This video contains footage from 1982 and 1984, when Koko was 11 and 13 years old, respectively. Koko's responses to Penny's spoken prompts are highlighted. During Koko's sign language acquisition phase, Penny consistently combined speech with signing. This upbringing in a bilingual setting enabled Koko to naturally grasp spoken English alongside sign language. Koko's understanding of spoken English developed organically, resembling the language acquisition process of human children, without the need for formal instruction.

How much of what we say do you think other animals understand?