Resident–intruder test

Aggressive behaviors were observed using the resident–intruder test. C57BL6/J mice (maintained at the NIG) that were younger than the subject males were used as intruders. An intruder mouse was placed into the home cage of the subject (resident) mouse, and the aggressive (attack bites and aggressive grooming) and nonaggressive social behaviors (sniffing and following) of the resident mouse were observed. The observation was performed for two consecutive days for 15 min a day, and the data were summed. All tests were recorded with a video camera and analyzed by an observer using tanaMove.