Having evolved in a jungle environment, those rats are capable of fighting anything. The rats began exerting themselves several breeding cycles into the siege. A rat jumps on my chest one night. On my back on my cot, I slap at him with my left hand while I try to shield my face with my right. He is grinning at me, I swear.
Rats love the sandbag walls.
Since the walls are several layers thick, the rats have a lot of room for their quarters. You can hear them in there screaming, eating, fucking, and kicking each others' asses. Rats are nasty - they are always fighting. Rats behave more logically during the siege than we do. They let their feelings out.