Adult female mice generally don't tolerate a new male (you usually have to set up mice colonies when they are young just weaned). It's been years ago but I remember reading an article (National Geographic maybe) about mice and as I remember it the new males release a chemical in their urine to make all the females abort so that they can get rid of the old male's babies and re-breed the females sooner. Knowing this the adult females try to kill or castrate any new male.
Interesting....Then that may just explain the death of the last male I added a couple of week's ago. I noticed they were fighting him almost immidiately when he was added, and two day's later I found him half eaten. He must have been attacked by all of the females at once because he was much bigger than they were and he was pretty much mutilated.