with the powers of optic fibers to funnel UV light to the skin. According to this theory, the skin was black to better absorb such energy without damage. Capturing this high-frequency electromagnetic energy would be a great adaptation for polar bears. This ability has attained the status of an Arctic legend, and contributed to the mystique surrounding the great white bears of the north. Unfortunately, this supposed adaptation has no basis in fact. Lavigne (1988) and Koon (1998) established unequivocally that the hair of polar bears, although transparent in the visible spectrum, absorbs UV light. If the hair of polar bears absorbs UV light, it does not efficiently transmit UV light. As UV light moves down the shaft of the hair, its energy is absorbed, preventing significant energy from being transmitted to the skin.
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.
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