Sherrill, C., Kavanagh, K. 2019. Fight Club: Using housing conditions to curb male cage aggression. Laboratory Animal Science Professional 7(1) (March), 38-40.

Those who have worked with C57BL/6 males know how aggressive they can be towards their cage mates. We poured over academic journals, reading every article we could find regarding male home cage aggression. Three articles piqued our interest and suggested some real-world techniques we could employ for our next study. Mirroring ‘traditional’ conditions [5 mice/cage with red igloo, a cotton nesting square, nesting material contained within a pouch, and an intermittent chewing device], fighting began at 6 wk in age. Unlike traditional conditions, however, we only observed 84 fight incidences versus 107, but no separations were required and no mice were euthanized due to fight wounds. This resulted in an average of 5.15 d between fight incidences, which is almost three times longer when compared to the traditional conditions. Our laboratory had a unique opportunity with these two studies in which simple changes resulted in a profound effect on male home cage aggression. Most importantly, we did not change their natural behavior: male mice are territorial and will inevitably fight. We did, however, reduce the frequency and severity of fighting. As such, we conclude that the best conditions to ameliorate fighting among male C57BL/6N mice are: house 4 per cage with cotton nesting square and gnawing stick, but do not provide igloos or nesting material pouches [only the material inside the pouches], and transfer some of the used nesting material to the new/clean cage. Additionally, we noted that there was no evidence of differences between cages containing siblings or unrelated males.