Koko, the beloved gorilla who learned to communicate in sign language has died in the US at the age of 46.

The Western lowland gorilla died in her sleep at the Gorilla Foundation’s preserve in California’s Santa Cruz mountains on Tuesday, the organisation said.

Koko was born at the San Francisco Zoo and Dr Francine Patterson began teaching the gorilla sign language that became part of a Stanford University project in 1974.

The gorilla, who was said to have an IQ of between 75 and 95, could understand 2000 words of spoken English and was able to communicate in more than 1000 signs.

Koko featured in many TV documentaries and in National Geographic twice – first in 1978 when a photo she took of herself made the magazine’s cover.

ON news of Koko’s death, National Geographic republished that cover story, written by Dr 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.”

Koko the gorilla, who appeared on our cover, could chat, tease, and even argue with scientists using sign language. She has died at the age of 46. pic.twitter.com/JX9vlFzpiI

— National Geographic (@NatGeo) June 21, 2018

In the story, Dr Patterson spoke of Koko’s ability to understand and respond to human emotion, and even argue.

The foundation said Koko’s capacity for language and empathy opened the minds and hearts of millions.

“Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication,” the Gorilla Foundation said in a statement.

“She was beloved and will be deeply missed.”

Footage of Koko on her 44th birthday went viral on YouTube when she was introduced to a litter kittens, where she picked out two babies, Ms Grey and Ms black, to join her family.

The video, which was published on October 13, 2015 has received more than 11 million views.

A Gorilla Foundation spokesman said it will honour Koko’s legacy with a sign language application for the benefit of gorillas and children, as well as other projects.

-with AAP