Polar bear
Based on Wikipedia: Polar bear
The Bear That Became a Whale
Here's something that might surprise you: the polar bear is technically a marine mammal. Not because someone made a bureaucratic error, but because this creature has become so thoroughly dependent on the ocean that scientists classify it alongside whales, seals, and dolphins. It's a bear that went to sea and never really came back.
The polar bear represents one of evolution's most dramatic recent transformations. Somewhere between one million and 600,000 years ago—scientists are still debating the exact timeline—a population of brown bears began adapting to life on Arctic sea ice. What emerged was something extraordinary: an apex predator perfectly engineered for one of Earth's harshest environments, capable of swimming for days through frigid waters, hunting seals through holes in the ice, and surviving temperatures that would kill most mammals in hours.
But this same specialization has become a trap. The polar bear's entire existence depends on sea ice, and that ice is disappearing.
Built for the Cold
To understand the polar bear, you have to understand just how completely it has been redesigned for Arctic life. Start with the fur, which isn't actually white at all.
The individual hairs are transparent and hollow. Light bounces around inside them and scatters back out, creating the appearance of whiteness—the same optical trick that makes snow look white when it's really just frozen water. This camouflage is so effective that a polar bear can approach within striking distance of a seal before being noticed against the ice.
But camouflage is almost a side benefit. The real engineering marvel is thermal. Those hollow guard hairs, combined with dense underfur about two inches thick, trap heat with remarkable efficiency. Ultraviolet light actually passes through the transparent outer hairs and gets absorbed by the black skin underneath, creating a kind of solar heating system. The bear then insulates that warmth with a layer of fat up to four inches thick.
This system works so well that overheating becomes a genuine concern. Polar bears have evolved a network of blood vessels and specialized muscle tissue specifically to dump excess heat when necessary. They'll also simply wade into frigid Arctic water to cool down—water that would give a human hypothermia in minutes.
A Different Kind of Bear
If you placed a polar bear skull next to a brown bear skull, you'd immediately notice the differences. The polar bear's is narrower, flatter, and more elongated—almost dog-like in its proportions. The neck is longer. The shoulders sit lower. The whole animal is built more like a distance swimmer than a forest omnivore.
The teeth tell an even clearer story. Brown bears are omnivores with teeth designed for grinding plants, cracking nuts, and occasionally eating meat. Polar bear teeth are different: longer canines, sharper cheek teeth, more spacing between them. These are the teeth of an almost pure carnivore, optimized for biting through seal blubber and tearing flesh.
The paws are enormous—proportionally larger than any other bear species. The front paws are broader than the back ones, which makes them excellent paddles for swimming. The undersides are covered in dense fur that provides both insulation and traction on ice. Small, sharp, hooked claws help the bear snatch seals and haul itself onto ice floes.
Even the kidneys have been modified. Polar bears eat an almost exclusively marine diet, which means consuming large amounts of salt. Their kidneys have evolved a specialized structure called a reniculate kidney, with multiple lobes that efficiently filter out excess salt—the same adaptation found in whales and seals.
The Largest Land Carnivore
Male polar bears typically weigh between 660 and 1,760 pounds, with the largest individuals approaching a full ton. The biggest polar bear ever recorded was shot in Alaska in 1960 and reportedly weighed 2,209 pounds—roughly the weight of a compact car.
Females are dramatically smaller, usually weighing between 330 and 660 pounds. This extreme size difference between males and females—what biologists call sexual dimorphism—is more pronounced in polar bears than in almost any other mammal. A large male can weigh four or five times as much as a small female.
But here's the thing about polar bear weight: it fluctuates wildly throughout the year. During the prime hunting season, when sea ice provides easy access to seals, polar bears gorge themselves and can increase their body mass by fifty percent. A pregnant female preparing for her winter denning might pack on enough fat to reach 1,100 pounds. Then, during the ice-free summer months when hunting becomes difficult, the bears can burn through those reserves at an alarming rate.
The Seal Hunter's Art
Polar bears are specialists in a way that few large predators are. Their primary prey is the ringed seal, a relatively small marine mammal that spends its life in and around Arctic sea ice. The bear's entire lifestyle—where it travels, when it's active, how it hunts—revolves around finding and catching these seals.
The most common hunting technique is called still-hunting, and it requires extraordinary patience. Seals must surface regularly to breathe, and they maintain breathing holes in the ice. A polar bear will locate one of these holes—probably by smell, since their olfactory system is enormously developed—and then simply wait. Sometimes for hours. Completely motionless.
When a seal surfaces, the bear strikes with explosive speed, using those hooked claws to snatch the seal and drag it onto the ice before it can escape. The whole attack takes seconds.
Other hunting strategies include stalking seals resting on the ice, approaching slowly while the seal sleeps and then rushing in for the kill, or waiting at the edge of ice floes for seals to swim past. In summer, when ice is scarce, polar bears will swim through open water looking for prey—they've been documented swimming continuously for days, covering hundreds of miles.
What the bear eats first is telling: the blubber. That thick layer of seal fat is so energy-dense that polar bears will often eat only the blubber and leave the protein-rich meat behind. In a cold environment where staying warm burns enormous calories, fat is the most valuable fuel.
Walrus and Whale
Seals make up the bulk of the polar bear diet, but they're not the only prey. Polar bears will hunt walruses, though this is dangerous—an adult walrus can weigh over a ton and has long tusks capable of killing a bear. Usually polar bears target walrus calves or animals that are sick or injured.
More surprisingly, polar bears occasionally hunt beluga whales. Belugas congregate in shallow coastal waters, and a bear patrolling the shoreline might find an opportunity to ambush one in shallow water. This is rare, but it happens.
When stranded on land during the ice-free season, polar bears become opportunistic scavengers and foragers. They'll eat seabird eggs, small mammals, berries, kelp—whatever they can find. But this terrestrial food is far less energy-dense than seal blubber, and bears that spend too long on land tend to lose weight steadily. Some have been observed eating garbage near human settlements, which brings them into dangerous conflict with people.
A Solitary Life
Polar bears are fundamentally solitary animals. Outside of mating season and mother-cub pairs, they generally avoid each other. The Arctic is vast and seals are scattered; there's little advantage to cooperation and plenty of incentive to avoid competition.
But they're not completely antisocial. When polar bears are forced onto land during summer, they sometimes congregate in loose groups, particularly near food sources like whale carcasses or garbage dumps. In these situations, a hierarchy emerges based mostly on size—large males dominate, followed by smaller males, then females without cubs, and finally mothers with cubs, who are most vulnerable and must be most cautious.
The breeding season runs from March through June. Males seek out females by following scent trails, sometimes traveling enormous distances. When a male finds a receptive female, he'll stay with her for days or weeks, mating repeatedly and guarding her from rival males. Fighting between males does occur, though serious injuries are relatively rare—most conflicts are settled through intimidation displays rather than actual combat.
The Maternity Den
Female polar bears have one of the most demanding reproductive strategies of any large mammal. After mating in spring, the fertilized egg doesn't immediately implant in the uterus—it floats free for months in a state of suspended development. Only in autumn, if the female has accumulated enough fat reserves, does the egg implant and pregnancy truly begin.
In late autumn, the pregnant female digs a maternity den in a snowbank, usually on land but sometimes on sea ice. She'll spend the entire winter in this den, not eating, not drinking, living entirely off her fat reserves while her body simultaneously sustains a pregnancy.
Cubs are born in December or January, tiny and helpless—about the size of a guinea pig, blind, and barely furred. The den provides a relatively warm, stable environment where they can nurse and grow. By spring, when the family emerges, the cubs will have grown enough to travel with their mother onto the sea ice.
Mother polar bears are intensely protective. Cubs stay with their mothers for up to two and a half years, learning to hunt and survive in one of Earth's most challenging environments. During this time, the mother won't mate again. This long period of maternal investment, combined with the high energy demands of reproduction, means female polar bears can only produce a limited number of cubs over their lifetime.
Names and Naming
The polar bear has accumulated names like a traveler collects passport stamps. Europeans between the 13th and 18th centuries called it the "white bear," the "ice bear," the "sea bear," or the "Greenland bear"—each name capturing some aspect of this strange creature they were only beginning to understand.
The Norse had two names: isbjørn, meaning "ice bear," and hvitebjørn, meaning "white bear." The Inuit, who have lived alongside polar bears for thousands of years and understand them far better than any European explorer ever did, call them nanook—a word that has passed into broader English through everything from documentary films to Led Zeppelin songs.
The Netsilik people of the Canadian Arctic have an even more sophisticated vocabulary, with different names for polar bears based on their sex, age, and behavior. An adult male is anguraq. A single adult female is tattaq. A pregnant female preparing to den is arnaluk. A newborn cub is hagliaqtug. A large adolescent is namiaq. And a bear in its winter dormancy is apitiliit.
The scientific name, Ursus maritimus, is Latin for "sea bear"—a recognition that this is no ordinary land animal. The Swedish naturalist Carl Linnaeus initially classified the polar bear as a variety of brown bear in 1758, but by 1774, the British naval officer Constantine John Phipps had formally described it as a separate species following his expedition toward the North Pole.
The Brown Bear's Cousin
The relationship between polar bears and brown bears is more complicated than it might appear. For a long time, scientists assumed polar bears descended directly from brown bears—that some population of brown bears wandered onto Arctic ice and gradually evolved white fur and marine hunting abilities. This seemed logical and straightforward.
Genetic studies have complicated this picture considerably. The two species did diverge from a common ancestor, probably somewhere between 600,000 and one million years ago. But they've also been interbreeding, off and on, ever since.
During ice ages, when glaciers advanced and retreated repeatedly, polar bear and brown bear populations would come into contact, mate, and produce hybrid offspring. These hybrids could reproduce successfully, passing genes back and forth between the two species. Scientists call this "gene flow," and it's left a complicated genetic legacy.
For example, brown bears on the ABC Islands of Southeast Alaska—Admiralty, Baranof, and Chichagof—carry up to nine percent polar bear DNA. Brown bears that once lived in Ireland, now extinct, carried even more: up to 21.5 percent. Meanwhile, all living polar bears seem to have inherited their mitochondrial DNA—the genetic material passed down through the maternal line—from those same Irish brown bears.
This interbreeding appears to have largely stopped around 200,000 years ago, though hybrids still occasionally appear in the wild today. They're sometimes called "grolar bears" or "pizzly bears," and as climate change pushes polar bears onto land more frequently, such encounters may become more common.
Genetic Specialization
The polar bear's genome tells a story of rapid, dramatic adaptation. Compared to brown bears, polar bears have fewer copies of olfactory receptor genes—the genes responsible for detecting smells. This makes sense: the Arctic has far fewer scent-producing organisms than a temperate forest. There's simply less to smell.
More striking are the dietary adaptations. Polar bears have fewer copies of the gene that produces amylase, an enzyme that breaks down starch. Brown bears need amylase because they eat plants, nuts, and berries. Polar bears, living almost entirely on seal fat, have little use for it.
Instead, polar bears have evolved enhanced versions of genes related to fatty acid metabolism. They can process and utilize enormous amounts of dietary fat without developing the cardiovascular problems that would kill most mammals on such a diet. Their circulatory system has been modified to handle this extreme nutritional strategy.
Even the polar bear's famous white coat has a genetic signature. Polar bears have more copies of genes involved in producing keratin—the protein that forms hair—particularly the variants associated with their distinctive fur structure.
Where They Live
Polar bears inhabit the Arctic and adjacent areas: Greenland, Canada, Alaska, Russia, and the Svalbard Archipelago of Norway. They've been recorded within sixteen miles of the North Pole. At the southern edge of their range, they reach James Bay in Canada and St. Matthew Island in Alaska.
Scientists have divided polar bears into at least eighteen subpopulations, each associated with a particular region of Arctic sea ice. These populations have somewhat different characteristics and face different challenges, though they're all part of the same species. The total global population is estimated at between 22,000 and 31,000 individuals.
The key to understanding polar bear distribution is sea ice. Polar bears prefer what's called annual sea ice—ice that forms each winter and melts each summer—particularly where it covers continental shelves or the spaces between Arctic islands. These areas, sometimes called the "Arctic Ring of Life," are biologically productive zones where seals are abundant.
The bears are especially drawn to places where ice meets open water: the edges of ice floes, the channels of open water called "leads," and the areas of open water surrounded by ice called "polynyas." These are the places where seals must surface to breathe, making them prime hunting grounds.
The Ice-Dependent Life
When the sea ice melts in summer, polar bears are forced onto land. This is not their preference. On land, they have difficulty hunting seals and must survive mostly on stored fat or whatever marginal food sources they can find. Some bears in areas like Hudson Bay spend four months or more on land each summer, slowly starving while they wait for the ice to return.
In other regions, like the Chukchi and Beaufort seas, the sea ice doesn't melt completely but rather breaks off and drifts north. Some bears follow this drifting ice, staying on the frozen surface even as it moves far from shore. Others—an increasing proportion since the 1980s—have begun spending summers on land instead.
A polar bear's annual range varies enormously depending on ice conditions. Some bears travel as little as 1,400 square miles in a year. Others cover 15,000 square miles or more. They can walk an average of seven miles per day, and they can run at speeds up to 25 miles per hour in short bursts, though they usually move at a leisurely pace of around three miles per hour.
Swimming is another matter. Polar bears are excellent swimmers, capable of sustained speeds around six miles per hour. They've been tracked swimming continuously for days, covering hundreds of miles of open water. But these marathon swims are exhausting and dangerous, particularly for cubs. As sea ice declines and open water increases, polar bears must swim farther and more often to find food.
The Warming Crisis
The International Union for Conservation of Nature, the organization that maintains the authoritative list of threatened species, classifies the polar bear as "vulnerable." This is the category below "endangered" and above "near threatened." It means the species faces a high risk of extinction in the wild.
The primary threat is climate change. The Arctic is warming faster than any other region on Earth—roughly four times faster than the global average. This warming is melting sea ice, and sea ice is the foundation of polar bear existence.
Less ice means less hunting opportunity. Polar bears need ice to hunt seals; without it, they cannot access their primary food source. As the ice-free season lengthens, bears spend more time on land, burning through their fat reserves while eating food that cannot replace what they've lost. Studies have documented declining body condition—bears are getting thinner—and declining reproduction rates in some populations.
Less ice also means longer swims. As the distance between ice floes increases, bears must swim farther to reach hunting grounds. Adult bears can survive these swims, but cubs often cannot. Drowning has become an increasingly documented cause of death for young polar bears.
Finally, less ice means more conflict with humans. When polar bears spend more time on land, they inevitably encounter people. They raid garbage dumps and food caches. They enter towns. Sometimes they attack and kill people. Often, they are killed themselves—shot by people defending their communities.
Other Threats
Climate change is the existential threat, but polar bears face other pressures too. Industrial pollution has contaminated Arctic ecosystems with persistent organic pollutants and heavy metals. These toxins accumulate in the food chain, becoming more concentrated at each level. As apex predators, polar bears end up with dangerously high levels of pollutants in their tissues, which may affect their immune systems, reproduction, and overall health.
Oil and gas development in the Arctic brings disturbance, habitat degradation, and the risk of catastrophic spills. A major oil spill in polar bear habitat could be devastating—oil destroys the insulating properties of fur, and bears that attempt to groom contaminated fur may ingest lethal amounts of petroleum.
Hunting is now regulated throughout the polar bear's range, but it continues. Indigenous peoples in Canada, Alaska, Greenland, and Russia have traditionally hunted polar bears for meat, fur, and other materials, and these subsistence hunts are generally allowed to continue under international agreements. Sport hunting still occurs in Canada and is controversial.
In Culture
Humans have been fascinated by polar bears for as long as they've encountered them. For Arctic peoples, the bear was a figure of spiritual significance—a powerful being that moved between the worlds of land, sea, and ice. Inuit hunters traditionally showed great respect for the bears they killed, observing rituals believed to honor the animal's spirit.
European culture has tended to view polar bears differently: as curiosities, as trophies, as symbols of the exotic Arctic. Live polar bears were brought to European menageries as early as the medieval period. In the modern era, they became staples of zoos and circuses, though keeping such large, active, intelligent animals in captivity has proven problematic.
Today, the polar bear has become the iconic symbol of climate change. Its white fur against blue ice, its dependence on a vanishing ecosystem, its charismatic appeal—all have made it the poster child for a warming world. Whether this symbolism will translate into the political will necessary to actually address climate change remains to be seen.
The Melting Future
The polar bear evolved to exploit a very specific ecological niche: the interface between Arctic sea ice and marine ecosystems. For hundreds of thousands of years, this niche was stable and reliable. Ice formed each winter, seals hauled out on it, and bears hunted them. The system worked.
That system is now breaking down. Within the lifetime of bears alive today, the Arctic may see ice-free summers—something polar bears have never experienced as a species. Some populations may adapt, finding new food sources or new habitats. Others will likely decline toward extinction.
The polar bear's story is, in a sense, a parable about specialization. The same adaptations that made it so spectacularly successful—the white fur, the fat metabolism, the seal-hunting skills, the tolerance for cold—have also made it vulnerable to change. When your entire existence depends on ice, the melting of that ice is an existential crisis.
There are roughly 25,000 polar bears left on Earth. Whether their descendants will still roam the Arctic a century from now depends largely on decisions being made right now, by people who live thousands of miles from the nearest ice floe. The bear that became a marine mammal, the white ghost of the Arctic, waits on shrinking ice for a verdict it cannot understand and cannot appeal.