dominance

Question
Previously we have had 2 guinea pigs for 5 years. When one of them died the other became extremely lethargic due to his loneliness so we decided to get a companion for him. We cleaned the cage thoroughly and made sure they both smelled the same and they were happy. But later the elder guinea pig died and the younger one was in a similar lethargic mood. We bought two new guinea pigs (from the same litter) and redid the whole process of introduction. For months the three have been getting along fine, but one day we noticed the original one was all scratched up on his back. The vet told us it was mites and had him treated, but afterwards we noticed that one of the younger ones keeps chattering his teeth at him and attacking. The small one continued to attack and bite (especially in the ears) the older one. We have tried separating the trouble maker but as soon as we place him back into the cage the fighting restarts. The vet said we should retry the process of introduction and whenever they start fighting we should spray the younger one with a weak water gun. However this did not change the smaller ones attitude. The other smaller one does not show any signs of aggression and lets himself be pushed around. We were thinking about castrating him, but this will probably not change his behavior. We do not know what to do, please help.

Answer
Well, the main thing I would tell you to do is separate the aggressive guinea pig, and don't put them back together.  I know that guinea pigs are social animals and that they enjoy the company of one another, but in this case, you are just putting your older guinea pig through a lot of stress by keeping him with the aggressive one.  Separating them, and keeping them separate is your best bet.  

This doesn't mean that the aggressive guinea pig has to be alone all the time.  When you take your piggies out for play time, all three can play together.  Being out of the cage may calm the aggressive guinea pig's behavior as he will not be cooped up in a cage with the one he does not get along with.

Definitely keep the aggressive one in a separate cage alone though, because you don't want to put your guinea pig through the trauma he felt when he was left alone before.  

I hope I could be of some help, and once again, don't let the aggressive guinea pig be alone all the time, make sure you let him have some social interaction with the other guinea pigs outside of cage, and make sure you supervise them to make sure there is no aggressive behavior.

Good luck.