WOODSIDE, Calif. (CBSDFW.COM/AP) — 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.

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

koko the gorilla Koko, The Gorilla Who Knew Sign Language, Dies At 46

Koko the gorilla and her lifelong teacher and friend, Dr. Penny Patterson. (credit: koko.org)

In a statement announcing Koko’s death, the foundation said, “Koko’s capacity for language and empathy has opened the minds and hearts of millions.”

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

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

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)