Sexing and separating baby mice

Question
QUESTION: Hi,
We started with two mice, which we were told were two males - wrong, so we ended up with nine mice.  We have found a home for one male and decided to keep another two males (hopefully they will get along).  We took two males back to the shop and also decided to keep another two females, with mom.  The pet shop sexed the mice and we had also had a go (even though we are not experts and agreed with this).  However, this morning, I caught one of the 'female' baby mice trying to mount mom; the babies are five weeks old today.  I have taken this mouse out, just incase but hope that I have done so in time!  On looking, this mouse does still appear to be a girl when sexing.  Can a female mouse get confused, or is it likely that this mouse is a male?  If the latter, what are my options?  Our niece is taking one this weekend and as one male and one female are the same colour, she could perhaps take this one instead.  However, this would leave us with three males together and I am not sure about this as an option.  I would be grateful of any advice.
Thanks

ANSWER: Dear Karen,

Let me start with an excellent link for how to sex mice of all ages:

http://www.thefunmouse.com/info/sexing.cfm

Here you can see clearly that there is different spacing between the genitals and anus of the two sexes of mice.  Hopefully that will solve that problem.

Female mice do mount each other as a show of dominance.  However, it would be quite bold of a 5 week old baby to try to show dominance over her mother.  If this is a male, it should not be sexually mature yet:  While females can be ready to mate at 4 1/2 weeks, most males are not ready until 6 weeks.  Most likely mom is not pregnant again.

There is unfortunately no guarantee that even two brothers will always get along.  Brothers or a father and son pair who have never been separated are pretty much the only cases in which males can get along for life, but even so it's not uncommon for them to begin to fight once they reach a certain age.  The rule for fighting is it's OK unless there is blood.  In that case each male must live alone; this is a sad truth for pet male mice.  A mouse who is alone should be as close to the other cages as possible because they do communicate in a high pitch that we cannot hear, and they do keep each other company.  A single mouse should also get at least an hour of quality people-time a day to make up for not having mouse friends. You can also probably handle the mice at the same time.

Because mice are much happier with friends, it would be best if your niece takes a male, since they usually have to be single mice anyway.  If she can take two or three females, and the males stay with you and in contact with each other, that would be even better. Females should never have to live alone.

I've tried to give you all the information you need to make your decision.  I wish you the best of luck and you and your niece much fun with your little buddies : ))

squeaks n giggles,

Natasha



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

QUESTION: Thanks for the response but I am a little confused.  The pet shop told us that our original mice was about four weeks old.  If this is true, then mom had her babies when she was about six weeks old.  If the gestation period is three weeks, if as it seems there was a mix up at the pet shop, it would seem that the male culprit, whether ours or in the pet shop, was therefore younger than six weeks.

Thanks greatly for your advice, I just have one additional question as to what to now do with the mouse that we have taken out of the cage presently, away from mom.  I guess perhaps we need to try and be sure of what sex it is.  If we are convinced it is a girl, could it go back with mom and sister?  If we think it is a male, I assume that it would be best not to put with the other two baby males, as it could cause a problem as has already been separated from them.  I understand that the two baby males we currently have may not get along and we that we may have to review this at some stage, fingers crossed that they do.  What if we are still not sure about the sex of this particular mouse?  Are the options to keep in a separate cage or ask the pet shop if they will take it back?
Thanks

Answer
Dear Karen,

Pet shops make an awful lot of mistakes (as you found out).  They probably were not accurate on the age of the babies.  Also, since girls are mature at 4 1/2 weeks, if there had been an older male in the cage before you saw them, he would have been the father.  I'm not going to guarantee that a male wasn't ready a little early, but 4 1/2 for girls and 6 for boys is usually pretty close. Still, I tell people to separate at 4 weeks just in case, for good reason!   

The females can be put back together.  Males generally cannot, though you can do it as a temporary measure when they are still small and see what happens.  Baby males don't fight yet.  If you have trouble you can return the mouse.  

It will soon be obvious what sex the little mouse is.  You will start to see very strong differences in the genitals in the next two weeks.  It's easier to see on little boys if you pull their tail up when you look at them.  

Best of luck and have fun :)

squeaks n giggles,

Natasha