A Hainan gibbon, gracefully perched on a tree in its natural habitat within the National Park of Hainan Tropical Rainforest. [Photo by the management bureau of the National Park of Hainan Tropical Rainforest]

Forty-two — that number may not sound particularly disturbing, unless we're talking about an entire species. And that's how many Hainan gibbons remain on this planet.

All of them live here — a small patch of forest called Bawangling in the National Park of Hainan Tropical Rainforest in China's southernmost province.

Huang Lubiao and Liu Huiqin are among the forest rangers who monitor these primates.

"When we monitor the gibbons, there are no trails. Wherever they go, we follow," said Liu Huiqin, gibbon monitoring team ranger with the Bawangling Branch Hainan Tropical Rainforest National Park Administration.

With powerful swings, a gibbon can travel nearly a kilometer through the treetops in just a minute.

Huang Lubiao, gibbon monitoring team ranger with the Bawangling Branch Hainan Tropical Rainforest National Park Administration, said, "They're incredibly fast. One swing and they're gone."

So rangers rely almost entirely on their sound. Their calls echo through the forest, telling patrollers where to run.

"Look, gibbons."

"Hurry, hurry."

Forty-two individuals may not sound like a lot for an entire species, but the story behind the Hainan gibbons is actually a conservation miracle, the recovery of a population that once fell to single digits.

They once populated much of Hainan Island. But decades of logging and hunting nearly wiped them out. By the late 1970s, only an estimated seven to nine individuals remained.

Liu said, "When human disturbance damages the forest they depend on, gibbons are pushed into smaller and smaller spaces."

As the scale of the loss became clear, China began to change course. National policies now put ecological protection ahead of short-term economic gain.

In 2021, the country established its first generation of national parks, including the National Park of Hainan Tropical Rainforest.

The goal is to create large, connected conservation zones, not the small, fragmented ones of the past.

"We planted trees to reconnect forest patches so the gibbons can move freely," Huang added,

But conservation today extends beyond boots on the ground.

At Sun Yat-sen University in Guangzhou, southern China's Guangdong province, scientists are using gibbon sound as a powerful monitoring tool.

Ma Haigang, a post-PhD researcher at Sun Yat-sen University, said, "Let me play you the Hainan Gibbons."

Each gibbon has its unique vocal features, essentially a vocal fingerprint.

By analyzing more than 1,800 hours of recordings, researchers developed the world's first full acoustic-AI monitoring system to identify individual gibbons and track their movements over the long term.

Ma said, "It helps us estimate the population size and determine whether new family groups have formed, or if males have been replaced. It allows for more effective monitoring of the species."

The recovery of the Hainan gibbon is not guaranteed. But this forest — and the people protecting it — are giving the species a fighting chance.

"Seeing the gibbon population grow — that gives meaning to our work," Liu said.

China's National Parks Law went into effect at the start of this year, marking a shift to a unified, law-based management system.

And ultimately, the measures of those policies are here — in protecting real families struggling to survive. And against all odds, they do.