Bouncing adult mice

Question
I have several female mice in a 20 gallon aquarium with a mesh lid. There are 5 females occupying it right now (3 adults - one which is VERY pregnant - and 2 weened adolescents at 5 weeks old).
I was wondering if it is normal for the adults to bounce around and "pop" like the popcorn stage. I find it happens especially much when I clean the enclosure or add new things to it (for example, I added the nesting box for the pregnant female today, just a small cardboard box with a toilet roll for entry and exit).

A secondary question; is it ok to have this many mice in the enclosure while the one female is so very pregnant? - I assure you they're all female, I separated the males when they were weaned. She came from the pet store rather pregnant about 2 weeks ago and hasn't given birth yet but shes absolutely huge now and looks like she'll have them soon. Shes become very tired lately, though she is still eating and drinking I find her sleeping most of the day by herself. I haven't seen her sides ripple yet while asleep so I'd imagine shes got a couple days left before birth.

Also, why do my mice push the bedding from where they sleep? they do this with toilet paper too from the nesting box (which I offered as soft bedding for the birth upcoming).

Answer
Hi Michelle,

Five mice in a standard 20 gallon aquarium should be just fine, but you may find them getting a bit cramped once the babies start to move around.  Typically I like to keep my pregnant mommas in their own homes alone, to reduce the chance that other mice might behave aggressively toward pups, but if the situation is stable now it would be better not to suddenly change mom's environment right before birth.  I trust your sexing - the only time I really suggest double checking is if someone is fighting...or mounting!  :)

Another thing to consider for the future meeces is to keep all new mice isolated for 3 weeks when they first come home - this can really help control incoming diseases (which are pretty common in pet shops, due to the frequently changing exposure and stress) and prevent one sick mouse from becoming many sick mice.  Again, it isn't a big deal right now, and it sounds like they are all doing wonderfully, but it can make a huge difference if/when something happens in the future, especially since moms and pups can't have traditional antibiotics.

As far as the popping - the popcorn stage can take quite a while to grow out of, so I'm not sure if it's normal for their age or not.  I don't consider a mouse an adult until about 2-3 months old, when they stop growing and changing.  If they are still popping after that, it may just be a learned habit that will never go away, but it's certainly not a bad thing.  In young mice, this response is an instinctual method to avoid predators swooping down - you might see it decrease a bit if you always approach them from the side (or one side of the aquarium, since they don't have side openings!), rather than from right above.  As long as no one is getting hurt by jumping from anywhere high, it's totally safe and just a little quirky.

Mice move the bedding around almost obsessively, especially when they are pregnant and nesting.  This is just them making themselves at home.  It could have to do with anything, including temperature, different scents from different mice, or just decorating the house!

Congratulations on your new litter, and let me know if I can help with anything else!  :)
-Tam