Physiological effects of housing density on C57BL/6J mice over a 9-month period.
The National Research Council has consistently recommended floor space for animals used in science and agriculture. For mice, the recommended floor space is 77.4 cm(2) (12 in(2)) for a 15-25 gm mouse. The Council noted that its recommendations were based on "best professional judgment" and encouraged alternatives that were data driven. As part of a continual effort of The Jackson Laboratory to ensure the health and well-being of production and research mice while promoting cost-effective, state-of-the-art research, several density-driven studies have been conducted by lab researchers. The objectives of this study were to determine the effect of housing density on variables related to mouse physiology and air quality in the cages and to assess the value of specific measured variables in such studies. The study discussed in this report monitored C57BL/6J mice in individually ventilated cages from weaning until 9 months of age. Housing densities were equivalent to 66.4 and 36.8 cm(2)/mouse (10.3 and 5.7 in(2)). Clinical physiological variables representing general health and well-being were measured. Hematological traits, plasma lipids and glucose, growth, bone mineral density and percent body fat did not differ between densities. In the more densely housed mice, however, adrenal glands were significantly smaller, heart rates were significantly lower, and food consumption was less. Cage air microenvironment was evaluated for ammonia, carbon dioxide, temperature and humidity in cages changed weekly or every 2 weeks. The cage microenvironment remained within acceptable limits at the higher density of mice at both cage-changing frequencies. The results suggest that mice housed for as long as 9 months at up to twice the density currently recommended by the National Research Council show no measurable adverse effects. Continued re-evaluation of the recommendation by measuring additional relevant variables of health and general well-being and studying additional strains is warranted.