Mortality
As with most other animal species, mortality in bears is greatest during the first year of life; mortality is about 40-50% between when cubs are born and when they leave their mother at 1.5 years old. In other words, about half of all cubs die before they are able to leave their mother’s side. About 80% of that mortality occurs when the mother is in estrous and is likely due to infanticide by male bears. Bears are known to kill other bears, at least young subadult bears, although this is relatively uncommon. There have been only a few documented cases of adult bears killing other adult bears, all of which were adult females with yearlings. It is possible that these deaths were the result of females vigorously defending their cubs against a male bear.
The bear population in Scandinavia is legally hunted, which means most adult mortality is caused by people. For our radio-collared bears, humans were the number one cause of death, with ~80% caused by some form of human activity (e.g., legal hunting, poaching, road accidents, etc.). We think that poaching, or illegal hunting, is likely an important source of mortality for Scandinavia’s bear population.
Sexually Selected Infanticide
Infanticide is when one animal kills another individual of the same species while they are young and still dependent on their mother. This behavior is ‘sexually selected’ if it is based on competition for sexual partners. This occurs when male reproductive success has the change to increase when young are no longer present.
For the male – Their reproductive success may benefit when they kill cubs provided that 1) he is not the father, 2) the mother comes into heat earlier than she otherwise would have and 3) he can be the father of her next litter. Thus, males may select to kill unknown cubs in order to boost their reproductive output.
For the female – Their reproductive success always suffers from
infanticide. Females can only produce so many litters during their life span, and their production and care is energetically costly. Thus, females employ a host of strategies to protect their cubs from males. This includes mating with multiple males to ‘hide’ paternity, physically defending their cubs, moving little to avoid detection, using areas with escape routes for cubs, such as trees, or using areas where males are unlikely to be.