Well guys, it ended up being a moot point. The little guy improved behavior wise (but was never "friendly"). However, he became super skinny...emaciated you could say, I could feel every bone in his body (even the vertebrae in his tail and his leg bones, hips, etc.). He had been skinny when we bought him, but me not knowing anything about rats thought that the small ones would be that way normally.

He was lethargic and breathing with a clicking noise and with each breath his head would "bob". Attempts to get him to gain weight by feeding individually (fatty foods, egg, olive oil with oats, etc.) failed and he started refusing food... So I did one of the hardest things I've ever done (although probably sounds silly to you guys). I euthanized him using CD. This was the first time I had done CD and I wasn't perfect at it. But at least the little guy isn't suffering any more. I now know that if I ever breed feeders for my snakes I will have to use a Co2 chamber. I don't think I could take doing CD multiple times.

After doing more research, I think he might have had megacolon because although he had no fat and extremely little muscle tissue, his stomach was a enlarged. He also did not gain weight like my other small rat purchased at the same time. I'm thinking if it wasn't megacolon it was another genetic condition that kept him from absorbing nutrients.

I was lucky enough to speak to someone who will give me a couple of male rats from a litter for adoption in a few weeks. I'm hoping that they will be healthy, and being used to being handled, will have a good temperament as well.

I have definitely been able to use your advice on my two other guys though. They do the taste / nipping and I will say "no" or make a noise loudly when they do this and tap them on the nose and so far, it seems that they've learned what is and is not acceptable. One of the two has never mouthed or nipped that I can remember, so if I do breed from these guys he may be the male I use.

Thanks again for all of your help!