Chimpanzees are one of the zoo's prime attractions, where visitors pay S$4.50 (US$2.57) to be photographed with the animals.

Ammann, a member of the American Zoo and Aquarium Association, said getting the chimpanzees to pose with strangers was against their nature.

"The amount of discipline needed can only be achieved with punishment and fear," he told the newspaper.

The WSPA's campaigns director Jonathan Pearce said the chimpanzees were being commercially exploited.

"They serve only to give visitors the opportunity of a cheap souvenir and the zoo the chance to raise more money," he said.

The zoo denied the accusations and said no force was used to train the three young chimpanzees who pose with visitors daily and are kept separately in cages.

Executive director Cheng Wen Haur said the photography sessions were aimed at giving visitors the opportunity to establish close contact with the animals.

"We hope that these chimpanzees, which act as ambassadors for the wilderness, will make the visitors think animals are so lovely," he said.

"And when they find out that they are actually endangered, they might be inspired to do something," he said.