The researchers found a new group of rare monkeys
The songs of one of the world's smallest and most endangered apes have been recorded for the first time in Vietnam.
Researchers say the recordings confirm that at least eight Eastern Black-crested Gibbons still survive in northern Vietnam.
The researchers, from the conservation group Fauna and Flora International (FFI), say there is an urgent need for more to be done to protect the apes.
They are one of five primate species in northern Vietnam that are on the brink of extinction, say conservationists.
The apes are also known as Hainan Gibbons because China's Hainan island is the only other place where they have already been found. Just 14 exist on the island.
Mating call
The FFI conservationists say their recordings were taken in Cao Bang province, on the border with China. They also interviewed village hunters and believe another six of the small apes are still alive in Bac Kan province.
However, the pressure from hunters is blamed for the apes effectively losing their voices - researchers have been unable to identify the Bac Kan group as they have stopped their signature early morning songs.
Males and females sing complex harmonies to mate and to mark territory.
The new discovery suggests there are now about 30 Eastern Black-crested Gibbons in the world.
But in a hopeful sign for the survival of primates in Vietnam, FFI says its researchers have also found a new group of the endangered Tonkin Snub-nosed Monkey, which is found in just one part of northern Vietnam.
FFI's Vietnam manager, Frank Momberg, says the country has a rich bio-diversity but its apes are at risk.
"If a primate species is lost, it will be from Vietnam," he says.
Mr Momberg says small apes have been overshadowed by previous campaigns to save the great apes, gorillas and orang-utans.
Losing interest
He blames a new shift to protecting landscapes and bio-diversity in general for the lack of attention paid by scientists and conservationists to the endangered smaller primates.
In Vietnam they are threatened by hunting and the wildlife trade, and by loss of habitat.
Some programmes to educate local villagers about the need to protect their wildlife are successful, but FFI says it is time for the rest of the world to help Vietnam.
Otherwise the song of the Eastern Black-crested Gibbon could be forever silent within five years.
