Group housing of male CD1 mice: reflections from toxicity studies

Owing to their naturally aggressive behaviour, male mice are often housed individually in toxicity studies. However, several publications advocate group-housing of mice to enable normal social behaviour and interactions between the animals. This refinement project aimed at facilitate group-housing in toxicity studies. A handling procedure, including key factors such as allocation into groups before sexual maturation, transfer of used nesting material into clean cages and avoidance of external changes, that makes group-housing of male CD-1 mice possible in long-term toxicity studies has been developed at Safety Assessment within AstraZeneca, Sweden. Observations on the effect on aggression/fighting in group-housed male mice following different procedures performed in toxicity studies have shown that temporary removal of animals from the group for blood or urine sampling does not affect the group dynamics. However, temporary removal of animals for mating leads to fighting if the animals are taken back to the original group. Treatment with test compound might affect the general condition of the animals and the social hierarchy could be changed. In such cases aggression/fighting might occur and the animals have to be separated. Our experience clearly indicates that group housing of male mice in long-term studies leads to more easily handled animals, as compared with individually housed mice.