Rats eating eachother

Question
I have a friend that had two hamsters (I know they're
different, but anyway) and they were both male. I suppose
the larger one ate the smaller one's penis...

I have a large rat, Sam, whom I have had for a while. I
just bought a smaller one, Paul, and when introduced, Sam
tried to flip Paul on his back and started doing something
to his "area." I don't know if this is normal rat behavior,
but if not, I would like to know ASAP!

Answer
I'm sorry for the delay getting back to you; I've been out with the flu.

So you're "supposing" the larger hamster ate the smaller hamsters penis based on what? Did the younger one die? Or..? I'm just wondering how you came to this conclusion. And actually hamsters and rats are extremely different - rats are much friendlier to their own than hamsters ever seem to be.. it's very easy to introduce rats when its done correctly.

Normal behavior when introducing usually includes some screeching and occasionally some puffy sideways dances, chasing each other around, flipping each other over on their backs and otherwise acting dominant. Rule of thumb is no blood, no foul. So if you don't see any blood - they're fine.