I find most original fiction to be both boring and unimaginative, but this was neither. It was very amusing and held my attention until the end.
I did have a little trouble understanding exactly what was going on when Shredder encountered the difference. It was a little too much in the rodent world for me to easily visualize.
I also noticed that you started a large proportion of sentences with the word "she." This can be a quite unwieldy when read aloud and is not normally considered good prose. On the other hand, I have seen best-selling authors do this, so I wouldn't worry about it too much. :)
Anyway, I really did enjoy the story. You have renewed my hope that I might, occasionally, keep from clicking away completely disgusted when I'm in the original fiction section.