Far out on the wind-polished sea ice north of Svalbard, a male polar bear moves with a slow, rolling gait. From a distance he looks white against white, which is exactly the point. The bear is not truly white at all. Each guard hair is transparent and hollow, scattering light, while the skin underneath is jet black to soak up what little sun reaches the high Arctic.

Scientists call him Ursus maritimus, the sea bear, and he is the largest living land carnivore. Adult males typically weigh between 300 and 800 kilograms and stretch 200 to 250 centimeters nose to tail, while females are roughly half that size at 150 to 300 kilograms. The species is noticeably sexually dimorphic, with males carrying broader heads and much heavier shoulders. Compared to a brown bear, the body is leaner, the neck longer, the skull flatter, all shapes that help it slip through water and thrust its head into seal breathing holes.

That marine life shows in every detail. A layer of blubber up to 11 centimeters thick sits under the fur, backed by a dense undercoat. The paws are enormous, often 30 centimeters across, with hairy soles that grip ice and spread weight like snowshoes. Even the claws stay short and curved, better for hooking seal skin than digging roots. Polar bears are officially classed as marine mammals because they spend so much of their year on the frozen ocean, not on land.

Their world circles the pole through five nations: Canada, the United States in Alaska, Greenland, Norway, and Russia. Nineteen recognized subpopulations follow the seasonal pulse of sea ice. Some bears stay near shorefast ice all winter. Others ride the pack for thousands of kilometers, drifting with floes from the Beaufort Sea toward Siberia. When summer melts the platform they depend on, many come ashore and wait, living largely off fat reserves.

Seals make that waiting possible. Ringed seals and bearded seals supply most of the calories, taken most often by still-hunting. A bear will locate a breathing hole, settle downwind, and wait motionless for an hour or more until a seal surfaces for air. It then strikes, drags the animal onto the ice, and eats the blubber first, sometimes leaving the rest for arctic foxes and gulls. Walrus, beluga carcasses, and the occasional bird or egg fill gaps, but fat-rich seal blubber is the fuel that lets females nurse cubs through long fasts.

Reproduction follows the ice calendar. Mating happens in April and May, but the fertilized egg does not implant until autumn. In October or November a pregnant female digs a maternity den into a snowdrift, often on land but sometimes on stable pack ice. One or usually two cubs are born blind in December or January, each weighing less than a kilogram. The mother does not eat or drink inside the den for months, converting her own fat into milk. Families emerge in March or April, and cubs stay with her for about two and a half years, learning where to find holes and how to read the ice.

Numbers are hard to pin down in such a vast habitat, but the IUCN lists polar bears as vulnerable, with a global estimate of roughly 22,000 to 31,000 animals. Not all subpopulations are faring the same. The Southern Beaufort Sea group off Alaska and western Canada has declined as ice-free seasons lengthen. The biggest long-term pressure is climate-driven sea ice loss, which shortens hunting time and forces bears ashore where they face more human contact, poorer nutrition, and lower cub survival. Pollution and industrial activity add stress, and the United States lists the species as threatened under its Endangered Species Act.

In the wild most bears live 15 to 18 years, though some females reach their late twenties. In zoos they can pass 30. They swim steadily for hours, have been tracked covering more than 100 kilometers without rest, and can smell a seal nearly a kilometer away. For Inuit communities who have lived alongside them for millennia, the bear is nanook, a figure of respect and caution woven into stories, hunting practice, and place names.

Watch that same male again as the light fades. He pauses at a pressure ridge, tests the air, then drops flat beside a dark hole. He will wait. The ice, the seal, and the bear have done this together for at least 120,000 years. What changes now is how long the platform holds each spring.