Polar Bear (Ursus maritimus) Fact Sheet: Diet & Feeding

Highly carnivorous diet

• Marine mammals (e.g., Stirling and Archibald 1977; Iversen et al. 2013; McKinney et al. 2013; Galicia et al. 2015; Galicia et al. 2016; McKinney et al. 2017; Laidre et al. 2018a; Smith and Stirling 2019; Florko et al. 2021; Michaux et al. 2021)
  • Primarily eat ringed seals, Pusa hispida
    • Greater than 50% of the prey base in most Arctic regions (Thiemann et al. 2008)
  • Also harp seals, hooded seals, bearded seals, and harbor seals
  • Occasionally: walruses, beluga whales, and narwhals
    • Viscount Melville Sound: beluga makes up about 20% of diet (Florko et al. 2021)
      • Highest consumption reported of any subpopulation
  • Polar bears are adapted to eat soft, high-calorie blubber (also, flesh but often leave meat for scavengers)
    • Primarily carnivorous, compared to brown and black bears (Rinker et al. 2019)
    • Diet constrained by skull morphology and dentition (Slater et al. 2010; Petherick et al. 2021)
      • Skull and teeth not well adapted for processing bone and tough plant foods
        • Smaller grinding surfaces on molars compared to brown bears
    • McKinney et al. (2013) noted polar bear's diet can shift to include more subarctic seal species with warmer temperatures and less sea ice [Greenland]
    • In recent years, observations of polar bears in Alaska broadening somewhat diet to include harder foods (Petherick et al. 2021)
      • Foraging on seal carcasses, at bowhead bone piles (discards of native subsistence hunting), and from human trash
• Birds and their eggs
  • Geese, eider, and other seabirds and waterfowl (Stempniewicz 2006; Prop et al. 2013, 2015; Rode et al. 2015; Laidre et al. 2018a; Barnas et al. 2020)
• Less common prey
  • In summer, if preferred food is unavailable, take land plants and lichen, seaweed/kelp, berries, small mammals, birds, and egg ((Lønø 1970; Iversen et al. 2013; Laidre et al. 2018a)
  • Opportunistically feed on whale carcasses caught by native subsistence hunters (Rogers et al. 2015; Atwood et al. 2016; Galicia et al. 2016; Pongracz et al. 2017b; Wilson et al. 2017; Bourque et al. 2020; Pagano et al. 2020; Florko et al. 2021; Griffen et al. 2022) and killer whale predation (Galicia et al. 2016)
    • E.g., bowhead whale remains in Northern and Southern Beaufort Sea, and Foxe Basin, Canada
  • In Svalbard, Norway, have been observed hunting reindeer (Iversen et al. 2013; see Stempniewicz et al. 2021)
    • One hunting event involved a polar bear chasing and overtaking a reindeer in water
  • Observed feeding on white-beaked dolphins trapped by ice floes (Aars et al. 2015)
  • In Canada, polar bears observed:
    • Infrequently scavenging on remains of land mammals and birds (Michaux et al. 2021)
    • Hunting caribou on the coast of Hudson Bay (Brook and Richardson 2002)
    • Fishing Arctic charr and fourhorn sculpin in Nunavut (Dyck and Romberg 2007)

Energy needs under harsh conditions

• High energy (metabolic) needs (Pagano et al. 2018b)
• Consume large amounts of fat when food is available
  • Gain majority of their body fat in the late spring and early summer
  • 90% of the energy eaten is assimilated directly to fat stores (Best 1977)
  • Metabolize fat when there is nothing to eat
• Can endure long periods without feeding (e.g., Pilfold et al. 2016)
• Often kill more than they can eat
  • Rarely cache food, unlike other bears
  • Large stomach can hold more than 70 kg (154 lb) of food
• Eating fat adds water to their diet
  • Water released when fat is digested (metabolized)

Nutrition

• Vitamin A content of liver ranges between 15,000 and 30,000 units per gram.
  • Phosphate, lipid and cholesterol contents relatively low.
• Bear milk is high in fat and protein but low in carbohydrates.
  • Polar Bears have richest milk of all bears
  • 35.8% milk fat when emerge from dens - 20.6% fat with yearlings
  • Sodium content is higher than that of other milks.

Hunting

• Use sit-and-wait tactics to hunt seals (e.g., Stirling 1974; Pagano et al. 2018b)
  • Stalk prey less often
• Locate birth lairs and breathing holes of Ringed Seals by smell and sound
  • Approach slowly to minimize their own sound
  • May break into lair immediately, or wait motionless and listening for several hours for seal to return
  • Walk cross current to winds to pick up smell (Togunov et al. 2017)
• Swim underwater to stalk seals lying at the edge of an ice floe
• Consume fat first
  • Tissue with most calories prioritized
• Prey selection may differ somewhat between male and female polar bears (Galicia et al. 2015)
  • Large adult males may be better able to hunt larger prey (e.g., large seals, beluga, etc.)
• Considerable competition between Polar Bears for a kill; larger bears will run off smaller bears
• Small size of prey probably eliminates any advantage in group hunting
  • But bears may scavenge together at a whale carcass or human garbage dump
• Wash and lick themselves while feeding; spend up to 15 minutes grooming afterward

Scavenging

• Subadults may scavenge kills left by larger individuals (e.g., Stirling 1974; Pilfold et al. 2014)
  • Scavenge more commonly than adults, which are more adept and experienced hunters
• Provides limited opportunity for weight gain/maintenance (Pagano et al. 2018b)

Food caching

(Stirling et al. 2020; Stempniewicz et al. 2021)

• After eating prey's fat and other high-value tissues, may bury carcass under snow, grass, or dirt
  • Conceals remains from scavengers
• Short-term; generally cache for only a few days
• Observed in Svalbard, Greenland, and Canada
  • But generally, an uncommon behavior
    • Fewer scavengers on sea ice than land; behavior may benefit polar bears less than other bear species

Cannibalism

• Thought to occur infrequently (Stirling and Ross 2011)
• Bears at risk are mainly cubs, but sometimes also adult females (Stirling and Ross 2011)
  • May be killed in a similar manner to how polar bears hunt seals (bites behind the head)
• Possible motivations (see refs in Stirling and Ross 2011; Ivanov et al. 2020)
  • Hunger
  • To avoid competition
  • For males, to increase breeding opportunities (infanticide)
• Most kills known to occur on land (Stirling and Ross 2011; Ivanov et al. 2020)

Tool use

(Stirling et al. 2021)

• Suspected for a small number of individuals
• Use rocks or ice to hunt seals and (rarely) walrus
  • See news story

Amstrup (2003)
Demaster & Stirling (1981)
Derocher et al (1993)
Derocher et al (2004)
Stirling (1993)