BIO 203

Life History and Reproduction

    Grizzly bears have an extremely low reproductive rate. This is caused by a variety of factors. Female grizzly bears reach sexual maturity between the ages of five and eight. With the lifespan of grizzlies being about 20-25 years, that would be comparable to a human female reaching sexual maturity at age 21 (Horejsi, 2008). With this large portion of their lifespan spent as sexually immature, reproduction rates are significantly lower than most terrestrial animals.
    The struggles of reproduction don’t end after reaching sexual maturity. Finding a mate is difficult for grizzly bears due to their largely solitary lifestyles (National, 2011). When a female is ready for a mate, she emits a scent which males can pick up from miles away. Male grizzlies have large territories, however, spanning up to 1,500 square miles (Horejsi, 2008). This range makes it difficult for males to detect their potential mates.
    Grizzly bears mate between the months of June and July (MacDonald, n.d.). Once a female breeds with a male, the fertilized egg doesn’t implant shortly after conception like in many mammals. Female grizzly bears experience delayed implantation in which implantation of the fertilized egg is held off until just before hibernation. If the female doesn’t have the proper nutrients during hibernation, she may miscarry. Once the fertilized egg implants, grizzly bears have a gestation period of six to eight weeks and give birth during hibernation (Teel, 2008).
     Another factor that influences grizzly bears’ low reproduction is the amount of offspring that female grizzly bears produce. An average litter yields two cubs, but may range anywhere from one to four cubs (MacDonald, n.d.). After the birth of a litter, the mother cares for her cubs for two to three years until the cub is old enough to live on its own. During this time, the mother teaches her young how to find food and dig a den (Austin, 2002). The mother will not breed while she cares for her cubs. Females may also wait up to three years after their cubs leave to breed again (Payton, 2001). Because of this slow rate of reproduction, the grizzly population is also slow at recovering from negative ecological factors (Horejsi, 2008).

U.S. Geological Survey

Other Interesting Facts