They are not very common compared to female rats. My boyfriend and I decided to get him 2 male rats instead of females when we decided to get back into owning rats, because I thought it would be better for him to have males and not have to experience the reoccurance with tumors females usually have. BUT his boys are now over 2 and both boys have tumors that will be removed soon. I've never had boys get tumors in the past, so I think these two boys are just "lucky". Boys can still get them, but they are definitely less common than females. Usually boys just get fatty tumors, that can be mammary or not.