Should I bring her back? Fighting.

Question
QUESTION: It turned out my mouse wasn't pregnant.
She was sick and had to go back to the pet store and had to be replaced.
My new mouse is very sweet her name is Butterscotch shes very tiny and my other mouse Oreo is much bigger.
Oreo won't stop fighting with Butterscotch. Oreo is the one starting all the fights. She even bit Butterscotch's tail and you can see where the bit is. Im afraid shes too aggresive for Butterscotch.
The people at the pet store told me to change up the cage I was going to put them in and clean it good, which I did but Oreo still went after her.
I tried putting one mouse in a different cage and one in a ball and then putting the mouse in the ball in the cage with the other so they could get to know each other but when I opened the ball they still fought.
Butterscotch wont stay in the cage with Oreo. She squeezes out of the bars. So I have them seperated right now.

What should I do?

ANSWER: Dear Tiffany,

I am sorry for your mouse troubles!  I hope she wasn't in pain.

Butterscotch should not be in a cage she can escape from.  With or without Oreo, she will escape  and likely get into trouble even if she does intend to return from her adventures.  I have to admit I have had a couple of instances where my mouse was getting out of the cage at night and exploring, unbeknownst to me, but it's dangerous.  For a little mouse sometimes you have to go with a solid cage such as a glass or plastic aquarium.  

Oreo is not only bigger than Butterscotch, she was there first and it's her territory.  You need to give Butterscotch an advantage.  Keep them in separate cages for 3 days (Butterscotch's cage must start out very clean), then put Oreo in with Butterscotch.  The cage will smell like Butterscotch and she will have the advantage of territory.  I hope you will see a lot of sniffing and maybe a little aggression from Butterscotch at first.  They are going to chase and squeak for a while until Butterscotch gives in and offers her throat to Oreo, saying "go ahead and kill me, I'm only a baby."  Big mice stop when a little mouse does this.  This might need to happen a few times.

If Oreo bites Butterscotch again, it sounds like they are not going to get along.  I have had some very sweet mice who simply could not get along with other mice.  If you keep them in separate cages, be sure to keep them close together.  They do communicate in little high squeaks that we can't hear, and they do not like to be alone.

Best of luck to the three of you!

squeaks,

Natasha



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

QUESTION: Oreo is still under the pet stores 15 day guarentee and she doesn't seem to be warming up to me at all either. Ever since her cagemate had to be taken back shes been acting alot different.
If she is still agressive to Butterscotch should I call the pet store and see about giving her back? I dont want her to be lonely but I also don't want her to fight with Butterscotch.

Answer
Dear Tiffany,

It sounds like maybe Oreo just isn't going to fit into your household very well.  When you bring her back, get a mouse the size of Butterscotch.  Put them in a completely clean cage with no toy unless it was washed carefully.  Thus neither mouse should have an advantage.  They may go through a period of chasing and squeaking, but if there is no bloodshed it is completely normal.  The squeaking is not pain or fear, it is communication.  Hopefully it won't be long before they have established their relationship to each other and settle down happily.  

Good luck!

Squeaks,

Natasha