Now this is a fascinating video(s) of 2 groups of domestic rats let loose in a controlled but feral environment. You can see the changes of these young PEW and black hooded rats as they revert to more wild behaviour. This film really helped me understand my pet rats more than most.
You can watch the film chapters on the left, or if you are dialup they have pics and a written description of each chapter.
http://www.ratlife.org/Home/0Main-frameset/Mainframeset.htm
Females are less aggressive overall because its a wild rats instinct to procreate and more females means more procreating. The males on the other hand are trying to have only their genes continue to the next generations plus they are the protectors of the colony.
Rats social hierarchy is fascinating even with large groups in cages.