The polar bear (Ursus maritimus) is the world’s largest land predator, specialized to thrive in the frigid Arctic environment. Their massive size and physiology enable them to dominate the northern food web. The public often wonders how these apex predators sustain themselves across the vast, icy landscape. Their specialized predatory strategy focuses almost entirely on the marine environment.
The Polar Bear’s Staple Diet
The survival of a polar bear depends on a diet rich in fat, necessary to maintain body temperature and fuel their large size. Their primary food source is the Arctic seal, specifically the ringed and bearded seals, which provide high-calorie blubber. Polar bears have a highly efficient metabolism, assimilating approximately 97% of the fat they consume.
Bears employ distinct hunting strategies on the sea ice. The most common method is “still-hunting,” where the bear waits motionlessly beside a seal’s breathing hole, or aglu. When a seal surfaces to breathe, the bear quickly attacks and pulls the animal onto the ice.
Another technique is stalking, where a bear crawls slowly toward a seal basking on the ice. They use a powerful burst of speed only for the final meters of the attack. During the spring feasting season, a successful hunt can provide enough blubber for a bear to consume up to 45 kilograms in one feeding. This stored fat sustains them through periods when hunting is impossible.
When Birds and Eggs Become Prey
Polar bears are opportunistic foragers and will consume birds and eggs when seals are unavailable. This terrestrial feeding occurs most frequently during ice-free summer months when bears are forced onto land. They target ground-nesting birds, such as geese, ducks, and seabirds, along with their nests.
Consumption involves raiding large nesting colonies where eggs and chicks are concentrated and easy to access. A study in the western Hudson Bay region showed that nearly one-third of polar bear scat collected during the ice-free season contained bird or egg remains. However, the nutritional reality of this alternative diet differs significantly from seal blubber.
Eggs and birds are primarily protein-based and lack the caloric density of marine mammal fat, offering minimal nutritional benefit. A single large seal provides energy equivalent to weeks of foraging for small prey like eggs and chicks. This land-based diet is an inefficient energy source and is insufficient to offset the energetic deficit a bear experiences while fasting.
Terrestrial Foraging and Seasonal Shifts
The shift toward foraging on land is directly tied to the seasonal loss of sea ice, which serves as the bear’s hunting platform. Rising Arctic temperatures cause the sea ice to melt earlier and freeze later, leading to a prolonged ice-free season. This forces polar bears ashore for longer periods, extending the time they must rely on stored fat reserves.
Foraging for terrestrial food represents a coping mechanism for an enforced fast, rather than a dietary preference. Bears must expend energy to find and consume these lower-value food items, depleting their reserves. Beyond birds and eggs, bears may also consume berries, grasses, or scavenge on terrestrial mammals like caribou.
The morphological structure of the polar bear’s jaw and teeth is specialized for consuming the soft, fatty blubber of seals. This makes them ill-suited for the tougher, lower-fat resources found on land. The increase in terrestrial feeding is a symptom of nutritional stress, demonstrating the difficulty bears face in adapting their specialized diet to a rapidly changing habitat.