The polar bear, Ursus maritimus, is a highly specialized mammal of the Arctic, spending most of its life on the sea ice. Although recognized as the world’s largest land predator, it is classified as a marine mammal due to its dependence on the ocean ecosystem for survival. The unique demands of its icy habitat have resulted in a precise, highly caloric diet entirely dependent on the availability of marine prey.
The Primary Calorie Source of the Arctic
The polar bear’s primary diet revolves almost entirely around seals, which provide the immense energy required to sustain its massive body size in freezing temperatures. The preferred prey species are the ringed seal (Pusa hispida) and, to a lesser extent, the bearded seal (Erignathus barbatus). These animals are abundant and provide the necessary platform for the bear’s hunting success: the sea ice itself. The largest proportion of a polar bear’s annual caloric intake occurs during the spring, when seal pups are born and are easier prey.
The nutritional goal of the polar bear is not simply to consume meat, but to access the energy-dense blubber layer of its prey. Seal blubber is a concentrated source of fat, which delivers approximately twice the energy per gram compared to lean muscle tissue. A single adult ringed seal can provide enough calories to sustain a polar bear for over a week.
When hunting is successful and seals are plentiful, an adult bear will often consume only the skin and the thick layer of blubber. The calorie-poor red meat is frequently left behind for other scavengers, such as Arctic foxes and ravens. This selective feeding ensures the bear maximizes its caloric intake while minimizing the consumption of protein, which requires more energy to metabolize. The ability to assimilate up to 97% of the fat it consumes is a testament to this unique dietary specialization.
Methods of the Apex Hunter
Polar bears rely on two hunting tactics, both executed from the platform of the sea ice. The first and most common technique is known as still-hunting, or surfacing-site hunting. Seals must maintain breathing holes, called aglús, in the ice, and a bear will use its powerful sense of smell to locate these openings.
The bear then waits motionless, sometimes for many hours, until a seal surfaces for air. When the seal appears, the bear erupts in a sudden, powerful lunge, often crushing the seal’s skull with a single bite or paw strike and pulling it onto the ice.
The second major technique is stalking seals that are resting, or hauled out, on the ice surface. The bear slowly inches forward while keeping its body low to the ice. Seals are vigilant, frequently lifting their heads to scan for danger, and the bear will instantly freeze in place whenever the seal looks up. This pattern of slow movement followed by stillness is repeated until the bear is within charging distance, typically about 20 feet, where it launches a final, explosive sprint to secure its meal.
Seasonal Scavenging and Opportunistic Feeding
During the summer months, when the sea ice melts, polar bears are forced ashore and must rely on opportunistic feeding. These periods can last for several months, and the bears will temporarily broaden their diet to include land-based resources. Such foods include:
- Seabird eggs and nestlings
- Berries like crowberries and blueberries
- Fish
- Small mammals
Scavenging also becomes a more frequent feeding behavior when seals are unavailable. Bears will feed on the carcasses of larger marine mammals, such as beached whale remains. While these alternative foods provide some sustenance, they are less energy-dense than seal blubber. The caloric intake from berries or grass is insufficient to sustain the bear’s high metabolic needs and large body mass over the long term. For this reason, these seasonal forays on land are considered a strategy to minimize energy loss while fasting, rather than a viable replacement for their primary marine diet.
Dietary Adaptations and Fat Requirements
The polar bear’s physiology is configured to handle its fat-heavy diet. This specialization is rooted in the concept of hyperphagia, where the bear gorges itself on blubber during the spring hunting season to build up fat reserves. These fat stores, which can account for up to 50% of its body weight, are essential for surviving the long periods of fasting when sea ice is gone.
The bear’s ability to process such high volumes of fat is unique among mammals. Genetic studies show that polar bears possess specific gene variants related to fatty acid metabolism and cardiovascular function. These adaptations allow them to efficiently convert dietary fat into body fat and energy without suffering from cardiovascular diseases.
This metabolic strategy allows the bear to enter a state often referred to as “walking hibernation” during the ice-free summer months. While not true hibernation, the bear significantly lowers its activity and metabolic rate to conserve its fat reserves, living off the energy accumulated during the spring feast. This ability to fast for months at a time, relying entirely on fat for energy and metabolic water, underscores the absolute necessity of the seal blubber in their diet.