Look at it this way: FIRST ----- It's not a causal relationship, more energy is available -> brains start growing. There must be evolutionary pressure for br

Look at it this way: FIRST ----- It's not a causal relationship, more energy is available -> brains start growing. There must be evolutionary pressure for brain growth in the first place. Otherwise larger brains are selected against because it's wasteful compared to what the competition does, even if there is more energy. Only if there is such a pressure does the availability of more energy make a difference. Other species put energy surpluses into things like elaborate but useless physiological features (interpreted as signalling their health to potential female mates), such as large tails or feathers that are either useless or sometimes even actually hinder and have no other function (that we found). So just "more energy" does not point towards any specific development, such as brain size. What happens with it, if anything, depends on other factors. If a predator species is doing just fine finding food and mates, no need to grow the brain, no matter how much energy there is. SECOND ------ A huge difference between herbivores and predators is the amount of time they spent on getting food. Herbivores spend pretty much all their time eating. "Meat eaters" spend most of their time resting. The additional energy is used to delay the time between meals. (Herbivore) meat grows a lot slower than plants. Alternatively, there would have to be a lot less predators, and they would have to roam greater distances - which may not work out energy wise when you add it all up. It would be interesting to do some calculations, what happens to energy if you tweak those variables, i.e. a predator species eating significantly more and using the additional energy to sleep less, what are the limitations, what happens to the system, where are the constrains? Humans eat meat and we also spend most of our time doing something (other than eating). Of course, we managed to get more nutrients and more energy by cooking, putting part of our digestion outside our bodies, so it's not limited to "energy dense meat" vs. "grass", we are much more flexible that way.