Body condition and growth dynamics of American black bears in northern Canada

We examined body condition and growth dynamics of wild American black bears (Ursus americanus) inhabiting interior regions of northern Canada, 1998-2009. Differences in body condition were unrelated to gender, but we found significant effects of age and season as well as an interaction between the two. We found a trend toward improved body condition in older bears that increased during summer and peaked in fall, with lowest values observed in spring. The von Bertalanffy growth function showed that males reached asymptotic body length 9.3% longer, and mass 29.3% heavier, than females. Our growth models indicated an association between sexual growth divergence and the onset of reproduction in females, together with more rapid and prolonged male growth. We suggest that sexual size dimorphism develops in part from constraints on female growth from high energetic costs of reproduction. In contrast, males experience no comparable energetic trade-off after reaching sexual maturity and apparently allocate available energetic resources to growth of larger body size, which benefits more competitive males in terms of increased reproductive success.