All gibbons sing at (or just before) sunrise. Their voices can carry for up to 2km (1 ¼ miles), so everyone knows where everyone else is in the forest.
Many gibbon couples sing morning duets – the only primates to do so. As well as defending their territory, this helps to strengthen their bond, and is hauntingly beautiful to human ears.
Each species of gibbon has its own distinct song, so there are twenty different gibbon songs to hear across Southeast Asia.