Andrew E. Derocher
Surviving in the frigid Arctic through months of winter darkness and roaming vast areas over the frozen oceans, few species are as charismatic and photogenic as the polar bear (Ursus maritimus) (see Figure 14.1). Sitting at the top of the Arctic marine food web, polar bears provide insight on the status, health and functioning of marine ecosystems over a variety of spatial and temporal scales. Having evolved from a grizzly/brown bear (U. arctos) ancestor in a rapid burst of evolution (Waits et al, 1999), polar bears evolved a life history pattern dependent on the sea ice. With this specialization, numerous morphological and physiological adaptations followed.
Polar bears differ vastly from their land-bound cousins and have abandoned over-winter denning except for pregnant females (Watts and Hansen, 1987). Being active in the icy winter means the bears must contend with air that would cool their body core and have thus evolved a narrower skull that helps to warm cold air before it reaches the lungs. The elongated skull also allows for heightened olfactory sensitivity important for finding prey. The large body size of polar bears is a further adaptation to the cold: larger animals have less surface area relative to their volume than smaller animals and this means they are better able to keep body heat from escaping. Add in a fur coat and a thick layer of fat and you have an animal perfected to deal with cold but unsuited to the heat. The shorter and more curved claws are well adapted to a predatory way of life and assist with the seizing of slippery prey eager to slide back into its watery domain. To deal with the variable conditions of the Arctic and the periodic scarcity of prey, all polar bears have a facultative ability to enter a fasting mode to conserve energy, a state that their ancestors only entered during denning (Ramsay et al, 1991).
Polar bears evolved to exploit a vacant niche on the sea ice of the Arctic as an obligate predator of seals. The hallmark white fur of polar bears serves as camouflage given that the prey of polar bears — ringed seals (Phoca hispida) and bearded seals (Erignathus barbatus) — are wary and have evolved means of avoiding the bears (DeMaster and Stirling, 1981). Polar bears and their prey have
Source: Photograph by A. E. Derocher.
co-evolved with the bears exploiting a variety of means to capture their prey and the seals evolving to avoid capture (Kingsley and Stirling, 1991). The sea ice is also crucial habitat for the seals. Both ringed and bearded seals rely on sea ice to give birth to and rear their young. Additional prey of polar bears includes harp seals (Phoca groenlandica), harbor seals (Phoca vitulina), walrus (Odobenus rosmarus), white whales (Delphinapterus leucas) and narwhal (Monodon monoceros). Polar bears are opportunistic and will sometimes feed on seaweed, berries, birds and other mammals although the energetic return is usually minimal (Smith and Sjare, 1990; Derocher et al, 1993; Derocher et al, 2002).
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