She adores playing with kittens, gently plucking them up and cradling them in her arms. When she has toothache, she simply calls for the dentist — and she isn’t at all intimidated by celebrities, having ordered Hollywood actors Robin Williams and Leonardo DiCaprio to tickle her.

However, Koko is not a little child, but a 44-year-old western lowland gorilla.

She is the world-famous ‘talking gorilla’, reportedly able to communicate with humans by using 1,000 words of sign language and capable of understanding 2,000 words of spoken English.

This week, the sad shooting of a gorilla in a U.S. zoo after a four-year-old boy fell into his enclosure has prompted an impassioned debate about the value we attach to our closest genetic relatives. Cincinnati Zoo officials insist they had to kill 450lb Harambe, like Koko a rare western lowland, in case any harm came to the boy.

Graduate student Penny Paterson with a young Koko on her back not long after they met in 1971 in San Francisco

After forming an inseparable bond, Ms Patterson began to teach Koko sign language as a means of communication

Koko has spent her entire life being cared for by humans after she had to be seperated from her mother in San Francisco zoo to be treated for a life-threatening illness

For many, the tragedy boils down to ethical questions. How much is the life of a gorilla worth against that of a human? And should an animal, no matter how endangered, automatically be sacrificed if there is any threat to one of us?

A deeply moving new BBC documentary about Koko will surely make those who are adamant that Harambe had to die a little less strident in their certainty.

Koko’s extraordinary story challenges some of our most basic assumptions about the fundamental differences between humans and other animals.

The BBC was allowed to spend a month watching the gorilla, who lives a life far removed from a concrete zoo compound in an affluent town near San Francisco.

She is still watched over by the same two people — the devoted Penny Patterson and her university friend Ron Cohn — who have raised her for the past 44 years.

Koko has spent her entire life with humans and, looking at the archive footage, her behaviour is so familiar, it’s sometimes hard to believe this isn’t someone in a very realistic gorilla suit. Not only does she leaf through magazines, she even licks her fingers before turning the pages.

Born in San Francisco Zoo in 1971, she was very young when she had to be permanently separated from her mother so she could be treated for a life-threatening illness.

I wanna be like you: Koko uses 1,000 signs to communicate and can even leaf through picture books. When she does so she even licked her fingers to turn the pages

Koko is also thought to be able to understand 2,000 words of spoken English

Patterson, a psychology PhD student investigating apes’ capacity for sign language, persuaded the zoo to lend Koko to her for research. The clever little creature was a fast learner, soon able to combine signs to ask for things.

Patterson and Koko were spending so much time together, they started to bond as mother and daughter — albeit a ‘daughter’ who soon had the strength of ten men.

‘You just don’t expect a gorilla to be that way,’ says Patterson in the documentary. ‘Everybody thinks King Kong, stupid, aggressive. She was small and sweet and creative. So it was like raising a kid.’

Eugene Linden, an animal language expert, recalls his first encounter with a young Koko after he was introduced by the sign language symbol for ‘friend’. They were standing by a closed door and he recalls: ‘She took a really hard look at me and signed “please”, “friend”, “open”, “hurry”. I was stunned. It was unlike anything I’d ever seen.’

Koko has even been credited with an ability to pun — intentionally confusing the words for ‘work’ and ‘rock’ — and play practical jokes. She once tied Patterson’s shoelaces together, then signed ‘chase’.

Brand new footage and pictures showing how the pair formed their inseparable bond will be shown on a new BBC documentary

Koko sits in the driver's seat of a car next to her trainer Ms Partterson, after striking up an inseparable bond

When she was four, the zoo demanded that Koko, as one of a now critically endangered species, should breed. Patterson would be allowed to keep her only if she found a potential mate. A young male, Michael, was procured from Vienna and, after joining Koko in California, was also taught sign language.

Aged seven, Koko was on the cover of National Geographic magazine — she took the photo herself by pointing the camera at a mirror — after Patterson published research claiming that not only did she have a vocabulary of more than 300 signs but, more controversially, she used them to express deep and complex emotions.

Some experts were far from convinced. Herbert Terrace, a prominent behavioural scientist who had conducted a similar sign language experiment with a chimp called Nim, believed the apes weren’t consciously communicating but merely copying the prompts from their human teachers.

He dismissed Patterson — who never married or had children of her own — as an ‘overzealous mother who is very proud of her surrogate children and tends to project meanings onto those children that may not be apparent to another observer’.

His criticism was, and remains, a blow to the credibility of talking ape projects.

Undaunted, Patterson soldiered on, moving the two gorillas to a new home in Woodside, California.

Koko has a maternal side, as she often signs the word baby. She is pictured caring for a young kitten

Koko caring for another young kitten. The animal mastered thousands of signs to communicate

Today, she argues that Terrace’s work with Nim in the Seventies, the focus of a 2011 documentary called Project Nim, was fundamentally flawed because — unlike her research — it involved an ever-changing roster of human carers who never really gained Nim’s trust.

‘Apes want to talk to someone they like,’ she says. Her argument has been bolstered by recent research which revealed that gorillas in the wild use 100 gestures of their own to communicate with each other.

Helpless to look after herself after a lifetime in captivity, 300lb Koko still lives in the same mobile home she grew up in, watching proprietorially as a team of carers cook her food, wrap her birthday presents and put on her favourite DVDs.

Patterson lives a few miles away and sees her almost every day, though she can monitor her continually via remote cameras linked to her home computer.

She and Cohn are often the only visitors in her enclosure, but the curious Koko insisted on getting to know the BBC camera crew for the new documentary (Patterson says she liked their British accents).As the cameraman discovered when Koko unbuttoned his shirt, Koko has a nipple fixation (which her carers discreetly try to conceal by claiming she is making the sign for ‘people’ when she is actually shouting ‘nipple!’).

Might this have something to do with her pangs for motherhood? Koko may have become an international celebrity — visited by stars including Sting and Blue Velvet actress Isabella Rossellini — but the flip side of her very human life is that she has never been able to become a mother.

A more recent picture of the gorilla, showing off her sign language, and her trainer giving a webcast in Dulles Virginia

As substitutes, she has been given dolls and kittens to nurse. When a kitten she adopted was run over by a car six months later, the hunt for a replacement — or as Koko put it, ‘Cat’ ‘Gorilla’ ‘Visit’ ‘Koko’ ‘Love’ — made news headlines around the world and even inspired a children’s book.

But a remorseful Patterson, now 69, admits that every birthday for years, Koko has asked for a real baby. She never mated with Michael, who died in 2000, nor has she done so with another male gorilla, Ndume, with whom she now lives.

Has Koko simply become too humanised ever to dream of mating with a gorilla?

Those who know her best appear to suspect this may be the case.

‘She knows she’s a gorilla and she does like Ndume, but she likes people, too, so she’s like in both worlds,’ says Cohn.

Koko also struck up a friendship with the late actor Robin Williams, and Ms Patterson says the gorilla was upset when she told her about his death in 2014

Asked in the programme if she has any regrets, Patterson becomes emotional and mentions not giving Koko a baby. It wasn’t Koko’s choice but her own, by bringing up her charge in isolation from other gorillas, she says.

Interestingly, the zookeeper who raised 17-year-old gorilla Harambe from a baby in Cincinnati Zoo warned this week against us ascribing too many human qualities or feelings to gorillas.

‘They do some things that are human-like, but they aren’t human,’ said Jerry Stones. ‘They’re very intelligent, but we need to let them be gorillas.’

As the BBC documentary concedes, it may never be possible to know for sure how much Koko communicates with humans. But for some people concerned about gorillas, that isn’t the point.

With demands growing for great apes to be accorded rights to protect them from mistreatment, animal rights campaigners will argue that anything which makes us a more compassionate species is to be welcomed. And it’s impossible to watch Koko and not feel compassionate.