Koko, the gorilla who mastered sign language, has died.

The Gorilla Foundation says the 46-year-old western lowland gorilla died in its sleep at the foundation’s preserve in California’s Sana Cruz mountains on Tuesday.

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Koko was born at the San Francisco Zoo, and Dr. Francine Patterson began teaching the gorilla sign language that became part of a Stanford University project in 1974.

The foundation says Koko’s capacity for language and empathy opened the minds and hearts of millions.

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Koko appeared in many documentaries and twice in National Geographic. The gorilla’s 1978 cover featured a photo that the animal had taken of itself in a mirror.

The foundation says it will honour Koko’s legacy with a sign language application featuring Koko for the benefit of gorillas and children, as well as other projects.

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