This is kind of related, but...the other day I was talking to a friend who owns two horses. I asked her how horses (and cows) can eat nothing but stray, hay, grass, and grain and yet they can pack on enormous amounts of lean muscle, whereas humans have to eat mainly animal protein to pack on the mass. She said that horses are "ruminids"(sp?) and have special enzymes, etc. that enable them to convert grass to protein that we humans don't have. Interesting stuff.