Polar Bears In Depth
Feeding Habits
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FIGURE 27.7. Bearded seals (Erignathus barbatus) are much larger than ringed seals, with adults weighing 350 kg. They are the second-most-important prey species for polar bears in many regions of the world. SOURCE: Photo by Steven C. Amstrup. Click image to enlarge.
Overall, polar bears are most effective predators of young ringed seals, perhaps because they are naive with regard to predator avoidance. In spring, polar bears may concentrate their predatory efforts on capture of new-born ringed seal pups (Smith and Stirling 1975; Smith 1980). In some areas, predation on pups is extensive. Hammill and Smith (1991) estimated that polar bears annually kill up to 44% of new born seal pups if conditions are right. Throughout the rest of the year, polar bears take seals predominantly from the first two year classes (Stirling et al. 1977a; Smith 1980). Whereas abundance of ringed seals may regulate density of polar bears in some areas, polar bear predation may regulate density and reproductive success of ringed seals in other areas (Hammill and Smith 1991; Stirling and Øritsland 1995).
Polar bears apparently digest fat more easily than protein (Best 1984). They seem to prefer the fatty portions of seals (and presumably other animals) to muscle and other tissues. Stirling (1974) reported that polar bears often remove the fat layer from beneath the skin of freshly killed seals and consume it immediately. Because over half of the calories in a whole seal carcass may be located in the layer of fat between the skin and underlying muscle (Stirling and McEwan 1975), a bear that quickly consumes most of the fat available has maximized its caloric return in the minimal amount of time possible. This may be important to all but the largest polar bears because there is considerable competition for kills. Younger and smaller bears often are driven away from their kills by larger bears.
A high-fat and low-protein diet apparently serves polar bears physiologically as well. They are very efficient at recycling nitrogenous products of catabolism, and can use metabolic water released from fat metabolism (Nelson et al. 1983). Digestion of protein requires water, whereas digestion of fat releases water. In a cold environment, free water is available only at the energetic cost of melting ice and snow. The lipophilic habits of the polar bear minimize energy expended to obtain water in winter (Nelson 1981).
Polar bears apparently digest fat more easily than protein (Best 1984). They seem to prefer the fatty portions of seals (and presumably other animals) to muscle and other tissues. Stirling (1974) reported that polar bears often remove the fat layer from beneath the skin of freshly killed seals and consume it immediately. Because over half of the calories in a whole seal carcass may be located in the layer of fat between the skin and underlying muscle (Stirling and McEwan 1975), a bear that quickly consumes most of the fat available has maximized its caloric return in the minimal amount of time possible. This may be important to all but the largest polar bears because there is considerable competition for kills. Younger and smaller bears often are driven away from their kills by larger bears.
A high-fat and low-protein diet apparently serves polar bears physiologically as well. They are very efficient at recycling nitrogenous products of catabolism, and can use metabolic water released from fat metabolism (Nelson et al. 1983). Digestion of protein requires water, whereas digestion of fat releases water. In a cold environment, free water is available only at the energetic cost of melting ice and snow. The lipophilic habits of the polar bear minimize energy expended to obtain water in winter (Nelson 1981).