Nguyen Van Hoan is no longer an enemy of the wildlife that he hunted in his hometown in the north-central province of Quang Binh, but a friend.
He was arrested in 1985 for hunting wild boars, including one weighing around 700 kilograms that he killed.
A court sentenced him to four years in jail, and he got a tattoo on his arm saying "will atone for all this guilt."
The 48-year-old has more than atoned by being a guide for 10 years now to international wildlife research groups around the province's national park Phong Nha-Ke Bang, a UNESCO heritage site since 2003 for its biodiversity, unique beauty and geodiversity.
Speaking to Lao Dong (Labor) newspaper he said after his release in 2000, he was puzzled about what to do with his tattoo.
"If I kept hunting, my children wouldn't have been able to look people in the eye."
He spent some time taking care of a herd of boars that had lost one member to his gun until a neighbor gave him another option.
"That man was working at Phong Nha Ke Bang and he asked me to work for the park.
"The park was new at the time and needed a person familiar with jungle paths to accompany scientific groups for biodiversity surveys."
As a man who used to spend most of his time in the jungle, Hoan has been a favorite guide for foreigners despite his poor English.
He first worked with the park's officials.
After a two-year internship, he managed to recognize the sounds of different monkey species and their genders, the smell of shit of different turtle species, and know where people can stop to eat or sleep.
Then he was assigned to take biologists from Russia, the US, France, Japan, Singapore, Denmark, and Germany.
Once he took them to a mountain in neighboring Laos for 20 days to look for the female yellow-cheeked gibbon and the grey-shanked douc langur.
He took another group deep into valleys when they wanted to study amphibians.
Hoan said he won kudos from the experts when he caught snakes, including dangerous ones, for them.
It took him a wink of an eye to catch a viper for a Russian student who wanted it as a sample for her study.
To catch a coral snake requires a v-shaped stick to pin its head on the ground. But once, when he had no stick, he took off his trousers and got the snake to crawl into it.
He has caught hundreds of snakes, including some later declared as unknown to Western science.
Hoan's knowledge of animals' habits and routines has helped scientists spot them as well as set camera traps.
"For example, most monkeys take a nap like humans [and so are not active in the afternoon]," he said.
His knowledge and devotion has made him famous among researchers who insist on meeting "Mr. Hoan" every time they come to the park.
Hoan is always available when summoned. Sometimes the experts go right to his house.
They pay him US$80 a day, not to mention rewards like $50 for a snake.
He has also received a certificate of merit from Germany's Frankfurt Zoological Society.
But he said these are not as important as the chance to live among wildlife and work with people who love them.
"I don't remember all the trips, but I have been impressed by the attitudes of the researchers coming here."
He said they release all the animals he helps them catch after taking photos.
"They always make sure no animal is killed for food, not even a crab or a snail.
"If they have to eat something, they would only agree to eat bamboo shoots and bananas," he said.
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