We have three male rats. We've had the oldest for some time now and
he is one of the calmest, sweetest rats we have ever owned. We got
two new, young rats mid way thru last year and now the oldest of these
two has reached full adult hood and is trying to be the dominant male.
The problem is that the old rat doesn't know how to defend himself
and comes away from fights with deep cuts all over his body while the
aggressor walks away with no more than a slightly chipped claw. The
aggressor is now in a cage by himself and is very lonely. We want to
put them all back together but we don't know how to stop the fights.

Can anyone help?

Anything would be appreciated.