Her instructor and caregiver, Francine Patterson, reported that Koko had an active vocabulary of more than 1,000 signs of what Patterson calls "Gorilla Sign Language" (GSL).
This puts Koko's vocabulary at the same level as a three-year-old human. In contrast to other experiments attempting to teach sign language to non-human primates, Patterson simultaneously exposed Koko to spoken English from an early age.
It was reported that Koko understood approximately 2,000 words of spoken English, in addition to the signs.
In 1978, Koko gained worldwide attention as she was pictured on the cover of National Geographic magazine. The cover picture was an image of Koko taking her own picture in the mirror. Koko was later featured on the cover of National Geographic in 1985 with a picture of her and her kitten, All Ball.