1. Animal Reproduction Laboratory, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan
2. Laboratory of Animal Behavior and Environmental Management, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan
3. Sumiyoshi Livestock Station, Field Science Center, Faculty of Agriculture, University of Miyazaki, Miyazaki 880-0121, Japan