mice - immaculate conception or delayed implantation

Question
Ok,
I work at a child care center we have two female mice that lived together...they escaped, and were found a couple weeks later and put back into a new cage...They came back from the adventure in a family way.  We let them keep the babies for 3 weeks (the kids got to hold them), once the eyes opened we sent them to a "breeder" we had 30 unwanted babies...

Now, 3 1/2 weeks later one mom has had babies, and the other is ready to pop!  Did the babies do it?  or could the original daddy do it through the bars of the cage?  Delayed implantation?

The guy at the pet store said that they can have up to 3 litters from one encounter...dear god tell me that is not true!

Thanks tons

Garnet Shah

Answer
Dear Garnet,

Pet store workers are, unfortunately, so often unknowledgeable about mice.  I have never heard of mice having more than one litter from one encounter, though I will pose the question to a vet I know, and I'll change this answer with a notification to you if she tells me differently.  

In any case, your situation is very common.  A little male wild mouse is getting into the cage through the bars.  This even happened to me recently!  Unfortunately, the first generation of tame/wild hybrid mice are usually hard to tame (they are fine at first but then get wilder).  Of the ten I had, just one became very tame.  

I assume you mean the second litter came 3 1/2 weeks after the first litter reached 3 weeks of age.  This means the babies were 3 1/2 weeks old when the mother mice conceived again.  If they had been 4 1/2 weeks old, my answer would be different.  This is the earliest that mice can become sexually mature.  

I suggest a solid cage for the mice.  If you'd rather keep the wire one (which is better for ventilation), carefully cover the cage with 1/4 inch galvanized steel mesh.  You can either make a cover to put on the cage at night, or attach the mesh to the cage and make the door mouse-safe as well.  

Best of luck.  You  may find one more litter after you cover the cage, because mice can mate on the day they give birth.  

squeaks n giggles,

Natasha