The discovery of a new breeding pair raises hope for the survival of the world's rarest primate, the Hainan Gibbon.
Ravaged by deforestation and poaching, the ape now lives only in a patch of forest on China's Hainan island.
In the 1950s, there were an estimated 2,000 left in the world, but numbers fell to fewer than 10 in the 1970s.
The latest census shows numbers have tripled to more than 30 gibbons, living in five separate family groups.
The fragile recovery follows decades of work by the Hainan Gibbon Conservation Project, run by the Kadoorie Farm and Botanic Garden in Hong Kong.
"I can't imagine how sad it would be if Hainan rainforest lost this beautiful animal and its wonderful song," said Philip Lo, senior conservation officer.
Gibbons are known for their ability to swing gracefully through the forest canopy, making loud, haunting calls to mark their territory. Breeding adults sing duets at dawn to enhance bonding.
Villagers heard the calls of two gibbons living in a part of the forest away from the main population late last year.
A male and a female were later spotted, and heard singing together, showing they had formed a stable bond.
The gibbon forms family groups consisting of one male, two females and their young offspring. The discovery of a fifth breeding pair in a separate fragment of forest is seen as highly significant.
Philip Lo said the species remained the rarest primate in the world, but there is hope it could overcome the risk of extinction and recover steadily. He described the success as "a piece of good news that could cheer up other dedicated conservation colleagues".
Conservation efforts have been under way at Hainan Bawangling National Nature Reserve since 2003, when the first full census found only 13 individuals living in two groups.
A comprehensive rescue programme was put in place, including patrols and monitoring, research into the apes' ecology and behaviour, and the planting of thousands of trees to provide food and shelter.
Nearly 20 gibbon species exist throughout the world, from northeastern India to Borneo. Most are under threat from the destruction of forests, hunting and illegal trade.
Two species of gibbon have recently disappeared in China and all surviving Chinese species, including the Hainan Gibbon (Nomascus hainanus), are classified as Critically Endangered by the International Union for the Conservation of Nature.
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