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 t o aggressive behaviour. From a welfare perspective, individual housing is n ot 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 proc edures has been repeatedly proposed as a means of reducing these peaks in a ggression. In this study, the aggression-modulating properties of olfactory cues were studied by investigating the effects of their source and distrib ution 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, howe ver, affected aggression to a marked degree: transfer of nesting material r educed 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 weig ht more rapidly than subordinate ones. We conclude that the transfer of nes ting material will reduce aggression, or at least slow down its development , and thus aid the reduction of social tension due to cage cleaning.