Group housed mice: are they really stressed?

Abstract

Group housing of male mice has been described as a stressful condition. In particular, subordinate male mice have been reported to show increased levels of stress-related behavioural and physiological parameters. In many studies, however, males are housed in groups of adult unfamiliar and genetically unrelated individuals. We review here a series of recent studies investigating the behaviour and the physiology of group housed male sibling Swiss CD1 mice. As adults, Dominants (D) and Subordinates (S) were subjected to the Open Field Test and the Free Exploratory Paradigm. Body-weights were monitored and relevant organs (adrenals, testis, spleen, thymus, preputials) weighed. Basal corticosterone level and immune competence were also determined. Results showed no difference between D and S in any parameters measured. Our data provide evidence that social status in a sibling context is not stressful for male mice because no differences were detected between dominants and subordinates. From a review of the literature, it was possible to conclude that what is stressful for a mouse is not the group-housing condition in itself but the lack of familiarity/ relatedness with respect to the cage mates. Furthermore, the majority of studies on social stress, are performed with animals having either no or limited experience of aggression, i.e. they lack a natural component of the social behaviour in this species. In contrast, we suggest that group housed siblings, living in a non-stressful condition and experiencing a non-deprived social setting, can be regarded as a valid control group. It is also suggested that living in a nonstressful condition, mice housed in same-sex sibling groups since birth, also provides the ideal social environment for the welfare of the laboratory housed mice.