Effects of housing density on weight gain, immune function, behavior, and plasma corticosterone concentrations in BALB/c and C57BL/6 mice

The Guide for the Care and Use of Laboratory Animals contains recommended housing densities for rodent species that are commonly used by the scientific community. However, at the time of the Guide's publication, housing density recommendations were based heavily on the professional judgment of qualified scientists. Some scientists therefore question whether rodents can be housed at greater densities, whereas others wonder whether the space currently provided for rodents is sufficient. The present study was designed to determine the effect of housing adult female BALB/c- and C57BL/6-mice in standard 75-in2 (484-cm2) ventilated cages at various housing densities (n = 2, 5, and 10 mice/cage). Measures of weight gain, plasma corticosterone, behavior, and immune parameters were evaluated at 7, 28, and 70 d after housing allocation. Housing BALB/c mice at 10/cage had negative effects on weight gain, corticosterone, behavior, and immune parameters. Housing C57BL/6 mice at 10/cage did not affect immune function or weight gain, although behavior and corticosterone showed statistical trends implying a negative effect Differences associated with housing densities of 2 and 5 mice/cage were less robust for all variables measured. We conclude that housing female BALB/c mice at 10 mice/cage (that is, at twice the Guide-recommended density) affects their physiology. We also conclude that mice vary in their responses in the parameters measured. These observations support the conclusion that it will be extremely challenging to scientifically determine an optimal cage density standard that can be uniformly applied across all mouse strains.