By leaving the males in all the time you put a lot of stress on the females as they typically will get pregnant right after giving birth.


What ends up happening is the mothers body has to decide to use what nutrients she has to either feed the already born (by producing milk) - which usually results in small future litters - or - her body focuses on the unborn and you get normal sized litters but the babies that should be nursing don't get enough milk and die or are very very small.

I recommend you rotate one male. Leave him in a tub with the girls for a week or 2, then rotate him to the next tub (again for a week or 2), then move him again and so on. Don't put him back in the first tub till those babies are weened - and the females have had a few weeks to recoup and regain weight.