The following is based on Project V13 promotional material and is not canon.
“It looked a like a brown furry blanket undulating across the ground. Make that covering the ground. All the ground. It was massive. A fuzzy flood. I’d heard stories of huge ant colonies, but this…Definitely not a snuggly blanket. I could hear the chirping. A high-pitched roar drowning out the gun fire – no mean feat. They just engulfed everyone and everything. Men turned into furry mounds, then collapsed and the chipmunks would disperse to another target.”— J. Hardy Murphy
The origin of these creatures is unknown, but they have been able to survive the conditions left behind by the Great War.
The mutated chipmunks appear identical to regular chipmunks, though particularly ill-tempered ones. They are hostile towards people. When a chipmunk spots someone, it will call out to its brethren. Any chipmunks in the vicinity that hear the call will congregate, coming together into a single mass to devour anything in there path. After eating, they disperse and return to their home.
↑Now there were two. Then a third joined them. All on their little haunches, all chirping away at me. The Furrball Trio. It was getting a bit loud and not particularly pleasant to listen to. And they looked angry. Well, as angry as a puff of fur can muster. The Armageddon Rag, Vol. 4, "J. Hardy and the Death Blankey"