I read all 14 volumes and I wonder. They said Titans eat humans because they want to kill them. I think instinctively Titans want to eat Titan Shifters in order to regain their humanity. The reason they ignored other life form is probably they instinctively know they don't posses Titan Powers but only Humans do. I mean it was proven that they do eat Titan Shifter. From the looks of it they can't tell which humans had Titan Powers or not.
Apparently Titans regurgitate human that don't posses it but I don't know what exactly happen to Ymir when she ate Marcel.
Did the Survey Corp theorize that yet?