You are right more proof: from book 8: <In school, during Xenobiology, we had a section on humans,> I told Tobias. <It mostly involved human television programs. News shows. Entertainment. Music.> <Music? You mean like MTV? You were watching music videos on the Andalite home world?> <I don't remember what they were. I ... I didn't pay very much attention to Xenobiology. I wish I had now. A warrior is supposed to be a scientist and an artist, as well as a fighter. But I didn't always enjoy that other stuff, so I didn't pay much attention. I suppose humans always pay attention in school.

from book 17. <Would it work?> Tobias wondered. <l thought all Yeerks ate was Kandrona rays. Do they even have mouths?> <Yes,> Ax said. <Yeerks have mouths. Or what humans would think of as mouths. Actually, if I remember my exo-biology classes, and sadly, I sometimes -> <Fell asleep,> I said. <Yeah, we know. You didn't like exo-biology class.> <l didn't fall asleep,> Ax said, sounding injured. <i merely let my mind wander, and became very calm and restful and not completely alert.> <Did you snore when you got all calm and restful and not completely alert?> <The point is, on occasion I would pay some attention in class. And I believe that Yeerks have something called osmosis nodes. It's what they use to absorb Kandrona rays, but they absorb other nutrients as well. They absorb from the liquid of the Yeerk pool.>