each of the cage has scree windows that lets them see the other yukkuris eating their poop. So they're making that face because they can see the other yukkuris eating their poop but the other yukkuris are oblivious.
Ah. The dillema. Make the yukkuri smarter, so that they can figure out they've been eating the shit of others too, or keep them dumb enough to work in other stories?