The polar bear, Ursus maritimus, is an apex predator in the Arctic ecosystem. Its ability to thrive in this harsh environment is directly linked to its specialized diet. The unique feeding habits of polar bears are central to their survival, allowing them to accumulate the substantial energy reserves necessary for insulation and sustenance in extreme cold.
Primary Food Sources
Polar bears primarily hunt seals, with ringed seals and bearded seals forming the majority of their diet. These seals are an ideal food source due to their high fat content, which is essential for polar bears to build their thick layer of blubber for insulation and energy storage. A single ringed seal can provide enough energy for a polar bear for up to eight days.
Polar bears employ several hunting strategies to catch seals on the sea ice. The most common method is “still-hunting,” where a bear waits patiently by a seal’s breathing hole in the ice. When a seal surfaces for air, the bear quickly pounces to secure its prey. Polar bears also stalk seals resting on the ice, using natural cover to approach before charging. The importance of sea ice is paramount, as it serves as the hunting platform from which polar bears access their primary prey.
Opportunistic Feeding and Seasonal Changes
While seals are their main food, polar bears exhibit opportunistic feeding behaviors, especially when seals are less available or during the ice-free season. They may scavenge on the carcasses of marine mammals like beluga whales, bowhead whales, or walruses, or occasionally prey on young walruses.
During periods of food scarcity, polar bears may consume terrestrial items such as birds, eggs, small rodents, reindeer, and some vegetation like berries or kelp. These alternative foods, however, are generally low in calories and do not provide the necessary fat content to sustain a polar bear long-term. As sea ice melts in summer, forcing bears ashore, they rely more on these opportunistic sources or their stored fat reserves until the ice returns.
Metabolic Adaptations for Their Diet
The polar bear’s body is adapted to process a high-fat diet. Their digestive system is highly efficient at assimilating large amounts of fat, digesting up to 97% of what they consume. This allows them to convert blubber into energy reserves. Up to half of a polar bear’s body weight can consist of fat.
Genetic studies have revealed adaptations that enable polar bears to thrive on such a diet without adverse health effects common in other species. Mutations in genes related to fat transport and fatty acid metabolism, such as APOB, help them manage high levels of cholesterol and triglycerides. This ability to efficiently metabolize fat provides the energy and insulation needed for their Arctic survival.
How a Changing Arctic Impacts Their Food
Changes in the Arctic environment affect the polar bear’s access to its primary food sources. The reduction in sea ice extent and duration, a consequence of warming temperatures, impacts their ability to hunt seals. Less sea ice means fewer hunting platforms, forcing bears to travel longer distances or spend extended periods on land.
Reduced access to seals leads to longer fasting periods, resulting in weight loss. Bears can lose an average of 21 kilograms over three weeks during these periods. Relying on opportunistic foods found on land does not provide sufficient calories to maintain their body condition, impacting their health and reproductive success. For instance, the energy content of milk in lactating females declines with increased time spent ashore, affecting cub survival.