'The orang-utan has such good visual memory,' says Dr Hanna Wirman, the developer of this computer game made specifically for orang-utans. They often have visual memories several times greater than humans' due to the intricate paths they have to carve out and remember in a forest of similar-looking trees.

And they will win even if the images are turned 180 degrees, because orang-utans are often upside down. 'They are sensitive at recognising images no matter how they are rotated,' Wirman said.

She created the game to highlight an area where orang-utans could beat humans: a kind of cross-species game play that, if implemented in zoos and wildlife rescue centres, could show visitors just how smart orang-utans - which share about 97 per cent of our DNA - are.

Wirman, who has a PhD in cultural studies from the University of the West of England in Bristol, has always been interested in games from the anthropological perspective. 'Play is the least controversial topic,' she said, swivelling in her chair at Polytechnic University's M-Lab, a digital entertainment laboratory, where she has been developing the orang-utan games for the past year under a project called Technology to Orang-utans for Understanding and Communicating cross-species for greater harmony (Touch).

Like humans, orang-utans need to play, which is why Touch evolved to provide stimulation for orang-utans in captivity. The PolyU group, along with local non-profit organisation Masarang Hong Kong, has provided the Hong Kong Zoological and Botanical Gardens with touch-screen games technology.

So far, the gardens have not taken it up, though they have been in touch about the enrichment technology since May 2010.

Wirman used to study women computer-game players or other players outside the mainstream, such as the elderly, to see how and if they played differently. Now she has moved on to designing games for non-human animals. ('It's a culturally learned thing to just say 'animal', but we're animals, too,' she said.)

'All mammals play,' Wirman said. 'Animals play in nature only when they're fully fed and all right. In captivity, they're mostly all right - so they need enrichment. They have their whole lives to play.'

As early as 1925, Robert Yerkes, a prominent American primatologist and psychologist, wrote: 'The greatest possibility for improvement in our provision for captive primates lies with the invention and installation of apparatus, which can be used for play or work.' This introduced the concept of environmental enrichment in captivity, at least in academia, for the first time.

Wirman says that in the decades since, most games with primates were organised by natural scientists or psychologists to study cognitive functions. But in the past few years, in the United States especially, zoos from Atlanta (whose orang-utans have been playing games on touch-screen computers since 2008) to Milwaukee (with three orang-utans playing iPad apps starting last year) have been charging ahead with technological enrichment. It can provide different kinds of games with no need for physical infrastructure, unlike physical games such as obstacle courses.

The Touch project strives for enrichment - meaning orang-utan happiness - as its goal, but is one of the first that actually allows humans to join in the game.

Two weeks ago, Wirman spent nine days in North Sulawesi, Indonesia installing the program for a pair of teenage male orang-utan twins, Is and Bento, at the Tasikoki Wildlife Rescue and Education Centre.

'[They were] so excited about the screens we brought them,' she said.

The games right now are simple, like the picture-matching memory game, and will remain so until she finds out more about what the orang-utans like or do not like. 'In any game design, there's no point to continue developing very complicated games if there's no user testing,' she said.

Wirman learned that orang-utans did not just touch the screens with their fingers but with their lips as well; they don't have the human preference for using hands.

In the future, games using motion-sensing technology (think Wii or Microsoft's Kinect) that involve full-body movement could be even closer to simulating what orang-utans do in nature.

Orang-utans, like humans, also have various personalities, so what works for Bento may not work for Is.

Dr Willie Smits, founder of Indonesia's Borneo Orangutan Survival Foundation, and a close collaborator with Wirman's project, has loftier goals for the technology as well. He wants orang-utans to be able to choose food on screen not only for themselves, but also to be able to press a button to virtually give food to another orang-utan, perhaps one they are interested in. 'Zoos wonder why in 50 per cent of cases they don't get offspring, as if matching orang-utans is one plus one,' Smits said. 'They have to fall in love.'

He also wants to use satellite techniques so orang-utans in rescue centres or zoos can connect and learn from their counterparts in the wild, who would be filmed in their natural habitat. He wants to provide technological stimulation for orang-utans all over the world.

Touch operates using PolyU's internal funding. It is looking for industry funding starting at HK$1 million per year for infrastructure and a small staff of researchers and programmers.

Wirman wants to develop this technology with other non-human animals as well. 'Enrichment shouldn't be on the basis of if someone is intelligent or not if they suffer in captivity,' she said.

Others are already doing this.

Pig Chase - an iPad game being developed at Wageningen University in the Netherlands - allows humans to virtually move a ball of light on the Apple tablet to a corresponding wall in a pig pen. If the pig touches the moving ball of light, virtual sparks fly and humans see a pig snout on their screen.

And Friskies, a cat food brand, has made free iPad games for cats that simulate mice on the screen: 'The bare glass screen on the iPad stands up to our cat's claws with no problems,' says Friskies' website.

Meanwhile, Wirman is still in touch with the two male orang-utans from Indonesia - through Skype.

They look at her on the screen. It's OK if one of them seems to be doing what looks like a human frown, lips turned down. An orang-utan has a long jaw. When his lips are curling far down, he is actually smiling.