Modulation of aggression in male mice: Influence of cage cleaning regime and scent marks

Group housing of male laboratory mice often leads to welfare problems due to aggressive behaviour. From a welfare perspective, individual housing is not a preferred solution to these problems - and so we sought other ways of reducing aggression between male mice. Aggression peaks after disturbances such as cage cleaning. Transfer of olfactory cues during cage cleaning procedures has been repeatedly proposed as a means of reducing these peaks in aggression. In this study, the aggression-modulating properties of olfactory cues were studied by investigating the effects of their source and distribution on aggression after cage cleaning in groups of male BALB/c mice. The physiological effects of aggression on individuals within a group were also monitored. Our results indicated that neither kinship nor distribution of urine marks affected aggression. Olfactory cues from nesting and bedding material, however, affected aggression to a marked degree: transfer of nesting material reduced aggression significantly, while transfer of sawdust containing urine and faeces seemed to intensify aggression. None of the physiological data revealed any differences between dominant and subordinate animals, nor any correlations with aggressiveness, except that dominant animals gained weight more rapidly than subordinate ones. We conclude that the transfer of nesting material will reduce aggression, or at least slow down its development, and thus aid the reduction of social tension due to cage cleaning.