so far knock on wood i have had no problems with breeding rats. I even leave the males in with them and they do just fine. Every now an again a baby will be eaten but thats to be expected with breeding them. Its gonna happen to everyone at some point. I leave them in the dark for the most part and give them plenty of food and water. So far no problems. I just picked up a few new breeding groups that have been breeding for a few months from a local so i will let you know how they do in the rat rack. The rats ive been breeding are 2 males and 4 females. I average about 13-18 rats per litter.