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.
A type of leaf-eating langur that has an unusually long and bushy tail with white hips.
Only about 300 Delacour's langurs are alive today, and experts fear they could be completely extinct if the current rate of decline continues.
Hunting has forced the animal into a few extreme strongholds, where steep limestone cliffs grant a little protection from poachers.
The red-shanked douc is a species of Old World monkey native to Indochina which lives in the forests of Vietnam, southern Laos and possibly northeastern Cambodia.
[2] The saola is one of the world's rarest large mammals, a forest-dwelling bovine native to the Annamite Range in Vietnam and Laos.