introducing baby mice to existing females

Question

Young mice   Hi Tamarah,

I have two adult female mice and I am trying to join them with my two new, smaller, young female mice. (I'm not exactly sure how old they are, but they are not yet full-grown.) There has not been any blood yet, but the fighting seems excessive and I am feeling really bad for how stressed and scared my new mice seem.

Originally, I bought only one, new mouse. I bought a new aquarium that was bigger and new toys for them. I have read many places on the internet that it is best to have three mice in anticipation of the death of the first, so that the remaining doesn't end up alone. I put the new mouse in the new house and then added in my existing girls. They chased her around non-stop. She got really stressed and kept just spinning around on the disc spinner because they couldn't get her as well there. After an hour or so, I offered her water and she drank a lot, which lead me to believe that she was super thirsty, but so scared that she wouldn't leave the spinner to drink. Plus, she wouldn't eat even treat food, which I figured was a sign of stress. So, I took the older mice out for the night. I decided that maybe having another young mouse for her to be friends with might help her be less stressed, so last night, I went and got a fourth mouse from the same store. I believe that they still remembered each other from the pet store, and they got along perfectly. I let them sleep through the next day by themselves. By the next day, the first younger mouse was eating normal and they both seemed happy.

Now, I'm trying to reintroduce the two older mice back into the cage. I did try the vanilla extract, but it doesn't seem to be enough to do the trick. It is soooo hard to watch them fight. The young ones are stationed on top of a piece of tubing where they older ones can't get them as well. (They can still fight a little bit, but don't seem to chase them the same way.) They've been down to the main part of the cage twice, and the older ones fight non-stop again. I feel so bad for them. They are stressed and can't access the food, water, or play on the toys out of fear.

At what point, should I separate them? The young ones are exposing their bellies and showing submission, but the older ones won't stop! As I said, no blood, but I think I have seen the "boxing" move a few times.

Most everything I have read online says to simply persevere and that they will eventually work out the pecking order, so long as there isn't blood. But, some have said that the boxing is a bad sign too. And some say to separate and then reintroduce if there is fighting. There is a lot of conflicting information out there are the best ways to introduce females and I'm not sure that I'm doing it right. Plus, in this scenario, the young ones are so obviously the submissive, scared ones. My older ones are having a blast in the new cage, with the new toys, etc. I think that might be part of why they are fighting when the new ones when they go into the main part of the cage because the older ones like all the new stuff so much and want to claim it as their own. Would making the cage temporarily more "boring" help or make it worse because they would be less distracted?

If the younger ones are too scared to properly drink water and eat food, how long do I give them to sort this out amongst themselves? I will offer the young ones the water bottle to make sure they don't dehydrate, but I can't do that 24/seven. At this point, I'm going to take out the older ones for the night to give the younger ones a chance to relax and because I can't supervise very well while I sleep and I'm scared for them. I don't want anyone dehydrating or getting blood drawn while I sleep!

What do you think is the best course of action at this point? Is it better to try to introduce only mice of the same size? I thought maybe younger ones would be easier to introduce, since the pecking order would be more obvious. Do I just need to worry less and let them work it out, other than watching for blood and making sure everyone drinks water? Thank you so much for your advice! I am new to having mice as pets and really enjoy them.

Answer
Hi Mahriah,

The information out there is very conflicting indeed, but what it basically boils down to is trusting your gut and what works, works!  I think making the cage more boring is a very good idea, and something I was actually going to recommend.  You can also spread the food out over the entire cage to help prevent guarding of the resources.  Boxing is a sign that they should be watched (because they feel seriously threatened, and a threatened mouse may behave unpredictably), but is not really a promise of aggression.  To the older mice, it is more about making sure the younger mice are "clear" on the pecking order, even though it seems awfully obvious to the younger ones!

I usually give them about 2 or 3 continuous days - by that point they should really be settled.  If you just feel like it is too dangerous, don't take a risk - keep them separated until they grow up a little bit, that is okay.  Then you can try again with a boring, freshly cleaned, vanilla-dabbed setting and see if the little ones can handle it with a little less stress.  If you think they aren't being violent, but are just chasing, then you can let them tough it out, but going longer than a couple of days when the younger ones are too scared to go to the water bottle is just not fair to them in my opinion.  One other thing you can try is reversing who you introduce - putting the younger ones in with the older ones, instead of introducing the older ones to a new cage that they might feel the need to sort of take over if that makes sense.  Sometimes a smaller space or flipping it around like that can make the difference.

I hope that made sense, but if not let me know and I'll be happy to help out more.  You are doing a really great job, though, and it sounds like you've already done your homework!  Best of luck,

-Tam