Suddenly agressive behavior

Question
I take an animal science course and we have 4 rats 2 males and 2 females. We got the males (brothers) in the beginning of the year and they were very friendly and social. They would spend long periods of time eating on my shoulder or sleeping in the hood of my sweatshirt. We got 2 females to breed them we put each male with 1 female. The actual breeding went great the males and females got along immediately and there was no problems. While with the female one of the males became super aggressive. He attack ANYONE that puts a hand in his cage and will bite people if they rest on arm on his cage or stand to close. he did bite for the first few days after we got him but he became very tame and seemed to love being out on peoples shoulders or just being held. We are afraid to put him back with his brother (who behaves fine) because of his sudden aressive nature. do you have any idea why he would have such a sudden and complete change? He has hurt several people and made me bleed at least 5 times so far. it has gotten to the point where I am the only one that will go near him

I have lots of experience with animals especially rodents and i have never seen such a sudden or dramatic change in a animal before. i feel bad for him because i really am the only one takeing care of him and im not there all the time so i fear he will go without food sometime because everyone else is afraid to put a hand in


Answer
You forgot one crucial bit of info that will help me answer your question.........How old are the rats, especially the age of the aggressive male?

I have had male rats that seem to get full of themselves when they have been used as a breeder. Sometimes these boys cannot be reintroduced to their former cagemates as they will fight for dominance, even if it means fighting the already established alpha.

Some males get hormonally aggressive when they reach 5 to 6 months of age.

Here's more info to help you figure it out.
http://www.ratbehavior.org/Aggression.htm

Feel free to e-mail me again if you need to.

spazrats
http://spazrats.tripod.com