This isn't so surprising, an adult Chimp has the same mental capacity as an eight year old human child, I remember in the 1960's an American documentary on TV made by a husband and wife team of zoologists who brought up a baby chimp called Washo in their home in the U.S. with their children, they dressed him in kids clothes and taught him and their children sign language so they could communicate with him, the Chimps intelligence was truly amazing and I've never forgotten it.