Laboratory mice are the most numerically significant vertebrate research model globally. Housing density significantly affects their welfare, with overcrowded conditions causing stress, aggression, and compromised scientific data quality.
Mouse housing density creates a direct tension between space availability and social complexity. Mice kept below minimum population sizes may show signs of social isolation stress, while mice in overcrowded conditions develop chronic stress responses, show increased aggression, and have elevated corticosterone levels that can confound research results. The Three Rs framework (Replace, Reduce, Refine) requires continuous improvement in housing standards as part of the refinement obligation. Current evidence suggests that housing conditions widely used in research institutions fall short of those that would produce optimal welfare outcomes. Nesting material is perhaps the single most important enrichment item for laboratory mice - it provides thermal insulation, hiding opportunities, and nest-building behaviour that is highly motivated in this species. Removal of nesting for experimental reasons should require scientific justification.