Fixed stare, open mouth, head bobbing. From a 1974 children’s book by Michael Boorer, The Life of Monkeys and Apes. I’m a big fan of the the fellow on the right, the siamang. I wish I had special vocal sacs.
Threat II: Gorilla Threat. The stages are hooting, symbolic feeding, hurling vegetation, chest beating, running sideways and ground thumping. I think I used to work with this guy, actually.