Koko, the gorilla who mastered sign language and showed the world what great apes can do, has died. She died Tuesday in her sleep at age 46. Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy," the [The Gorilla Foundation] said. The western lowland gorilla was born at the San Francisco Zoo in 1971 and began to learn sign language early in life. Researchers moved her to Stanford in 1974 and established The Gorilla Foundation, a non-profit organization that works to preserve and protect gorillas. Koko and The Gorilla Foundation later moved to the Santa Cruz Mountains. She liked to read and be read to. She purred at parts of books she particularly enjoyed. She was very maternal toward kittens, and has had several throughout her lifetime. Her "tenderness" showed people how loving a gorilla can be, the foundation said. Koko made famous friends like Fred Rogers, who appeared on TV as Mr. Rogers, and Robin Williams. She used her sign language skills to communicate with them. She was said to have understood some 2,000 words of spoken English, and could usually keep up with conversations. Koko appeared in several documentaries and twice on the cover of National Geographic. The first cover featured a photo she'd taken of herself in a mirror. The foundation will continue its work on conservation and preservation of gorillas with continued projects, including a sign language application featuring Koko.