Is being fat natural or unatural?

If there is an exceptional amount of food around, then other animals gorge on it, like the animals you quoted. The fact is that we in the western world have the amount we can gorge on and the third world suffers. I do belive that people in the middle ages gorged when the could, but would burn the fat off when there was no food. Unfortunately we never have the chance to burn it off in our priviliged worlds!