Koko the Gorilla: She Learned More Than 1,000 Signs. She Also Asked for a Kitten.

In 1972, a one-year-old lowland gorilla named Hanabi-Ko was born at the San Francisco Zoo. She would become better known by the name her trainer gave her: Koko. Over the next 46 years, in a research facility in Woodside, California, Koko would be taught American Sign Language by a Stanford graduate student named Francine Patterson, and by the time she died in 2018, she had a working vocabulary of more than 1,000 signs and could understand approximately 2,000 words of spoken English. She used language to express preferences, emotions, and needs no scientist had previously measured in a non-human primate.

In 1983, Koko signed the word for cat. She wanted a pet. The researchers gave her a kitten. She named him All Ball.

What Koko Could Do With Language

Koko's language use was the subject of ongoing scientific study and significant debate throughout her life. What was documented: she used signs to request specific objects, express displeasure, describe pain, and make jokes. She signed combinations of words to describe things she had no direct sign for, calling a ring a "finger bracelet" and a mask an "eye hat." She used signs in new combinations to express novel concepts rather than retrieving memorized labels.

Her emotional range was documented in peer-reviewed journals. After All Ball was killed by a car in 1984, Koko signed combinations including "cry" and "sad" and stopped eating for several days. When shown photographs of mourning humans, she selected signs related to sadness without prompting. The researchers at the Gorilla Foundation, the organization Penny Patterson founded to support Koko's work, published findings showing her using past and future tense signs, indicating an understanding of time beyond the immediate present.

The Scientific Debate

Not all researchers accepted the Gorilla Foundation's interpretations of Koko's signing. Critics, including comparative psychologist Herbert Terrace who had run a similar project with the chimpanzee Nim Chimpsky, argued that great apes in language studies often imitate their trainers and respond to subtle cues rather than generating truly novel linguistic output. The debate was about where imitation ends and language begins, and it was never definitively resolved.

What was broadly accepted: Koko learned to use symbols to communicate wants and preferences. What remained contested: whether this constituted language in the way humans use it. The distinction matters scientifically, but the practical evidence of Koko's behavior, her grief over All Ball, her requests for more kittens over the years, her apparent humor, and her preferences for specific people, was difficult to dismiss regardless of how it was classified.

Koko's Later Years and Legacy

Koko lived at the Gorilla Foundation's Maui Animal Research and Conservation Center in California until her death on June 19, 2018, at age 46. She was given two subsequent kittens over the years, which she named Miss Black and Miss Grey, and was documented cradling them gently, a behavior she had apparently learned from watching human caretakers. Her final weight was approximately 280 pounds. She was given routine veterinary checkups, cognitive assessments, and daily enrichment until the end of her life.

The question Koko raised, whether the capacity for something like language exists across species rather than being uniquely human, was taken seriously enough by linguists, cognitive scientists, and philosophers that it shaped how animal cognition research was designed for decades after.

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Sources

• Smithsonian Magazine — Koko, the Gorilla Who Learned Sign Language, Dies at 46 — death announcement, vocabulary statistics, and her time at the Gorilla Foundation
• National Geographic — Koko, the Famous Gorilla Who Used Sign Language, Dies — account of her language abilities, kittens, and scientific significance
• Nature — Koko, the Gorilla Who Talked to Humans — scientific context on the debate over ape language and Koko's documented behavior
• The Gorilla Foundation — Koko's Story — full account from the organization that cared for her, including her sign vocabulary and grief response after All Ball's death

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