Koko, a beloved gorilla who learned to communicate with humans and then stole their hearts, has died.

The Gorilla Foundation said the 46-year-old celebrity ape — a western lowland gorilla — died in her sleep earlier this week at the organization’s preserve in Northern California. The Gorilla Foundation, a nonprofit that works to study and protect great apes, said in a statement that Koko will be most remembered “as the primary ambassador for her endangered species.”

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

The gorilla was born at the San Francisco Zoo on Independence Day in 1971, according to the Gorilla Foundation, and named Hanabi-ko, which means “fireworks child” in Japanese, though she was mainly known by her nickname, Koko.

It was in San Francisco where the newborn gorilla met a budding psychologist, Francine “Penny” Patterson. By the next year, Patterson had started teaching the animal an adapted version of American Sign Language, which she dubbed “Gorilla Sign Language,” or GSL. Video footage from that time shows Patterson playing games with the young gorilla and trying to teach her a new way to communicate.

It grew into a decades-long relationship that revealed a deeper side of Koko and her apparent ability to understand — and the enchanting animal gained widespread attention because of it.

Koko was featured in National Geographic twice — the first time in 1978 when a photo she took of herself made the magazine’s cover. This week, National Geographic republished that cover story, written by Patterson, along with an editor’s note stating: “Current research paints a more complicated picture of primate sign language than was understood in the 1970s. We are presenting this article as originally published; the science within may not be up-to-date.”

In the story, Patterson wrote about Koko’s ability to understand and respond to human emotion — even argue, revealing her ornery side. Patterson described a moment between one of Patterson’s assistants, Cathy Ransom, and Koko in which the assistant pointed to a poster of the gorilla and asked her about it.

Cathy had asked Koko, “What’s this?”
“Gorilla,” signed Koko.
“Who gorilla?” asked Cathy.
“Bird,” responded a bratty Koko, and things went downhill from there.
“You bird?” asked Cathy.
“You,” countered Koko.
“Not me, you are bird,” rejoined Cathy, mindful that “bird” can be an insult in Koko’s lexicon.
“Me gorilla,” asserted Koko.
“Who bird?” asked Cathy.
“You nut,” replied Koko, resorting to another of her insults. (For Koko, “bird” and “nut” switch from descriptive to pejorative terms by changing the position in which the sign is made.)
“Why me nut?” asked Cathy.
“Nut, nut,” signed Koko.
“You nut, not me,” Cathy replied.
Finally Koko gave up. Plaintively she signed, “Damn me good,” and walked away signing, “Bad.”

Some experts, however, have questioned Patterson’s methods and Koko’s abilities. As Slate reported four years ago: “In the past few decades there has been a spirited debate about whether apes are using language in the same way humans do.”

In 1980, according to Slate, Herbert Terrace, a psychologist who worked with a chimpanzee in a language study at Columbia University, expressed skepticism about apes’ way with words.

Nim’s signing, and that of the other signing apes as well, appears to be motivated more by a desire to obtain some object, or to engage in some activity, than a desire to exchange information for its own sake. First the ape tries to obtain what it wants directly — without signing. When reminded by its teacher that it must sign, the ape often signs until the teacher complies with its request. The critical question is whether the ape is generating sentences or simply running on with its hands until it gets what it wants.
Careful scrutiny of the ape’s utterances favor the latter interpretation.

In any case, Koko was a unique ape that connected not only with humans but also with other animals.

She loved baby dolls. And kittens.

And she built relationships with Mister Rogers, Betty White and Robin Williams.

When Williams died in 2014, the Gorilla Foundation said Koko took it hard.

After Patterson told Koko that Williams had died, the foundation said, “Koko was quiet and looked very thoughtful.” Later, the organization said, “Koko became very somber, with her head bowed and her lip quivering.”

In its 2014 article on apes, Slate further questioned whether Koko really understood the tragedy, noting: “Was Koko really mourning Robin Williams? How much are we projecting ourselves onto her and what are we reading into her behaviors? Animals perceive the emotions of the humans around them, and the anecdotes in the release could easily be evidence that Koko was responding to the sadness she sensed in her human caregivers.”

Following Koko’s death this week, the Gorilla Foundation said she leaves a legacy.

“Koko’s capacity for language and empathy has opened the minds and hearts of millions,” the statement read. “She has been featured in multiple documentaries and appeared on the cover of National Geographic twice. The first cover, in October of 1978, featured a photograph Koko had taken of herself in a mirror. The second issue, in January of 1985, included the story of Koko and her kitten, All Ball. Following the article, the book Koko’s Kitten was published and continues to be used in elementary schools worldwide.

“Her impact has been profound and what she has taught us about the emotional capacity of gorillas and their cognitive abilities will continue to shape the world.”