Polar bears possess unique adaptations allowing them to thrive in sea ice environments. Their existence is intricately linked to the frozen marine landscape, which serves as their primary hunting ground. Summer presents a considerable challenge for these animals as their icy habitat diminishes. Understanding their activities during this warmer season is important for comprehending their survival strategies.
Summer Habitat Shifts
As Arctic sea ice melts during the summer, polar bears face distinct habitat choices: some follow the retreating ice northward, remaining on residual ice to hunt seals, though in a more fragmented environment. Other polar bears move ashore onto land, particularly where sea ice completely disappears. This shift to land often means a period of forced fasting, as their primary prey, seals, are not readily available terrestrially. Land-based summer retreats for polar bears include coastal areas or archipelagos. In some areas, polar bears are spending approximately 30 more days on land compared to a few decades ago due to changes in ice conditions.
Dietary Adaptations and Fasting
The primary challenge for polar bears in summer is limited access to their main prey, seals, due to reduced sea ice. Many polar bears enter a period of forced fasting during the ice-free season, relying on thick fat reserves accumulated during the spring. These fat stores are acquired during late spring and early summer when prey is abundant, providing about two-thirds of their annual energy.
Land-based food sources, such as berries, bird eggs, or carrion, are scarce and offer limited nutritional value for a polar bear. While bears have been observed consuming terrestrial foods, these diets are insufficient to sustain them or prevent weight loss. Studies show that bears lose approximately 1 kilogram of fat per day during the ice-free period. Physiologically, polar bears can develop insulin resistance during fasting, an adaptation that helps maintain blood sugar levels and preserve muscle mass over extended periods without food. However, this “adaptive fasting” is not always sufficient to prevent protein loss, particularly as fasting periods lengthen.
Energy Conservation and Behavior
To endure the warmer summer months and periods of food scarcity, polar bears employ various energy conservation strategies. They significantly reduce their activity levels, often resting frequently and minimizing movement. When not hunting, polar bears may spend up to 87% of their time resting. This reduced activity helps them conserve energy, as their bulky build and gait already require more energy to move than many other mammals.
Their thick fur and blubber, while excellent for insulation in cold Arctic conditions, can lead to overheating in summer temperatures. To counteract overheating, polar bears seek shade, lie in snow patches if available, or enter the water to cool down. They may spend more time in cool coastal waters and dig day beds in the sand to dissipate heat. Unlike their active hunting periods on the ice, when they might spend up to 40% of their time hunting, their summer behavior is characterized by relative lethargy.
Climate Change and Summer Survival
The increasing duration and extent of sea ice melt, driven by global warming, directly impact polar bears’ summer activities and their long-term survival. Arctic regions are warming faster than the global average, leading to sea ice forming later in the fall and breaking up earlier in the spring. This results in longer ice-free periods, which force polar bears to endure extended fasting, sometimes for a month longer than in previous decades.
Extended fasting periods lead to reduced body condition and lower reproductive success in polar bears. Studies indicate that female bears produce smaller litters, and cub survival rates decrease with longer periods of food deprivation. The increased energy expenditure required to travel further for food or to reach remaining ice also contributes to their declining health. If ice-free periods continue to lengthen, it could lead to significant polar bear mortality.