But making things harder, Call would sometimes shake only the empty cup, which made no noise. In this case, the apes would still pick the other one, thus operating on the basis of exclusion. From the absence of sound, they guessed where the grapes must be. Perhaps we are not impressed by this either, as we take such inferences for granted, but it is not all that obvious. Dogs, for example, flunk this task. Apes are special in that they seek logical connections based on how they believe the world works.
