The polar bear (Ursus maritimus) is the apex predator of the Arctic, a massive carnivore adapted to its frigid, marine environment. Their existence is linked to the sea ice, which serves as their primary platform for hunting. These bears, with males sometimes weighing over 1,700 pounds, possess a specialized diet that provides the energy required to survive in one of the planet’s harshest climates.
The Primary Prey
The staple food source for the polar bear is the seal, particularly the ringed seal (Pusa hispida), which is the most common and widely distributed prey across the Arctic. Ringed seals are small enough for most bears to overpower and are abundant throughout the year, making them the most reliable target. Bearded seals (Erignathus barbatus) serve as a secondary, larger prey source, though these are typically only successfully hunted by larger, adult male bears.
Seals are necessary for the polar bear’s survival due to their extremely high caloric density, which is unmatched by other potential food sources. Polar bears are hypercarnivores, meaning their diet is overwhelmingly meat-based, and they have evolved to metabolize fat much more efficiently than protein. When a successful hunt occurs, the bear typically consumes only the blubber and skin of the seal, leaving the less energy-dense meat for scavengers. A single polar bear can consume up to 100 pounds of blubber in one sitting, quickly building the fat reserves needed to sustain it through periods of fasting.
Hunting Strategies and Nutritional Needs
The polar bear’s success hinges on its ability to use the sea ice as a hunting platform, employing patient, low-energy strategies tailored to capturing seals. The most common technique is known as “still-hunting” or “ambush predation,” where the bear locates a seal’s breathing hole, or aglu, using its powerful sense of smell. The bear crouches motionless for hours, waiting for the seal to surface for air, at which point it lunges to capture the prey with a powerful strike.
Bears also employ a stalking method, targeting seals that have hauled out onto the ice to rest or bask. The bear uses the uneven terrain to approach, freezing its movement whenever the seal lifts its head to scan the surroundings. Once within close range, the bear bursts into a short, explosive charge to overwhelm the seal before it can escape into the water. The success of these techniques is linked to the bear’s need to accumulate fat reserves. A single bear may consume over 12,000 calories per day, and the high-fat seal diet allows them to quickly gain the weight necessary for surviving the lean summer months.
Opportunistic and Seasonal Feeding
While seals form the basis of the polar bear diet, the bears are also highly opportunistic, particularly during ice-free periods when their primary prey is inaccessible. When forced onto land, they may scavenge on the carcasses of marine mammals, such as bowhead or beluga whales, that have washed ashore. These scavenging events provide a significant, though infrequent, caloric boost.
Other land-based foods include the eggs and nestlings of ground-nesting birds like snow geese and ducks, and occasionally even small land mammals like caribou. Polar bears have also been observed grazing on vegetation, including berries, grass, and kelp. These supplemental food sources, however, are low-energy and insufficient to replace the high-fat seal diet needed to maintain body mass. They serve only as a temporary measure until the sea ice returns and hunting can resume.
Dietary Shifts in a Changing Arctic
The reduction of Arctic sea ice, which melts earlier in spring and freezes later in autumn, is fundamentally altering the polar bear’s traditional diet and hunting season. This shrinking platform limits the time bears have to successfully hunt seals, forcing them to spend longer periods on land and rely more heavily on their fat reserves.
The increased reliance on terrestrial and opportunistic foods due to ice loss does not provide a viable long-term solution for the population. While some polar bears have demonstrated a flexible foraging behavior by increasingly consuming caribou and bird eggs, these resources lack the energy density of seal blubber. The energetic stress caused by this dietary shift results in weight loss and reduced reproductive success, as females struggle to maintain the body condition necessary to raise cubs. The shift away from the seal-based diet reflects a changing ecosystem and poses a major threat to the polar bear’s ability to sustain itself.