Brother mice living together

Question
Hi Natasha,
I have a nice sized cage with six males in it. Its big enough for all of them, its not huge. I can't afford really big cages but its nice. One male, he's the biggest is picking on my other males. they've always gotten along before, there all brothers and they've never been separated in there whole lives. the big mouse's name is Duke. Duke was sniffing some of my males then they would run away from him and Duke would chase them the whole time they would squeak a lot. He's not hurting them, he's just sniffing them so i don't understand whats wrong. they run away from him but only when he sniffs them. if he's just sitting there the other males are fine. he never actually hurt them, i checked them all for any sign of aggression, no blood, cuts, or bite marks on any of them. is he just playing? is he just trying to dominate them?
please help me, I'm really worried.
from Hadley

Answer
Dear Hadley,

You are lucky that these guys have been getting along so well! And maybe it will continue.  A little chasing and squeaking is OK. As you know, blood is not OK. It is also not OK if the chasing and squeaking is nonstop; if anyone is blocked from nest, wheel, food, or water; or if someone becomes depressed, lethargic.

So you will monitor them closely. As you do, pay attention to see if there is any brother that the aggressive one never bothers. If you need to separate them, it would be great to have one brother he can live with. However, be careful experimenting, because you don't want to end up with a third cage because you try another brother; it doesn't work; and then the other brothers won't accept that one again. Keep the cages next to each other for now.

Best of luck.

squeaks,

Natasha