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.

READ MORE: 'I'm Worried I'm Going To Kill My Parents' Man Told 911, Pointed Fake Gun At Fort Worth Police Before Being Killed

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 and her lifelong teacher and friend, Dr. Penny Patterson. (credit:

READ MORE: Dallas City Council Unanimously Approves Major Ethics Reforms

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.

MORE NEWS: Harris County Jail, Texas' Largest, Under Renewed Scrutiny After Report Of Sexual Assault Of Sergeant

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