Koko, the western lowland gorilla that began to learn sign language in the San Francisco Zoo and eventually became a celebrity beloved by millions, passed away in her sleep Tuesday at the age of 46.

Instructor and trainer Dr. Penny Patterson began teaching Koko a modified version of human sign language at an early age. In time, Koko came to use more than 1,000 signs of what [f500link]Patterson called “gorilla sign language” and could understand about 2,000 words of spoken English.

“Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy,” the Gorilla Foundation said in a statement. “She was beloved and will be deeply missed.”

Through sign language, Koko was able to convey emotions such as empathy and grief. After reading picture books about kittens, Koko asked for a pet kitten of her own. When the kitten was later killed after being struck by a car, Koko “discussed” the death for several days afterward. She also learned to play the recorder, surprising scientists who thought the regulating breath was unique to humans.

Koko was featured on the cover of National Geographic magazine twice and made memorable appearances with other celebrities such as Robin Williams, Betty White, and Fred Rogers.

In a 1978 National Geographic article recounting some of her conversations with Koko, Patterson talked about the time a reporter asked Koko: “Are you an animal or a person?”

Koko, Patterson recalled, replied instantly: “Fine animal gorilla.”