1. Center for Northeast Asian Studies & Graduate School of Life Sciences, Tohoku University, 41 Kawauchi, Aoba, Sendai, Miyagi 980-8578, Japan
2. Hahajima Branch of Ogasawara Ranger Office, The Ministry of the Environment, Shizukazawa, Hahajima, Ogasawara, Tokyo 100-2211, Japan
3. Japan Wildlife Research Center, Koutoubashi, Sumida, Tokyo 130-8606, Japan