New Mice Fighting

Question
I recently got 3 male mice. The woman at the pet store who sold them to me told me that it was one of her best groups considering all 3 got along well together without fighting and were from the same litter. I took them home and everything was fine but now a week later one of the mice (AMJ) won't leave one of the other mice (PJ) alone. AMJ has no problem with the third mouse, only PJ. If PJ is in a house, AMJ has to chase him out. If PJ is eating, AMJ chases him away. I was wondering if there was something that set off this fighting and if there is anyway to discourage it. The fighting is not non-stop just randomly throughout the day and only lasts about 10 seconds at best. I check PJ for injuries every single time and so far he has not received any. I'm wondering if eventually it will stop or just get worse and I should remove AMJ from the cage and get him his own. Any ideas what to do? :x

Thanks for your time.

Answer
Dear Michelle,

It is very rare that boys, even littermates, don't fight. A couple of things might have started this change. If they are young, they are maturing. If they were living with others, the group dynamic has changed. The new cage and toys are also something to fight over. And the move was unsettling.

When I give advice about separating mice, one rule is no blood. But I also recommend separation if someone can't get to nest, wheel, food, or water. So it depends on how often this happens. Once or twice a day may be ok. Repeated harassment is problematic. The third reason to separate is if a mouse is depressed- sluggish, inactive, quiet, sits in the corner or the wheel for periods of time. If these conditions don't  occur and the chasing is only sporadic it is OK.

As you likely know, male mice usually have to live alone. If two of the mice just can't get along, you will have to separate them. It is up to you to decide who to take out - remember, it isn't the aggressor's fault, he is just being a mouse the best way he knows how. So try to figure out who will be happiest, without thinking of punishing AMJ. Keep the two cages very close but not in physical contact. A new mouse would definitely make things worse.

I hope the boys can resolve their differences.

Squeaks,

Natasha