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Brown Bear (Ursus arctos) Fact Sheet: Reproduction & Development

Courtship

  • Polygamous
    • Several males may mate with one female.
    • Females may mate with more than one male.
  • Female estrus is about 10-30 days
    • Period is split by a short time of no receptivity.
  • Breeding activity occurs mid-May to July.

Reproduction

(Craighead et al 1995)

  • Lowest reproductive rate of any land mammal in North America. (Schoen 1999)
    • Females don't breed until 5-6 yrs.
    • Average of 2 cubs/litter
    • 3-4 years between litters
  • In a study of Yellowstone brown bears, dominant males did not have sole access to estrus females.
    • Surprisingly, only 27.6 % of all matings observed were between alpha or beta males and females
    • Rest of matings were between lower-status males and females
    • Alpha males usually preoccupied with fighting
    • Alpha males spend more time checking fertility status of females (by sniffing)
      • Perhaps acquire access to females at the best time for fertility
    • Each cub in a litter can have a different father
      • 33% of all litters studied had multiple sires.
  • Females have delayed implantation of embryo after fertilization
    • Allows for timing of birth with favorable environmental conditions.
    • Normally implantation is in November, around 6 weeks before birth.
  • Females and males enter individual dens
  • Females line den floor with mosses and grasses; give birth while hibernating
  • Reproductive biology similar to that of Black Bears.

Gestation & Birth

Gestation

  •  6-7 months; occasionally 9 months (Kurt 1990)


  Birth

  • January-March (Schwartz et al 2003)
  • Most litters 2 cubs; up to 4 cubs known. (Craighead et al 1995)
  • Birth weight: 400-500 g (14-18 oz) (Kurt 1990)
  • At birth: closed eyes, little hair, helpless (altricial)

Life Stages

Infant (< 1 year old)

  • Remain with mother in den after birth 
  • By 3 months weigh 15 kg (33 lb)
  • During 1st summer, young have white chest patch of fur

Juvenile

  •  Young nurse 1.5 - 2.5 years.

Subadult

  • Young may remain with mother up to 3 or 4 years.  

 Adult

  • Sexual maturity: males - 5.5 years; females - 3.5 years
  • Females breed at ages 3 to 8; varies depending on locality and food supply.

Longevity

  • About 25 years. (Pasitschniak-Arts 1993)
    • 20 yrs. in wild; 30 in captivity (Kurt 1990) but possibly 50.
  • A European Brown Bear born at the Leipzig Zoo, Germany lived 39 years. (Weigl)
  • A Hokkaido Brown Bear died at 39 years at the Odawara Zoo in Japan.
  • A female at Yellowstone National Park gave birth to cubs at age of 22.5 years (Craighead et al 1995)
    • Two years later juveniles were weaned.
  • Average lifespan at Yellowstone National Park is only 6 years.

Mortality

(Pasitschniak-Arts 1993)

  • Human hunters and human-caused habitat disturbances
  • Severe winters, malnutrition and canabalism cause some deaths
  • Very rarely, tigers in Russia attack bears (and vice versa)
  • Rarely, bears are gored by large ungulates.
  • Infanticide most common cause of natural death in a study of 100 bears in Sweden (Morner et al 2005)

Page Citations

Craighead et al (1995)
Kurt (1990)
Pasitschniak-Arts (1993)
Schoen (1999)
Schwartz et al (2003)

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