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

Mating system

(Biddlecombe et al. 2019, and as noted)

• Polygynous, like other bears; possibly serial monogamous
  • Drives sexual dimorphism (e.g., Derocher et al. 2010)
• 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)

Seasonality

• 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

Pregnancy

• 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

Birth

• 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)

Adoption

• 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

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

Infant

• 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

Cubs

• 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

Adults

• 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

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)

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.

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 in Beaufort Sea, Alaska.

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

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)