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Polar Bear (Ursus maritimus) Fact Sheet: Reproduction & Development


Mating system

(Biddlecombe et al. 2019, and as noted)

  • Polygynous, like other bears; possibly serial monogamous
  • Older (larger) and more experienced males may have more access to female mates
    • Once fully grown (about 10 to 12 years of age), males in a population have similar mating success (Rode et al. 2010)
  • Intense competition between males for mates (as a female may breed only every few years)
    • Fights can result in injury (bite wounds, broken teeth, etc.)
  • Some females have been observed to mate with more than one male in a breeding season (Ramsey & Stirling 1986)
    • Cubs in the same litter may have different fathers (Zeyl et al 2009)
      • Multiple paternity documented in other bear species (Onorato et al. 2009)

Breeding habitat

  • No territorial behavior but male deters male challengers (Stirling et al. 2016)
  • Male has smaller breeding range than female (Laidre et al. 2013)

Courtship behavior and copulation

  • Males locate females in estrous by following their tracks
  • Pursues female determinedly (Stirling et al. 2016)
    • May fast while courting a female
    • Male may separate a female from her cub
  • During courtship, ongoing physical and likely vocal interactions between pair members (Stirling et al. 2016)
    • May last a week
  • A pair may remain together up to 2 weeks (e.g., Stirling et al. 2016)



    Interbirth interval
    • Females typically give birth every 3 years (range: 1 to 4) (Ramsay and Stirling 1986)
    • Females only breed again when they've separated from their young
  • Reproductive age
    • See "Adults," below


  • Breed seasonally
    • Flexible reproductive cycle
  • Mating: typically March/April to early May
    • May extend into June (Smith and Aars 2015; Stirling et al. 2016)
    • Females that lose cubs may mate again later in the year

Gestation & Birth


  • Ovulation
    • Induced by mating (Stirling 2009)
  • Implantation of fertilized egg
    • Usually in autumn; depends on female's body condition and environmental factors
  • Gestation
    • 195–265 days
  • In Western Hudson Bay, pregnant females stay on land for up to 8 months
    • Do not hunt/feed while in maternal dens (fasting period)
      • Female's fat reserves are critical for survival of mother and cubs during this period
        • 93% of the energy used for maintenance and reproduction come from body fat
      • Females lose up to 43% of body weight

Maternity dens

  • Timing
    • Typically dug in late autumn (but as early as September/October)
  • Location
    • Females dig in snow drifts, slopes on land or in peat banks, or on sea ice (Amstrup and Gardner 1994; Wiig 1998)
    • Usually within 1–10 km of coast (rarely beyond 20–50 km), depending on location (e.g., island vs. mainland habitat)
    • Often dig on slopes of 20–40 degrees, where snow is 1–3m deep
  • Temperature
    • Inside den area may be 30 degrees Celsius warmer than outside
  • Structure
    • May be single chamber with short tunnels, or complex system with several tunnels and chambers


  • Late November to early January
    • Most births occur by mid-December

Litter Size

  • 2 cubs most common (approximately 70% of births)
    • 1 or 3 cubs less common
    • 3-cub (e.g., Regehr et al. 2018) and 4-cub litters reported but more rare (Derocher 1999)
  • Litter size may be reduced by 1–2 cubs by age 1 (Derocher and Stirling 1996; Regehr et al. 2018)


  • Reported but uncommon (Malenfant et al. 2016a)
  • Generally occurs during first year of a cub's life (Malenfant et al. 2016a)
  • Cub adopted by an unrelated mother bear (Malenfant et al. 2016a)
    • Possibly results from cub misidentification, or when a female's cubs die and she takes on care of other young

Life Stages


  • Born blind and with sparse hair (Wiig et al. 2015)
    • Grow rapidly, feeding on mother's rich milk
  • Weight: 600–700 g (1 lb 5 oz – 1 lb 9 oz)
    • Young are some of the most underdeveloped (altricial) of all placental mammals
      • Mother's weight is about 500 times greater than that of cub's
  • Newborn cubs usually fed while mother lies down and encircles cubs with forepaws, pressing them to nipples for nursing (Harington 1968)
  • Mother and cubs emerge from dens in March or April and migrate to sea ice (e.g., Yee et al. 2017)
    • Cubs weigh about 10–15 kg (22–33 lb)
  • Polar bear milk is very rich (Jenness et al 1972)
    • Similar in fat content to seal and walrus milk


  • May remain with mother until 2.5 years of age
  • Weaned at about 2.5 years old (Ramsay and Stirling 1988)
    • Nurse for up to 2 years
  • Young bears appear to learn to habitat use from their mothers (Lillie et al. 2018)
  • When leaving dens, cubs don't have enough body fat to swim without potentially dying from hypothermia
  • Young-of-the-year mortality is high
    • First-year survival rate was 44% in one study of 200 cubs studied in Hudson Bay (Derocher and Stirling 1996)
    • Male bears of all species sometimes kill cubs; by six months, cub can outrun adult male


  • Peak reproductive years
    • About 10–20 years old (Derocher et al. 2010; Biddlecombe et al. 2019; Richardson et al. 2020)
  • Females
    • Sexually mature at 4–5 years, on average (e.g., Wiig et al. 2015)
      • Minimum breeding age: 3.5 yr
      • Maximum: 21 yr
        • Most females no longer have cubs by age 20
    • Female adult weight reached at 5 yr
  • Males
    • Sexually mature around 6 years, on average
      • Minimum breeding age: 3–4 yr
      • Maximum: 19 yr
    • Male adult weight at 8–10 yr

Typical Life Expectancy

Wild populations

  • 15–18 years old (Polar Bears International 2023)
    • Few live past age 25 (Derocher 2012)

Managed care

  • Median life expectancy
    • Males: 20.8 years (AZA 2023)

Finding a Mate

Across huge stretches of sea ice, how do polar bears find a mate?

San Diego Zoo Wildlife Alliance scientists found that they follow their nose...

© San Diego Zoo Wildlife Alliance. All rights reserved.

Female and Cubs

Polar bear mom and cubs

A female polar bear typically gives birth to two cubs.

Polar bear cub mortality is high in their first year of life.

Image credit: Steve Amstrup of United States Fish and Wildlife Service, USFWS. Licensed under Public Domain via Wikimedia Commons.

Polar Bear Cub

 Polar bear cub

Polar bear cub in Beaufort Sea, Alaska.

Image credit: © Alan D. Wilson. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Page Citations

Amstrup (2003)
Atkinson & Ramsay (1995)
Jenness et al (1972)
DeMaster & Stirling (1981)
Onorato et al. (2009)
Ramsey & Stirling (1986)
Stirling (1993)
Zeyl et al (2009)

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