They become excited when they come across rare species. When they caught sight of a critically endangered helmeted hornbill in Malaysia, one of the locals said that it was only the third time he had seen the bird over the past 15 years.

"People used to kill the bird for its casque, which is a precious ornamental material. Someone may think it's just a bird, but you will be touched when you know the story behind it," Zheng says.

"This kind of connection between human beings and animals is not uncommon during our travels."

Yang once raised a baby chimpanzee for three years before it was sent on to another zoo in 2003. It so happened that when he guided a group of tourists around Shanghai Zoo last year, the chimpanzee recognized him through glass－and greeted him by moving back and forth.

"If an animal was once raised by me and then sent away, I rarely see them again to avoid recalling old memories," Yang says.

"I was very happy and surprised that he still remembered me. When I left, he still followed me with his eyes."