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

Courtship

  • March to late May: paired bears observed in field.
  • Most females are induced ovulators (don't ovulate until stimulated by mating).
  • Males locate females in estrous by following their tracks.
  • No territories.
  • A pair may remain together approximately 2 weeks.
  • Some females have been observed to mate with more than one male in a breeding season (Ramsey & Stirling 1986)
    • Members of the same litter have now been documented by gentetic studies to have different fathers (Zeyl et al 2009).
    • Other bears are known to give birth to litters fathered by more than one male. (Onorato et al. 2009)

Reproduction

  • Typically reproduce every 3 years
  • Females only breed again when they've separated from their young

Gestation & Birth

Pregnancy

  • Implantation of fertilized egg depends on female's body condition and environmental factors; occurs probably in November
  • Gestation (conception to partuition) is relatively long: 195-265 days
  • Pregnant females stay on land for 8 months, all the while food-deprived while they are in maternal dens, giving birth, raising cubs;
    • Female's fat reserves are critical for survival of mother and cubs.
    • Females loose up to 43 % of body weight during fasting
    • 93% of the energy used for maintenance and reproduction come from body fat.

Females dig maternity dens

  • Often on slopes of 20-40 degrees where snow is 1-3m deep.
  • Temperature within may be 20 degrees warmer than outside.
  • May be single chamber with short tunnels or complex system with several tunnels and chambers.
  • Usually within 8 km of coast (rarely beyond 48 km).

Birth: Late November / early January; most births occur by mid-December.

Litter Size

  • 2 cubs in approximately 70% of births; 3-cub births in about 6% of observations
  • 4-cub litters reported but extremely rare

Life Stages

  Infant

  • Blind at birth but not hairless
  • Weight: 600-700 gms (1 lb 5 oz - 1 lb 9 oz).
    • Young are some of the most undeveloped of all placental mammals.
    • Mother's weight is about 500 times greater than that of cub's
    • A newborn dog or cat is 10-20% mother's weight.
  • Newborn cubs fed usually by mother lying down, encircling cubs with forepaws to press them against the nipples. (Harington 1968)
  • When cubs about one month, mother sits upright, holds young to her breast with forepaws.
  • Emerge from dens in March or April, weighing 10-15kg (22-33 lb)
  • Polar Bear milk is richest of all bears', similar to seal and walrus milk. (Jenness et al 1972)

 Cubs

  • Weaned at about 24-28 months (nurse for at least 1 yr)
  • May remain with mother until 2 1/2 years of age
  • At time of leaving dens, cubs don't yet have enough body fat for swimming in arctic water.
  • First year survival rate for 200 cubs studied in Hudson Bay region was only 44% (Derocher and Stirling 1996)
  • Male bears of all species sometimes kill cubs; by six months cub can outrun adult male.

 Adults

  • Females sexually mature 4-8 years.
    • Minimum breeding age: 3.5 yrs.
    • Maximum: 21 yrs.
  • Peak reproductive condition between 10-19 yrs.
  • Most females stop having cubs by age 20.
  • Males sexually mature around 6 yrs.
    • Minimum breeding age 3 yrs.
    • Maximum: 19 yrs.
  • Female adult weight reached at 5 yrs.
  • Male adult weight at 8-10 yrs.

Longevity

  • Most live to between 15 and 18 years in wild.
  • A few reach 25-30 years.
  • Oldest female recorded in wild: 32 years (Stirling 1998)
    • Some reports of females living in wild well past their reproductive years.
    • Other studies suggest females have life-long ability to reproduce; more research needed. (Amstrup 2003)
  • Oldest male: 28 years.
  • Longevity record for a Polar Bear raised in managed care is 42 years for a female at a zoo in Winnipeg, Canada.

Finding a Mate

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

New research by San Diego Zoo Global shows that they follow their nose...

© San Diego Zoo Global. All rights reserved.

Female and Cubs

Polar bear mom and cubs

A female polar bear typically gives birth to one or two cubs.

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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