fighting rats

Question
I currently have 4 male rats. I purchased one from a local Petco at about 18 weeks old. The other three I got from seperate breeders at around 9 weeks old. I put them together and they got along fine for about the first three weeks. Then the younger ones started bullying the older one. They bit his neck so many times he was bleeding and they constantly attack him. Why have they just now started this and how do I get them to stop?

Answer
Sounds like they've all hit puberty and are now picking on the odd-man-out. Have you considered having them neutered? Have you noticed its during any particular time of day, or is it just random? Is it one more than the other two or do they all just gang up on him?