1. Laboratory of Veterinary Ethology, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
2. Present address: Wildlife Damage Management Group, Central Region Agricultural Research Center, National Agricultural Food Research Organization, 2-1-18 Kannondaira, Tsukuba, Ibaraki 305-8666, Japan
3. Technical Research Laboratory, Ikari Shodoku Corporation, 1-12-3 Akanehama, Narashino-shi, Chiba 275-0024, Japan