Occasionally, with aggressively territorial male rats will fight with other rats, leading to injuries. These injuries are rarely fatal.
A1: Not unless they're cannibals. A2: Sometimes two or more rats who don't get along may kill and/or eat each other, and if a female rat who just gave birth is stressed or disturbed while or just...
to make the exterminators think they got all of them when most of them had escaped.
There are really just regular Brown Rats also called the Norwegian Rat, but they were just "mutated" at the lab.
To mate most likely.