Effects of cage density on inbred mice

The amount of space that should be allocated to each laboratory mouse in a cage is specified by the Guide for the Care and Use of Laboratory Animals (Guide) to ensure the humane treatment of the research animals. For the last few decades, there have been significant improvements in mouse husbandry and caging, suggesting a need to re-evaluate the housing density for current conditions. Many recent studies that examined the effect of housing density indicate that laboratory mice can be housed at about double the housing density as suggested by the Guide (Davidson et al.; Fullwood et al.; McGlone et al.; Paigen et al.); in some studies, investigators found that mice were less aggressive and exhibited better survival when housed at twice the recommended density (Fullwood et al.; McGlone et al.).

The Jackson Laboratory scientist Leah Rae Donahue and her staff performed an additional study with five commonly used inbred strains (129S1/SvImJ, A/J, BALB/cByJ, C57BL/6J, and DBA/2J). This study investigated the health and well-being of the mice using housing densities that were approximately 2, 2.6, and 3 times greater than the recommendation found in the Guide. Their results indicate that for 22 of the 27 traits measured, increased housing density had no significant effect throughout 3- and 8-month timeframes  (Table 1) (Morgan et al.).

Table 1. Traits unaffected by housing density

General	Clinical chemistry	Behavioral
Alopecia	Cholesterol	Fighting & tail biting
Areal bone mineral density	Eosinophil count	Light-dark test
Body weight	Hematocrit	Open field test
Heart & testes weights	Hemoglobin	Whiskering & barbering
Mortality	Lymphocyte count
Systolic blood pressure	Monocyte count
	Neutrophil count
	Platelet count
	Red blood cell count
	Reticulocyte count

Table 2. Traits affected by increased housing density

Traits	Strains affected
	129S1/SvImJ	A/J	BALB/cByJ	C57BL/6J	DBA/2J
Kidney weight	♀ ♂	♀ ♂	♀ ♂	♀ ♂	♀
Adrenal weight	♂	♀	♀	♀	♀ ♂
Heart rate	♂	♂	(-)	♀ ♂	♀
Percent fat	♀ ♂	♀	(-)	(-)	♂
♀: female mice; ♂: male mice; (-) no change in either male or female mice.

Interestingly, however, four trait values were found to be significantly affected as housing density increases for the five strains tested (Table 2).

In general, kidney weight, adrenal weight and heart rate were all decreased in response to decreased cage density. The percent body fat was, however, increased in three of the strains tested. Strains that showed an increase in body fat may be explained by increased cage temperature that is closer to the thermoneutral zone for mice. Housing density seemed to affect males and females differently within a strain. For example, the kidney weight of females was more likely to show a significant decrease than in the males; female C57BL/6J in particular showed the most consistent decreases. Different strains also responded differently; heart weight and percent fat were not altered in BALB/cByJ by increasing the density. Notably, measurements for all 4 traits were within physiological ranges, and the difference between the values at the lowest and highest densities was less than the difference between males and females within the same strain.

Adrenal weight and heart rate are often used as measures of stress (Naidu et al.; Sterlemann et al.; Bernberg et al.; Gilmore et al.), and adrenal weight is used to reflect degree of chronic stress induced hormone production. The trend in decreased adrenal weight and heart rate observed in this study suggests that mice housed more densely were experiencing less stress. These results are consistent with two previous studies demonstrating a reduction in heart rate with increased housing density (Nicholson et al.; Van Loo et al.).

Comparison to other studies

This study concurs with many previous studies indicating that many different strains of mice can be housed at twice the density recommended by the Guide. Furthermore, many previous results demonstrated positive health benefits for more densely housed mice, including enhanced immune response (Fullwood et al.; McGlone et al.) and reduced anxiety behavior (Davidson et al.; McGlone et al.).