Mortality Rates of North American Bears

The age structures of 39 populations of three species of North American bears were analyzed. Estimated mortality rates of cubs in their first year were 30-40% for brown bears and 25-30% for black bears. Apparent subadult mortality rates derived from living animals (15 -35% an- nually) were higher than those of adults. Apparent mean annual mortality rates of subadult and adult females combined were 17.2, 16.8, and 18.8% for black, brown, and polar bears respectively. Comparable values for males were 25.5, 23.0, and 22.6% annually. Because hunting appears to be the major mortality factor in most North American bear populations, interpretation of age structures is facilitated by explicitly incorporating the effects of hunting and its associated biases in the analyses. The simple model proposed to accommodate the hunter-bear interaction clarifies differences in age distributions between species and between sexes within species. Most of the differences in sex-specific mortali- ty rates are a product of differential vulnerability related to home range size and method of hunting.