The polar bear, Ursus maritimus, is the Arctic’s apex predator, existing as the most carnivorous member of the bear family. This marine mammal has evolved to specialize in the harsh, ice-covered environment of the North, where its diet is almost exclusively tied to the sea. The bear’s survival hinges on its ability to hunt from the platform of sea ice, making its life cycle and caloric intake directly dependent on the availability and stability of frozen ocean water. Its diet reflects a physiological need to consume large amounts of fat to sustain its massive body size and endure long periods of fasting.
Highly Specialized Diet: Focusing on Seals
The primary food source for the polar bear is the seal, serving as the dietary staple across the Arctic. The most common prey is the ringed seal (Pusa hispida), which is smaller and more accessible year-round. Polar bears also hunt the larger bearded seals (Erignathus barbatus), which offer a substantial caloric payoff due to their greater size and thicker blubber layer. While other species like harp or ribbon seals are consumed regionally, ringed and bearded seals form the core of the diet.
Seals provide the high fat content required for survival in the Arctic cold. A successful hunt allows the bear to consume energy-dense blubber, which it prioritizes over lean meat. When seals are abundant, a bear may eat only the skin and blubber, leaving the protein-rich carcass for scavengers. This focus on fat makes the polar bear a hypercarnivorous predator.
Hunting Strategies on the Ice
The polar bear employs specific hunting methods that rely entirely on the presence of sea ice as a stable platform. One common and energy-efficient strategy is still-hunting, where the bear locates a seal’s breathing hole, or aglu, using its powerful sense of smell. The bear waits patiently at the aglu for the seal to surface for air, sometimes remaining motionless for hours. When the seal emerges, the bear launches a rapid attack to pull the animal onto the ice.
Another technique is stalking, where the bear quietly pursues seals resting or basking near the water’s edge. Using its white coat for camouflage, the bear moves slowly, freezing its position when the seal scans for danger. Once close, the bear quickly closes the distance and pounces before the seal can escape.
Polar bears also hunt seal pups in their snow dens by crashing through the roof with their forepaws. The spring months, particularly April through June, represent the peak hunting season when seal pups are born. These young seals are relatively easy prey, allowing the bears to accumulate the majority of their annual fat reserves during this short period.
Nutritional Imperative: The Need for Fat
The polar bear’s survival depends on accumulating a thick layer of blubber, which serves as insulation and an energy reserve for long periods of food scarcity. A healthy polar bear can reach a body composition where up to 50% of its weight is fat, a level detrimental to most mammals.
This high-fat diet is possible due to unique metabolic adaptations. Genetic studies show key genes related to lipid metabolism enable the bear to process high levels of dietary fat without developing cardiovascular issues. The bear’s digestive system is highly efficient, capable of assimilating up to 97% of the fat consumed from seal blubber.
Reliance on fat storage allows bears, particularly pregnant females, to survive extended fasting periods of eight months or more while denning. The breakdown of fat reserves also provides metabolic water, a necessary adaptation since fresh water is largely unavailable in the Arctic. Furthermore, focusing on blubber keeps their diet naturally low in protein, preventing the kidney and liver stress associated with processing lean meat diets common in other carnivores.
Opportunistic Feeding and Fallback Foods
When the sea ice melts during the summer and access to seals becomes limited, polar bears may be forced onto land, where they resort to opportunistic feeding on fallback food sources. These foods include scavenging on carcasses, such as the remains of bowhead whales or walruses that wash ashore. Bears also consume terrestrial items like bird eggs, young birds, small mammals, berries, and vegetation.
While these items provide temporary sustenance, they are nutritionally poor substitutes for energy-dense seal blubber and cannot sustain the bears long-term. Bears typically lose body mass during these ice-free periods, surviving primarily on the fat reserves accumulated during the spring hunt. Terrestrial foods are insufficient to meet the massive caloric demands of the polar bear population. The increasing duration of the ice-free season, which forces bears onto land for longer periods, poses a nutritional challenge because these fallback foods do not allow bears to build the necessary fat stores.