The adult male C57BL/6 physical target intruder mouse (P) is placed into the divided cage with a resident adult male retired breeder CD1 aggressor mouse (A1), while the witness intruder mouse (W) is placed across the divider. Critically, the witness mouse can be either sex and adolescent or adult. After a 10-minute stress interaction involving physical aggression of the resident (A1) against the physical target (P), the witness mouse (W) is moved to a new cage, where it is housed across a divider from a different resident male retired breeder CD1 aggressor mouse (A2), while the physical target intruder mouse (P) is placed across the divider from the original aggressor mouse (A1), and the mice are co-housed for 24-hours. This process then repeats for a total of 10 consecutive days.