Mice and rats are the most widely used laboratory animals and are profoundly social species whose welfare depends critically on housing conditions.
Single housing of social laboratory animals represents an ethical failure and a scientific one simultaneously. Isolated mice and rats are stressed, show abnormal behaviours, and produce experimental data that may not accurately reflect normal physiology. The argument that aggression requires single housing is often used as a justification where management solutions including environmental enrichment, appropriate group sizes, and careful introduction protocols would allow social housing. Institutional Animal Care and Use Committees and their international equivalents are increasingly requiring justification for single housing rather than treating it as a default.