HAIKOU－Around 6 am every day, the ape "songs" ring through the rainforest on the tropical island province of Hainan.

For conservationists, the sounds made by the hairy tenors and sopranos may be an indication Hainan gibbons, the world's rarest primates, are increasing in number thanks to an improved environment.

The latest data from the province's forestry department suggest there are 33 gibbons living in at least five families, a threefold rise in the population from the 1970s.

The black-crested apes can only be found in the Bawangling National Nature Reserve on Hainan Island. They live in rainforest trees over 10 meters tall and rarely set foot on the ground, making captive breeding difficult.

The apes are famous for producing melodious whistle-like sounds to mark territorial boundaries and attract mates. Researchers also use the sounds to identify different families.

Numbering more than 2,000 in the 1950s, the species was pushed to the brink of extinction due to excessive logging, burning of forests for farming and poaching. In the late 1970s, Bawangling had fewer than 10 Hainan gibbons living in two families.

To save the apes from extinction, the local government established the Bawangling reserve in the 1980s and launched afforestation drives. Since 2005, Hainan's forestry department has planted more than 300,000 trees to provide food for the gibbons.

Forest authorities also teamed up with conservationist groups.

Li Fei and Bosco Chan from Hong Kong's Kadoorie Farm and Botanic Garden are researchers and volunteers dedicated to protecting threatened gibbons.

In 2005, the conservation organization and the reserve formed a team to track the gibbons and record their habits for research and protection purposes.

"Hainan gibbons play an important role in the rainforest food chain. They feed on juicy fruits, bird's eggs and insects and disperse the seeds through their feces and (disposing of) the leftovers," Li said.

With their arms longer than their legs, the gibbons can easily fetch fruit and swing swiftly between the branches, which allows them to live exclusively in trees.

However, this arboreal lifestyle means if the forest is damaged, either by human activity or nature, their population is at risk.

In 2014, Typhoon Rammasun caused massive landslides in the habitat of the gibbons. Chan said their researchers built a rope bridge, equipped with infrared cameras, to let the apes safely cross a 15-meter-wide gully caused by landslides.

Initially, the gibbons were suspicious of the structure. Some curious apes moved a few meters along the rope and went a little further the next day. Finally, 176 days after the bridge was built, the cameras recorded the first crossing. Researchers said the bridge is now frequently used by the gibbons to move around the area.

Experts said the gibbon population is still too small and faces serious gender imbalance. However, they are optimistic about the population doubling within 15 years, provided there is no major natural disaster and the protective measures are enhanced.

On Aug 29, inspectors from the Hainan Tropical Rainforest National Park administration spotted a female gibbon cuddling its baby in Dongbengling, Baisha Li autonomous county. Experts later confirmed that a new family had been formed and their habitats in the reserve were expanding.

Li's team is also excited about the recent discovery of two adult female gibbons who left their families and started living alone.

"An adult male is needed for the formation of a new family, which is likely to happen this year or next year. It will be the sixth family of the species," he said.

Xinhua