Group-housed male C57BL/6J mice are highly prone to aggression and severe fighting/barbering, which leads to physical injury and alters stress hormone baselines, compromising our behavioral assay results (e.g. open field, elevated plus maze). Simply separating the mice into single housing can cause social isolation stress, which also confounds behavioral testing.

To mitigate male cage-mate fighting without resorting to single housing, we implemented a combination of strategies: 1) We preserve odor cues during cage changes by transferring a handful of clean, unsoiled nesting material from the old cage into the clean cage rather than completely washing all odors away. 2) We added crinkle paper and cardboard tunnels to increase environmental complexity, giving submissive mice places to hide. 3) We avoid mixing males from different litters after weaning, keeping sibling cohorts together. This significantly reduced aggression, fighting, and barbering behavior, while maintaining social housing and stable behavioral baselines.