Reconstructing the dietary history of Japanese black bears through stable isotope analysis of hair

The population of Japanese Asiatic black bears ( Ursus thibetanus japonicus ) has been increasing in Japan, leading to more human–bear conflicts, including crop damage and an increase in the lethal removal of bears. However, it remains difficult to verify whether captured bears were directly involved in such damage. This study focused on the carbon and nitrogen stable isotope ratios in guard hair, based on the premise that growing hair records dietary intake, to develop a method for reconstructing the dietary histories of captured individuals. Feeding experiments with captive Japanese black bears revealed that guard hair began to grow in June, elongated relatively consistently, ceased growing at the end of October, and did not grow during the winter denning period. New hair began to grow in June of the following year, whereas hair from the previous year was shed between July and September during molting. We then evaluated whether this temporal framework could be applied to wild individuals. By comparing temporal changes in the carbon isotope ratios of guard hair from GPS-tracked bears that used cornfields, we found that the timing of their use of cornfields, where maize, a C 4 plant, was available, corresponded well with the changes in hair carbon isotope ratios. Bears that visited cornfields more frequently showed higher carbon isotope ratios in hair. These findings indicate that the hair growth processes observed in captive bears can be applied to wild bears, enabling reconstruction of their dietary histories.