Do you have evidence for this? I ask you cite evidence the they could not extract enough amines and nitrogen containing compounds from plant matter alone. There is no evidence that nitrogen fixation occurs at levels capable of sustaining ruminants: http://journalofanimalscience.org/content/41/4/1161.full.pdf html So they have to be getting all their nitrogen from compounds in the plants, not gut flora, which at best process, convert and release these compounds from the plant matter. No please cite evidence of the amount of daily protein uptake the brain requires. This is circumstantial, it is not evidence that brains require a lot of protein for simple maintenance. Again proof of nothing only correlation.