group intro for elderly mouse

Question
Hi Tamarah,

I have two mice that are between a year and a half and 2 years old. One of my mice, Lupe has a terminal cancer. I have taken her to the vet and she is currently on "hospice" care until her tumor begins to cause her pain, at which point I will opt to put her to sleep with my mouse vet. My other mouse, Maggie is healthier, but still appears elderly and is also overweight at about 80 grams. She is fairly slow moving, but I still believe her to be a happy mouse.

My question is actually about Maggie, my healthier mouse and what to do when her friend passes away. I don't want her to live alone because I don't want her to become depressed. My boyfriend has three mice and I would ideally like to combine her with them. However, I am worried that the introduction will not go smoothly- the reason is that my boyfriend's mice were originally bought to live with mine. We tried to introduce the two new mice to Lupe and Maggie not long after I bought them. I actually consulted AllExperts on that introduction because it did not go smoothly. Maggie was the primary aggressor and she drew blood from the young mice, so at that point, we decided to just have two enclosures.

Is it possible to have a successful group intro for Maggie if she previously had been very aggressive? How does aging change the social hierarchy? If she has always been the dominate mouse, would she still be dominate with the younger, more active mice? Maggie is also much larger than my boyfriend's mice, who weigh between 35-50 grams. I assume that it is worth trying to integrate her, but what if it's not successful? Will Maggie be happier if their enclosures are near by, so she at least talks with other mice? Or does that just stress her more? (We don't live together.) The other thing is that Maggie really isn't too socialized, either, so she doesn't really draw comfort from socializing with me. She doesn't mind being handled, but she doesn't seek it out.

Thank you for your help! It makes me so sad about Lupe and it is awful to also have to worry so much about Maggie after Lupe passes.

Answer
Hi Mahriah,

Sorry to hear the news about little Lupe.  Not all female mice draw comfort from other females, as you've seen with Maggie in the past, so it isn't necessary to stress her out more by trying again (in my personal opinion).  Chances are that if she did not get along with them before, she probably won't at an older age, when she is more used to a certain hierarchy and routine.

I would recommend interacting with the two of them more frequently, so that when Lupe passes on, Maggie will be better able to lean on you for emotional support.  You may wish to try introducing everyone again, but I worry about stressing Lupe if things do not go well, and if there is any biting, potentially having to deal with injuries, too.  I don't think that having them near each other would have any benefits; I think you may be right that it may cause undue stress.

In the end it is up to you, but be prepared to immediately separate them should anything go wrong.  Chasing and squeaking for the first day is normal, but biting, boxing, or scratching would be cause to abort the mission!  You can also begin by introducing them in a neutral space, rather than in one cage or the other, and by providing lots of hiding places.  Whichever you decide to do, I wish you the best of luck, and let me know if I can help in any other ways!

-Tam