Mouse losing fur on neck and back

Question
I have two mice (each about three months old).  One has begun losing fur around its neck and back.  They tend to clean each other and sometimes, I think they chew a bit on each other because I hear them squeal.  I can't see if the cleaning or nibbling is causing the fur loss or maybe something else.  Have you had this problem before?

Answer
Hair loss is quite common in old mice but yours are still in their prime. Hair loss can be down to a number of problems. For example it may be environmental, stress related, a poor diet or an illness. They may have an allergy to the bedding or product you clean the cage with or are stressed out by a noise an electronic device is giving out.

As there are so many possibilities try to narrow it down to when the hair loss started and note if you did anything different. If you are still concerned take them to the vet. Please feel free to ask any more questions.