Abstract
Murine aggression has profound implications on animal welfare and husbandry. This report examines how three distinct combinations of environmental enrichment - wheel, igloo and tunnel; wheel, igloo, and tunnel with nesting; and tunnel with nesting - affect aggressive behaviour in CD-1 male mice. We found that combining wheel/igloo/tunnel enrichment with nesting or replacing the wheel/igloo with two tunnels while maintaining the nesting enrichment reduced aggression. These findings not only suggest how enrichment can improve the welfare of aggressive male mice but also emphasise the need for further research to determine the optimal combination of enrichment.