Effect of housing density on reproductive parameters and corticosterone levels in nursing mice.

The Guide for the Care and Use of Laboratory Animals (Guide) recommends minimum floor space per mouse based on weight, with no other factors considered. We conducted a randomized experiment to evaluate the effect of housing density on reproductive indices and corticosterone levels in lactating mice. Female mice matched for age, strain, and date-of-pregnancy were housed individually. At parturition the dams were randomly allocated to have litters culled or remain intact. The experimental group had litters culled to meet the Guide's space density requirement. Litters of the second group were maintained as the numbers born to each dam. Fecal corticosterone levels (first-generation mice only), growth, and weaning weights were measured for mice in all cages; in addition, the reproductive behavior of progeny generated under both housing conditions was assessed to determine whether a space x litter size interaction affected subsequent reproduction. The growth rates for pups from culled litters were significantly greater than those from intact litters. The first-generation pups showed no statistically significant differences in fecal corticosterone or reproductive parameters. The second-generation pups showed no statistically significant differences in growth rates. The results of the study suggest that a strict interpretation of space requirements as listed in Table 2.1 of the Guide is not warranted for lactating dams with litters.