\(Ursus\) \(maritimus\), commonly known as the polar bear, is the largest terrestrial carnivore and a highly specialized marine mammal, standing as the apex predator of the Arctic. Its existence is fundamentally tied to the presence of sea ice, which serves as the platform for hunting and travel across its frigid habitat. The bear’s immense size and unique physiology result from adaptation to extreme cold and periods of extended food scarcity. This specialized lifestyle necessitates a diet with an exceptionally high caloric density, meaning the polar bear’s survival depends on specific prey.
The Primary Prey: Ringed and Bearded Seals
The preferred and most significant food source for the polar bear is the seal, with ringed seals (\(Pusa\) \(hispida\)) forming the staple of their diet across the Arctic. These smaller seals are the most abundant marine mammals in the polar bear’s range and are accessible because they maintain breathing holes in the sea ice. Polar bears rely on these seals to provide the bulk of the energy needed for survival, growth, and reproduction.
Bearded seals (\(Erignathus\) \(barbatus\)) are also a substantial part of the diet, particularly for larger, older male bears. While less numerous than ringed seals, bearded seals offer a significantly greater biomass per successful hunt. The preference for both species is rooted in their thick layer of blubber, which supplies the necessary energy for the bear to thrive in the Arctic climate.
Specialized Hunting Techniques
Polar bears employ several strategies that rely on the sea ice to successfully capture their primary prey. The most common method is “still-hunting,” where a bear uses its powerful sense of smell to locate a seal’s breathing hole, or \(aglu\). The bear waits motionless, sometimes for hours, poised over the hole until a seal surfaces to breathe.
Once a seal emerges, the bear uses a sudden, explosive pounce or strikes with its paw to drag the seal onto the ice. Another effective technique is stalking, which targets seals resting on the ice near the water’s edge. The bear crawls slowly forward, freezing instantly whenever the seal raises its head to scan its surroundings.
The bear’s white fur provides excellent camouflage, allowing it to close the distance before launching a rapid charge. In the spring, bears use “den-raiding” to target seal pups born in snow dens. They locate the hidden dens by scent and violently collapse the snow roof by pouncing with both forepaws, capturing the vulnerable pup inside.
The Nutritional Imperative of Fat
The polar bear’s dietary focus on seals is due to a profound metabolic requirement for high-calorie fat. Polar bears are physiologically adapted to consume a diet that consists of two-thirds fat, unlike many other carnivores that rely heavily on protein. They often selectively eat only the blubber and skin of a seal, leaving the lean meat for scavengers like the Arctic fox when seals are abundant.
This preference avoids a condition known as protein poisoning, where the liver and kidneys are stressed by processing excessive protein. Seal blubber provides the high caloric density required to maintain massive body mass and generate the energy needed to survive the Arctic cold. A polar bear can assimilate 97% of the energy available in the seal blubber it ingests.
The accumulated fat reserves are vital for insulation and, more importantly, for surviving long periods of fasting. Pregnant females, for instance, must fast for up to eight months while denning and nursing their cubs. The ability to rapidly store and efficiently utilize this highly caloric fat is a fundamental adaptation.
Seasonal and Backup Food Sources
During the late summer and early autumn, when the sea ice melts or retreats, polar bears in many regions are forced onto land and must rely on backup food sources. These opportunistic items include bird eggs, nestlings, small mammals like lemmings, and vegetation such as berries and kelp. Scavenging on the carcasses of stranded marine mammals, such as bowhead or beluga whales, can also provide a temporary, substantial windfall of fat and protein.
While a single whale carcass can offer the caloric equivalent of hundreds of ringed seals, these events are unpredictable and localized. The energy from birds, eggs, or plants is too low in caloric density to sustain the bear’s large body mass over long periods. These backup foods help a bear survive a short-term fast, but they are nutritionally inadequate to replace the consistent, high-fat input provided by seals.