They definitely eat something other than veggie cakes after taming. I know this because mine haven't starved. Trough has berries, raw meat, and spoiled meat in it so I'm not sure which is being eaten. Definitely worth the effort if you have trouble getting cp.
If you are considering breeding these, go slam your hand in a car door a few times. It will be much less painful. Literally one veggie cake per percent of growth....and it grows slow.. Digests roughly one point of food per second, so feed a cake about one for every 7 mins (during event boost) and it will gain one percent of growth.