Having spent almost 40 years at Rockhampton Zoo, Cassius was well known to many visitors as a gentle and inquisitive resident. Perhaps less well known is his distinct contributions to science. He demonstrated capacities we did not know about chimpanzees; he changed our understanding of what humans share and what sets us apart from even our closest animal relatives.

When I heard about Cassie and his brother Ockie in 2001, my collaborators and I at the University of Queensland started visiting the pair on more or less annual trips, testing their cognitive abilities and comparing them to those of other animals, and to young children. The dedicated zoo staff have been wonderfully supportive throughout, and it was fantastic to see the troop and the enclosure expand over the years. I was very sad to hear of his recent decline, and it was bittersweet to have been able to spend his last hours with him.

I recalled that after Ockie died in 2013, I had sat next to a bewildered Cassie observing the burial. He was such a curious, sensitive,........

© Psychology Today