:( I know... cancer is awful, both in animals and in humans.
Ya, I heard about mice being territorial. Hamsters are as well (I've had a few when I was a kid). As long as rats are in same-sex groups they're fine, until one starts thinking they can fight for the alpha position, lol.
Ugh, cats and their instincts. I know it's their nature to hunt, but it still makes me mad when I see a cat go after some poor defenceless animal and harm it, leaving it there, and then casually walk back to its home and get a big bowl of food, etc. (>_<)