"I don't believe any animal holds grudges because they lack rational thought. Lots of people humanize their pet's actions when really they're just being animals."

I completely disagree with this. I think the line between what some think is "human" behaviour and "animal' behaviour is the same. Just a different perpective. I absolutely see my boys having "rational" thought. If they do this it will result in that so do they chose to do this or not? Calvin knows that if he eats Smudge's food, he'll be scolded. But he always does. Until he hears my footseps and knows he's been nauhty. Scampers and hides. Then tries to find forgivness by being cute. That's rational.
As far as grudges, I'd come home after a business trip WITH a sitter mind you , and find the cabinets and draws in the kithchen open and counter tops cleared of everything!dishes, knife racks, open fridge doors ..you name it. It was pretty clear they made their sentiments clear. NEVER happend when I was home.