This may merely be a campfire story. I have no proof of its veracity. But I knew an old hunter/trapper who would shoot an American Red Squirrel on sight. He said that male red squirrels were known to go up a tree, catch a gray squirrel asleep in his nest, and bite through the gray squirrel’s testicles; effectively castrating him. The old guy swore it was true. FWIW.
I’ve heard similar stories, but thought it was vice-versa...the grays after the reds.