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.

The use of aromatic plants and their essential oils for ectoparasite treatment is a field of growing interest. Several species of birds regularly introduce aromatic herbs into their nests putatively to reduce parasites. The behaviour is most often seen in cavity nesting birds and after nest building has finished. The plants are included in a non-structural manner and are often strongly aromatic. Various different hypotheses have been proposed regarding the function of this behaviour; from the plants altering some non-living factor in the nest (crypsis, water loss and insulation hypotheses) to them being involved in mate selection (mate hypothesis) or even having a beneficial effect, direct or indirect, on chicks (drug or nest protection hypothesis, NPH). Many studies have been carried out over the years observing and experimentally testing these hypotheses. This review focuses on studies involving the most popular of these hypotheses, the NPH: that plants decrease nest parasites or pathogens, thereby conveying positive effects to the chicks, allowing the behaviour to evolve. Studies providing observational evidence towards this hypothesis and those experimentally testing it are discussed.