ORANG-UTANS at Singapore Zoo just keep on breeding: there are now 23 of the species, making it the world's largest captive collection. The zoo started with just a few orang-utans, most of them previously kept as pets and confiscated by the authorities. They adapted to the spaciousness and comfort of their new home - and set about mating prolifically. Orang-utans, which originate in the jungles and forests of near-by Indonesia, were also taught to become more people-friendly by doing tricks and posing for photographs. The zoo claims to be the only place in the world where visitors can have breakfast with an orang-utan, or shake hands with one. Training animals in zoos and circuses has become less fashionable in recent years but Singapore zoo-keepers defend their training policy. They say the success of the breeding has meant the animals can be loaned and given to other zoos, and making them accessible to the public increases awareness of the need to save threatened species. ''We have a successful training programme which takes account of the fact that they are highly intelligent animals,'' said show co-ordinator Vinodh Ayathan. ''They can work things out easily; if you're not careful you can end up being trained by them. ''It's like being a teacher with kids. ''We are not trying to make fun of the animals but there are limitations as to what we can do. ''It is impossible to replicate wildlife. They enjoy what they do - it's like going to work in the wild for them; we do things that keep them occupied.'' The animals are rotated in a series of daily shows, which include having breakfast with tourists, performing various tricks and posing for photographs. If the adult animals became annoyed, they could easily maim or kill humans but, so far, there have been no incidents. ''We try to reduce their aggression level,'' Mr Ayathan said. ''They have tremendous power in the arms and feet. They could tear off someone's head and their bite could crush bones.'' The fact that none of the orang-utans has turned nasty suggests that the zoo is keeping them happy. When not working, they are allowed to roam free in a large forest area, designed to replicate the jungles of home. Each of the 23 orang-utans - including one animal which is pregnant - has a distinctly different personality. The keepers know each and every one by name and by nature. ''Some aren't scared of anything,'' Mr Ayathan said. ''Others are very, very, shy and some of them are very, very, flighty.''