Polar bears, known scientifically as Ursus maritimus, are large carnivores inhabiting the Arctic. As apex predators, they are uniquely adapted to their icy environment. Their thick layer of fat and dense fur enable them to thrive in frigid temperatures. These adaptations are fundamental to their specialized diet, allowing them to efficiently hunt and consume the high-calorie foods essential for survival in a marine ecosystem.
The Primary Prey
The diet of a polar bear is primarily composed of seals, with the ringed seal being their most common and important prey species across their circumpolar range. These seals are an ideal food source due to their abundance and manageable size. Ringed seals, along with bearded seals, are particularly rich in blubber, a dense layer of fat that provides significant energy and helps insulate the seals.
Polar bears rely on this high-fat content to meet their energy needs and maintain their own insulating fat layer, which can comprise up to half of their body weight. An adult polar bear can consume up to 45 kilograms (100 pounds) of seal blubber in a single feeding session.
When seals are plentiful, polar bears often prioritize eating only the blubber and skin, leaving the remaining carcass for scavengers like Arctic foxes, ravens, or other bears. Other seal species, such as bearded, harp, hooded, and ribbon seals, also contribute to their diet when available, though typically to a lesser extent than ringed seals.
Hunting Techniques
Polar bears employ various strategies to secure their prey, with their hunting success often linked to the presence of sea ice. One primary method is “still-hunting,” where a bear patiently waits motionless by a seal’s breathing hole in the ice or at an ice edge. When a seal surfaces for air, the bear quickly seizes it, often pulling it onto the ice. This method can require hours of waiting. Polar bears use their powerful sense of smell to locate these breathing holes, even those hidden under snow, and can detect seals from a mile away or hear them under layers of snow.
Another technique involves stalking seals resting on the ice, known as basking. The bear uses its white fur for camouflage, slowly crawling forward and freezing in place whenever the seal lifts its head. Once within charging distance, typically about 6 meters (20 feet), the bear uses its explosive speed to pounce before the seal escapes. Polar bears also locate seal birthing lairs hidden beneath snowdrifts, breaking through the snow to access vulnerable pups.
Secondary and Seasonal Foods
While seals form the core of their diet, polar bears are opportunistic feeders, consuming other food sources when seals are scarce or inaccessible, such as during the open-water season when sea ice is diminished. These secondary foods include carrion, such as whale or walrus carcasses that wash ashore. Larger adult male bears may hunt bearded seals and young walruses, though adult walruses are generally too formidable.
Other occasional food items include birds, bird eggs, and small mammals like rodents or reindeer. Polar bears consume goose eggs and even caribou, especially in areas like Western Hudson Bay where they spend more time on land.
Vegetation, such as berries, grasses, and marine algae (kelp), is also consumed, but these plant materials offer limited nutritional value and are not a significant part of their energy intake due to the bears’ digestive system being highly specialized for animal matter. These alternative food sources primarily act as a supplement, helping bears to bridge periods of food scarcity until their primary prey becomes available again.
Dietary Needs and Adaptations
The polar bear’s specific diet is directly linked to its physiological requirements for surviving in the Arctic. Their large body size, which can reach up to 800 kg (1,760 lbs) for males, and the need for insulation in extreme cold necessitate a diet exceptionally rich in fat and calories. Fat is an energy-dense nutrient, containing almost twice the calories of protein, making seal blubber an efficient fuel source.
Polar bears possess a digestive system highly efficient at processing large quantities of fat. Studies of their genome reveal specific genetic adaptations related to fatty acid metabolism and cardiovascular function. These adaptations enable them to maintain a high-fat diet without developing the cardiovascular diseases that would afflict other species under similar dietary conditions.
This ability to metabolize fat so effectively also aids in water production, as breaking down fat yields metabolic water, a significant advantage in an environment where fresh water can be scarce or frozen.