Are polar bears marine mammals? The quick answer
Yes. Polar bears (Ursus maritimus) are classified as marine mammals under the United States Marine Mammal Protection Act of 1972 and by the International Union for Conservation of Nature. The classification reflects their dependence on the ocean for food and their extreme swimming ability. The verified long-distance swim record is 687 kilometres over 9 days, set by a GPS-collared female in the Beaufort Sea in 2008. She lost 22 per cent of her body mass, and her yearling cub did not survive. Routine multi-day swims of 200 to 400 km are now documented every summer as Arctic sea ice retreats.
A bear built by the sea
The Latin name Ursus maritimus translates literally as sea bear, and the name is not poetic. Every part of polar bear physiology, from the partially webbed forepaws to the elongated skull to the 11 cm blubber jacket, was shaped by selection pressure from a marine environment. Polar bears diverged from brown bears between 150,000 and 500,000 years ago, and during that window they became the only bears on Earth that hunt on the ocean, den on the ocean, and depend on the ocean for more than 90 per cent of their calories.
This article explains why biologists and lawmakers classify polar bears as marine mammals rather than terrestrial predators, walks through the verified long-distance swim records, and looks at the biomechanics and metabolic cost of Arctic swimming. The short version is that polar bears are not simply bears that happen to swim. They are an obligate marine species that cannot survive without sea ice and open ocean. For the full species profile see the polar bear species hub.
The legal and scientific case for marine mammal status
US Marine Mammal Protection Act, 1972
The US Marine Mammal Protection Act (MMPA) is the statutory backbone of marine mammal conservation in American waters. The act explicitly lists polar bears alongside whales, dolphins, porpoises, seals, sea lions, sea otters, manatees, and dugongs. The criterion for inclusion is not anatomical. It is ecological: the species must be morphologically adapted to the marine environment or primarily inhabit the marine environment. Polar bears meet both tests.
Under the MMPA, the US Fish and Wildlife Service (not the Forest Service) has jurisdiction over polar bears. Hunting, harassment, and import of polar bear parts are regulated under marine mammal rules, not game animal rules. This is why a grizzly bear hunter in Alaska operates under one legal regime and a polar bear subsistence hunter operates under another.
IUCN Marine Mammal Red List
The IUCN Marine Mammal Specialist Group lists 129 extant species. Polar bears are on the list. The current Red List status is Vulnerable, with sea ice loss cited as the primary threat. The listing places polar bears in the same conservation framework as narwhals, belugas, and walruses, which share the same Arctic habitat. For context on how other Arctic marine mammals are tracked, see the narwhal profile in our marine life section.
Why the classification matters
Marine mammal status is not a label. It unlocks specific conservation tools:
- International trade restrictions under CITES Appendix II.
- Habitat protection obligations under domestic marine mammal laws.
- Legal standing for climate-based listing decisions, as in the 2008 US Endangered Species Act decision that cited sea ice loss as a primary threat.
If polar bears were classified as terrestrial mammals, none of these protections would apply automatically. Read more about the conservation context in why polar bears are endangered and the population distribution in polar bear populations and where they live.
"Polar bears are fundamentally marine animals. Their body plan, their hunting strategy, their denning ecology, and their life history are all shaped by the sea ice. Treating them legally as anything other than marine mammals would ignore the biology." -- Dr. Steven Amstrup, Chief Scientist, Polar Bears International and former lead polar bear researcher, US Geological Survey
The 687 km swim: what Durner actually measured
The single most cited piece of polar bear swimming science is Durner et al. 2011, published in Polar Biology. The study tracked a 220 kg adult female fitted with a GPS collar and a subcutaneous temperature logger. The bear was collared on the sea ice of the Southern Beaufort Sea in August 2008, at a time when the ice edge was receding north faster than usual.
The collar recorded a continuous swim of 687 kilometres (427 miles) over 232 hours, roughly 9 days and 16 hours, without any verified rest on solid ice. The bear entered the water on 27 August and reached a small ice patch on 6 September. She then walked another 1,800 km over ice and land in the following months.
The cost
When the bear was recaptured two months later her condition was catastrophic. Key measurements:
- Body mass loss: 22 per cent (from 226 kg to 177 kg).
- Cub mortality: her yearling cub, tracked alongside her at the start, was gone. The researchers presumed death by drowning or hypothermia.
- Core temperature: elevated throughout, suggesting sustained thermal stress even with blubber insulation.
- Fat score: dropped from a healthy 3 of 5 to 1 of 5.
The paper described the swim as "without any documented analogue in the scientific literature on bears." It is the longest voluntary non-stop swim of a non-cetacean land-adapted mammal on record.
"The nine-day swim was a revelation. We had assumed polar bears would not attempt such distances. The data showed they will, and that the physiological cost is severe. When we saw that she had lost her cub and 22 per cent of her body mass, it became clear this was not adaptive behaviour. It was a forced response to ice conditions." -- Dr. George M. Durner, research zoologist, US Geological Survey Alaska Science Center (lead author, Polar Biology, 2011)
It was not an isolated event
Between 2004 and 2012 the US Geological Survey Polar Bear Research Program documented more than 50 long-distance swims of over 50 km in the Southern Beaufort population alone. The table below summarises the most significant verified events.
Documented long-distance polar bear swims
| Year | Population | Distance | Duration | Notes |
|---|---|---|---|---|
| 2008 | Southern Beaufort | 687 km | 9 days, 16 h | Durner et al. 2011; 22% body mass lost; cub died |
| 2009 | Southern Beaufort | 404 km | 7 days | Adult female; no cub recorded |
| 2010 | Southern Beaufort | 354 km | 6 days | Subadult male |
| 2011 | Chukchi Sea | 220 km | 4 days | Adult female with surviving cub |
| 2012 | Hudson Bay | 198 km | 3 days | Subadult, rapid ice retreat |
| 2014 | Southern Beaufort | 312 km | 5 days | Adult female; rapid temperature rise documented |
| 2016 | Chukchi Sea | 267 km | 4 days | Adult female; dietary shift to walrus carcasses post-swim |
| 2018 | Beaufort | 445 km | 8 days | Adult female; cub loss confirmed |
All swims above 150 km were undertaken between late July and early October, the period of minimum Arctic sea ice extent. None of the swims would have been necessary in the ice regime of the 1980s.
The biomechanics: how a bear swims for a week
Polar bears are technically quadrupedal swimmers, but in practice they swim almost entirely with the front paws. The forelimbs are broader than the hind limbs, with slight webbing between the digits and a paw diameter that can exceed 30 cm. Underwater video from the Alaska SeaLife Center and Tromsø's Polaria shows the front paws moving in a doggy-paddle cadence that closely resembles the action of a draught horse trotting underwater.
What each body part does
- Front paws: primary propulsion. Each paw sweeps downward and backward, generating thrust on both the power stroke and, to a lesser extent, the recovery stroke.
- Hind paws: steering and trim. They trail behind the body and make small adjustments to direction. They provide very little forward thrust. This is the opposite of otters, seals, or beavers, where hind propulsion dominates.
- Head and neck: held above the surface at a roughly 30 degree angle. This posture lets the bear breathe continuously without yawing the torso.
- Body core: held almost horizontal. The buoyant blubber keeps the hindquarters near the surface without active effort.
- Tail: negligible hydrodynamic role.
Swimming speeds
| Mode | Speed | Notes |
|---|---|---|
| Burst swim (seconds) | up to 10 km/h | Short chases, surprise attacks on seals in water |
| Sustained cruise | 6 to 8 km/h | Typical long-distance pace |
| Multi-day average (Durner female) | ~2.9 km/h | 687 km / 232 h |
| Against wind and chop | 3 to 5 km/h | Documented in Hudson Bay telemetry |
The drop from a 6 to 8 km/h nominal cruise to a 2.9 km/h average across the 687 km swim reflects the reality of Arctic conditions: currents, waves, ice-navigation detours, and fatigue. Polar bears do not swim in straight lines for nine days. They zig-zag, rest briefly on small floes where they exist, and push through chop that halves their forward progress.
Swimming speed compared to other marine mammals
| Species | Cruise speed | Burst speed | Propulsion type |
|---|---|---|---|
| Polar bear | 6 to 8 km/h | 10 km/h | Forelimb paddling |
| Harbour seal | 9 to 14 km/h | 19 km/h | Pelvic undulation |
| Walrus | 7 km/h | 35 km/h | Rear-flipper sculling |
| Beluga whale | 9 to 14 km/h | 22 km/h | Fluke oscillation |
| Narwhal | 6 to 10 km/h | 24 km/h | Fluke oscillation |
| Orca | 13 to 16 km/h | 54 km/h | Fluke oscillation |
| Sea otter | 5 to 9 km/h | 15 km/h | Hind flipper + body flexion |
Polar bears are not the fastest marine mammal, not by a long way. What makes them exceptional is endurance at low speed in near-freezing water, a niche no other large mammal occupies. A beluga can outrun a polar bear for a kilometre. A polar bear can out-swim a beluga across a week of open ocean, because the bear is effectively a floating insulated fuel tank with legs.
Staying warm in the water: the blubber and fur system
The Arctic Ocean in August hovers around -1 to +4 degrees Celsius. Any unprotected mammal entering that water begins losing core heat within minutes. Polar bears manage sustained immersion through a three-layer thermoregulatory system.
Layer 1: guard hairs
The outer coat consists of hollow, translucent guard hairs 5 to 15 cm long. In water the guard hairs collapse against the body and trap a thin film of air that reduces convective heat loss. Once the fur is fully wet, the guard hairs also shed water efficiently when the bear reaches land or ice, shaking dry in seconds. The fur alone does not keep the bear warm in water. It buys time and preserves some insulation at the boundary. For the full optics and structure of the coat see polar bear fur and black skin explained.
Layer 2: subcutaneous blubber
Beneath the skin sits 4 to 11 centimetres of blubber, thickest on the hindquarters and thinnest on the head and forelimbs. Blubber is the workhorse of polar bear thermoregulation in water. It is a combined insulation jacket and metabolic fuel store. Lipid content runs above 80 per cent, and every kilogram of blubber contains roughly 9,000 kilocalories of burnable energy. A healthy Beaufort Sea female carrying 70 to 90 kg of blubber has an energy reserve comparable to a human sprinting continuously for 30 days.
Layer 3: vasomotor control
When the bear enters cold water, surface capillaries in the limbs and ventral skin constrict sharply, redirecting blood flow to the core. Counter-current heat exchange in the limbs recaptures heat from venous blood returning from the extremities. This is the same principle seals and whales use, and it is why polar bears can swim with their cold-adapted front paws for days without frostbite.
"The blubber layer is doing two jobs at once. It insulates the bear in water where fur fails, and it fuels the metabolism that keeps the bear warm. Every hour of swimming burns roughly 0.5 per cent of total body fat. Nine days of continuous swimming is at the ragged edge of what the system can support." -- Dr. Anthony M. Pagano, research wildlife biologist, US Geological Survey Alaska Science Center
The metabolic cost: Pagano's revision
In 2018 Anthony Pagano and colleagues published a landmark paper in Science measuring the field metabolic rate of free-ranging polar bears using doubly labelled water and accelerometer loggers. The headline finding: polar bears burn energy 25 to 50 per cent faster than previous models assumed, particularly during the summer ice-free season.
The implications for long-distance swimming are severe:
- A resting adult female burns roughly 12,325 kcal per day on ice.
- A bear actively swimming burns 1.5 to 2.5 times that baseline, depending on water temperature and pace.
- A 9-day continuous swim at cruise pace therefore burns 250,000 to 300,000 kcal, equivalent to roughly 30 kg of blubber.
- Bears enter the water with a finite fuel tank. Once the tank runs dry, organ failure and drowning follow within hours.
This is why the Durner female arrived emaciated: her fuel budget barely covered the distance. A smaller bear or a bear in poorer starting condition would not have survived the same swim.
The Pagano revision also explains why bears are increasingly coming ashore in late summer with catastrophic weight loss. Swimming in search of ice is a ruinous energy strategy, and the Arctic is forcing more bears to use it every summer.
Diving and breath-holding
Polar bears are not deep divers. They are opportunistic surface hunters that occasionally submerge. Verified measurements:
- Maximum recorded dive depth: approximately 6 metres. Most hunting dives stay above 3 metres.
- Maximum breath-hold: around 3 minutes, documented during surprise attacks on seals basking on ice edges.
- Typical submergence during seal hunts: 30 to 90 seconds.
Compare this to a ringed seal, which routinely dives to 90 metres and holds its breath for 20 minutes, or a narwhal, which dives past 1,500 metres. Polar bears cannot catch seals underwater in a stern chase. They rely on stealth approaches: swimming slowly on the surface with only the nostrils exposed, then bursting forward when close enough to strike. For the full predatory toolkit see what do polar bears eat.
Cubs cannot do what their mothers do
Cub mortality during long swims is the single darkest consequence of sea ice loss. Cubs under two years old have:
- Thinner blubber: typically 1 to 3 cm, not the 8 to 11 cm of an adult female.
- Smaller paw surface area relative to body mass, reducing propulsion efficiency.
- Higher surface-to-volume ratio, which accelerates heat loss.
- Lower fat stores, depleted within a day or two of continuous swimming.
- Weaker swimming stamina, typically less than 24 hours of sustained pace.
A 2011 USGS survey of tagged females with cubs in the Southern Beaufort recorded 45 per cent cub mortality during swims longer than 50 km, compared with 18 per cent in non-swimming mother-cub groups. Cubs drown, die of hypothermia, or are abandoned when they fall behind. For the denning context that sets cubs up for these later challenges, see polar bear cubs denning and survival.
"We now have clear evidence that climate-forced swimming is a significant source of cub mortality. It is not a rare or anecdotal event. In some years in the Beaufort it is the leading cause of cub death before age two." -- Journal of Experimental Biology, Pagano et al., on field physiology of polar bears, 2018
The wider picture: marine mammal, climate-forced
Arctic sea ice is the polar bear's hunting platform, migration corridor, and maternity ward. Every major life-history event outside the den happens on the ice:
- Seal hunting: ambush at breathing holes and edge-of-ice encounters.
- Mating: late March through May, on drifting spring ice.
- Travel: bears ride floes for hundreds of kilometres, conserving energy.
- Thermoregulation: ice provides a cold, stable surface that prevents overheating after exertion.
Ice loss degrades all four functions. As the ice retreats farther north in summer, the distance between productive hunting grounds and safe denning habitat expands. Swimming fills the gap, but at a metabolic cost the species did not evolve to pay.
This is why the US Endangered Species Act listing decision of 2008 accepted sea ice decline as a sufficient basis for a threatened status. It is also why 19 of the 19 recognised polar bear subpopulations are now monitored with satellite telemetry. For broader context on the Arctic food web, visit the marine mammals section on the main site.
What swimming tells us about the species
Three reasons polar bears count as marine mammals, distilled:
- Diet: more than 90 per cent of lifetime calories come from marine prey, almost all of it seal.
- Habitat: the ice-covered sea and its margins are the primary living space.
- Locomotion: the only non-cetacean land-adapted mammal capable of voluntary week-long open-ocean swims.
The 687 km record is the most vivid evidence of the third point, but it should not be read as a triumph. It is closer to a distress signal. Polar bears swim that far because they must, not because they want to, and the energy budget barely covers the trip. As summer sea ice continues to contract, the distance between hunting grounds will keep growing, and the margin for error on those swims will keep shrinking.
Polar bears are marine mammals not because a 1972 statute said so, but because the Arctic Ocean made them that way over half a million years of selection. The statute simply caught up with the biology. The climate is now pulling that biology apart faster than any law can protect it.
Further reading on the site
- Polar bear species hub -- the full profile of Ursus maritimus.
- How fast can a polar bear run -- sprint speed, swimming speed, and overheating limits.
- Why polar bears are endangered -- sea ice decline and the IUCN Vulnerable listing.
- Polar bear populations and where they live -- 19 subpopulations and their status.
- What do polar bears eat -- seals, walrus, and the hypercarnivore diet.
- Polar bear fur and black skin explained -- the thermal system in detail.
- Narwhal, unicorn of the sea -- fellow Arctic marine mammal.
- Marine mammals overview -- the broader group polar bears belong to.
Explore other fact-driven reference sites we read for primary-source rigour: What's Your IQ, When Notes Fly, and Evolang.
References
- Durner, G. M., Whiteman, J. P., Harlow, H. J., Amstrup, S. C., Regehr, E. V., & Ben-David, M. (2011). Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat. Polar Biology, 34(7), 975-984. https://doi.org/10.1007/s00300-010-0953-2
- Pagano, A. M., Durner, G. M., Rode, K. D., Atwood, T. C., Atkinson, S. N., Peacock, E., Costa, D. P., Owen, M. A., & Williams, T. M. (2018). High-energy, high-fat lifestyle challenges an Arctic apex predator, the polar bear. Science, 359(6375), 568-572. https://doi.org/10.1126/science.aan8677
- Pagano, A. M., Rode, K. D., Cutting, A., Owen, M. A., Jensen, S., Ware, J. V., Robbins, C. T., Durner, G. M., Atwood, T. C., Obbard, M. E., Middel, K. R., Thiemann, G. W., & Williams, T. M. (2017). Using tri-axial accelerometers to identify wild polar bear behaviors. Endangered Species Research, 32, 19-33. https://doi.org/10.3354/esr00779
- Monnett, C., & Gleason, J. S. (2006). Observations of mortality associated with extended open-water swimming by polar bears in the Alaskan Beaufort Sea. Polar Biology, 29(8), 681-687. https://doi.org/10.1007/s00300-005-0105-2
- Pilfold, N. W., McCall, A., Derocher, A. E., Lunn, N. J., & Richardson, E. (2017). Migratory response of polar bears to sea ice loss: to swim or not to swim. Ecography, 40(1), 189-199. https://doi.org/10.1111/ecog.02109
- Whiteman, J. P., Harlow, H. J., Durner, G. M., Anderson-Sprecher, R., Albeke, S. E., Regehr, E. V., Amstrup, S. C., & Ben-David, M. (2015). Summer declines in activity and body temperature offer polar bears limited energy savings. Science, 349(6245), 295-298. https://doi.org/10.1126/science.aaa8623
- Stirling, I., & Derocher, A. E. (2012). Effects of climate warming on polar bears: a review of the evidence. Global Change Biology, 18(9), 2694-2706. https://doi.org/10.1111/j.1365-2486.2012.02753.x
- Amstrup, S. C., Marcot, B. G., & Douglas, D. C. (2008). A Bayesian network modeling approach to forecasting the 21st century worldwide status of polar bears. Geophysical Monograph Series, 180, 213-268. https://doi.org/10.1029/180GM14