• the age at which females first reproduce
• the number of young produced per litter
• the number of years between litters.
  

Population productivity depends on all of these factors, plus the rate that cubs survive to breeding age.

The average age at which black bear females first give birth varies in North America from 3 to 6 years of age (Kolenosky and Strathearn 1987). Most litters number 2 or 3 cubs and females reproduce every 2 years on average. The percentage of females with litters varies considerably from year to year, not only because there are 2 years between litters, but largely because of nutritional differences in a given year.

Differences in diet and nutrition also account for regional variations in reproductive rates. Females with access to garbage in Minnesota produced first litters at 1-2 years earlier than those without supplementary food sources (Kolenosky 1990). The age of first reproduction for female black bears in the southeastern United States, where acorn mast is available, is 1-2 years earlier than for those in Ontario or Interior BC.


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BLACK BEAR SUPER MOTHERS

Kolenosky (1990) found that a small percentage of the female black bear population makes a significant contribution towards producing young. Twenty-five percent of the females in the study produced 66% of the litters. The characteristics of these super mothers were:

• permanently established home ranges
• above-average size and weight, longevity
• low vulnerability to mortality including hunting

They were nearly always accompanied by cubs and therefore not legally hunted.

Black bears have one of the lowest reproductive rates of any North American land mammal. They have a comparatively slow reproductive maturity, prolonged reproductive cycle and relatively small litter sizes. However, some black bear populations have increased at rates as high as 25% annually (MacCallum 1999). Some of the reasons are related to the limited mortality of cubs between 0.5 and 1.5 years due to maternal care and the precocious nature of cubs of this age. Nutrition has been related to cub survival, but losses of entire litters to starvation are not common.


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MORTALITY

Black bear mortality occurs as a result of a number of factors both natural and human-caused. These include starvation, which is a factor for all ages of bears but particularly of young cubs. Cubs are dependent on their mothers and will starve if she is unable to acquire sufficient nutrition to support them. The cubs may also starve if they are lost from the mother from any cause, including death or abandonment. Black bears live dangerous lives and can die from accidents (falling from trees), fighting, predation, infanticide and cannibalism. Human-caused mortality for black bears includes collisions with vehicles, removal of nuisance bears, illegal killing by any means and hunting.


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PREDATION

Black bears may be both predator and prey. Black bears can be prey for other predators, particularly grizzly bears. In addition, black bear cubs and adult females can be cannibalized by male black bears. Black bears can be significant predators of young deer, elk, moose and caribou in some areas of North America, particularly where they do not have competition from other predators, such as grizzly bears and wolves.

The great variation in the percentage of moose calves killed by black bears in Alaska can be attributed to the different densities of black bears, grizzly bears and wolves.

The role of black bears in regulating or limiting ungulate populations is complex and depends on many factors, including the presence of other predators. Black bears rarely kill adult moose, although 15% of the adult caribou that died in a study near Quesnel Lake, BC were killed by black bears (Ballard 1994). Moose have several strategies to offset the impact of predation on their young. Female moose tend to all give birth at about the same time, which reduces the risk that predators will be able to kill all or even most of the young. Moose also have high twinning rates and low adult mortality. There are also other overriding factors that control moose populations, including disease, habitat quality and quantity and weather.


Reproduction  |  Super Mothers  |  Mortality  |  Predation