agressive mouse

Question
I got a new mouse about 4 months ago and at first the other mice were a bit aggressive but they learned to except her after i completely washed all of their toys and houses and introduced each one of them to them in a separate cage. recently the new mouse has become aggressive. she has been biting my eldest mouse, who is now on antibiotics and she has recently appeared aggressive toward the other 2 mice. i hear them squeaking and i she is always involved but i dont know if she is just accidentally pulling their hair during cleaning or if she actually is hurting then on purpose. is there any way that i could get her to be less aggressive? should i try doing what i did before or is there something new, i really dont want to have to separate her, i dont know what else to do if she wont stop biting the other mice.

Answer
Dear Sara,
thank you for your question.
Some mice will forcibly groom other mice, sometimes so often that the mice loose hair, typically around the snout. It's called barbering and if your aggressive mouse doesn't do anything else, I would just leave the group alone. Some mice will stop that behaviour after some time.

If she is biting and hurting other mice, I would try another introduction. Separate the aggressive mouse for a week or so and then slowly reintroduce her to the group. let them meet in the bathtub or in an empty cage. If that works, they can move back into the /cleaned) normal cage, but they shouldn't have any second levels, toys or furniture yet. Add one piece of furniture per day if things stay peaceful for a week. You can offer hay for them to hide in in the meantime.

If that doesn't work, it's probably best to look for another home in another group for the mouse. It's unusual for a female mouse to be that aggressive and in all cases I know of, they only showed that behaviour in one particular group and were much more peaceful in another group.
I hope I was of some help to you
Jennifer