'Barbering' is a unique condition seen in mice who are housed together. Mice, like chickens, develop a pecking order. As a sign of dominance, one mouse will literally chew the fur and whiskers off of the more submissive mice. This may result in having only one mouse in a group of mice having all of its hair - the dominant one. Overcrowding, age at weaning, and diet may influence the amount of barbering, which may also have a learned-behavior component. Some genetic strains of mice may be more prone to 'barber.'
Clearly, the way to prevent this condition is to separate the mice. If only the dominant mouse is removed, another (the second-in-command) may assume the role of dominant mouse and also barber the subordinate mice. Barbering is seen more commonly in groups of female mice. Male mice will tend to fight instead.