Mice are preferentially kept in stable, harmonious groups. Introducing newcomers to a group or removing one or more animals, disrupts the social structure, requiring it to be re-established, which can lead to stressStress An animal’s biologic response in trying to deal with a threat to its internal equilibrium (homeostasis) by internal and external stressors. A failure to do so may lead to ‘distress’ resulting in an inability to cope, leading to poor welfare. and conflicts.

Male aggression

Dominant males are strongly territorial, even in the absence of females. This can give rise to serious fighting (see picture), which renders social housing of males virtually impossible in some strains. Cage enrichment, particularly the availability of hiding places, may sometimes diminish aggression.

Scents

Scented substances, pheromonesPheromones Scented substances that have a signaling function for the animal species that secretes them., and urine markings play a role in social behaviourSocial behaviour The gamut of behaviours displayed by an animal in the presence of other animals that are either not displayed when solitary, or are not displayed in the same manner.. Scents clearly influence the measure of aggression. Both the absence of (the smell of) nesting material and the presence of urine/fecesFeces Stool/droppings. Also called faeces. scents can lead to increased aggressive behaviour.