Increased brown bear predation on sika deer fawns following a deer population irruption in eastern Hokkaido, Japan

By the 1970s, brown bears (Ursus arctos) in Hokkaido, northern Japan, were opportunistic omnivores that mainly depended on plant materials. Because the sika deer (Cervus nippon) population irrupted in eastern Hokkaido in the 1990s, we expected that brown bears might prey on sika deer fawns. First, we developed a simple and cost-effective method of monitoring possible bear predation on deer fawns by analyzing the widths of deer hairs remained in bear scats. Based on hair thickness standards, we distinguished the brown bear consumption of deer fawns from adults by analyzing bear scats (n = 108) collected during the deer birthing season (late May–late July) in 1999–2008. To evaluate the importance of fawns to bears, we compared the occurrence of fawn and adult deer hairs in bear scats among three periods (I, 1999–2000; II, 2003–2005; III, 2006–2008) in eastern Hokkaido. The occurrence of fawn hairs in bear scats increased from 12.5 to 27.3 % in volume and from 6.3 to 33.6 % in frequency from period I to period III, whereas adult hairs in scats decreased from 42.8 to 26.1 % in volume and from 34.4 to 22.7 % in frequency during the same time. These data suggest that bears increasingly preyed on deer fawns after the deer population irruption and decreasingly used adult carcasses because of the enforcement of deer carcass treatment by the Hokkaido government.