[Vaevictis Asmadi]

lwcamp is right about the brain being a hungry organ. The theory about human evolution is that our ancestors couldn't have larger brains until they started eating significant amounts of meat. Meat has denser energy than plants and is also easier to digest, allowing us to have smaller guts and shorter intestines.

Well pure autotrophs shouldn't consume anything, right? Even an Earthly carnivorous plant will only consume what it needs for minerals it can't get in the soil. So it should only matter for mixed autotroph/heterotroph creatures like lichen, and then I think a big factor is what % of their carbon is primary production and what % is consumed.