Pair of mice wont stop fighting

Question
Dear Natasha,
I bought two mice yesterday. The employee who helped me said I was getting a female and male. They were fine on the way home in one of those little cardboard carrying cases, but when I put them in the aquarium together, the big one attacked the little one and made it bleed. It happened several times so I ended up putting a divider in the aquarium. I had a male and female before, and they never fought, so today I checked the big one to make sure it was a female, and I believe it is a male. (Both mice appear to have the same distance between gentials/anus) They have not had one moments peace together, so do you think I should see if I could return the big aggresive one and possibly buy a new one that's the same size as the little one? (If so, should I try getting a female or male?) Or should I simply seperate them?
Thanks in advance!
-Carol

Answer
Dear Carol,

Probably the last thing you want as a new mouse owner is a male and female together, because before you know it you may have more mice than you can handle!  People say rabbits breed a lot, but mice are far more prolific.  If you are serious about breeding, check out the following previous post of mine:

http://en.allexperts.com/q/Mice-3824/2008/10/Breeding-Mice-1.htm

It sounds to me like your two mice are not going to get along.  If it is two males, this is no surprise.  Males generally have to live alone because they fight.  Thus you would simply separate them, but keep the cages close so they can communicate.  If you have females, and want females, which is the easiest choice, I agree that you should return the aggressive mouse.  Female mice are not usually that aggressive.  You should be able to have two girls who get along.

When new mice are introduced, you can expect the chasing and squeaking, but you are correct that it's not ok for one mouse to bite the other.  My rule of thumb is, no blood.  One way to help the introduction along if you get a new mouse is to put a tiny dab of REAL vanilla essence on their butts and necks.  This confuses them and makes them smell the same.  If one mouse is bigger or more aggressive, start by putting that mouse in a cage where the smaller mouse has been living for at least a day.

Best of luck and have fun!

squeaks n giggles,

Natasha