To the psychologist these particular studies are worthy of thought because they show that monkeys and apes are capable of dealing with symbols. The psychological chasm between man and the rest of the animal world yawns widest at just this point. The outstanding example is language. Words are written or spoken symbols representing things or events. They are also something more important.
Words are the ground stuff, the indispensable raw material, of thought.
Try to do it sometime without using words, and you will discover that even silent thought depends heavily upon language. Now, as far as we have been able to determine, no other animal has a true language. To be sure, many species are capable of rudimentary communication, but not all communication necessitates symbolic language. For instance, the shrill trumpeting of one frightened elephant may send a whole herd stampeding in panic through the jungle. But this represents the contagious spread of a vague though powerful emotional state rather than the transfer of a specific thought or idea.