Nibbler the mouse

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 separated the boy. He spends allot 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 lonely boys in separate cages which is the last thing I want -- and I would never consider bringing a mouse back to the pet shop once I've bought 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 mutual ground but that's all.
I really appreciate your time.
:) Katy

Answer
I'm sorry the pet shop put you in this position. Although it won't make you feel better this sort of thing happening is frighteningly common.

Your mouse really needs a buddy as mice are social creatures and need the company of other mice. Try to find a male around the same age and sex ideally a litter brother. You can take him to a pet store to see which of the boys he gets along with. Make sure the cage is big enough for them to have their own territory, the bigger the better. Include lots of hiding places such as tubes and boxes.

It is very important that you watch them when they are first introduced.  I highly recommend keeping the cage with you as much as possible for the first few days. If you are going to out separate them until you get back, just to be safe. It is normal to hear squeaks from the cage and they will probably chase each other around a lot. You should separate them immediately if there is any serious fighting or blood drawn.

Keep the cage away from the females cage and if you are still having trouble try feeding them plain boiled rice for a few weeks as they may be fighting over the best bits of food.

You may not need to get him a buddy if your other mouse is pregnant. If they were together for 24 hours then there is a high chance your female is pregnant. He will happily live with his sons providing the cage is big enough.

The most important thing is to watch them, I cannot stress this point enough. Keep the cage in your bedroom for the first few nights and if theres any squeaking just tap on the cage until they stop. If they don't you may want to investigate. I know it seems like a lot of work but it's just to be sure neither ends up injured or dead.

Please feel free to ask any more questions. Good luck.