This article is part of a series that explores life along the Hu Line, an imaginary diagonal line across China that has vast demographic, environmental, and political significance.

YUNNAN, Southwest China — The humid air hung in the valley like a harbinger of impending rain. Zao Zhengwang stood in front of his house, quietly listening to the gibbons calling from the nearby mountain.

Nearly every morning when the weather is clear, Zao and the other villagers can hear the apes’ characteristic “Woohoo, woohoo” call, like a greeting from an old neighbor. Sometimes, when the locals forage in the mountains, they can even see the gibbons hanging from the treetops in the distance.

Zao remembers once saving a baby gibbon after poachers had trapped it. Hoping it would join its fellow wild gibbons, he brought it to the mountains and returned each day to feed it fruit. For months, the young gibbon came running at the sound of Zao’s voice — but one day, it was gone. Zao believes it was once again trapped by a poacher or killed by another animal. “It’s like losing your own child,” he says.

Residents of Sudian, a Lisu ethnic minority village on the border between China and Myanmar, call these gibbons jiami. Experts had been studying them for 10 years and used to believe that they belonged to the eastern hoolock gibbon species found in the border zone where China, India, and Myanmar meet. But in January, scientists announced that molecular genetic analysis had revealed they were actually a new, as yet unidentified species that diverged from other apes about 500,000 years ago, close to 300,000 years before humans are believed to have migrated out of Africa. The new species is now called the Skywalker hoolock gibbon and is critically endangered.

Scientists believe there might be fewer than 200 of these gibbons living in the wild in China, while there are no estimates of the population in neighboring Myanmar. For comparison, the number of giant pandas left in the wild is estimated at more than 1,800.

Sudian is located near the southern end of the Hu Line, an imaginary line that divides China into two parts: a tightly packed eastern section where almost 94 percent of the population lives on just over a third of the country’s land, and an expansive, sparsely populated western portion where the remaining 6 percent of citizens live. Flora and fauna are coming under increasing threat along the line as the incursion of outside influences — urbanization, tourism, and even poachers — gains momentum.

This part of the country is where the Eurasian and Indian tectonic plates collided, giving rise to the Gaoligong Mountains — a treasure trove of biodiversity and, therefore, an ecologically critical part of the Hu Line.

Stretching from its northern origins in China down south to Myanmar, this long, narrow mountain range boasts dense tropical forests, canyons, snow-covered mountains, and enormous variations in elevation — resulting in a wide range of climates and habitats. More than 5,000 plants and 200 animal species have been documented here, and new species are identified almost every year.

Researchers believe that many species remain undiscovered, but field observation in the area is difficult, even with state-of-the-art equipment. Zoologist Fan Pengfei from Sun Yat-sen University led the team that discovered the new gibbon species and was the first Chinese researcher to “habituate” wild gibbons — a process that takes stamina and patience. “They are not going to sit there motionless for you to observe,” he says.

For 18 months, Fan quietly followed a group of western black crested gibbons in Yunnan’s Wuliang Mountains, hoping they would become accustomed to his presence. Then, on a rainy morning in 2005, he lost the gibbons he’d been observing. Disappointed, he sat on the ground and began to eat an apple.

When he turned his head, however, he found that the gibbons were watching him from a nearby tree. Before, the gibbons would scatter whenever they spotted him, but this time they were just resting and grooming each other, as if he wasn’t there at all. “I was just eating an apple, but I couldn’t hold back my tears,” he recalls.

The first time Fan was able to observe the gibbons up close, he stayed with them until evening. For a zoologist, this constituted huge progress. He later drew on this experience during research in the Gaoligong Mountains, where he managed to habituate three groups of gibbons — allowing him to identify the differences between them and the Skywalker hoolock gibbon.

Ethnic minority groups living in the Gaoligong Mountains, including the Lisu and Jingpo, have a long tradition of hunting gibbons for their alleged healing powers. Poaching is one of the main reasons that gibbon populations are low today, says Fan. Although the government confiscated hunting rifles in the area 10 years ago — and the hunting of protected animals has been illegal under the nation’s wildlife protection law since 1989 — he says he still encounters poachers during his fieldwork. The gibbons’ loud calls, used to mark their territory and find a mate, make it easy for poachers to track them.

In Sudian, however, villagers never hunted the gibbons, as they believe that the call of a gibbon after noon signals imminent misfortune — such as the death of a villager. “The elders in our family wouldn’t allow us to hunt them,” Zao says. Fan is certain this is the reason gibbons can still be found in the area. Zao used to hunt bears and wild boar but was recruited as a ranger by the local forestry bureau in the 1980s, tying him even closer to the apes.

Today, the biggest threat to the gibbons is the fragmentation and loss of their habitats. Although they can live at higher altitudes than any other ape species — with habitats as high as 2,700 meters above sea level — this may not be by choice. Research shows that they prefer tropical rainforests at lower elevations, where food is abundant. Many of these areas, however, have been developed into villages, roads, and farms, leaving the apes no choice but to head for higher ground.

Whenever he talks to students or the media, Fan mentions a well-known ancient poem, “Departing From Baidi in the Morning”: “From both banks, the steady sound of shrieking monkeys fills the air. Our little boat has already carried me past thousands of hilltops,” he recites. To Fan, the poem exemplifies how serene the area used to be and suggests that when the poem was penned, gibbons may have lived well beyond the borders of Yunnan province. Today, they’re restricted to three counties in Yunnan, and two gibbon species are believed to be extinct.

In the spring, following the announcement of the new Skywalker species, Fan took a group of volunteers to the mountains to track the gibbons’ calls and thereby estimate their population numbers. They are still collecting data, but Fan says that initial findings suggest the population is in further decline.

Two years ago, Fan and other conservationists founded Cloud Mountain Conservation, an organization that advocates for the protection of gibbons by educating locals, holding lectures in schools, and conducting further research on the apes.

The area has become like a second home to Fan, who points out that the Lisu minority’s traditional houses have no front doors — guests come and go freely.

“How many places like this still exist in China,” he asks, “where you can stand outside your door and hear the apes calling?”

Correction: A previous version of this story misstated the year zoologist Fan Pengfei lost the gibbons he had been observing in the Wuliang Mountains as 2015. It was 2005.

Translator: Clemens Ruben; editor: Denise Hruby.

Over the coming weeks, Sixth Tone will publish stories, videos, photo galleries, and social media posts that chronicle our road trip across China along the Hu Line, as well as an interactive multimedia platform in the fall.

(Header image: A Skywalker hoolock gibbon climbs a tree in the Gaoligong Mountains, Yunnan province, May 26, 2011. VCG)