Ready for your close-up, Nénette?

France's newest female movie star is having a bad hair day. She is lying on her back, quite naked, playing with her toes. Her long red hair is covered with scraps of straw. She stares primly at her visitors and then, with a morose, bored expression – worthy of a Hollywood celeb – looks away.

Meet Nénette, 40, the oldest inhabitant of the oldest zoo in the world. The female orang-utan, long a favourite of generations of Parisian schoolchildren, plays the undisputed leading role in a new film by France's most successful documentary-maker, Nicolas Philibert.

In 2002, Mr Philibert charmed audiences all over the world with Etre et Avoir, a laconic, lyrical movie about a single-class primary school in a mountain village in the Auvergne. His latest film, Nénette, follows, for almost all of its 70 minutes, the facial expressions, and slothful movements, of an ageing orang-utan in her glass-fronted cage at the menagerie in the Jardin des Plantes in the centre of Paris. There are continuous takes of the animal's face which last for five minutes or more.

Nénette's human visitors, and minders, are seen only as vague reflections in the glass of her cage. Her amusingly and disturbingly human expressions – boredom, curiosity, puzzlement, but mostly boredom – are accompanied by the dotty or admiring comments of the humans. "She is the same age as daddy". "She doesn't have much space but rents are expensive in Paris." "She looks sad," says an elderly female voice. "Maybe she has also lost her husband."

By the end of the movie, the glass-front of the cage has become both a barrier and a mirror. Is Nénette really one of us? Or are we – the zoo visitors and the movie audience – projecting ourselves on to the unfathomable ape?

Mr Philibert, who has won critical acclaim and prizes all over the world for previous movies, is almost as inscrutable as Nénette. Unlike some documentary-makers, his style is not to bully the audience with tendentious editing. He leaves all options open. "I don't like to tell the audience what to think," he said. "I like just to discover what is before me."

The idea for the movie came when he visited the Jardin des Plantes, and its celebrated – and controversial – 200-year-old menagerie in 2008. He set out to make a 15-minute short film and became more and more engrossed with Nénette.

"The film is as much about ourselves as it is about orang-utans," he said. "Nénette is a mystery. We don't know what she thinks or if she thinks at all ... She is a receptacle for our fantasies. She is a projection screen ... The monkey house where she lives is almost like a confessional. When they talk about Nénette, people talk about themselves..."

During a brief visit by The Independent yesterday, Nénette, the unknowing film star, was in typical form. She spent most of her time slumped on a broad shelf, occasionally playing with her toes or her fur. Whether awake or apparently asleep, she always kept one hand on a thick rope, as if preparing for a rapid getaway.

Nénette was born in the jungles of Borneo in 1969 but has lived in the menagerie of the Jardin des Plantes, on the left bank of the Seine, not far from Notre Dame Cathedral, since 1972. Of the four orang-utans in the menagerie, including her son, Tubo, she is the only one to have been born in the wild. Since the death last year of the menagerie's celebrated, sex-mad 146-year-old giant turtle, Kiki, Nénette is the zoo's oldest inmate.

Kiki was popular with visitors because of his constant amorous and noisy assaults on the other turtles, despite his great age. Nénette is popular, despite doing not very much at all. She has a reputation for being cantankerous and – unlike the other orang-utans – takes little interest in the clothed apes on the other side of the plate glass.

The menagerie at the Jardin des Plantes (Botanical Gardens) is the oldest, largely unreconstructed zoo in the world. It was created in 1793, in the midst of the French Revolution, to receive the royal collection of animals from Versailles. In 1871, when the Prussian army besieged the French capital, the zoo animals were butchered for their meat.

Up to a few years ago, the menagerie was, frankly, a disgrace. Until 2004, generations of bears had lived for almost two centuries in two narrow, stone-lined holes, equipped with a dank pool, a dark den and a log. Big cats, including lions and a Siberian tiger, lived in old-fashioned, cramped, black wrought-iron cages, built in the mid-19th century.

After years of complaints from the French league of animal rights, the larger animals have now been moved to more spacious quarters in the French countryside. The small zoo – still the target of complaints by French animals' rights groups – says that it has "reorientated" its activities from entertainment to the "preservation of endangered species". The cages for Nénette and the other orang-utans are relatively spacious. There is also an outdoor area which they can reach when the weather is warm enough.

Mr Philibert's film does not ignore the moral issues involved in keeping wild animals in captivity. A poetic, concluding voice-over by a French actor, Pierre Meunier, draws attention to the deep scratches torn in the cage walls by the orang-utans, "apparently in frustration at their captivity". Is Nénette's moroseness and boredom, Mr Meunier asks, natural to orang-utans or the product of her unnatural surroundings?

The film-maker Mr Philibert, while shunning simple conclusions as usual, says that he was surprised to find that even Nénette's keepers were critical of zoos. "Her principal carer told me that there was no more anti-zoo person in the world than himself ... It is a complicated issue but people like him have helped to push zoos from being mere collections of captive animals to helping to preserve threatened species," he said.

Orang-utans are a very threatened species. They are the only type of ape which is found exclusively in Asia. Their remaining habitat, constantly reduced by logging, is in the rainforests of Borneo and Sumatra. "We are exterminating them by cutting the forests to make room for intensive agriculture, especially to produce palm oil for creams, beauty products and food," said Mr Philibert. "Every time you buy a bar of chocolate from Nestlé you are contributing to the demise of the orang-utan."

So are we destroying our relatives? The film-maker does come close to taking sides in the human/ not-so-human debate in the last seconds of his film. We are abruptly shown an unusually animated Nénette at feeding time. She is handed a yoghurt in a small plastic carton which she eats sitting up using a white plastic spoon. Nénette is also given several closed bottles of tea. She unscrews them between finger and thumb with a delicacy which escapes some humans when opening bottles of beer.

Mr Philibert will be grateful that the human traits of the orang-utan do not extend to extreme litigiousness. After the worldwide success of his school documentary, Etre et Avoir, in 2002, the "star" of the movie, an apparently mild-mannered teacher, sued him – unsuccessfully – for a share of the profits. So did the parents of some of the children. Nénette, one presumes, is unlikely to do the same.

Under threat: 'Man of the Forest'

The name orang-utan means "man of the forest" in Malay. The animals' "human" characteristics include an ability to use tools, mostly to open fruit. Their hands are similar to humans', with four long fingers and an opposable thumb (they also have four long toes and an opposable big toe, meaning they can grasp things with both their hands and their feet). Orang-utans can grow to 5ft high.

Like all the great apes, the orang-utan is now threatened with extinction in the wild, but although it is not the rarest – that distinction belongs to the mountain gorillas of Central Africa, which number only about 700 – it may be the most threatened of all.

The threat comes from destruction of their forest habitat on the two big Asian islands where they now survive, Sumatra (part of Indonesia) and Borneo (which contains part of Indonesia, and also Sarawak and Sabah). The forest is disappearing on a vast scale because of logging, burning, wholesale conversion of forest to agricultural land and oil palm plantations, and fragmentation by roads.

The orang-utans of each island, formerly considered sub-species, are now regarded as species in their own right. The Sumatran orang-utan ( Pongo abelii) is the worst hit and considered as Critically Endangered in the Red List of the International Union for the Conservation of nature; the most recent estimate suggests that only about 7,300 animals are left, in the northern part of the island.

The Bornean orang-utan ( Pongo pygmaeu) is currently assessed as Endangered; the last population estimate, from 2003, was between 45,000 and 69,000 animals remaining, but since recent trends are steeply down in most places, it is thought that the current numbers are now below these figures.

Michael McCarthy, Environment Editor