Polar bears (Ursus maritimus) are the largest extant species of bear and land carnivore by body mass, uniquely adapted to thrive in the harsh Arctic environment. While closely related to brown bears, polar bears have evolved into primarily carnivorous hunters, with a diet centered around marine mammals. This dietary specialization, coupled with the challenges of a changing Arctic landscape, makes understanding the polar bear's diet crucial for conservation efforts.
Dietary Specialization and Primary Food Sources
Polar bears are hypercarnivores, with the most carnivorous diet among bear species. Their diet is primarily composed of ice-living seals, providing the energy-rich blubber necessary for survival in frigid temperatures. The skull morphology and dentition of polar bears are specifically adapted for this carnivorous lifestyle. Compared to brown and black bears, polar bears possess sharper teeth that are more suited for cutting meat.
The most commonly consumed prey is the ringed seal, which is abundant and relatively small, making it easier for even smaller bears to overpower. Bearded seals are also a significant part of their diet, although adult bearded seals are larger and more likely to escape. Occasionally, polar bears may also prey on harp seals, hooded seals, spotted seals, ribbon seals, and even young walruses.
Polar bears obtain around two-thirds of their energy for the entire year during the spring, when ringed and bearded seals are abundant across the entire polar bear range. Seals are nutritionally high in fat, and polar bears' digestive systems have evolved to use high quantities of fat to build up insulation to survive in the cold.
Hunting Strategies and Adaptations
Polar bears are patient and strategic hunters, employing various techniques to capture their prey. They often use sit-and-wait tactics, waiting near seal breathing holes in the ice. They also stalk seals hauling out on the sea ice, approaching slowly with their head and neck lowered to minimize visibility. Polar bears walk across the current to winds to pick up the smell. When close enough, they charge at high speed to catch the seal before it escapes into the water.
Read also: Learn how spa therapy can enhance your overall health.
During the spring, female polar bears with pups may stalk ringed seal birthing lairs, using their powerful sense of smell to locate the hidden pups. They destroy the roof of the lair with their hind legs, exposing the seal and pups.
Polar bears prefer to hunt for seals on thick, multi-year sea ice as a platform. The polar bear can stay underwater with its nose exposed.
Alternative Food Sources and Opportunistic Feeding
While seals are the primary food source for polar bears, they are opportunistic feeders and will supplement their diet with other available resources, especially when preferred prey is scarce. They eat whale and walrus carcasses and search for bird eggs and other food sources.
In summer, if preferred food is unavailable, polar bears may consume land plants, lichen, seaweed/kelp, berries, small mammals, birds, and eggs. They have also been observed feeding on white-beaked dolphins trapped by ice floes and hunting reindeer in Svalbard, Norway. Polar bears opportunistically feed on whale carcasses caught by native subsistence hunters and those resulting from killer whale predation.
In recent years, observations of polar bears in Alaska show a broadening of their diet to include harder foods. Due to climate change, the Arctic ice is melting faster, and polar bears have been finding alternative food sources to compensate for the loss of hunting time. Consequently, scientists have discovered that in times of great scarcity, polar bears may resort to supplementing their food sources by eating other polar bears.
Read also: Facial oil: Benefits and how to use
Polar bears will scavenge on the remains of land mammals and birds. Subadults may scavenge kills left by larger individuals. Arctic foxes routinely follow polar bears and scavenge scraps from their kills. The bears usually tolerate them but will charge a fox that gets too close when they are feeding.
Diet Variations Among Polar Bear Populations
Prey selection can vary somewhat between male and female polar bears. Subadults may scavenge kills left by larger individuals.
The Impact of Climate Change on Polar Bear Diet
Climate change poses a significant threat to polar bears and their diet. The decline in sea ice extent and thickness reduces the time polar bears have to hunt seals, forcing them to go without food or expend more energy getting food by swimming further distances.
Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. For this reason, foods that polar bears consume during the ice-free season may become increasingly important in alleviating nutritional stress from lost seal hunting opportunities.
As sea ice retreats earlier in the spring and forms later in the autumn, polar bears are running into problems. This has forced many polar bears to go without food or expend more energy getting food by swimming further distances.
Read also: The role of alpha-keto acids in metabolism.
Terrestrial Diet and Shifting Food Resources
As the Arctic environment changes, polar bears are increasingly relying on terrestrial food sources during the ice-free season. Studies of polar bear scat have revealed a diverse range of land-based foods in their diet, including marine algae, grasses, berries, fish, small mammals, caribou, and waterfowl.
Polar bears currently appear to be exploiting increasingly abundant resources such as caribou and snow geese and newly available resources such as eggs. This opportunistic shift is similar to the diet mixing strategy common among other Arctic predators and bear species.
Reports of polar bears exploiting land-based prey have become more common in recent years. For example, consumption of eggs and young from nesting colonies of waterfowl across the Arctic is increasingly pervasive, and predation on larger land mammals, such as caribou, had been reported.
Polar Bear Fasting and Energy Balance
Polar bears are often referred to as "fasting" while ashore during the ice-free period. Although the term may apply to some polar bears, extension to the majority of the western Hudson Bay population seems inappropriate given multiple observations to the contrary. While ashore, polar bears are in a negative energy balance, reportedly surviving primarily on their fat reserves, although supplementary, terrestrial foods are also consumed when available.
Conservation Implications
Understanding the polar bear's diet and how it is changing in response to climate change is essential for effective conservation strategies. Monitoring their food sources, hunting success, and reliance on alternative prey can provide valuable insights into their adaptability and resilience.
By studying the current terrestrial diet of polar bears, scientists can assess how they are reacting to climate change and identify potential food resources that may help them survive in a warming Arctic. This information can inform management decisions aimed at protecting polar bear populations and their habitat.