Polar bear
The polar bear (Ursus maritimus) is the largest terrestrial carnivoran species, specialized for life on Arctic sea ice where it hunts marine mammals as its primary prey.[1] Native exclusively to the circumpolar Arctic, it depends on seasonally varying pack ice over shallow continental shelves to access ringed seals (Pusa hispida) and bearded seals (Erignathus barbatus), consuming up to 45 kg of blubber in a single meal to sustain its high-energy needs during periods of fasting.[2] Adult males average 400–600 kg in weight and up to 3 m in length, with females smaller at 150–250 kg; their adaptations include dense, insulating fur, a thick blubber layer for buoyancy and thermal regulation, enlarged paws for propulsion on ice and water, and physiological tolerances for prolonged swimming over 100 km.[3][4] As the Arctic's apex predator, polar bears exhibit low reproductive rates, with females denning on land or ice to birth one to three cubs every two to three years after delayed implantation.[1] Despite classification as vulnerable by the IUCN Polar Bear Specialist Group owing to projected sea ice reductions impacting hunting access, empirical surveys indicate a global population of approximately 22,000–31,000 individuals across 19 subpopulations, with several—such as in the Chukchi Sea—demonstrating stability or growth rates near maximum intrinsic levels, challenging earlier forecasts of imminent collapse.[5][6] These trends reflect the species' resilience amid environmental variability, though ongoing monitoring underscores uncertainties in long-term viability tied to ice dynamics rather than immediate empirical declines.[7]Taxonomy and Evolution
Naming and Etymology
The scientific binomial Ursus maritimus for the polar bear was established by British explorer Constantine John Phipps in 1774, based on specimens encountered during his expedition toward the North Pole, marking the first formal description of the species as distinct from other bears.[8] [9] The name combines Latin roots: ursus, meaning "bear," and maritimus, meaning "maritime" or "of the sea," chosen to highlight the animal's affinity for ocean-adjacent habitats, extensive swimming capabilities, and reliance on sea ice for hunting.[9] [1] An alternative scientific name, Thalarctos maritimus, incorporating Greek thalassa ("sea") and arktos ("bear"), was proposed in the 19th century but was abandoned by polar bear researchers in 1971 in favor of retaining Phipps' original designation for consistency with historical taxonomy.[9] Common English nomenclature as "polar bear" derives from its exclusive Arctic distribution near the North Pole, while indigenous Arctic peoples employ terms reflecting ecological and cultural reverence, such as the Inuit nanuq or nanook, denoting an animal of great respect and power.[10] [8] Other regional designations include the Alaskan Inuit ah tik tok ("those that go down to the sea"), Norwegian isbjørn ("ice bear"), and Russian "white bear," each underscoring observed traits like coloration, habitat, or behavior.[11] [10]Phylogenetic Classification
The polar bear (Ursus maritimus) is classified in the genus Ursus of the subfamily Ursinae within the family Ursidae, order Carnivora, infraclass Eutheria, class Mammalia, phylum Chordata, kingdom Animalia.[12] This placement reflects its membership in the bear family, which comprises eight extant species divided into two subfamilies: the basal Tremarctinae (containing the spectacled bear, Tremarctos ornatus) and the more derived Ursinae (encompassing all other bears).[13] Within Ursinae, U. maritimus forms a clade with the brown bear (Ursus arctos), supported by nuclear and mitochondrial DNA analyses that position this pair as sister taxa relative to other ursines such as the American black bear (Ursus americanus), Asiatic black bear (Ursus thibetanus), sloth bear (Melursus ursinus), and sun bear (Helarctos malayanus).[14][15] Phylogenomic studies indicate that the divergence between polar and brown bears occurred relatively recently, with estimates ranging from 343,000 to 1.6 million years ago depending on the genomic markers and modeling approaches used; mitochondrial DNA often yields older dates (up to 1.3–1.6 million years ago), while whole-genome analyses incorporating admixture suggest younger splits around 479,000–343,000 years ago.[16][17] This timeline aligns with Pleistocene glacial cycles, during which ancestral brown bear populations likely adapted to Arctic marine environments, leading to the polar bear's specialization.[18] However, the phylogeny is complicated by recurrent gene flow: polar bears exhibit introgression from brown bears (accounting for ~8–10% of their genome in some populations), and vice versa, with evidence of multiple hybridization events over the last 100,000–600,000 years that blur strict monophyly in certain datasets.[19][20] In broader Ursidae phylogeny, the Ursinae radiation from a common ancestor occurred around 6.3 million years ago in the Pliocene, with the sun bear branching as sister to the U. arctos–U. maritimus clade among the most recent divergences.[11] Despite hybridization, U. maritimus maintains distinct morphological, ecological, and genetic signatures justifying its species status, including fixed adaptations for sea-ice hunting absent in brown bears; claims of polar bears being merely an ecotype of brown bears lack support from comprehensive genomic evidence showing independent evolutionary trajectories post-divergence.[21][22]Fossil Record and Evolutionary Origins
The polar bear (Ursus maritimus) is phylogenetically classified within the genus Ursus, forming a sister species to the brown bear (Ursus arctos), with both diverging from a common ancestor in the bear family Ursidae.[19] Genetic analyses indicate that the polar bear lineage separated from brown bears, with divergence time estimates varying widely from approximately 340,000 years ago to 4–5 million years ago, reflecting ongoing gene flow and admixture events that complicate precise dating.[23] Ancient DNA from a 130,000–115,000-year-old polar bear specimen positions it near the branching point between modern polar and brown bears, supporting a relatively recent evolutionary split followed by independent adaptations, such as those for a marine diet, which occurred within about 20,000 years.[24] [25] Fossil evidence for polar bears is sparse compared to their brown bear relatives, with no confirmed remains predating the late Pleistocene. The oldest verified polar bear fossils consist of a jawbone from Svalbard, Norway, dated to 130,000–110,000 years ago via geological context and radiocarbon methods, doubling the age of previously known specimens.[26] [27] This find aligns with genetic data suggesting rapid morphological evolution post-divergence, including dental adaptations for seal predation, but contradicts deeper genetic divergence estimates by indicating that distinct polar bear morphology emerged no earlier than the Eemian interglacial.[25] Subsequent Holocene records show a hiatus in fossils from 8,000–6,000 years ago, resuming with remains dated to about 5,700 years ago, consistent with post-glacial recolonization of Arctic habitats.[28] Hybridization has played a significant role in polar bear evolution, with genomic evidence revealing recurrent admixture from brown bears into polar bear populations, particularly during glacial periods, which introduced adaptive alleles for fat metabolism and insulation.[29] This gene flow, peaking in the last ice age, explains discrepancies between mitochondrial DNA (suggesting an Irish-origin matriline predating the last glacial maximum) and nuclear genomes showing multiple introgression events.[30] Such interbreeding underscores that polar bears maintain genetic cohesion despite occasional hybridization, with no fixed derived alleles in key adaptive genes arising de novo in the Pleistocene fossil record.[31] Overall, the combined fossil and genetic data portray polar bears as a specialized Arctic lineage arising from brown bear stock through isolation-driven adaptations amid fluctuating Pleistocene climates, rather than a deeply ancient independent clade.[32]Physical Characteristics
External Morphology
Polar bears (Ursus maritimus) exhibit pronounced sexual dimorphism, with adult males significantly larger than females, making them the largest extant species of bear and carnivoran land mammal.[33] Adult males typically measure 2.0–2.5 m in body length, stand 1.3–1.6 m at the shoulder, and weigh 350–650 kg, though exceptional individuals reach up to 800 kg or more.[34] [35] Adult females are smaller, with body lengths of 1.8–2.1 m, shoulder heights around 1.0–1.2 m, and weights of 150–290 kg.[35] [3] The body is stocky and elongated, adapted for terrestrial and aquatic locomotion, featuring a long neck, relatively small head with a pronounced "Roman" nose, short rounded ears, and a short tail measuring 7–12 cm.[36] [34] The overall build supports efficient swimming and walking on ice, with a streamlined profile that minimizes drag in water.[3] Fur consists of a dense undercoat and longer guard hairs, totaling up to 11.5 cm in length on the body, which appear white due to light scattering despite being translucent and lacking pigment; the underlying skin is black to absorb heat.[1] Coloration varies from pure white in winter to yellowish or brownish in summer due to algae or staining, providing camouflage on snow and ice.[37] Paws are oversized relative to body size, reaching 30 cm in diameter, with partially webbed forepaws for propulsion in water and elongated hind paws functioning as rudders; five non-retractable claws per paw, 5–7 cm long, aid in gripping ice and prey.[36] [34] Soles feature small dermal papillae for traction on slippery surfaces.[34] Small ears and tail reduce surface area for heat loss in the Arctic environment.[35]Internal Physiology and Adaptations
Polar bears maintain a core body temperature of 37°C (98.6°F) through integrated physiological mechanisms that conserve heat in subzero environments while dissipating excess during exertion. A subcutaneous blubber layer up to 11 cm (4.3 in.) thick acts as primary insulation, supplemented by vasoconstriction that minimizes peripheral blood flow to extremities, thereby reducing conductive heat loss. During prolonged swimming, this vasoconstriction allows intra-abdominal temperatures to drop rapidly—at rates up to 5°C per hour—to as low as 22°C, reflecting adaptive hypothermy without compromising viability.[38][4][4] Their circulatory system incorporates countercurrent heat exchange in the limbs and nasal passages, where arterial blood is preheated by cooler venous return, preserving core warmth at the expense of peripheral cooling. Genetic analyses reveal positive selection on cardiovascular genes, enhancing vascular function and preventing thrombosis despite chronic exposure to high circulating lipids from a fat-dominant diet. These adaptations enable sustained activity in air temperatures as low as -50°C (-58°F) without frostbite, though they also predispose bears to overheating during pursuits exceeding 10 km/h (6.2 mph), prompting panting and behavioral pauses.[39][16][40] Metabolically, polar bears exhibit a basal rate 20-30% lower than that of brown bears (Ursus arctos), facilitating extended fasting—up to 180 days in pregnant females—via efficient protein-sparing and reliance on lipid reserves exceeding 50% of body mass. During seasonal land periods without sea ice, they downregulate activity and enter a hibernation-like state, reducing daily energy expenditure to 1.0-1.5 times basal levels while maintaining muscle mass through urea recycling. This contrasts with true hibernators, as polar bears avoid profound torpor, instead sustaining alertness for den defense.[41][42][43] Digestive adaptations support a hyperlipidic diet comprising 80-90% fat from seals, with assimilation efficiencies of 97% for lipids and 84% for proteins, far exceeding those of omnivorous ursids. Genomic expansions in fatty acid oxidation genes (e.g., APOB, SLC27A1) and aortic function enable rapid clearance of triglycerides, averting atherosclerosis despite plasma levels that would induce pathology in humans. Pregnant females further adapt by mobilizing these reserves for lactation, producing milk with 30-40% fat content to provision cubs during 3-4 months of nursing in dens. Bone physiology counters fasting-induced resorption: pre-denning hyperphagia elevates formation markers, maintaining density without the osteoclast activation seen in brown bears.[44][45][45]Habitat and Distribution
Geographic Range
The polar bear (Ursus maritimus) exhibits a circumpolar distribution confined to the Arctic Circle, encompassing marine and coastal habitats surrounding the North Pole where annual sea ice persists. Its range is limited by the southern extent of perennial sea ice, spanning approximately 23 million square kilometers of land and ocean across five polar nations: Canada, Russia, Greenland (under Danish sovereignty), Norway (primarily Svalbard archipelago), and the United States (Alaska).[46][47][1] Polar bears occupy diverse Arctic ecoregions defined by sea ice dynamics, including divergent ice zones in the central polar basin (e.g., northern Canada to Russia), convergent ice areas near the North Pole, seasonal ice habitats along southern peripheries like Hudson Bay, and the Canadian Arctic Archipelago's stable ice cover. The International Union for Conservation of Nature (IUCN) Polar Bear Specialist Group delineates 20 discrete subpopulations within this range, reflecting adaptations to local ice conditions and prey availability; these include the Chukchi Sea (Alaska-Russia), Southern Beaufort Sea (Alaska-Canada), Baffin Bay (Canada-Greenland), and Barents Sea (Norway-Russia). About 60% of the global population resides in or shares Canadian territories.[48][49][50] Subpopulation boundaries are informed by satellite telemetry, genetic studies, and mark-recapture data, though some overlap occurs due to occasional movements; for instance, the Kane Basin subpopulation straddles Greenland and Canada, while the Laptev Sea group is primarily Russian. Vagrants rarely extend south of 60°N latitude, with historical records limited to Iceland and Newfoundland but no established breeding populations outside core Arctic seas.[51][52]Preferred Environments
Polar bears (Ursus maritimus) primarily prefer habitats consisting of annual sea ice over continental shelf waters in the Arctic, where high densities of ringed seals (Pusa hispida) and bearded seals (Erignathus barbatus), their main prey, are accessible for hunting from stable ice platforms.[53] The quality of this habitat is determined by sea ice type, extent, and proximity to productive marine ecosystems, with bears selecting areas of first-year ice at concentrations of 50-80% during active foraging periods from late fall through late spring.[54] [55] Empirical resource selection analyses indicate that polar bears avoid thick multi-year ice in deep-water basins, which supports lower prey densities, favoring instead dynamic ice edges, leads, and polynyas over shallower depths less than 300 meters that enhance seal haul-out opportunities.[56] Seasonal variations show strongest selection for sea ice in winter and spring, when bears aggregate near breathing holes and lairs of seals, while summer preferences shift toward remaining ice floes or coastal land if ice retreats offshore.[57] In regions like the Chukchi Sea, habitat selection patterns have remained consistent despite sea ice declines, prioritizing ice-covered shelf areas for energy-efficient hunting.[58] For reproduction, pregnant females seek stable snow-covered sea ice or coastal terrestrial sites for maternity dens, with preferences varying by subpopulation; for instance, in the Southern Beaufort Sea, a shift from sea ice to land denning has occurred as offshore ice destabilizes earlier.[59] Overall, polar bears exhibit limited adaptation to ice-free periods, relying on fat reserves accumulated during ice-abundant seasons, and avoid prolonged terrestrial habitats except when forced by ice melt.[60]Responses to Arctic Environmental Shifts
Polar bears exhibit behavioral flexibility in response to diminishing Arctic sea ice, primarily by extending fasting periods on land during ice-free seasons, relying on accumulated fat reserves from prior hunts rather than continuous access to ringed and bearded seals on sea ice platforms.[61] In regions like the Chukchi Sea, bears have increasingly utilized land-fast ice and coastal areas for hunting when offshore ice retreats earlier, maintaining relatively stable body condition compared to more ice-dependent subpopulations.[62] Observations indicate longer swimming distances, sometimes exceeding 60 miles between ice floes, which elevates energy expenditure and drowning risks, particularly for subadults and females with cubs.[63] Some polar bears have adapted by shifting to alternative habitats, such as glacier ice in Svalbard during low-ice summers, providing temporary hunting platforms inaccessible to seals but allowing opportunistic predation.[64] High winds and storm events can drive bears landward from degraded ice edges, prompting scavenging of bird eggs, berries, or human refuse, though these terrestrial foods yield insufficient calories to offset marine mammal energy deficits, leading to weight loss and reduced reproductive success in affected individuals.[65][4] Subpopulation responses vary markedly; southern groups like Western Hudson Bay experience earlier ice breakup, correlating with lower cub survival rates—declining from 1.0 in the 1980s to 0.7 by 2015—due to maternal denning disruptions and extended land confinement.[66] In contrast, divergent subpopulations such as those in the Arctic Basin, with access to persistent multi-year ice, show minimal impacts, sustaining densities of 1-2 bears per 1,000 km² without evident declines.[67] Overall, while sea ice extent has decreased by approximately 13% per decade since 1979, global polar bear numbers have remained stable at 22,000-31,000 since the 1973 hunting moratorium, contradicting early models forecasting two-thirds losses by 2050 from uniform ice projections that overlooked regional variability and bear plasticity.[68][50][7]Behavioral Ecology
Social Organization
Polar bears (Ursus maritimus) are predominantly solitary, with social interactions limited to specific contexts such as familial bonds, brief mating associations, and temporary gatherings at concentrated resources.[69] [70] Adult individuals maintain large home ranges that rarely overlap except opportunistically, reflecting their adaptation to a patchy, ice-based environment where sustained group living offers minimal advantages over independent foraging.[69] The principal enduring social unit comprises a female and her dependent offspring, typically one to three cubs born in December or January within a maternity den. Cubs remain with the mother for about 2.5 years, during which they acquire hunting, swimming, and navigational skills through observation and imitation, fostering independence before dispersal.[69] Females exhibit high parental investment, grooming and protecting cubs vigorously, though adult males may infanticide cubs to bring females into estrus, prompting defensive responses from mothers.[69] Subadult bears, aged 2-5 years post-dispersal, often wander solitarily while establishing foraging proficiency.[70] During the breeding season from March to June, males seek receptive females via scent trails and vocalizations, forming transient pairs that copulate multiple times over 1-2 weeks before parting; males do not provide post-mating care, and females den alone for gestation.[69] [70] Intense male-male competition for mates involves displays of size and strength, with fights causing injuries like lacerations or broken bones in up to 20% of encounters observed in some subpopulations.[69] Aggregations of 10-100 individuals occur sporadically at predictable food bonanzas, such as bowhead whale carcasses from Inuit harvests (yielding up to 20-30 tons of remains annually in areas like Hudson Bay) or historical dumpsites, particularly during ice-free periods when bears concentrate on shore.[70] These gatherings reveal a fluid dominance hierarchy prioritizing access to carcasses, where the largest adult males (often exceeding 500 kg) feed first through intimidation or combat, displacing females, subadults, and smaller males; adult females with cubs may secure peripheral shares but face exclusion risks.[69] [71] Hierarchy enforcement relies on body size and prior residency rather than kinship, minimizing chronic alliances but enabling efficient resource partitioning amid scarcity.[69] Play-fighting among subadults and occasional non-kin affiliations among males (lasting days to weeks) suggest latent social flexibility, though these do not constitute stable groups.[70]Hunting Strategies and Diet
Polar bears (Ursus maritimus) maintain a diet dominated by marine mammals, with ringed seals (Pusa hispida) serving as the primary prey due to their abundance, predictable behavior at breathing holes, and high blubber content that supplies up to 90% of caloric intake from fat. [72] [73] Bearded seals (Erignathus barbatus) rank as the second most important prey, comprising significant biomass in stable ice conditions, while harp, hooded, and harbor seals contribute opportunistically based on regional availability. [44] [74] In quantitative assessments from stable isotope and fatty acid analyses, ringed seals accounted for approximately 45% of consumed biomass across subpopulations, underscoring their foundational role in sustaining polar bear energy demands exceeding 12,000 kcal daily for active adults. [74] [75] During periods of sea ice absence, such as summer in ice-free regions, bears shift to suboptimal alternatives including scavenging whale carcasses, consuming seabirds, eggs, and limited terrestrial vegetation like grasses and algae, which fail to meet hypercarnivorous nutritional requirements and result in fasting or body condition decline. [76] [72] Polar bears preferentially target blubber and skin over lean muscle, discarding much of the latter to scavengers, as the lipid-rich layer provides efficient energy storage adapted to intermittent feeding cycles where individuals may go weeks without meals. [77] Hunting relies on Arctic sea ice as a hunting platform, with still-hunting at seal breathing holes comprising the most energy-efficient and frequently observed strategy; bears position motionless for hours or days near cracks or lairs, lunging to seize surfacing prey with powerful forelimbs. [78] [79] Stalking entails slow, camouflaged approaches toward seals hauled out for thermoregulation, culminating in a short sprint to capture, though success diminishes with distance over 50 meters due to seals' alertness. [79] [80] Less common aquatic tactics involve submerged swims to ambush from below, exploiting seals' vulnerability at water edges, while scavenging supplements diet during low-ice periods but yields lower nutritional returns. [72] These ice-dependent methods yield variable success rates, typically 5-10% per attempt for still-hunting, highlighting the bears' specialization for ambush predation over sustained pursuit. [81]
Reproduction and Parental Care
Polar bears (Ursus maritimus) typically mate from late March to early May, with males searching widely for receptive females.[82] Following copulation, the fertilized embryo experiences delayed implantation, remaining unattached in the uterus for several months until conditions favor pregnancy, which occurs in autumn if the female has amassed sufficient body fat.[83] [82] This adaptation synchronizes birth with winter denning and spring prey availability, as implantation depends on the female's nutritional status.[82] Pregnant females, having fasted since summer, excavate maternity dens in snow or coastal terrestrial sites during October or November.[82] Cubs—usually 1 to 3, with twins most common—are born from late November to January after an active gestation of about 2 months, though the total period from mating spans roughly 8 months.[84] [83] Newborns weigh 450–700 grams (1–1.5 pounds), measure about 30 centimeters (12 inches) in length, and are blind, hairless or thinly furred, and toothless, relying entirely on maternal milk rich in fat for initial growth.[84] [83] Within the den, mothers cease eating and enter a state of voluntary fasting lasting 4–8 months, conserving energy while nursing cubs that grow rapidly to 9–14 kilograms (20–30 pounds) by the time the family emerges between late February and April.[85] [86] Emergence coincides with increasing seal pup availability on sea ice, prompting initial family foraging near the den site before dispersal.[87] Post-emergence, cubs remain dependent on their mother for 2–3 years, during which she teaches essential skills such as swimming, hunting ringed and bearded seals via still-hunting at breathing holes, and navigating sea ice.[83] [88] Weaning occurs around 2 years of age in most subpopulations, after which the female expels the cubs and resumes estrus, with interbirth intervals averaging 3 years (range 1–4) contingent on cub survival and maternal condition.[82] [89] Females reach sexual maturity at 4–6 years, but first reproduction often delays until 5–8 years due to the extended dependency period.[89]Movement Patterns and Mortality Factors
Polar bears demonstrate movement patterns that are predominantly dictated by the seasonal dynamics of Arctic sea ice, which serves as their primary platform for hunting seals. Satellite telemetry studies have recorded individual bears traversing distances exceeding 3,500 km across the polar basin, reflecting nomadic behaviors synchronized with ice formation in autumn and melt in spring.[90] Home range sizes, estimated via minimum convex polygons from radio-collared bears, average approximately 72,263 km², varying by sex, reproductive status, and regional ice conditions.[91] Adult males typically exhibit broader roaming patterns than females with cubs, who restrict movements during maternity denning on land from October to March, emerging in spring to follow receding ice edges for foraging.[92] Subadult bears, aged 2–5 years, undertake long-distance dispersal to avoid competition and establish territories, often swimming between ice floes or land masses over tens to hundreds of kilometers.[93] In regions with persistent sea ice, such as parts of the Chukchi Sea, bears maintain year-round ice-associated movements, whereas in seasonally ice-free areas like Hudson Bay, individuals migrate onshore during summer, reducing daily travel distances and concentrating in coastal zones.[94] Recent tracking data indicate that declining sea ice has enlarged home ranges for ice-dependent bears by 64% between 1999–2016 compared to 1986–1998, as animals expend more energy seeking prey over greater expanses.[95] Local sea ice concentration and persistence strongly predict migration timing, with bears advancing or delaying onshore movements in response to ice availability rather than fixed calendars.[96] Mortality factors for polar bears emphasize nutritional limitations over predation, as adults face no significant natural enemies and primarily succumb to starvation from failed hunts or extended fasting periods without access to seals.[97] Subadults experience the highest starvation rates due to hunting inexperience and displacement from kills by dominant adults, while cubs perish mainly from maternal malnutrition or abandonment if nursing mothers lack sufficient fat reserves.[98][99] Infanticide by intruding adult males occurs sporadically, targeting dependent cubs to bring females into estrus, but its contribution to overall cub mortality is unclear relative to starvation, with documented cases in areas like Svalbard and Hudson Bay.[100][101] Drowning represents a minor but observable cause, particularly for family groups during prolonged swims (exceeding 100 km) between fragmented ice floes, as noted in aerial surveys following storms where cub mortality reached 45% in affected cohorts.[102] Human-induced mortality includes regulated subsistence and sport hunting under international quotas, averaging hundreds annually across range states, alongside infrequent defense-of-life killings.[101] Natural survival metrics show first-year cub rates at 0.55 (assuming maternal survival) and 0.35 from birth to age two in Svalbard subpopulations, with adults maintaining high longevity—often 20–30 years—barring nutritional shortfalls.[103] Polar bears exhibit robust baseline survival, with most non-human deaths traceable to energy deficits rather than disease or other pathologies.[104]Population Dynamics
Historical Population Trends
Prior to the intensification of commercial hunting in the 19th and early 20th centuries, polar bear populations were likely more abundant across their Arctic range, though precise historical estimates are unavailable due to limited systematic surveys. Commercial exploitation for pelts, meat, and train oil reduced numbers steadily, with records indicating thousands harvested annually in regions like Svalbard, where over 30,000 bears were killed in the century preceding 1973. By the 1940s–1960s, technological advances such as aircraft-assisted and snowmobile-based hunting accelerated declines, particularly in Canada, Alaska, and Greenland, leaving many subpopulations at critically low levels.[105] In 1965, an international conference of polar bear specialists estimated the global population at 5,000–19,000 individuals, with consensus figures toward the lower end for surveyed areas in Canada (around 10,000), Alaska (under 1,000), and Greenland (a few thousand), while Soviet stocks were poorly quantified but presumed higher. These estimates reflected severe depletion from unregulated harvest, prompting early conservation discussions. The absence of reliable pre-1950 global data underscores the challenges of retrospective assessment, but anecdotal and harvest records suggest populations were substantially higher a century earlier.[106] The 1973 Agreement on the Conservation of Polar Bears, ratified by Canada, Denmark (for Greenland), Norway, the United States, and the Soviet Union, banned sport and commercial hunting while allowing limited indigenous subsistence quotas, marking a causal turning point. This led to documented recoveries in most of the 19 recognized subpopulations by the 1980s–1990s, with global estimates rising to 20,000–25,000 bears, as evidenced by aerial surveys, mark-recapture studies, and harvest monitoring. For instance, Canadian subpopulations, which comprise about two-thirds of the total, expanded from lows of 5,000–8,000 in the 1960s to over 12,000 by the 1990s. Such rebounds were attributed primarily to reduced human harvest rather than environmental factors, highlighting hunting as the dominant historical driver of decline.[107][108][109]Current Subpopulation Estimates
The IUCN Polar Bear Specialist Group (PBSG) recognizes 20 distinct subpopulations of polar bears, distributed across Arctic sea ice regions, with estimates derived primarily from aerial surveys, mark-recapture studies, and genetic analyses. As of the PBSG's October 2024 status report, abundance estimates with associated uncertainty are available for 16 subpopulations, while four (Arctic Basin, East Greenland, Kara Sea, and [Laptev Sea](/page/Laptev Sea)) remain data deficient due to logistical challenges in remote areas. The global population is estimated at 26,000 individuals (95% confidence interval: 22,000–31,000), based on aggregating these subpopulation figures, though this total has not been comprehensively updated since 2016 and excludes unquantified groups.[110][111] Subpopulation sizes vary widely, from small isolated groups like Viscount Melville Sound (161 bears) to larger ones such as the Chukchi Sea (2,937 bears). Trends assessed over short-term (10–20 years) or long-term (30+ years) scales indicate stability or increases in several areas with managed harvesting, contrasted by declines in others linked to sea ice reduction or overharvest. For instance, the Southern Hudson Bay subpopulation increased to 1,119 (95% CI: 860–1,454) in 2021 surveys, reflecting recovery from prior lows, while Western Hudson Bay declined to 618 (95% CI: 425–899) over the same period.[110][111] Many estimates rely on data over a decade old, introducing uncertainty from environmental variability and methodological differences across surveys.[110]| Subpopulation | Estimate (95% CI unless noted) | Trend (Short/Long-Term) | Survey Year(s) |
|---|---|---|---|
| Baffin Bay | 2,826 (2,284–3,367) | Data deficient | 2012–2013 |
| Barents Sea | 2,644 (1,899–3,592) | Likely stable | 2004 |
| Chukchi Sea | 2,937 (1,552–5,944) | Likely stable | 2016 |
| Davis Strait | 2,015 (1,603–2,588; CRI) | Likely decreased | 2017–2018 |
| Foxe Basin | 2,585 (2,096–3,189) | Likely stable | 2009–2010 |
| Gulf of Boothia | 1,525 (949–2,101) | Likely stable | 2015–2017 |
| Kane Basin | 357 (221–493) | Likely increased | 2013–2014 |
| Lancaster Sound | 2,541 (1,759–3,323) | Data deficient | 1997 |
| M'Clintock Channel | 716 (545–955; CRI) | Likely increased | 2014–2016 |
| Northern Beaufort Sea | 980 (825–1,135) | Data deficient | 2006 |
| Norwegian Bay | 203 (115–291) | Data deficient | 1993–1997 |
| Southern Beaufort Sea | ~900 (606–1,212; 90% CI) | Likely decreased | 2010 |
| Southern Hudson Bay | 1,119 (860–1,454) | Likely stable | 2021 |
| Viscount Melville Sound | 161 (93–229) | Data deficient | 1992 |
| Western Hudson Bay | 618 (425–899) | Very likely decreased | 2021 |