Polar Bears: Marine Mammals and Swimming Habits

The polar bear is the only bear species classified by scientists as a marine mammal. Unlike grizzlies or pandas, Ursus maritimus spends most of its life on drifting sea ice, hunts almost exclusively in or beside the sea, and depends on marine prey for most of its diet. The polar bear is the largest land carnivore on Earth today, the heaviest member of the bear family and one of the few large predators that actively treats humans as prey under the right circumstances.

This guide covers every aspect of polar bear biology and ecology: size and strength, adaptations to cold, hunting behaviour, diet, reproduction, social life, conservation status and the relationship between polar bears and the humans who share the Arctic.

Etymology and Classification

The scientific name Ursus maritimus was coined by Constantine Phipps in 1774 after his voyage toward the North Pole. It literally means 'maritime bear' and reflects how surprised early European explorers were to find such a large mammal living on sea ice rather than on land. In Inuktitut the polar bear is called nanuq , a word carrying deep cultural weight across Inuit communities. Russian and Scandinavian languages use variants of isbjorn or belyy medved meaning 'white bear'.

Genetic analysis places polar bears firmly inside the brown bear family tree. The two species are separate sister lineages: a 2010 study estimated the split at around 150,000 years ago, a 2012 study at around 600,000 years ago, and a 2022 study at over one million years ago.

Any straightforward family tree is impossible because of ancient gene flow between the two lineages.

Modern polar bears retain enough genetic similarity to brown bears that hybrids, known as pizzly or grolar bears, are fully fertile and have been documented in the wild since at least 2006.

Size and Physical Description

Polar bears are the largest land carnivores alive today. Size differences between males and females are extreme, a pattern biologists call sexual dimorphism.

Males:

• Length: 2.0-2.5 metres from nose to tail
• Shoulder height: 1.3-1.6 metres on all fours
• Standing height: up to 3 metres on hind legs
• Weight: typically 350-590 kg, reported record 1,002 kg (Alaska, 1960)

Females:

• Length: 1.8-2.0 metres
• Weight: typically 150-295 kg, rising to as much as 500 kg when heavily pregnant
• Generally half the mass of a comparable male

Cubs at birth:

• Weight: 500-700 grams, about the size of a guinea pig

Polar bears are built for long-distance travel on ice and in water. Their bodies are more elongated than brown bears, with longer necks and narrower heads. The skull is narrower, flatter and smaller than that of the brown bear. Their feet are large, spreading weight across thin ice and functioning as paddles during swims. Small papillae (bumps) and fur between the toes provide traction on ice.

Their famous white coat is one of the most frequently misunderstood features in biology. Each hair is not pigmented white. Each hair is a hollow transparent tube. Visible light scatters off the inner walls of the shaft and bounces out in every direction, producing a diffuse white or cream appearance. The skin beneath is jet black.

In ultraviolet photographs polar bears appear almost black, because the fur transmits UV rather than reflecting it. Under humid, dirty, or captive conditions the coat can take on yellow, green, or even pinkish tints when algae grow inside the hollow hair shafts.

How big is a polar bear compared with a grizzly bear?

A polar bear is usually much heavier. Polar Bears International puts adult males at 350 to 590 kg (772 to 1,300 lb) and adult females at 150 to 295 kg (331 to 650 lb). In Yellowstone, the National Park Service records an average adult male grizzly at 187 kg (413 lb) and an average adult female at 122 kg (269 lb), and the heaviest known male in the Greater Yellowstone Ecosystem weighed 324 kg (715 lb).

That means a typical adult male polar bear outweighs even the heaviest grizzly on record in Yellowstone. Standing on its hind legs, a polar bear can reach about 3 metres (roughly 10 feet), according to Polar Bears International. Grizzly weights vary from one population to another, and the Yellowstone figures describe one inland population.

Built for Extreme Cold

Polar bears are adapted to extreme Arctic cold. Every major feature of their biology serves one of two goals: keep heat in, or find enough food to generate heat.

Insulation layers:

• Outer guard hairs: about 15 cm long, hollow, water-shedding
• Dense underfur: about 5 cm long, traps a still layer of warm air against the skin
• Subcutaneous fat reaches 5 to 10 cm thick in healthy adults and can be thicker still.

Thermal regulation features:

• Small ears and short tail: minimise surface area for heat loss
• Compact body shape: favourable volume-to-surface ratio

Bears insulate so well that the animals can overheat from physical effort, which is why polar bears hunt almost exclusively through ambush and patience instead of chase.

Hunting and Diet

Polar bears are hypercarnivores whose diet consists entirely of meat, with their food source narrowed almost exclusively to marine mammal fat augmented by minimal muscle protein. Blubber from seals provides the bear's primary food and energy source.

Primary prey:

• Ringed seal ( Pusa hispida ): most common
• Bearded seal ( Erignathus barbatus ): larger, more energetic payoff
• Harp seal and hooded seal: seasonal, regionally important

Secondary and opportunistic prey:

• Walrus (usually juveniles or carcasses)
• Beluga whale (usually trapped in ice or stranded)
• Narwhal (usually when trapped at breathing holes or stranded)
• Seabirds, eggs, small mammals
• Reindeer in parts of Svalbard
• Whale carcasses when they drift ashore

Hunting techniques:

1. Still-hunting at breathing holes. A polar bear locates a seal's breathing hole by scent, sits motionless beside it for up to several hours, and strikes the instant a seal surfaces. This is the primary hunting method from winter to early spring.
2. Stalking on ice. The bear moves across the ice belly-first, using ice ridges for concealment and then bursts into a short sprint at the end. Hunting success hinges on the seal not detecting any movement.
3. Aquatic stalking. In open water, the bear dives and swims slowly beneath a hauled-out seal, surfacing beneath it.
4. Lair raids. Mothers with cubs target newborn ringed seal pups that are hidden in snow lairs above the ice. The bear uses scent to locate the lair, then crashes through the roof with its paws.

Fat reserves allow polar bears to survive for months without eating.

Life Cycle and Reproduction

Polar bear reproduction runs on a strict seasonal schedule keyed to Arctic ice cycles. Mating takes place on the sea ice, mostly between March and May. Males roam enormous distances searching for females in oestrus, guided almost entirely by scent trails. Competition for mates is intense, and males often carry scars from fighting.

Once mated, females enter a form of reproductive pause called delayed implantation . The fertilised egg floats in the uterus without developing until the fall. Implantation, and pregnancy proper, only begins if the female has enough fat reserves to support gestation and nursing. If not, her body reabsorbs the embryo. This is one of the strongest signals of how climate-dependent polar bear reproduction is: a bad summer hunt means no cubs the following year.

Denning cycle:

• August to October: pregnant female constructs and enters a maternity den
• Mid-September to mid-October: implantation, birth follows in the den during winter
• Winter: female and cubs remain in the den
• Late February to early April: family emerges, then heads out to the sea ice

Cubs are born blind and tiny, with woolly hair. They weigh roughly five hundred to seven hundred grams. They nurse on rich, fatty milk. By the time they emerge from the den in spring, cubs weigh ten to fifteen kilograms.

Cubs stay with their mother for about two and a half years, learning hunting techniques, ice navigation and the social geography of the local population. Survival to independence is difficult: cubs can starve if their mothers cannot kill enough prey, and adult males kill cubs.

Most litters contain two cubs.

Movement, Range, and Swimming

Polar bears travel extraordinary distances. Polar bears may travel areas as small as 3,500 square kilometres to as large as 38,000 square kilometres in a year, depending on ice conditions, while drifting ice allows them to move further. Most of this movement is passive: bears ride drifting ice floes, letting currents carry them long distances, then correcting by swimming or walking when needed.

Travel data:

Metric	Value
Typical daily walking range	about 12 km (average)
Swim speed	up to 6 km/h
Longest recorded swim	687 km (continuous, 9 days)
Longest recorded dive	about 3 minutes
Annual range (reported extremes)	3,500 to 38,000 km^2

The 687-kilometre continuous swim, tracked in the Beaufort Sea off Alaska and reported in 2011, was recorded on a tagged female who also lost twenty-two per cent of her body mass and her yearling cub during the journey. Long-distance swimming is becoming more common as sea ice recedes further from coastlines, and research indicates that these swims are energetically expensive enough to reduce body condition and reproductive output.

Polar bears can dive for as long as three minutes. They swim with the front paws paddling and the hind legs trailing as rudders. Their fur traps enough air to provide buoyancy, and a thick layer of subcutaneous fat provides further insulation and flotation.

Populations and Subpopulations

Scientists divide the global polar bear population into at least eighteen recognised subpopulations distributed across the circumpolar Arctic. These subpopulations interbreed only at their margins and exhibit measurable genetic and behavioural differences.

Geographic distribution by country:

Country	Key subpopulations
Canada	Western Hudson Bay, Foxe Basin, Lancaster Sound
Russia	Kara Sea, Laptev Sea, Chukchi Sea
United States	Chukchi Sea, Southern Beaufort Sea
Greenland	East Greenland, Baffin Bay
Norway	Barents Sea (Svalbard)

Subpopulation trends vary. Foxe Basin and Kane Basin appear stable or slightly increasing. Western Hudson Bay, Southern Hudson Bay and Southern Beaufort Sea are likely in decline. Several subpopulations in the Russian Arctic have insufficient monitoring to assess trends. Total global population is estimated at 22,000 to 31,000 individuals as of the most recent assessments.

Conservation Status and Threats

The IUCN Red List classifies polar bears as Vulnerable . The species is listed on CITES Appendix II, which regulates international trade and protected under the 1973 Agreement on the Conservation of Polar Bears signed by the five range states.

Primary threats:

• Sea ice loss. The September minimum extent of Arctic sea ice has declined by about thirteen per cent per decade relative to the 1981-2010 average, according to NOAA figures cited by NASA. Reduced ice means less time hunting seals, longer forced fasts, lower body condition, lower reproductive output and higher cub mortality. A 2008 study predicted that two-thirds of the world's polar bears may disappear by 2050, while a 2016 study projected a likely decline of more than 30 percent over three generations.
• Pollution. Polar bears sit at the top of a marine food chain that concentrates persistent organic pollutants, including PCBs, PFAS, mercury and flame retardants, in fatty tissue. Cubs receive a toxic load through milk. These compounds affect immune function, hormone balance and reproduction.
• Oil and gas development. Spills in Arctic seas are difficult to contain and cleanup under ice. Chronic noise and infrastructure disturbance can displace bears from key feeding or denning areas.
• Shipping. Reduced ice has opened new shipping routes through polar bear habitat, bringing collision risk, noise and new pollution sources.
• Human-bear conflict. Bears drawn into settlements by hunger are increasingly killed in defence-of-life-and-property incidents. Communities in Churchill and parts of Nunavut run active bear patrols.
• Hybridisation. Warming drives grizzly bears northward into polar bear habitat. Hybridisation is genetically interesting but may dilute polar bear-specific adaptations over generations.

Several conservation measures exist but none address the primary driver. Hunting is regulated by quotas under national laws and Indigenous co-management agreements. Protected areas cover a meaningful fraction of denning habitat. Captive breeding has produced animals for display but has no realistic role in species recovery.

The long-term outlook for polar bears depends almost entirely on Arctic sea ice trajectory. Sea ice depends on atmospheric carbon dioxide. No local conservation measure can compensate for global greenhouse gas emissions.

Polar Bears and Humans

Polar bears have anchored Inuit culture for thousands of years. Hunters treated the bear with spiritual respect as an equal, and subsistence rules contained ecological wisdom that western science did not grasp for centuries. Today, modern Inuit communities hold hunting rights protected by national and international agreements, with quota setting done collaboratively between governments and hunters' organizations.

Among non-Indigenous people, the polar bear's reputation is mixed. Early Arctic explorers feared the species, not without reason. Documented attacks are rare (73 recorded between 1870 and 2014, with 20 fatalities), but in most of them the bear behaved as a predator, and the attackers were most often adult males in poor nutritional condition.

Protocols at Arctic field stations and communities include firearm training, rubber slugs, flares, bear fences and strict rules about food storage.

Polar bear tourism generates meaningful income for communities like Churchill, Manitoba, where purpose-built vehicles carry visitors to observe bears gathering during the autumn ice freeze-up. Done well, tourism provides economic incentives for conservation. Done badly, it habituates bears to human presence with dangerous consequences.