I keep 1.4 in concrete tubs on the rack i made and once i know that theyre all prego i move the male out and leave all the females there till birth. I have had times where i dont catch it or im off on the timing and males are left with the females but so far havnt seen anything wrong as far as the male killing the babies or anything but accidents do happen and have seen inadverdent trampling so i keep an eye on it to avoid this. I use alot less space by using racks then using 40 gal breeder tanks which take up way too much space in width and id have to use several of them for all the rats i have so the rack in the end comes cheaper as well.