PM listeners' sounds: Singing gibbons

Listener Tom Blomley says he heard this sound every day as dawn was breaking in a Sumatran rainforest in Indonesia where he was working in 2013. He writes: "Apparently the males 'sing' to the females in the hope that the females will sing back. You can hear the males urging on the female and I am sure you can hear the sound of appreciation and relief when the female replies with her song."