The western lowland gorilla appeared in many documentaries showing her mastery of 1,000 words of sign language. On hearing of her death, fans took to social media to describe the impact she had on humanity.
Followers of Koko, the gorilla who captured the hearts of millions by mastering sign language, mourned her passing on Thursday at the California-based Gorilla Foundation where she lived.
Born Hanabi-ko (Japanese for "Fireworks Child") at San Francisco Zoo in 1973, Koko became one of the most famous research subjects into how apes use language.
She reputedly learned more than 1,000 words in American Sign Language, and became the subject of many books and television shows.
Icon for interspecies understanding
"Koko touched the lives of millions as an ambassador for all gorillas and an icon for interspecies communication and empathy," the foundation said in a statement on its website.
Her impact is "profound" and "what she has taught us about the emotional capacity of gorillas and their cognitive abilities will continue to shape the world," the statement went on.
Within a few hours, news of Koko's death had been shared more than 25,000 times on the foundation's Facebook page, where hundreds of fans sent their condolences.
"So intelligent and sweet," wrote Kim Donohue, from Washington state. "She really meant a lot to me but I am grateful she passed easily in her sleep."
Marion Burford, from Shropshire in the UK, wrote: "I watched several documentaries about her and thought what a wonderful example of love and compassion she was to the human race."
Another fan, Kimberley Morton, from the US state of Kentucky posted: "What a role she played in our humanity. She taught us that gorillas are gentle, compassionate and extremely intelligent."
A fan in Robin Williams
Among her many media appearances, perhaps most famous was her introduction to the late actor Robin Williams in 2001. The pair were filmed tickling each other, and in one shot, Koko playfully tried on Williams' glasses.
The star of Mrs. Doubtfire described their interaction as "mind-altering," while other actors including Oscar-winning Leonardo DiCaprio and Star Trek's William Shatner were similarly impressed upon meeting her.
Animal psychologist Penny Patterson began teaching sign language to Koko when the gorilla was just a year old. The project later moved to Stanford University.
Koko was among a handful of primates who could use sign language, along with Washoe, a female chimpanzee, and Chantek, a male orangutan. Her keepers said she understood some spoken English, too.
First online gorilla chat
In 1998, Koko took part in the first "interspecies" chat, relaying comments such as "I like drinks" via a human interpreter to tens of thousands of online participants.
In 2004, Koko used sign language to describe a pain in her mouth, and used a pain scale of 1 to 10 to show how badly it hurt, according to a story by the Associated Press.
Despite these efforts, some critics argued that Koko's signs were not spontaneous but prompted by Patterson, and questioned the extent to which apes use language in the same way as humans.
The foundation says it plans to publish a sign language mobile app featuring Koko for the benefit of gorillas and children.
Mourning rituals in the animal kingdom
Gorilla mums carrying around their dead babies, dogs that won't leave their owner's grave - it's heartbreaking to see how animals grieve when a beloved one dies. DW looks at how animals cope differently with their pain.
Image: picture-alliance/dpa/F. Gentsch
Inseparable
Gorilla mum Gana at Münster Zoo couldn't accept her baby Claudio's death. For days, she carried his dead body around and defended it against the zookeepers. This isn't unusual for great ape mothers who lose offspring, researchers say. Some moms won't even let go their baby's dead - and in the meantime, mummified - body for weeks.
Image: picture-alliance/dpa/F. Gentsch
Ocean burial
Orcas, dolphins and other sea mammals also carry around their dead offspring for a while - not an easy task underwater. Researchers observed dolphin moms trying to balance their dead baby's body on their beaks, and diving after it when it finally sank to the ground. When adult dolphins die, their companions guard the dead bodies for days as well.
Image: Public Domain
Death watch
Elephants are famous for their excellent memory - no wonder they mourn especially long and intensively for their dead. A group of elephants congegrate around a dead elephants body in a kind of wake. Even elephants from other groups come to say goodbye to the deceased. Elephants have also been seen visiting the bones of other dead elephants, and touching the skeletons with their trunks.
Image: picture alliance/WILDLIFE/M. Harvey
Grooming in consolation
Baboons show strong signs of stress when a close companion dies. The concentration of stress hormones in their blood rises, researchers have shown. To cope with the loss, baboons tend to seek out their friends. They dedicate themselves to grooming, which helps to lower their stress hormones.
Image: picture alliance/chromorange
Calling to say goodbye
When a crow dies, other crows summon members of their species, and together they gather around the carcass. They'll also typically stop eating for some time after a death. The effects of grief are especially evident in birds that spend their entire life with one partner - like geese or songbirds. Effects sometimes extend to the remaining partner stopping eating, and eventually dying itself.
Fish often remain unusually still after a fellow dies in the same aquarium. Researchers say this behavior is probably due to stress hormones released into the water by the dying fish. Few studies have looked into whether fish actually mourn their dead, but this seems conceivable - at least for fish that live in pairs, like the French angelfish.
There are also cases of animals grieving for members of other species. Take, for example, "Muschi" the cat and "Mäuschen" the Asiatic black bear. The pair became inseparable at Berlin Zoo - and when the bear died, the cat refused to leave her companion's enclosure. She stayed there, meowing mournfully.
Image: picture-alliance/dpa/A. Rüsche
Guarding at the grave
People grieve deeply when they lose their beloved dog. The same goes for dogs who lose their masters. The German shepherd "Capitan" for many years stood guard at the grave of his master at the Villa Carlos cemetery in Argentina.
