Does koko really understand sign language

Her caregiver, Francine "Penny" Patterson, reports that Koko is able to understand more than 1,000 signs of what Patterson calls "Gorilla Sign Language" (GSL).[2] 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. Reports state that Koko understands approximately 2,000 words of spoken English. 

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