Barbering
The Science
Barbering is the partial loss of hair (alopecia) due to over-grooming by cage mates or even the rat itself (self-barbering). It is thought to be brought about from boredom, dominating, stress, or lack of suitable chewing materials.
Symptoms
Most typically, this kind of alopecia is usually found around the muzzle, head, and abdomen. There should be no signs of inflammation or scabs, with the skin appearing completely unharmed, but the skin may become increasingly pigmented.
Diagnosis
There should be no signs of broken and/or damaged skin, and hair will appear broken and chewed. Scrapings of the skin should be taken and examined to rule out other more harmful causes of alopecia (parasites, infections, etc). Try to discover whether it is self inflicted or another cage mate.
Treatment
It is generally harmless, as no actual physical damage is being done, and should require no treatment.
Prevention
Make sure there are plenty of objects in the cage to keep the rats from getting bored, as well as suitable material for them to chew on. Smaller colonies or bigger cages could also help.