The polar bear, Ursus maritimus, is the largest living land carnivore and the only bear classified as a marine mammal. It is a very recent species that diverged from the brown bear, it hunts almost exclusively on sea ice, and because the ice is the thing it needs, it has become the standard emblem of climate change, which has made its actual status harder rather than easier to discuss clearly.
Large males run 350 to over 600 kilograms, females roughly half that. The body is longer in the neck and narrower in the head than a brown bear's, which suits reaching into seal holes, and the feet are very large, spreading weight on thin ice and acting as paddles.
The fur is not white. The guard hairs are unpigmented, hollow and translucent, and they scatter light, which is what makes the coat appear white or cream. Beneath them the skin is black. A long-repeated claim that the hairs act as fibre optics piping light to the skin was tested and is false; they conduct ultraviolet no better than other mammal hair.
Insulation comes mostly from fat, up to about ten centimetres of it, rather than from fur. Polar bears are so well insulated that overheating is a real constraint, and they cannot run far without stopping.
The liver is dangerous to eat. It concentrates vitamin A to levels that cause acute hypervitaminosis in humans, a fact learned repeatedly and expensively by Arctic expeditions.
Polar bears split from brown bears within roughly the last 500,000 years, which is extremely recent for such a distinct animal, and the genomic relationship between the two is more tangled than a clean split.
Brown bears on the ABC Islands of southeast Alaska carry mitochondrial DNA nested inside the polar bear lineage, evidence of ancient interbreeding. The current reading is that gene flow ran mainly from polar bears into brown bears, and that the polar bear lineage itself has stayed comparatively distinct.
Hybrids are fertile, which is unusual and reflects how recent the split is. Wild hybrids have been confirmed in the Canadian Arctic since 2006. Whether increasing hybridisation is a meaningful threat or a marginal curiosity is unresolved, though the ranges are converging as brown bears move north.
The rapid adaptation shows in the genome: strong selection on genes involved in fat metabolism and cardiovascular function, which is what an animal eating a diet of almost pure blubber requires.
The polar bear is a specialist on ringed and bearded seals, and the method is still-hunting: waiting motionless at a breathing hole, sometimes for hours, and striking when a seal surfaces. Bears also stalk seals hauled out on the ice and break into birth lairs to take pups.
All of this requires ice as a platform. A polar bear cannot catch a seal in open water, where the seal is faster and more manoeuvrable. Bears swim well and far, with tracked individuals crossing hundreds of kilometres, but swimming is transport, not hunting.
Blubber is what they are after; bears often eat the fat and leave the rest, which feeds Arctic foxes and gulls that follow them. When ice is absent they come ashore and eat what they can find, including berries, birds' eggs, vegetation and carrion, but terrestrial food does not come close to meeting the energy budget of an animal this size. A fasting bear on shore is losing condition, not subsisting.
This is where public argument outruns the evidence in both directions, so the numbers are worth stating carefully.
The global population is currently estimated at roughly 22,000 to 31,000, across nineteen subpopulations. That is higher than mid-twentieth-century estimates, and the increase is real. It is also almost entirely explained by the end of unregulated hunting: aircraft and ship-based trophy hunting was removing large numbers until the 1973 International Agreement on the Conservation of Polar Bears restricted it, and the population recovered from that. Comparing today's figure with 1960 measures the effect of a hunting treaty, not the effect of sea ice.
The subpopulation trends are what matter, and they are mixed. Western Hudson Bay, the most studied, has declined substantially since the 1980s, with measurable falls in body condition, cub survival and the date of ice breakup all moving together. Southern Beaufort Sea has declined. Several subpopulations are stable, a few may be increasing, and a large number remain data-deficient because surveying bears across the Arctic is extremely expensive.
The mechanism connecting ice to bears is well established and does not depend on a current population trend: a longer ice-free season means a longer forced fast, and there is a limit past which a female cannot carry enough fat to produce cubs. Projections of severe decline this century rest on that, not on counts to date.
A 2022 finding added a genuine complication. A previously undescribed subpopulation in southeast Greenland hunts from glacial mélange, the mix of ice discharged by glaciers, rather than from sea ice. It shows the species has more flexibility than assumed. It is also a few hundred animals in a very specific fjord setting, and there is not much Arctic coastline like it.
The IUCN lists the species as Vulnerable.
The polar bear became a symbol, and symbols are brittle. Because it was used as the face of climate change, every fluctuation in its numbers is now enlisted as proof or refutation of something much larger. The animal itself is a recently evolved, highly specialised predator that needs a hunting platform which is present for fewer weeks each decade, and that statement is both less dramatic and better supported than most of what is said about it.
