Testing for a predicted decrease in body size in brown bears (Ursus arctos) based on a historical shift in diet

A recent study found a historical decline in the proportion of meat in the diet of brown bears (Ursus arctos L., 1758) in the Hokkaido Islands, Japan. Because feeding habits are strongly correlated with the body size of animals, the shift in diet should have led to a decrease in the size of these bears. To predict the effects of this dietary shift on the skeletal size in bears, we correlated the femur length in Hokkaido brown bears with the carbon and nitrogen stable isotope values from bone samples and predicted the historical change in their body size. The variation in the femur lengths of the male and female subpopulations was positively correlated with their delta N-15 values, but not with their delta C-13 values, and the explanatory power of the constructed model was higher in males than in females. Based on the model and the delta N-15 values for historic and modern bears, the skeletal size of bear subpopulations in eastern Hokkaido was estimated to have decreased by 10%-18% for males and 8%-9% for females. Our results suggest that a historical dietary shift caused the decrease in the size of the Hokkaido brown bears.