HAIKOU, Oct. 28 (Xinhua) -- It first sounds like a whistle or birdsong, and within minutes the solo turns into a chorus with a melody reminiscent of the waltz "The Blue Danube."

Around 6 a.m. every morning, the ape songs ring to awaken the primeval rainforest in China's tropical island province of Hainan. For conservationists, these indicate the comeback of the hairy tenors and sopranos: Hainan gibbons.

Known as the world's rarest primates, Hainan gibbons are increasing in number thanks to an improved environment. Latest data from the province's forestry department suggests there are 33 gibbons living in five families, a threefold rise in 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.

These apes are famous for producing melodious whistle-like sounds to mark territorial boundaries and attract mates. Longtime researchers also use them to identify different families.

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

To save them 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 Kadoorie Farm and Botanic Garden (KFBG) 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.

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

With arms longer than legs, the gibbons can easily fetch fruits and swing swiftly among branches which allows them to live exclusively in trees. This arboreal lifestyle, however, means they are easy victims of forest damage caused either by human activity or nature.

In 2014, typhoon Rammasun caused massive landslides in the habitat of the gibbons. Chan said KFBG 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 on the ropes for some meters, trying to go 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 told Xinhua that the gibbon population is still too small and faces serious gender imbalances. 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.