Rodent Laboratory Welfare: Refinements for Mice and Rats

Mice and rats account for over 80% of all laboratory animals used worldwide, yet their welfare requirements including social housing, environmental complexity, and humane endpoints receive less attention than their numerical dominance warrants.

Key Facts

Mice are the most used laboratory animal globally, with an estimated 60-70 million used annually in scientific procedures, followed by rats at approximately 10-15 million
Standard laboratory housing of mice in small groups in plastic shoebox cages provides minimal environmental complexity relative to the natural behavioral repertoire of mice as burrowing, nesting, foraging animals
Social isolation of mice causes measurable neurological changes in pain processing systems that can confound research results dependent on accurate pain responses, creating a scientific as well as welfare case for social housing
Burrowing and nesting are highly motivated behaviors in mice that are prevented by standard bedding depths insufficient for burrow construction and inadequate nesting material
Humane endpoint refinement, establishing earlier criteria for when animals in distress should be humanely killed rather than continued in procedures, has reduced per-animal suffering in studies with high severity procedures

Welfare Considerations

Laboratory rodent welfare improvements have the potential to benefit the largest number of laboratory animals of any intervention in biomedical research. The extreme numerical dominance of mice and rats in research means that even modest per-animal welfare improvements represent enormous aggregate benefit. Social housing improvements, environmental enrichment including running wheels, nesting material, and burrowing substrate, and earlier humane endpoints collectively represent validated welfare improvements with either neutral or positive effects on most research outcomes. The resistance to rodent welfare improvements often cited by researchers — concerns about confounding variables and standardization — has been addressed by evidence showing that enriched environments produce more robust and generalizable research findings rather than less reliable ones. Regulatory requirements for enriched housing for rodents have expanded in the EU and other jurisdictions.

What You Can Do

Support funding for research into environmental enrichment and housing improvements for laboratory rodents that maintain scientific validity
Advocate for mandatory environmental enrichment requirements for laboratory rodents in research institution animal care and use committee standards
Donate to organizations including the NC3Rs that specifically fund research into refinement improvements for the most widely used laboratory species
Engage with medical research charities to ensure they require high animal welfare standards from the institutions they fund
Support the development and adoption of standardized housing and enrichment protocols for laboratory rodents that balance welfare and scientific validity