Keeping littermates

Question
QUESTION: Two weeks ago I bought a pair of fancy mice and was told they were both female (the store clerk would not allow me to handle the mice to check for myself).  Upon returning home, I noticed that one was indeed female, but one was unquestionably male.  Within ten minutes, the male tried mating with my little girl (Sienna), so back to the store he went, as I don't have room for baby mice.  A week later I bought a new female (Scarlett) to keep her company.  On the first day, Sienna would occasionally hump Scarlett, but I know this can be a dominance thing, and she hasn't done it since they've bonded.  But I noticed yesterday that Scarlet is getting distinctly fat.  I thoroughly examined them both, compared Sienna to Scarlett, and am still 99.9% convinced that they are BOTH girls.  Also, I've only had Scarlett for 5 days, which seems awfully early to be able to see pregnancy swelling.  I'm thinking that the pet store sold me a pregnant mouse, but would like to know if I'm correct about 5 days being too soon to see pregnancy swelling.  I'm going to keep Scarlett regardless, but I can't keep a whole litter!  Thank you for your help.

ANSWER: I am very sorry this happened, what the store clerk did is disgraceful and should be out of a job. It probably won't make you feel any better but this is a VERY common problem. I have had so many questions regarding this sort of thing, mice already pregnant, mice turning out to be a different sex etc. You are not alone!

The clerk obviously doesn't know much about mice and doesn't appear to care. Unfortunately Scarlet may have been kept with males in the past week or two and therefore would be pregnant. Mice usually start to show at the end of their second week and go 'pear shaped' in the middle of their third week. You sound like you know what you are doing (unlike the store clerk) and are probably right about their gender and the pregnancy.

If you do end up with babies (I suspect you will) and don't want to keep them, I strongly advise you give them to friends and family or another pet shop.

On the bright side babies can be fun and won't cost much extra to raise.

If you need help if and when the babies come or have any more questions please don't hesitate to ask.

---------- FOLLOW-UP ----------

QUESTION: Scarlett actually had her babies before you had a chance to respond to the question!  She had her babies on Wednesday the 9th, and they are all doing quite well.

She had nine pups, six males and three females.  Naturally, I've fallen madly in love with them all, and now I'm trying to figure out if there's a way I can avoid giving them up.

I've only ever kept my rodents in pairs.  This will not be a problem if I can manage to get another tank for the boys, as there are six of them (and hopefully aggression will be at a minimum since they are littermates and there will be no females to compete for).  However, I'm worried about the girls.  As I mentioned, I have one other girl in addition to Scarlett, and that's Sienna (who has been a wonderful nursemaid to the pups).  This means that if I kept the three girls, there would be five total, and I'm not sure how the dynamic would work with an uneven number of females.

Alternatelly, my mother might want to keep a pair of the baby girls, which would still leave me with an uneven mix (three girls to the tank).

Could this be a problem?  As I said, I've never had an uneven amount of rodents living together, so I'm not sure how the social dynamic might change, and if the new littermate(s) being Scarlett's children might make it better or worse.

Thanks in advance.

Answer
You can keep any number of mice together as long as the cage is big enough. For mice the more the merrier so you can keep all the girls together.

You are right, the boys should get along fine as they are brothers. Again, make sure the cage is big enough so they don't fight for territory and provide plenty if hiding places such as tubes or boxes.

I'm glad shes raised a healthy litter, please feel free to ask any more questions.