The polar bear, Ursus maritimus, is the world’s largest land carnivore, adapted to the harsh Arctic environment. This immense predator is classified as a marine mammal because its existence is intrinsically tied to the sea ice, which serves as its primary hunting platform. Its specialized diet is a direct reflection of its icy habitat, providing the extreme caloric intake necessary to survive the frigid temperatures of the far North. The polar bear’s life cycle revolves around its ability to locate and consume high-fat marine prey to build up sufficient energy reserves.
The Primary Diet Ringed and Bearded Seals
The polar bear’s survival hinges on a diet composed almost exclusively of seals, particularly the ringed seal (Pusa hispida) and the bearded seal (Erignathus barbatus). These prey species are essential because of the exceptional concentration of fat stored in their blubber layer. This blubber provides the polar bear with a highly energy-dense meal, which is metabolized much more efficiently than protein in the extreme Arctic cold.
When a polar bear successfully catches a seal, it will typically consume the blubber and skin first, often leaving the leaner carcass meat for scavengers like Arctic foxes and gulls. This selective consumption is crucial for the bear to rapidly accumulate fat reserves. An adult bear can consume up to 100 pounds of blubber in a single feeding session. Due to their size, bearded seals contribute a disproportionately large amount of biomass to the bear’s diet, even if ringed seals are numerically more frequent prey.
Specialized Hunting Tactics and Metabolic Needs
Acquiring this high-fat diet requires highly specialized hunting strategies linked directly to the presence of sea ice. The most common technique is “still-hunting,” where the bear uses its powerful sense of smell to locate a seal’s breathing hole, or aglu, in the ice. The bear then waits, sometimes for hours or even days, patiently poised over the hole, to ambush the seal when it surfaces for air.
Another successful tactic involves stalking seals that are resting on the ice, a process that relies on the bear’s excellent camouflage and careful movement to minimize vibrations that could alert the prey. When a bear is within about 20 feet of the seal, it bursts forward with explosive speed to secure the meal before the seal can escape into the water.
These hunting efforts are driven by the polar bear’s immense metabolic needs, which are significantly higher than previously assumed for a predator that sometimes employs a “sit-and-wait” strategy. Studies indicate that a polar bear’s field metabolic rate can be 1.6 times greater than predicted, and this high energy expenditure necessitates consuming vast quantities of fat-rich prey. The physiological need to consume large amounts of fat in a short window, known as hyperphagia, allows the bear to build the fat reserves needed to cover the energy deficit created by its high metabolism and the long fasting periods.
Seasonal and Opportunistic Food Sources
When sea ice recedes in the summer, preventing access to their primary seal diet, polar bears are often forced onto land and must rely on opportunistic food sources. These fallback items include terrestrial foods like bird eggs, chicks, small mammals such as arctic foxes or lemmings, and various types of vegetation, including berries and grasses. They also scavenge on the carcasses of larger marine mammals, such as beluga or bowhead whales, that wash ashore.
While these varied foods provide some temporary nutritional support, they are generally protein-rich and fat-poor, meaning they cannot replace the necessary caloric density of seal blubber. For instance, consuming bird eggs or vegetation does not provide the massive fat stores required to sustain a polar bear long-term. This opportunistic feeding is a survival strategy during the ice-free season, but it is insufficient to maintain the body condition achieved through seal hunting.
Ecological Role in the Arctic Food Web
The polar bear occupies a distinct position as the apex predator in the Arctic marine food web. By primarily preying on seals, the polar bear helps to regulate their populations, which in turn influences the balance of the ecosystem below them. Their specialized diet and immense size mean they have virtually no natural predators, a defining characteristic of an apex species.
The remains of their seal kills also provide a food source for a host of smaller scavengers, including Arctic foxes and ravens, linking the polar bear to the terrestrial food chain. This position at the top of the food web makes the species an indicator of the overall health of the Arctic ecosystem. The polar bear’s reliance on sea ice for hunting means that environmental changes impacting its platform directly threaten its ability to fulfill this crucial ecological role.