Seems like there's an environmental competent to weight gain as well. Wild animals have also been putting on weight. So have lab animals which have had the same controlled diets and activity levels for decades.
A Chemical Hunger opens up with a list of mysteries related to the obesity epidemic. I don’t endorse the list – for one, “wild animals are becoming obese” seems to have been pretty much made up (see the fourth point in this comment), and all evidence we have that “lab animals are becoming obese” is exactly one (1) unreplicated paper co-authored by a guy that has been involved in numerous controversies regarding his conflict of interest with the processed food and restaurant industries.[8]
