Ambrose, N., Morton, D. B. 2000. The use of cage enrichment to reduce male mouse aggression. Journal of Applied Animal Welfare Science 3, 117-125.

Aggression was significantly reduced by the addition of novel enrichment [cardboard box, softwood block, cardboard tube]. This reduction was expected as the provision of environmental enrichment for naive animals would decrease the familiarity of the cage and, therefore, lead to reductions of aggression. ... Even a simple enrichment aid such as a glass water bottle can significantly reduce postcage-cleaning aggression compared with mice kept in a barren cage.