The population of milu deer has seen a revival at the Dafeng Milu National Nature Reserve in Jiangsu province.[Photo by Zhou Gukai/For China Daily]

"I will never forget the scene," Ding recalled. "I was so relieved that all my efforts had paid off."

But he hesitated and stepped back.

Milu were kept in captivity for centuries. They should be free to roam, he thought.

From then on, Ding gave up fencing and began observing the milu through binoculars in a natural habitat. He always took a notebook and a flashlight, recording details of their feeding and other activities.

Keeping his distance resulted in a lot of discoveries. For example, milu prefer to stand against the wind to keep out the cold; they can eat about 100 kinds of plants, both land and aquatic; and their short cries signal fear.

His discoveries enabled the release of more captivebred milu deer into the wilderness. Soon, the first wild milu was born in the Dafeng reserve. On that day, Ding celebrated with a few cups of wine to welcome a "new family member".

Ding's real family often complained about his obsession.

"For the past 30 years, he has spent more time there than in his home," his wife, Cao Caiping, said.

His biggest regret is missing his father's funeral because of his work.

In 2014, Ding retired, but he continued writing essays and books, and sharing his experience with younger generations. His daughter studied milu conservation for her doctorate.

Ding said he is deeply grateful that people have joined him to care for the species. The team of milu keepers has grown from nine in 1986 to 80 this year, including a dozen with university degrees.

Plus, millions of students have learned about milu deer through their textbooks.

After the death of the buck, Ding and a colleague wrote a poem:

The beautiful land, your ancestors' home, Lives do not end, left for a long time, Fun in the wild, unforgettable days, My dear friend, rest in peace.