Aggression screening/Resident Intruder (RI) test

Aggression screening was performed as previously described by utilizing the resident intruder (RI) test⁹. After a minimum of one week of habituation to home cages, experimental mice were exposed to a novel C57BL/6J intruder for 5 min daily over 3 consecutive days. Each intruder presentation was performed in the home cage of the experimental mouse between 12–3 PM daily under white light conditions. During RI sessions the cage top was removed to allow for unobstructed viewing and video recording of sessions. The duration and number of screening sessions were selected to prevent induction of stress- and anxiety-related behaviors in experimental CD-1 or F1 hybrid mice⁹. All RI sessions were video recorded with a digital color video camera. Two blind observers recorded (1) the latency to initial aggression and (2) the total duration of aggression. The initiation of aggression was defined by the first clear physical antagonistic interaction initiated by the resident mouse (usually a bite), not including grooming or pursuit behavior. Aggression was considered completed when the resident mouse had reoriented away from the intruder following the initiation of attack. This definition allows for slight breaks (less than 5 s) in continuous physical interaction within an aggressive bout, assuming the resident mouse has remained oriented towards the intruder throughout. Resident mice were defined as AGGs if they initiated aggression during all three screening sessions, while NONs were defined as those that showed no aggression during any screening sessions. Aggression screening was halted if an intruder showed any signs of injury in accordance with our previously published protocols^{9, 54}.