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

• 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.

(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

•  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)

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.

Wild populations

• Typical life expectancy not well known
  • Might reach 25 years (Pasitschniak-Arts 1993)

Managed care

• Median life expectancy (AZA 2023)
  • Males: 21.9 years
  • Females: 26.8 years

(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)

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