Male mice prefer to live on their own

Around the world, guidance for the management of laboratory mice recommends social housing (1-3). However, fighting in male mice represents a major animal welfare and scientific issue. Despite the risks of stress and injury and associated welfare costs of incage aggression, we currently prioritise group housing. In this study we first validated a trans-species behavioural biomarker of affective state for mice using a chronic corticosterone model of depression. We next used the same methods to quantify affective state in male mice, group or singly housed in conventional versus individually ventilated cages (IVCs). The results revealed group housed male mice are in a negative affective state similar to that induced by chronic corticosterone treatment. IVC housing was also associated with a more negative affective state. These studies provide an objective assessment of the welfare costs of different housing conditions for male mice.