I thought I'd put this here since these are pet rats ^__^

Anyway, I have a pair of male rats that are brothers. They've lived together literally all their lives. They're both dumbos, one blue capped, one black capped. Tonight I was watching television in the living room when I suddenly heard a horrible racket from the back room, squeaking and chattering. I rushed in and found Momo and Ringo rolling around yelling and screeching. They stopped when I slammed the cage door open, and came up to it. I could tell just from body language that Ringo (the black capped) had been the aggressor; Momo basically ran up to me going "Help!" while Ringo sat sullenly in the cage, fur puffed up, teeth chattering angrily. I checked Momo out and found one bleeding bite near the base of his tail; otherwise, he's OK. I cleaned him up and put him in the cage with the retired girls (no worries, they're all well past getting preggers and well acquainted with him) and went back to see if I could figure out what was up with Ringo. He was obviously still upset but he came up and sniffed me semi-normally; however when I reached in to try and check him for damage he bit me, not play-biting, but serious, drew-blood biting. Ouch!

So the question is, why is he acting this way, and what can I do about it? I've read that male rats will sometimes become more aggressive around 6 months of age and neutering can help with that, but Ringo and Momo turned a year old back in August, so I'd think they'd be past any testosterone-fuelled behavior changes by now. The only other thing I can think of is that it might be some kind of mental disorder? Anybody with some ideas or experience with this, please help! I really don't want Ringo to have to live in isolation for the rest of his life, or have him put down if his sudden aggression becomes unmanageable!