Its a shame to have lost two mice at the same time.
How many do you have?
I find that mine tend to become ill at the same time, and a couple of times I've had mice from different tanks die suddenly. I put it down to some contagious disease which the vast majority are unaffected by, the deaths occur in immunity weak individuals which otherwise seem healthy. You say the buck was a pet shop mouse approaching a year of age, depending on his breeding that might be his natural life length so it may have just been a coincidence.
How many do you have?
I find that mine tend to become ill at the same time, and a couple of times I've had mice from different tanks die suddenly. I put it down to some contagious disease which the vast majority are unaffected by, the deaths occur in immunity weak individuals which otherwise seem healthy. You say the buck was a pet shop mouse approaching a year of age, depending on his breeding that might be his natural life length so it may have just been a coincidence.
