Is he infertile?

Question
QUESTION: Hi,
I bought a mouse from the pet shop who was supposed to be a friend for my girl, but it ended up being a boy rather than a girl which was what I was told. He is black and has a cute white lightning bolt mark on his forehead ( maybe I should have called him Harry Potter). :) Anyway, I've since brought another girl as a friend and seperated the boy. He spends alot of his time running around trying to get there attention from inside his cage and stands up watching them longingly. When ever my guinea pigs run around he rushes out of bed and tries to pull them into his cage. :'(
I've been thinking of getting another boy for him as a friend, but i'm worried the two will fight and i'll end up with two lonley boys in seperate cages which is the last thing I want -- and I would never consider bringing a mouse back to the pet shop once i've bourght it. Could you please give me any advice on how I can introduce the two so they can live happily together? I know to introduce them on mutral ground but that's all.
I really appreciate your time.
:) Katy

ANSWER: Dear Katy,

Male mice not raised together almost never get along.  Thus you would indeed end up with yet another cage if you bought another male.

It's too bad that males fight.  They do so because each one really wants a very large territory and no mouse cage will satisfy this need such that they could share.  So male mice usually have to live alone.

It is important that his cage be close to the females'-- which it sounds like it is-- so they can still communicate.  They squeak all the time in registers too high for us to hear.  It is also important that you be his mouse friend.  That means he needs intensive 'people-time' at least an hour a day!  You can probably sometimes hold all three together.  You should interact with him to his face if he will allow it.  He will become very attached to you-- and you to him, I guarantee!  

If he was in the cage with the girl for more than 5 seconds ; ) she is pregnant.  Well, if they were ever left alone together, she may be pregnant.  If this turns out to be the case, you might like to check out these posts in my archives about pregnancy, birth, and raising babies (pardon any redundancy):

http://en.allexperts.com/q/Mice-3824/2008/7/pregnant-1.htm

http://en.allexperts.com/q/Mice-3824/2008/2/pregnancy-infancy-1.htm

http://en.allexperts.com/q/Mice-3824/2008/8/mouse-reproduction.htm

http://en.allexperts.com/q/Mice-3824/2008/2/mouse-babies.htm

Best of luck with all the mice!  Do write back with further questions.

squeaks n giggles,

Natasha

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

QUESTION: Hi,
Thank you very much for your help. :)
He was in there with one of the girls when I first bought him from the petshop he must have been four to six weeks old. He was in there a cupple of weeks but that was months ago and she never became pregnant. Do you think he's infertile or was just too young at the time?
All the best and thanks again.
:) Katy

Answer
Dear Katy,

Mice can be sexually mature at 4 1/2 weeks at the earliest.  Thus he might not have been ready at 6 weeks old.  I think he just didn't figure it out yet.  It's unusual to have an infertile mouse-- the only ones I know of are ones whose genitals have misformed or who 'miss' when they mate (the sperm does not go inside the female). It is nice to have an infertile male, of course, because then he can live with females, but it's not worth taking the chance if you don't want babies!

squeaks n giggles,

Natasha