Laboratory conditions matter: social housing in male and female laboratory mice in a multicenter study approach.

Despite numerous studies and literature reviews, the social housing of laboratory mice is still under debate. Aggression and social stress can lead to physiological and behavioral alterations, especially in male mice, whereas single housing is a potential source of discomfort, particularly in female mice. This multicenter study aimed to assess the impact of single versus group housing on severity in both sexes and to examine whether a general recommendation could be made. C57BL/6NRj mice from the same litters of a commercial breeder were housed either singly or in groups of four across four different laboratory sites. Here we assessed fecal corticosterone metabolite concentration, aggressive behavior, wound scores and observed behavioral readouts on locomotion, stress coping in the forced swim test, burrowing, nesting and anxiety. Differences due to social housing conditions were not very pronounced. Instead, we found that, despite harmonization efforts, the differences in outcomes across the four laboratory sites were substantial. Furthermore, the individuality of the mice appears to have an additional influence on the results. These findings suggest that optimal housing is not generalizable but depends on the specific laboratory conditions. Beyond meeting the minimum housing standards as set by legislation, we should actively embrace refinement as a guiding principle and strive to create housing conditions that not only meet scientific needs but also continuously improve the welfare of the animals entrusted to our care.