I am wondering how it is possible for some herbivores to reach such a great size when their diet consists of what appears to be nutrient-light grasses and leaves. Where does the fat of a hippo or elephant come from, for example? Many humans will point to water retention for weightiness instead of the much more likely chip retention, but I'm supposing that isn't so for our wild animals? What's going on?
I think you'll find that grasses and other plant matter contains sugars. In human physiology the liver is able to convert food of any other group (protein, carbohydrate) into fats for storage in the body. I guess the same thing goes for hippos.
I was thinking more about this and realised that plants are at the very bottom of the whole food chain. So somewhere up the food chain fat has to be synthesized from sugar otherwise it wouldn't exist. But more interesting is that the same has to happen with protein. An animal that eats some form of meat may get enough protein from its food but herbivores have to create their protein and again I think that is accomplished in the liver.