lol thats probably part of it. It certainly explains why the ALBINO rat is the one who is so careful. I give the girls most of their treats through the door, and stick my fingers through the bars to tickle their faces or bellies so often that for the most part they don't mistaken my finger for a snack.
I'm too wary for my fingers because of their eyesight, lol. Although little Dione is Ok, you can put your fingers in at her and she'll just test with her teeth gently and when she sees it's not food, she leaves you alone, hehe.