After the death of Belle, I noticed some behaviors with the remaining female mice that could be construed as mourning.
One mourning-like behavior I noted with the mouse, "Little" Quimby, was that within a short time Belle was removed from the nest, Quimby was frantically rushing around the cages and tubes like she was looking for someone. Quimby would also go sit up in the top wheel and insist on sleeping there by herself. She also would not put up as much resistance when I would try to pick her up.
Other mouse mourning-behaviors in the case of Dixie was that Dixie would isolate herself from the rest of the group. Sitting alone, sleeping alone and generally not wanting to interact.
Pudding, the mother of all the mice, also exhibited some behaviors. Belle and Pudding had come into my life together as a pair, sisters as they were, from out of the same pet shop at the same time. Pudding had also been nonchalant and independent. But after Belle's leaving Pudding started to be more snuggly, coming to the cage bars or doors and wanting to interact.
I am worried that there are some negative behaviors. One example is "Big" Quimby appearing to be aggressive with some of the smaller, younger mice. At first I thought it might have been over food or turns running on the wheel. But the behavior appeared more prevalent after Belle's death.
In the meanwhile I am planning to keep an eye on them and little details such as their eating habits and how they deal with cage cleanings.