Polar Bears Can Swim for 9 Days Straight.

In the winter of 2004, researcher Ian Stirling at Environment Canada retrieved a GPS collar from a female polar bear that had walked 687 kilometers across the Beaufort Sea in a single season, searching for sea ice stable enough to hunt from. The ice she needed was retreating faster than she could follow it. She survived. Many do not.

Ursus maritimus is the only bear species that is considered a marine mammal. It spends more of its life on sea ice than on land, hunts almost exclusively in the marine environment, and has been shaped by evolution into one of the most specialized large predators on Earth. That specialization, which has made it supremely effective in its environment for hundreds of thousands of years, is now the source of its vulnerability.

The Body Built for Cold

The polar bear’s adaptations to Arctic conditions are layered and mutually reinforcing. The fur, which appears white but is actually transparent and hollow, scatters light in a way that produces the white appearance while providing insulation. Beneath the fur is a layer of black skin that absorbs solar radiation. Beneath the skin is a layer of fat up to 11 centimeters thick that provides both insulation and an energy reserve during the months when hunting is impossible.

The paws are enormous, up to 30 centimeters across, functioning as snowshoes on soft snow and as paddles in water. The front paws are partially webbed. Polar bears are strong swimmers, capable of sustained swimming at speeds of around 10 kilometers per hour, and individuals have been tracked swimming continuously for over 9 days across open water. The fat layer provides buoyancy as well as insulation.

Researcher Andrew Derocher at the University of Alberta, who has studied polar bear physiology and ecology for over three decades, has documented the metabolic flexibility that allows polar bears to survive extended fasting periods. During summer, when sea ice retreats and hunting becomes impossible, polar bears enter a state called walking hibernation, reducing their metabolic rate and living off fat reserves while remaining mobile. A large male can fast for up to 8 months without eating, though at significant cost to body condition.

The Hunt

Polar bears hunt primarily by still-hunting, a technique of extraordinary patience. A bear locates a seal’s breathing hole in the ice, often by smell through a meter of snow and ice, and waits. Motionless. Sometimes for hours. When the seal surfaces to breathe, the bear strikes with a single blow of its forepaw, stunning or killing the seal instantly, then hauls it out of the water.

The olfactory capability required for this is remarkable. Researcher Malcolm Ramsay documented polar bears detecting seal breathing holes buried under a meter of compacted snow, and detecting the birth lairs of ringed seals, where pups are born in snow caves above the ice, from distances of over a kilometer. The bear’s sense of smell is estimated to be several hundred times more sensitive than a human’s.

Ringed seals are the primary prey, and the relationship between polar bears and ringed seals is one of the most tightly coupled predator-prey relationships in any ecosystem. Ringed seal population dynamics directly determine polar bear body condition, reproductive success, and survival rates. When seal populations decline, polar bear populations follow within a few years.

The Sea Ice Dependency

Everything about the polar bear’s hunting strategy depends on sea ice. The ice is the platform from which bears hunt. Without it, they cannot access the seals that live in the water below. This dependency is absolute and non-negotiable in a way that distinguishes polar bears from most other large predators, which can shift prey or habitat when conditions change.

Arctic sea ice extent has declined by approximately 13% per decade since satellite records began in 1979. The summer minimum, the point of least ice coverage each year, has declined even faster. Stirling’s long-term data from the western Hudson Bay population, one of the most studied in the world, shows that the ice-free season has extended by approximately three weeks since the 1980s. Bears are coming ashore earlier and leaving later, spending more time fasting and arriving at the hunting season in poorer body condition.

The consequences are measurable. Researcher Steven Amstrup at Polar Bears International has modeled the relationship between sea ice loss and polar bear population decline across all 19 recognized subpopulations. His projections, published in Nature in 2010 and updated subsequently, suggest that two-thirds of the world’s polar bears could be lost by 2050 under business-as-usual emissions scenarios.

The current global population is estimated at 20,000 to 25,000 individuals. The species is listed as Vulnerable on the IUCN Red List, though several researchers argue that the trajectory of sea ice loss makes a more severe listing appropriate.

What Remains

Polar bears have survived previous periods of climate change in Earth’s history, including warmer interglacial periods when Arctic sea ice was reduced. But the rate of current warming is faster than anything in the geological record of the past several million years, and the question is not whether polar bears can adapt to a warmer world in principle, but whether they can adapt fast enough given the speed of change.

Stirling, who has watched individual bears across decades of fieldwork, has described the experience of seeing animals he has known for years arrive at the hunting season visibly thinner, their body condition declining in ways that are directly traceable to the shortening of the ice season. The data and the individual animal are the same story, told at different scales.