Social group composition in laboratory mouse housing significantly affects welfare, with all-male groups presenting specific management challenges that affect welfare outcomes.
Mouse social housing welfare illustrates the complexity of laboratory animal welfare in practice. Mice need social contact for welfare, but group-housed males frequently fight and injure each other, creating acute welfare problems. The solution is not single housing but thoughtful social group management: early introduction, appropriate group size, space provision, and enrichment that reduces competition. Where single housing is experimentally required, the welfare obligation is to minimize duration and provide compensatory enrichment. The '3Rs' principle of refinement means continuously improving housing to reduce welfare harm even when alternatives to single housing are not available.