For most of her life, a western lowland gorilla named Koko was among the best-known non-human animals on the planet. She appeared twice on the cover of National Geographic. She sat with Robin Williams and taught Mister Rogers a sign for love. And she was said to speak, after a fashion, in a modified sign language that let her tell the people around her when she was happy, hungry, or sad.

Over nearly five decades, Patterson and the Gorilla Foundation she co-founded advanced a single claim: that Koko had learned more than 1,000 signs and understood roughly 2,000 words of spoken English. Koko was born at the San Francisco Zoo on 4 July 1971 and died in June 2018 at the foundation’s preserve in Woodside, California. Patterson, an animal psychologist, began working with her in 1972, when the gorilla was about a year old, adapting American Sign Language into what she called Gorilla Sign Language.

One story did more than any other to fix Koko in the public mind, and it was about a kitten. She was given a grey Manx she named All Ball. When the cat was hit by a car and killed in the mid-1980s, Patterson reported that Koko grieved, producing signs her caretakers read as sad, bad, and cry, and making a low hooting sound. A children’s book, Koko’s Kitten, followed in 1985 and stayed in print for decades.

It is an account almost designed to be believed, because it flatters something we badly want to be true. The harder question, and the one that occupied linguists and animal-cognition researchers for years, is whether Koko was using language at all, or whether the people watching her were supplying the meaning themselves.

What the record actually shows

Patterson published a handful of peer-reviewed studies on Koko in the late 1970s, and her 1979 Stanford dissertation documented that the gorilla could produce a set of signs adapted from ASL. That part is not seriously disputed. Koko clearly learned to associate hand shapes with objects and actions, and she plainly grasped some nouns, verbs, and adjectives.

Everything after that is where the disputes begin. From the late 1970s onward the project had no university or government affiliation, and the large later vocabulary attributed to Koko was recorded largely inside the foundation, not set out in independent, replicable publications. The figure of 1,000 signs is the foundation’s own count. The scientific consensus that formed over the following decades is narrower than the headlines: Koko used many signs and understood a good deal, but she did not demonstrate the grammar or syntax that linguists treat as the defining feature of human language.

That distinction is easy to lose. A gorilla stringing together sad, bad, and cry looks, on the page, like a sentence.

Whether it functions as one is a separate matter.

The Clever Hans problem

Behind the most persistent objection sits a name and a horse. In early twentieth-century Berlin, a horse called Clever Hans appeared to do arithmetic by tapping out answers with his hoof. He was in fact reading tiny, unconscious changes in the posture and expression of the humans watching, stopping the moment they relaxed. The animal was genuinely clever, just not in the way everyone thought.

Herbert Terrace, a Columbia psychologist, put that worry at the centre of the ape-language debate. He had run his own project with a chimpanzee pointedly named Nim Chimpsky, and when he reviewed the videotape frame by frame he concluded that the apes were largely being prompted: a handler would form a sign, and the animal would echo it moments later. His 1979 results landed as a shock, and in December 1982 he set out the case against the gorilla project directly, in an essay for the magazine The Sciences titled “Why Koko Can’t Talk”.

Others went further. The semiotician Thomas Sebeok sorted the whole field into fraud, self-deception, and the careful work of Terrace. Susan Goldin-Meadow, a psychologist, later noted that in studies of the bonobo Kanzi, who communicated by pointing to symbols on a keyboard, only a small fraction of that use was commentary; the overwhelming majority was functional, a way of asking for food or toys rather than exchanging ideas.

Underneath all of this sits the interpretation problem. When Koko was said to have coined finger-bracelet for a ring she had no word for, that reading depends on a human deciding the combination was inventive and not accidental. When she signed sad, there was no independent way to confirm she meant sadness. The person doing the translating was also the person most invested in the result.

Reading grief into the signs

Grief is where the stories become most affecting and least testable. Patterson reported that Koko fell silent for days after All Ball’s death, and that years later, having overheard that Robin Williams had died, the gorilla grew withdrawn. There is a widely repeated account of Koko, shown a skeleton and asked where animals go when they die, signing something read as a comfortable hole.

Those observed behaviours are not in doubt. Gorillas, like elephants and other primates, do respond to the death and absence of those they are bonded to, and there is a serious body of field research on how they behave around the dead. What is in question is the leap from an animal’s distress to a coherent account of mourning delivered in signs. The first is well supported. The second rests almost entirely on how a devoted caretaker chose to narrate an ambiguous moment.

Why the story survives the doubts

Anyone who has lived with an animal knows the pull. A 2014 Slate piece on the ape-language research made the point plainly: we cannot help reading intelligence and feeling into the creatures we know best, and they, in turn, learn to watch us closely for the cues that get them what they want. Koko sat at the exact meeting point of those two habits.

Scientifically, the fallout was severe. After Terrace’s results and a combative 1980 conference that borrowed the Clever Hans name, funding for great-ape language work largely dried up, and the field never really recovered. The parrot researcher Irene Pepperberg took the lesson early, describing her work with the parrot Alex as vocal communication rather than language, precisely to avoid the trap Koko’s supporters had fallen into.

What was left was not proof that a gorilla could talk. Even before the project ended, the argument was being made that its value lay elsewhere. Writing in Science in 2010, the journalist Jon Cohen suggested that what Koko offered was less a case for language than a closer view of an intelligence quite unlike our own. The journal’s obituary, published after her death in 2018, credited her with changing how the human world thought about animal emotion and intelligence, a genuine legacy and a more modest one than the books had promised.

What the argument is really about

Koko turns out to be less a story about what a gorilla could say than about what people were prepared to hear.

The gorilla was real, and so were the signs. Her intelligence, in its own register, was real too. What none of it settled was the translation.

Uncertainty was never mainly about her. It was about the gap between an animal producing a sign and a human deciding what that sign meant, a gap that only widens the more we love the animal on the other side of it. Part of why the debate never resolved is that language itself has no firm border, so no single measurement was ever going to settle it.

Half a century on, the most honest thing to say is that we do not know how much of the conversation with Koko was hers, and how much was ours. She spent her life being asked to prove something about us. She was not in a position to tell us how much of it she meant.