The gorilla's trainer was talking to a group of people with her back to the gorilla. You could sense him getting angry because she was ignoring him. I've never seen an animal make a fist as what appeared to be a sign of anger. Finally, he turned and sat down with his back to the group so they couldn't see his face. Since the gorilla is isolated (no other gorillas in the zoo), he normally looks very sad and bored, but this time, he looked very angry.