Implications of a high-energy and low-protein diet on the body composition, fitness, and competitive abilities of black (Ursus americanus) and grizzly (Ursus arctos) bears

Plants are not ideal foods for bears yet many populations are largely vegetarian. Implications of this diet on the body composition, fitness, and competiveness of black (Ursus americanus Pallas, 1780) and grizzly (Ursus arctos L., 1758) bears have had limited field investigation. The analysis of scats of grizzly and black bears from the Flathead valley, British Columbia, suggest seasonal dietary differences between species, but >85% of the summer diet of both species were fruits that are low in protein. Body composition measurements showed bears loose fat during spring, gained fat during summer, and grizzly bears were leaner than black bears. Individual black bears gained mass up to 2.7 times faster than theory predicted. Bears rapidly gained fat but lost lean tissues while feeding on fruit, suggesting that lean tissues were used to buffer seasonal protein shortages. Comparisons among populations of grizzly bears without access to salmon revealed the amount of meat in the diet was positively related with adult female mass but negatively related with bear density. Bears have the behavioural and phenotypic plasticity which enables populations that focus their foraging on plants to have small but fat females and live at higher densities than populations that focus more on obtaining terrestrial meat.