Why Herbivores Will Never Rule the World

I heard an interesting evolutionary theory the other day: Herbivores are incapable of evolving to sentience. The idea is that a big brain is an advantage if you have to outsmart your food, otherwise it's a waste of energy (it takes a lot of calories to keep a brain operating, calories which could be better spent growing horns, or genitals, or children, or whatever. But, if not being smart means you don't eat, well, that changes things. You just don't have to be that clever to sneak up on a bush.