To him, the day he first took care of the animals was significant. Guo said, "It was January 21st, my birthday. From that day I started my years with monkeys."

The first monkeys they looked after were golden monkeys. Later, he also spent seven years taking care of a Chimpanzee.

One day, when he was staring at his buddies in the cages, he suddenly realized that he was only a warden of an animal's prison. He started to write articles and make speeches to call on people to pay more attention to animal and environmental protection.

In 1998, Guo Geng joined David's Deer Park in Beijing as an animal expert and volunteer. It is the new home of milu, which went extinct in China a century ago.

Guo said, "This species of deer was first made known to Western science in the 19th century, by Father Armand David, a French missionary working in China. At the time, the only surviving herd was in a preserve belonging to the Chinese emperor. After Father David publicized their existence, a few animals were illegally transported to European countries for exhibition purposes, and bred there. When the last deer died in China, only 18 milu survived in England.

Artificial insemination technology had not been used until the year 2007.

Guo Geng built a graveyard for extinct animals within this park, with 145 species of animal's tombstones topple unto each other like dominos. Facing the 100-meter-long graveyard, Guo is worried.

Guo said, "Hundreds of species have gone extinct on the Earth, and soon human beings will meet their dooms day. I designed the graveyard to repent to the animals. I also wrote an epitaph and inscribed on a tablet on April 22nd, 1999, which is Earth Day."

In 2000, Guo Geng's devotion received international recognition and he won the "Earth Award".

More and more animal species are settling down in the park, and more and more people come and listen to Guo Geng's speech and read his books. This progress has been exciting to Guo and his fellow volunteers, and it encourages them to keep going on.